Post on 21-May-2018
NEW MATERIALS FOR OPTICAL SENSING OF EXPLOSIVES
COPOLYMERS CONTAINING 2-VINYL-4,6-DIAMINO-1,3,5-TRIAZINE
AND CO-CRYSTALS OF ELECTRON RICH AROMATIC MOLECULES AND
1,3-DINITROBENZENE
by
STEVEN KEITH MCNEIL
DAVID E. NIKLES, COMMITTEE CHAIR
MARTIN G. BAKKER CHRISTOPHER S. BRAZEL
SHANLIN PAN SHANE C. STREET
A DISSERTATION
Submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in the Department of Chemistry
in the Graduate School of The University of Alabama
TUSCALOOSA, ALABAMA
2013
Copyright Steven Keith McNeil 2013 ALL RIGHTS RESERVED
ii
ABSTRACT
This dissertation focuses on the development of electron rich polymers with an affinity
for nitroaromatics. Thin polymer films of the electron rich polymers could be applied in an
optical waveguide sensor to detect nitroaromatics by changes in the optical properties of the
polymer thin films. Charge transfer complexes between electron rich aromatic reagents and
electron deficient nitroaromatics were produced providing an understanding of the
intermolecular interactions between the electron donor and electron acceptor.
Electron rich copolymers were synthesized with 2-vinyl-4,6-diamino-1,3,5-triazine
(VDAT) using a published literature procedure. The polymerization procedure was extended to a
variety of electron rich monomers, resulting in the production of a number of electron rich
copolymers. Thin films of the copolymers were spin coated and their optical properties were
characterized by spectroscopic ellipsometry before and after exposure to a nitroaromatic vapor.
The exposure to the nitroaromatic vapor allowed the formation of complexes with the electron
rich copolymers and the nitroaromatic molecules, creating a change in the optical properties of
the polymer films. This refractive index change after exposure to a nitroaromatic demonstrated
the possibility of these films to be applied in an optical waveguide sensor for explosive detection.
Co-crystals were grown between electron rich donors and the electron deficient 1,3-
dinitrobenzene by the slow evaporation method. When the electron donor solution and electron
acceptor solution were combined in a crystallization dish, significant color changes were
observed. The interaction between the electron donor and electron acceptor were characterized
using analytical techniques.
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DEDICATION
To my wife, Jeanna, who gives me support, encouragement, and love.
To my father and mother, Steve and Pam, who give me guidance and encouragement.
To my sister, Megan, who cares and supports me unconditionally.
To my family and friends.
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LIST OF ABBREVIATIONS AND SYMBOLS cm/s
centimeters per second
km/s
kilometers per second
cal/g
calories per gram
TNT
2,4,6-trinitrotoluene
NG
nitroglycerine
RDX
tetranitro-triazacyclohexane
HMX
tetranitro-tetracyclooctane
Tetryl
tetranitro-N-methylamine
picric acid
2,4,6-trinitrophenol
NH4NO3
ammonium nitrate
O
oxygen
N
nitrogen
C
carbon
PbN6
lead azide
AgN3
silver azide
NaN3
sodium azide
DMNB
2,3-dimethyl-2,3-dinitrobutane
°C
degrees Celsius
v
o-MNT
2-nitrotoluene
EGDN
ethylene glycol dinitrate
PETN
pentaerythritol tetranitrate
NC
nitrocellulose
TATP
triacetone triperoxide
HMTD
hexamethylene triperoxide diamine
$
dollars
km2
square kilometers
AT
anti-tank landmine
AP
anti-personnel landmine
U.N.
United Nations
mm
millimeters
kg
kilograms
g
grams
Composition B
RDX + TNT
C-4
RDX based explosive
1,3-DNB
1,3-dinitrobenzene
2,4-DNT
2,4-dinitrotoluene
2,6-DNT
2-6-dinitrotoluene
2,4-DNB
2,4-dinitrobenzene
pg/mL
picograms per milliliter
2-ADNT
2-amino-4,6-dinitrotoluene
4-ADNT
4-amino-2,6-dinitrotoluene
vi
%
percent
°
degrees
GPR
Ground Penetrating Radar
GHz
gigahertz
IR
Infrared
EIT
Electrical Impedance Tomography
XBT
X-Ray Backscatter
keV
kilo electron volts
cm
centimeters
min
minutes
m2
square meters
LIDAR
Light Detection and Ranging System
MMDS
Microbial Mine Detection System
NQR
Nuclear Quadrupole Resonance
MHz
megahertz
AM
amplitude modulation
H
hydrogen
ppt
parts per trillion
kV
kilovolts
TNA
thermal neutron analysis
FNA
fast neutron analysis
PFNA
pulsed fast neutron analysis
PFTNA
pulsed fast thermal neutron analysis
vii
NRA
nuclear resonance absorption
Cl
chlorine
P
phosphorus
S
sulfur
Si
silicon
γ
gamma
n
thermal
MeV
mega electron volts
IMS
Ion Mobility Spectrometry
Ni
nickel
V/cm
volts per centimeter
MS
Mass Spectrometry
THz
terahertz
UV
ultraviolet
NIR
near-infrared
LIBS
Laser-Induced Breakdown Spectroscopy
ng
nanogram
pg
picogram
pH
negative log of hydrogen concentration in a solution
DNA
Deoxyribonucleic acid
sec.
seconds
mins.
minutes
hrs.
hours
viii
MZI
Mach-Zehnder interferometer
L
liter
π
pi
Δn
change in refractive index
ppm
parts per million
n
refractive index
L
path length
λ
wavelength
nm
nanometers
He
helium
Ne
neon
VDAT
2-vinyl-4,6-diamino-1,3,5-triazine
Co.
company
HPLC
High-performance liquid chromatography
ACS
American Chemical Society
M.W.
molecular weight
AIBN
2,2'-azobisisobutyronitrile
PVDAT
Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
g
grams
D.I. H2O
deionized water
mL
milliliter
mmols
millimols
Na2S2O8
sodium persulfate
ix
MeOH
methanol
PS-co-PVDAT
Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
DMSO
methyl sulfoxide
CaH2
calcium hydride
EtOH
ethanol
PMMA-co-PVDAT
Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
MMA
methyl methacrylate
PMMA
Poly(methyl methacrylate)
PS
Polystyrene
PMA-co-PVDAT
Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
PMA
Poly(methyl acrylate)
MA
methyl acrylate
P2VP-co-PVDAT
Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
2-VP
2-vinylpyridine
(w.t)
weight
NaCl
sodium chloride
P2VP
Poly(2-vinylpyridine)
PAM-co-PVDAT
Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
PAM
Poly(acrylamide)
PVK-co-PVDAT
Poly(N-vinylcarbazole)-co- Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
DMF
dimethylformamide
PVK
Poly(N-vinylcarbazole)
PS-co-PVK
Polystyrene-co-Poly(N-vinylcarbazole)
x
PMMA-co-PVK
Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole)
≈
approximate
PVI-co-PVDAT
Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
VI
1-vinylimidazole
PS-co-PVI
Polystyrene-co-Poly(N-vinylimidazole)
PMMA-co-PVI
Poly(methyl methacrylate)-co-Poly(N-vinylimidazole)
MDAT
2,4-diamino-6-methyl-1,3,5-triazine
2-NT
2-nitrotoluene
3-NT
3-nitrotoluene
PNT
4-nitrotoluene
NB
nitrobenzene
9-VC
9-vinylcarbazole
9-EC
9-ethylcarbazole
CBZ
carbazole
10-M
10-methylphenothiazine
PHZ
phenothiazine
FTIR
Fourier transform infrared spectroscopy
mg
milligrams
KBr
potassium bromide
1H NMR
proton nuclear magnetic resonance
13C NMR
carbon nuclear magnetic resonance
D1
relaxation delay
TD
time domain
xi
NS
number of scans
CDCl3
deuterated chloroform
DMSO-d6
deuterated methyl sulfoxide
Tg
glass transition temperature
DSC
Differential Scanning Calorimetry
TGA
Thermogravimetric analysis
°C min-1
degrees Celsius per minute
Td
decomposition temperature
SEC
size exclusion chromatography
THF
tetrahydrofuran
mg mL-1
milligram per milliliter
mL min-1
milliliter per minute
RI
refractive index
VASE
variable angle spectroscopic ellipsometry
ψ
psi
Δ
delta
SiO2
silicon dioxide
Å
angstrom
k
extinction coefficient
in.
inch
UV/Vis
ultraviolet/visible
cm-1
wave numbers
NH2
amine functional group
xii
Tm
melting endotherm
GPC
Gel Permeation Chromatography
δ
ppm
s
singlet
d
doublet
dd
double of doublets
td
triplet of doublets
m
multiplet
q
quartet
PS
Polystyrene
Tc
ceiling temperature
Mn
number average molecular weight
Mw
weight average molecular weight
Mz
Z-average molecular weight
PDI
polydispersity index
r
reactivity ratio
α
alpha
M-1cm-1
molar absorptivity
pm
picometers
M
molarity
vas
asymmetric vibration mode
vs
symmetric vibration mode
S.P.
splitting pattern
xiii
Int.
integration values
NO2
nitro functional group
Z
number of formula units
Ñ
complex index of refraction
j
√-1
c
speed of light
ν
velocity
φi
angle between the incidence light and the material
φt
angle of reflection
Ep
electric field vector parallel to the plane of incidence
Es
electric field vector perpendicular to the plane of incidence
rs
perpendicular wave Fresnel reflection coefficient
rp
parallel wave Fresnel reflection coefficient
ts
perpendicular wave Fresnel transmission coefficient
tp
parallel wave Fresnel transmission coefficient
β
film phase thickness
δ1
phase difference before the reflection
δ2
phase difference after the reflection
RP
parallel wave total reflection coefficient
RS
perpendicular wave total reflection coefficient
tan Ψ
ratio of the magnitudes of the total reflection coefficients
ρ
the complex ratio of the total reflection coefficients
MSE
mean square error
xiv
A,B,C
Cauchy parameters
b.p.
boiling point
MEK
methyl ethyl ketone
≤
less than or equal to
rpm
revolutions per minute
PVI
Poly(vinylimidazole)
PVI-co-PVA
Poly(vinylimidazole)-co-Poly(vinylaniline)
P4VP
Poly(4-vinylpyridine)
PTFE
Polytetrafluoroethylene
μm
micrometers
O.K.
Oklahoma
U.S.
United States
Comp. B
Composition B
ng/L
nanograms per liter
pg/L
picograms per liter
Temp.
temperature
VK
9-vinylcarbazole
%T
percent transmittance
d
distance traveled
p-wave
wave parallel to the plane of incidence
s-wave
wave perpendicular to the plane of incidence
λ
wavelength
Lit.
literature value
xv
ACKNOWLEDGEMENTS
First and foremost, I am appreciative of my research advisor, Dr. Nikles. Without your
help, patience, and guidance, I would have never developed into the scientist that I am today.
Through the years, you have taught me how to understand and manage research problems,
encouraging me to think outside of the box. Your guidance and advice has helped me reach this
milestone in my scientific career. It was an honor and a privilege to work with you on this
research project. Thank you for everything you have done for me.
I would also like to thank my Nikles's group colleagues, Dr. Jeremy Pritchett, Clifton,
Amanda, Lei, Greg, Dr. Medhat Farahat, Adam, Todd, and Dr. Jackie Nikles, for your support,
assistance, and friendship throughout my time here at the university. You all were always willing
to lend a helping hand whenever I needed anything, and it was a privilege to work with all of
you. You all were my scientist support net, helping me to collect or interpret data and were
helpful in discussing ideas and possible applications.
To the Nikles's undergraduates and high school students, Cameran, Margaret, Morgan,
Lindsey, Jesse, John, Ben, Kim, Hamilton, Kirsten, and Jerome, thank you for keeping the lab
interesting during the semesters. There was never a dull moment with all of you in the lab.
To my committee members and chemistry faculty, thank you for your guidance and
teaching. Because of you, my knowledge and experience in chemistry reached levels I did not
originally realize were achievable.
Lastly, thank you to the chemistry department and the University of Alabama for giving
me this opportunity to further my education.
xvi
CONTENTS
ABSTRACT .................................................................................................................................... ii
DEDICATION ............................................................................................................................... iii
LIST OF ABBREVIATIONS AND SYMBOLS .......................................................................... iv
ACKNOWLEDGEMENTS ...........................................................................................................xv
LIST OF TABLES ....................................................................................................................... xxi
LIST OF FIGURES ................................................................................................................... xxvi
LIST OF SCHEMES.............................................................................................................. xxxvii
CHAPTER 1: Introduction .............................................................................................................1
1.1 Explosives ................................................................................................................1
1.1.2 Types and Properties of Explosives .........................................................................3
1.1.3 Economic and Human Cost from Explosives ..........................................................6
1.2 Landmine Overview .................................................................................................6
1.2.1 History of Landmines ..............................................................................................7
1.2.2 Landmines: The Problem .........................................................................................7
1.2.3 Landmines: Human and Economic Costs ................................................................8
1.3 Classification of Landmines ..................................................................................10
1.3.1 Anti-Tank Landmines ............................................................................................10
1.3.2 Anti-Personnel Landmines.....................................................................................10
1.3.3 Chemical Signatures of Landmines .......................................................................11
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1.4 Landmine Detection Methods ................................................................................14
1.4.1 Manual Detection Method .....................................................................................14
1.4.2 Electromagnetic Detection Systems ......................................................................15
1.4.3 Acoustic and Seismic Detection Systems ..............................................................19
1.4.4 Biological and Biomimetic Systems ......................................................................20
1.4.5 Bulk Explosive Landmine Detection Systems .......................................................24
1.4.6 Chemical Landmine Vapor Detection Systems .....................................................27
1.5 Explosive Detection Techniques ............................................................................28
1.5.1 Bulk Explosive Detection Systems ........................................................................28
1.5.2 Spectroscopic Explosive Detection Systems .........................................................32
1.5.3 Olfactory Explosive Detection Systems ................................................................35
1.5.4 Chemical Sensors for Explosive Detection ............................................................36
1.5.5 Explosive Sensors Summary..................................................................................38
1.6 Mach-Zehnder Interferometer Optical Waveguide Sensor ....................................39
1.7 Research Objectives ...............................................................................................41
CHAPTER 2: Experimental ...........................................................................................................42
2.1 Sources of All Chemicals .......................................................................................42
2.2 Polymer Syntheses .................................................................................................43
2.2.1 Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVDAT) ..............................................43
2.2.2 Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PS-co-PVDAT) .........44
2.2.3 Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMMA-co-PVDAT) .............................................................................................45
2.2.4 Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMA-co-PVDAT) ................................................................................................47
xviii
2.2.5 Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (P2VP-co-PVDAT) ................................................................................................49
2.2.6 Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PAM-co-PVDAT) ................................................................................................50
2.2.7 Poly(N-vinylcarbazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVK-co-PVDAT) .................................................................................................52
2.2.8 Polystyrene-co-Poly(N-vinylcarbazole) (PS-co-PVK) ..........................................54
2.2.9 Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole) (PMMA-co-PVK) ..........56
2.2.10 Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVI-co-PVDAT) ...................................................................................................57
2.2.11 Polystyrene-co-Poly(N-vinylimidazole) (PS-co-PVI) ...........................................59
2.2.12 Poly(methyl methacrylate)-co-Poly(N-vinylimidazole) (PMMA-co-PVI) ...........60
2.3 Co-Crystals with Nitroaromatics ...........................................................................61
2.3.1 General Co-Crystal Procedure with Nitroaromatics ..............................................61
2.3.2 2,4-Diamino-6-methyl-1,3,5-triazine (MDAT) Co-Crystals with Nitroaromatics ................................................................................................................................61
2.3.3 2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) Co-Crystals with 1,3-Dinitrobenzene ................................................................................................................................62
2.3.4 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-Dinitrobenzene ...........................62
2.4 Instrumentation ......................................................................................................65
CHAPTER 3: Polymer Characterization .......................................................................................69
3.1 PVDAT Characterization .......................................................................................69
3.2 PS-co-PVDAT Copolymers Characterization .......................................................73
3.3 PMMA-co-PVDAT Copolymers Characterization ................................................82
3.4 PMA-co-PVDAT Copolymers Characterization ...................................................91
xix
3.5 P2VP-co-PVDAT Copolymers Characterization ..................................................95
3.6 PAM-co-PVDAT Copolymers Characterization .................................................100
3.7 PVK-co-PVDAT Copolymers Characterization ..................................................105
3.8 PS-co-PVK Copolymers Characterization ...........................................................107
3.9 PMMA-co-PVK Copolymers Characterization ...................................................109
3.10 PVI-co-PVDAT Copolymer Characterization .....................................................115
3.11 PS-co-PVI Copolymer Characterization ..............................................................119
3.12 PMMA-co-PVI Copolymer Characterization ......................................................122
CHAPTER 4: Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor .......................................127
4.1 Ellipsometry Overview ........................................................................................127
4.2 Data Analysis .......................................................................................................133
4.3 Cauchy Model ......................................................................................................134
4.4 PS-co-PVDAT Films ...........................................................................................134
4.5 PMMA-co-PVDAT Films ...................................................................................156
4.6 P2VP Polymer Film .............................................................................................169
4.7 Commercial Polymers Films................................................................................172
4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine .............................182
4.9 Polymer Thin Films Summary .............................................................................198
CHAPTER 5: Co-Crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules .....................................................................................200
5.1 1,3-Dinitrobenzene Crystals (1,3-DNB) ..............................................................201
5.2 9-Ethylcarbazole (9-EC) Co-Crystals with Nitroaromatics .................................206
xx
5.3 9-Vinylcarbazole (9-VC) Co-crystals with 1,3-DNB .........................................213
5.4 Carbazole (CBZ) Co-Crystals with 1,3-DNB ......................................................218
5.5 Phenothiazine (PHZ) Co-Crystals with 1,3-DNB ................................................226
5.6 10-Methylphenothiazine (10-M) Co-Crystals with 1,3-DNB ..............................234
CHAPTER 6: Conclusions and Future Works.............................................................................247
REFERENCES ............................................................................................................................251
APPENDIX .................................................................................................................................259
xxi
LISTS OF TABLES
Table 1.1.2.1 Common explosives' vapor pressures ......................................................................5
Table 1.2.1 List of countries with estimated unexploded landmines ..........................................9
Table 1.3.2.1 Examples of the variety of AP mines based on their material, color, fuse, explosive charge, and weight .................................................................................12
Table 1.5.1.1 Neutron analysis techniques ..................................................................................31
Table 1.5.5.1 Explosive sensors limit of detection ranges ...........................................................39
Table 2.2.2.1 Experimental amounts and conditions for PS-co-PVDAT polymerizations .........45
Table 2.2.3.1 Experimental amounts for the PMMA-co-PVDAT copolymers and PMMA polymerizations ......................................................................................................47
Table 2.2.5.1 Experimental amounts for P2VP-co-PVDAT copolymers and P2VP polymerizations ......................................................................................................50
Table 2.2.6.1 Experimental amounts for PAM-co-PVDAT copolymers and PAM polymerizations ......................................................................................................52
Table 2.2.7.1 PVK-co-PVDAT copolymers and PVK experimental amounts for free radical polymerizations ......................................................................................................54
Table 2.2.8.1 Experimental amounts for the PS-co-PVK polymerizations .................................55
Table 2.2.9.1 Experimental amounts for PMMA-co-PVK copolymers polymerizations ............57
Table 2.3.1.1 Co-crystals experimental reagents, solvents, and descriptions of crystals ............64
Table 3.2.1 Polystyrene and PS-co-PVDAT 20 mol % VDAT copolymer 13C NMR peaks ...76
Table 3.2.2 Thermal decomposition temperatures, Td (10% weight loss for PS and PS-co- PVDAT copolymers) .............................................................................................79
xxii
Table 3.2.3 Glass transition temperatures for PVDAT, PS-co-PVDAT copolymers, and PS ................................................................................................................................81
Table 3.2.4 PS-co-PVDAT copolymers GPC data ...................................................................82
Table 3.3.2 Thermal decomposition temperatures for PMMA and the copolymers of PMMA and PVDAT ...........................................................................................................88
Table 3.3.3 The glass transition temperatures for PMMA, PMMA-co-PVDAT copolymers, and PVDAT ...........................................................................................................90
Table 3.3.4 Molecular weights for the PMMA-co-PVDAT copolymers determined by GPC ................................................................................................................................90
Table 3.5.1 P2VP and P2VP-co-PVDAT copolymers glass transition temperatures (°C) .....100
Table 3.6.1 PAM and PAM-co-PVDAT copolymers glass transition temperatures (°C) ......104
Table 3.7.1 PVK and PVK-co-PVDAT copolymers glass transition temperatures (°C) ........106
Table 4.4.1 The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds .................................................................................140
Table 4.4.2 The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution .................................................142
Table 4.4.3 The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film ............................................144
Table 4.4.4 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film ..............................................146
Table 4.4.5 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film ...........................................148
Table 4.4.6 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds .................................................................................................................151
xxiii
Table 4.4.7 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds .................................................................................................................153
Table 4.4.8 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds .................................................................................................................155
Table 4.5.1 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds ...................................................................160
Table 4.5.2 The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes ..............................................................................................................................162
Table 4.5.3 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................164
Table 4.5.4 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................166
Table 4.5.5 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes .................................................................................................................168
Table 4.6.1 The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds. ........171
Table 4.7.1 The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds ..............................................................................................................................175 Table 4.7.2 The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for 5 seconds .............................................................................................................................178
xxiv
Table 4.7.3 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds ..................................................181
Table 4.8.1 The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours ...............186
Table 4.8.2 The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours ....................................................................189
Table 4.8.3 The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours ............................................................................................................192
Table 4.8.4 The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds ................................................195
Table 4.9.1 Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor ..............................................................................................199
Table 5.2.1 1H NMR peak positions, splitting patterns, and integration values of the 1,3-DNB crystals, 9-EC co-crystals, and 9-EC crystals ......................................................208
Table 5.2.2 Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1,3-DNB co-crystals ...............................................209
Table 5.2.3 Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3- DNB .....................................................................................................................213
Table 5.3.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals .....................................................215
Table 5.3.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and 9- VC co-crystals ......................................................................................................217
Table 5.3.3 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals melting points .............217
Table 5.4.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, CBZ co-crystals, and CBZ crystals .......................................................220
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Table 5.4.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals ...................................................................................................221
Table 5.4.3 1,3-DNB crystals, CBZ crystals, and CBZ co-crystals melting points ................225
Table 5.5.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, PHZ co-crystals, and PHZ crystals ........................................................227
Table 5.5.2 NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals..................................................................................................229
Table 5.5.3 Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals .....233
Table 5.6.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals .....................................................235
Table 5.6.2 NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals............................................................................................237
Table 5.6.3 Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals ...241
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LIST OF FIGURES
Figure 1.3.3.1 Chemical structures of common explosives and TNT impurities found and used in landmines .........................................................................................................13
Figure 1.6.1 MZI consisting of a polymer waveguide with two optical paths ...........................40
Figure 3.1.1 The FTIR spectrum of PVDAT recorded in KBr ..................................................70
Figure 3.1.2 PVDAT TGA curve ...............................................................................................71
Figure 3.1.3 PVDAT DSC curve ...............................................................................................72
Figure 3.2.1 FTIR spectra for the PS-co-PVDAT copolymers ..................................................73
Figure 3.2.2 PS-co-PVDAT 20 mol % 1H NMR (360 MHz, DMSO-d6) spectrum ...................75
Figure 3.2.3 PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum (500 MHz, DMSO-d6) .....76
Figure 3.2.4 TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer ..................79
Figure 3.2.5 PVDAT, PS-co-PVDAT copolymers, and PS DSC curves ...................................80
Figure 3.3.1 FTIR spectra for the PMMA-co-PVDAT copolymers ..........................................83
Figure 3.3.2 1H NMR spectrum in DMSO-d6 for PMMA-co-PVDAT (20 mol %) ..................84
Figure 3.3.3 13C NMR spectrum in DMSO-d6 for the PMMA (80%)-co-PVDAT (20%) copolymer ..............................................................................................................86
Figure 3.3.4 TGA curves for PMMA and the copolymer containing 10 mol % VDAT ............88
Figure 3.3.5 PMMA and PMMA-co-PVDAT copolymers DSC curves ....................................89
Figure 3.4.1 FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ...............91
Figure 3.4.2 1H NMR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ..........92
Figure 3.4.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-PVDAT 20 mol % VDAT copolymer ..............................................................................................93
xxvii
Figure 3.4.4 DSC curve for the PMA-co-PVDAT 20 mol % VDAT copolymer ......................94 Figure 3.5.1 FTIR spectra for P2VP and P2VP-co-PVDAT copolymers ..................................95
Figure 3.5.2 1H NMR spectrum for the P2VP-co-PVDAT 20 mol % VDAT copolymer in DMSO-d6 using the 360 MHz spectrometer ..........................................................97
Figure 3.5.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the P2VP-co-PVDAT 1 mol % VDAT copolymer ..............................................................................................98
Figure 3.5.4 P2VP and P2VP-co-PVDAT copolymers DSC curves .........................................99
Figure 3.6.1 PAM and PAM-co-PVDAT copolymers FTIR spectra .......................................101
Figure 3.6.2 The 1H NMR spectrum (360 MHz, D2O) for the PAM-co-PVDAT 20 mol % VDAT copolymer ................................................................................................102
Figure 3.6.3 The 13C NMR spectrum (500 MHz) recorded in D2O for the PAM-co-PVDAT 20 mol % VDAT copolymer .....................................................................................103
Figure 3.6.4 DSC curves for the PAM and PAM-co-PVDAT copolymers .............................104
Figure 3.7.1 The FTIR spectra for PVK and PVK-co-PVDAT copolymers recorded in KBr pellets ...................................................................................................................105
Figure 3.7.2 DSC curves for PVK and PVK-co-PVDAT copolymers ....................................107
Figure 3.8.1 The 1H NMR spectra for the PVK and PS-co-PVK copolymers recorded in CDCl3 (360 MHz) ............................................................................................................108
Figure 3.9.1 The FTIR spectra for KBr pellets containing PVK and PMMA-co-PVK copolymers ...........................................................................................................109
Figure 3.9.2 The 1H NMR spectrum for the PVK homopolymer recorded in CDCl3 using the 360 MHz spectrometer .........................................................................................110
Figure 3.9.3 The 1H NMR spectra for PMMA-co-PVK 50 and 20 mol % copolymers recorded in CDCl3 (360 MHz) ............................................................................................111
Figure 3.9.4 The 13C NMR spectrum for PVK recorded in CDCl3 (500 MHz) .......................113
Figure 3.9.5 The 13C NMR spectrum for the (50:50) PMMA-co-PVK copolymer ( 500 MHz, CDCl3)..................................................................................................................114
Figure 3.9.6 The DSC curves for the PMMA-co-PVK copolymers ........................................115
xxviii
Figure 3.10.1 The FTIR spectra for PVI homopolymer (black) and PVI-co-PVDAT 20 mol % VDAT copolymer (red) ........................................................................................116
Figure 3.10.2 The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer ..............................................................................................................................117
Figure 3.10.3 The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz) ............................................................................................................118
Figure 3.11.1 The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature ..............................................................................................................................119
Figure 3.11.2 The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz) ............................................................................121
Figure 3.11.3 The 13C NMR spectra for the PS-co-PVI copolymer and the homopolymer (PVI) ..............................................................................................................................122
Figure 3.12.1 The FTIR spectra for the PMMA-co-PVI copolymer and the PVI homopolymer recorded in KBr pellets ........................................................................................123
Figure 3.12.2 The 1H NMR spectra for the homopolymers, PVI and PMMA, and the PMMA- co-PVI copolymer containing 20 mol % vinylimidazole ....................................124
Figure 3.12.3 The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz) ..................................................................125
Figure 3.12.4 The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer ............................................................126
Figure 4.1.1 Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013) ......................................................................................128
Figure 4.1.2 Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013) ......................................................................................................129
Figure 4.1.3 Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013) ..............................................................................................................131
Figure 4.1.4 Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions .................................................................131
xxix
Figure 4.4.1 The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds ....................139
Figure 4.4.2 The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds ................................................................................................................141
Figure 4.4.3 The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds .................................................................................................................143
Figure 4.4.4 The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds ..............................................................................................................................145
Figure 4.4.5 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds ..............................................................................................................................147
Figure 4.4.6 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..............................................................................................................................150
Figure 4.4.7 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds .................................................................................................................152
Figure 4.4.8 The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds ..............................................................................................................................154
Figure 4.5.1 The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds .........................159
Figure 4.5.2 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3- DNB for sixteen minutes .....................................................................................161
Figure 4.5.3 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% toluene solution exposed to NB for twenty-five minutes .................................................................................................................163
Figure 4.5.4 The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes .................................................................................165
xxx
Figure 4.5.5 The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes .......................................................................167
Figure 4.6.1 The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..................170
Figure 4.7.1 The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds ..............................................................................................................................174
Figure 4.7.2 The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT .........................................177
Figure 4.7.3 The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds .........................................................................180
Figure 4.8.1 The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours ....................................................................................................................185
Figure 4.8.2 Refractive index curves for a polystyrene/10-M film spin coated from 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours .....................................................................................188
Figure 4.8.3 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for three hours ............................................................................................191
Figure 4.8.4 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for ten seconds ...........................................................................................194
Figure 5.1.1 Image of 1,3-DNB crystals ..................................................................................201
Figure 5.1.2 1H NMR spectrum of 1,3-DNB crystals (360 MHz, CDCl3) ...............................202
Figure 5.1.3 The FTIR spectrum of the 1,3-dinitrobenzene crystals .......................................203
Figure 5.1.4 Electronic absorption spectrum of the 1,3-DNB crystals in acetonitrile .............204
Figure 5.1.5 Diffuse reflectance spectrum of the 1,3-DNB crystals ........................................205
Figure 5.2.1 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals .................................................................................................................206
xxxi
Figure 5.2.2 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB) ............................................................................................................207
Figure 5.2.3 FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve) ........................................209
Figure 5.2.4 Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile ..................................210
Figure 5.2.5 Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black) .................................211
Figure 5.2.6 Diffuse reflectance spectra for the 9-EC crystals (black) and the co-crystals of 9- EC and 1,3-DNB (red) .........................................................................................212
Figure 5.3.1 Co-crystals of 9-VC and 1,3-DNB.......................................................................214
Figure 5.3.2 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3) .......................................................................215
Figure 5.3.3 FTIR spectra of KBr pellets containing either 9-VC crystals (red) or the co- crystals of 9-VC and 1,3-DNB (black) ................................................................216
Figure 5.4.1 Crystals of carbazole (A) and co-crystals of carbazole and 1,3-DNB (B) ..........218
Figure 5.4.2 1H NMR spectra of CBZ crystals, CBZ co-crystals with 1,3-DNB, and 1,3-DNB crystals (360 MHz, CDCl3) ..................................................................................219
Figure 5.4.3 FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co- crystals with 1,3-DNB (red) .................................................................................221
Figure 5.4.4 Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue) ..........223
Figure 5.4.5 Comparison of the electronic absorption spectrum in acetonitrile for the co- crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for 1,3- DNB crystals and the spectrum for the CBZ crystals (black) ..............................224
Figure 5.5.1 Images of PHZ co-crystals (A) and PHZ crystals (B) .........................................226
Figure 5.5.2 1H NMR spectra (360 MHz, CDCl3) of the 1,3-DNB crystals, PHZ crystals, and the co-crystals made from PHZ and 1,3-DNB .....................................................227
Figure 5.5.3 FTIR spectra for KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue) .....................................................................................228
xxxii
Figure 5.5.4 Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue) ....................................................................................................................230
Figure 5.5.5 Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black) ...............231
Figure 5.5.6 Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red) .......................................................................................232
Figure 5.6.1 Images of 10-M co-crystals (A) and 10-M co-crystals with 1,3-DNB (B) ..........234
Figure 5.6.2 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3) ...............................................................235
Figure 5.6.3 Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co- crystals containing 10-M and 1,3-DNB (red) ......................................................236
Figure 5.6.4 Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue) .....................................................................................238
Figure 5.6.5 Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black) ..................................................239
Figure 5.6.6 Diffuse reflectance spectra for the 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red) ...................................................................240
Figure 5.6.7 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii ......................................................................................................................242
Figure 5.6.8 Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance .....243
Figure 5.6.9 Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts ....244
Figure 5.6.10 The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..................244
Figure 5.6.11 View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..............................................................................................................................245
xxxiii
Figure 5.6.12 Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue ....................................................246
Appendix Figure 1 VDAT 1H NMR (360 MHz, DMSO-d6) ..................................................259
Appendix Figure 2 PS-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3) ...........................................................................................260
Appendix Figure 3 PS-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3)......................................................................................................261
Appendix Figure 4 PS-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3)......................................................................................................262
Appendix Figure 5 Polystyrene 1H NMR (360 MHz, CDCl3) spectrum ................................263
Appendix Figure 6 Polystyrene 13C NMR (500 MHz, CDCl3) spectrum ...............................264
Appendix Figure 7 PS-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3)......................................................................................................265
Appendix Figure 8 PS-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3)......................................................................................................266
Appendix Figure 9 PS-co-PVDAT 10 mol % VDAT 13C NMR spectrum (500 MHz, CDCl3) ..................................................................................................................267
Appendix Figure 10 VDAT 13C NMR spectrum (500 MHz, DMSO-d6) .................................268
Appendix Figure 11 Polystyrene TGA curve ............................................................................269
Appendix Figure 12 PS-co-PVDAT 1 mol % VDAT copolymer TGA curve .........................270
Appendix Figure 13 PS-co-PVDAT 5 mol % VDAT copolymer TGA curve .........................271
Appendix Figure 14 The PS-co-PVDAT 10 mol % VDAT copolymer TGA curve ................272
Appendix Figure 15 The PS-co-PVDAT 1 mol % VDAT copolymer GPC data .....................273
Appendix Figure 16 The PS-co-PVDAT 5 mol % VDAT copolymer GPC data .....................273
Appendix Figure 17 The PS-co-PVDAT 10 mol % VDAT copolymer GPC data ...................274
xxxiv
Appendix Figure 18 The PS-co-PVDAT 20 mol % VDAT copolymer GPC data ...................274
Appendix Figure 19 FTIR spectrum of the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................275
Appendix Figure 20 FTIR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer .................................................................................................................276
Appendix Figure 21 FTIR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer ..................................................................................................................277
Appendix Figure 22 FTIR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer ..................................................................................................................278
Appendix Figure 23 The 1H NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer ............................................................................................279
Appendix Figure 24 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................280
Appendix Figure 25 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix ..........................281
Appendix Figure 26 The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT in CDCl3 using the 500 MHz spectrometer .................................................282
Appendix Figure 27 The 1H NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................283
Appendix Figure 28 The 13C NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer .............................................................................................284
Appendix Figure 29 The 13C NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................285
Appendix Figure 30 The 13C NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................286
Appendix Figure 31 The 13C NMR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer in DMSO-d6 using the 500 MHz spectrometer ......................287
Appendix Figure 32 TGA curve for the PMMA-co-PVDAT 1 mol % VDAT copolymer ......288
xxxv
Appendix Figure 33 The TGA curve for the PMMA-co-PVDAT 5 mol % VDAT copolymer ..................................................................................................................289
Appendix Figure 34 The TGA curve for the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................290
Appendix Figure 35 GPC curve and data for the PMMA-co-PVDAT 20 mol % VDAT copolymer ...............................................................................................291
Appendix Figure 36 The GPC curve and data for the PMMA-co-PVDAT 10 mol % VDAT copolymer ...............................................................................................292
Appendix Figure 37 The GPC curve and data for the PMMA-co-PVDAT 5 mol % VDAT copolymer ...............................................................................................293
Appendix Figure 38 The PMA 1H NMR spectrum (360 MHz, CDCl3) ...................................294
Appendix Figure 39 The PMA 13C NMR spectrum (500 MHz, DMSO-d6) ............................295
Appendix Figure 40 The P2VP 1H NMR spectrum (360 MHz, CDCl3) ..................................296
Appendix Figure 41 The P2VP-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6) .............................................................................297
Appendix Figure 42 The P2VP-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6) .............................................................................298
Appendix Figure 43 The P2VP 13C NMR spectrum (500 MHz, DMSO-d6) ............................299
Appendix Figure 44 The P2VP-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 180 - 110 ppm ............................................300
Appendix Figure 45 The P2VP-co-PVDAT 20 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 190 - 110 ppm ............................................301
Appendix Figure 46 The PAM polymer 1H NMR spectrum (360 MHz, D2O) ........................302
Appendix Figure 47 The PAM-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz) ....................................................................303
Appendix Figure 48 The PAM-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz) ....................................................................304
Appendix Figure 49 The PAM-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz) ....................................................................305
xxxvi
Appendix Figure 50 The Poly(acrylamide) (PAM) 13C NMR spectrum (500 MHz, D2O) ......306
Appendix Figure 51 The PAM-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).......................................................................................307
Appendix Figure 52 The PAM-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).......................................................................................308
Appendix Figure 53 The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer ........................................................................309
Appendix Figure 54 The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz) .................................................................310
Appendix Figure 55 The DSC curve for the PVI homopolymer ..............................................311
xxxvii
LIST OF SCHEMES
Scheme 1.1 Classification of explosives .........................................................................2
Scheme 2.2.1.1 Free radical polymerization of PVDAT .....................................................43
Scheme 2.2.2.1 Free radical polymerization for the synthesis of PS-co-PVDAT random copolymers .................................................................................................45
Scheme 2.2.3.1 Free radical polymerization for the synthesis of PMMA-co-PVDAT random copolymers ....................................................................................47
Scheme 2.2.4.1 Free radical polymerization for the synthesis of a PMA-co-PVDAT random copolymer .....................................................................................48
Scheme 2.2.5.1 Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers .................................................................................................50
Scheme 2.2.6.1 Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers .................................................................................................52
Scheme 2.2.7.1. Free radical polymerization for the synthesis of PVK-co-PVDAT random copolymers .................................................................................................54
Scheme 2.2.8.1. Free radical polymerization for the synthesis of PS-co-PVK random copolymers .................................................................................................56
Scheme 2.2.9.1 Free radical polymerization for the synthesis of PMMA-co-PVK random copolymers .................................................................................................57
Scheme 2.2.10.1 Free radical polymerization for the synthesis of a PVI-co-PVDAT random copolymer ..................................................................................................58
Scheme 2.2.11.1 Free radical polymerization for the synthesis of a PS-co-PVI random copolymer ..................................................................................................60
Scheme 2.2.12.1 Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer ..................................................................................................61
1
Chapter 1
Introduction
1.1 Explosives
Explosives are defined as energetic materials which react to produce rapid oxidation of
products accompanied by the generation of heat, light, or gas.1 Explosives are classified by
structure and performance, and fall into two broad categories, low and high explosives, with high
explosives being further classified into additional categories. Scheme 1.1.1 shows the
classification for high and low explosives. Low explosives, which include propellants and
pyrotechnics, burn at relatively low rates (cm/s) and are capable of producing heat, light, smoke,
gas, or sound with propellants.1 High explosives are capable of detonating at high velocities (1 to
9 km/s) and produce vast amounts of energy (400 to 1,200 cal/g).1 High explosives are further
classified into primary and secondary explosives based on their ability to detonate. Primary
explosives are very susceptible to initiation and are used as a source for igniting secondary
explosives. Secondary explosives, which consist of nitroaromatics and nitro-amines, are
prevalent for military and industry uses, since they are designed to detonate under specific
conditions.
2
Scheme 1.1.1. Classification of explosives.2
Explosives
High Explosives Low Explosives
Pyrotechnics Propellants Primary Explosives
Secondary Explosives
Military Explosives
Industrial Explosives
3
1.1.2 Types and Properties of Explosives
There are several different types of manufactured explosive materials designed to
detonate, but the most common are organic based compounds. The organic based secondary
explosives fall into two categories based on their structure: aromatic and aliphatic. Aromatic
explosives contain a benzene ring, and the aliphatic explosives do not. A variety of aromatic
explosives exists due to the fact one or more molecular subgroups can be substituted for a
hydrogen atom on the benzene ring. Aliphatic explosive materials primarily consist of aliphatic
nitrate esters (─ONO2), aliphatic nitramines (─N─NO2), and nitro-aliphatics (─NO2). The most
common aliphatic explosives are the nitrate esters. Examples of aromatic and aliphatic
explosives are: 2,4,6-trinitrotoluene (TNT), nitroglycerine (NG), tetranitro-triazacyclohexane
(RDX), tetranitro-tetrazacyclooctane (HMX), tetranitro-N-methylamine (Tetryl), and 2,4,6-
trinitrophenol (picric acid).
Even though most explosives are organic based materials, inorganic explosive materials
also exist and are composed of fuels and oxidizers. A common inorganic explosive is ammonium
nitrate (NH4NO3), which is commercially produced as a fertilizer. A common inexpensive
explosive is created when NH4NO3 is combined with other explosives or fuels. Fulminates
(Metal─ONC) are pure inorganic primary or initiating explosives, but are seldom used due to
their susceptible detonation and poor storage characteristics. Azides are another type of inorganic
explosives, which are the salts of hydrazoic acids. The most common explosive in the azide
family is lead azide (PbN6), which can be used for initiating explosives. Other azides exist such
as silver azide (AgN3) and sodium azide (NaN3), which are classified as low explosives since the
combustion reaction is less reactive than that of high explosives.
4
Most explosives contain carbon, hydrogen, oxygen, and nitrogen, many of which are
richer in oxygen and nitrogen than carbon and hydrogen. The oxygen:nitrogen ratio is an
important characteristic that many anomaly detectors rely on for detecting explosives. Explosives
have a wide range of vapor pressures at ambient temperatures shown in Table 1.1.2.1. The low
vapor pressures of explosives make detection by vapor methods difficult. Detecting explosives
with low vapor pressures require vapor sensors to either sample large volumes of air or have low
limits of detection. To improve the detection of low vapor pressure explosives, 2,3-dimethyl-2,3-
dinitrobutane (DMNB) is incorporated in some explosives as a marker. DMNB contributes to
locating explosives either by vapor pressure or by odor contamination due to its high vapor
pressure, high permeability, and no known industrial applications.3
5
Table 1.1.2.1. Common explosives' vapor pressures.3
Class Explosive Vapor pressure at 25 °C (Torr)
Acid salt Ammonium nitrate NH4NO3 5.0 x 10-6
Aliphatic nitro Nitromethane 2.8 x 101 2,3-Dimethyl-2,3-dinitrobutane DMNB 2.1 x 10-3
Aromatic nitro
2-Nitrotoluene o-MNT 1.5 x 10-1 4-Nitrotoluene PNT 4.1 x 10-2
2,4-Dinitrotoluene 2,4-DNT 2.1 x 10-4
2,4,6-Trinitrotoluene TNT 3.0 x 10-6
2,4,6-Trinitrophenol Picric acid 5.8 x 10-9
Nitrate ester
Ethylene glycol dinitrate EGDN 2.8 x 10-2
Nitroglycerin NG 2.4 x 10-5
Pentaerythritol tetranitrate PETN 3.8 x 10-10
Nitrocellulose NC N/A
Nitramine Tetranitro-N-methylamine Tetryl 5.7 x 10-9
Tetranitro-triazacyclohexane RDX 1.4 x 10-9
Tetranitro-tetrazacyclooctane HMX 1.6 x 10-13
Peroxide Triacetone triperoxide TATP 3.7 x 10-1 Hexamethylene triperoxide diamine HMTD N/A
6
1.1.3 Economic and Human Cost from Explosives
The development and use of explosive materials has been considered beneficial both
socially and economically, but these materials have conversely been used to create terror, chaos,
destruction, and human fatalities. Between 1999 and 2009, there were approximately 74,000
causalities in 119 countries due to landmines, explosive remnants from previous wars, and
improvised explosive devices.4 The Unabomber, Ted Kaczynski, mailed packages containing
small explosives. On April 19, 1995, the Alfred P. Murrah federal office building in Oklahoma
City, O.K. was bombed, killing 167 and causing property destruction totaling $652 million.5 On
December 25, 2009, a Nigerian man on a flight from Amsterdam to Detroit attempted to ignite a
hidden explosive device consisting of a mixture of powder and liquid that passed airport security.
On May 1, 2010, a car bomb was discovered near Times Square, in New York City, after smoke
was observed coming from the vehicle. The bomb ignited, but did not detonate, preventing any
harm to the surrounding environment. These few examples listed above illustrate how easily
explosives can be utilized and how potentially detrimental they can be to innocent civilians.
1.2 Landmine Overview
A landmine can be generally described as an explosive device that is placed under, on, or
near the surface of an area and is designed to detonate by the contact of a person or vehicle.6
Landmines are constructed such that when detonated, they are capable of causing either
immediate injury or death to individuals, while also having the ability to disable military
vehicles. Landmines are indiscriminate weapons primarily used to target opposing enemies, but
in other instances affect innocent civilians. A significant problem with landmines is that once
laid, the mines remain in place long after conflicts and remain a danger.
7
1.2.1 History of Landmines
Anti-tank (AT) landmines first originated in the First World War as a means of disabling
advancing tanks. These mines were effective against military vehicles, but were relatively large,
easily noticeable, and easily removed. To combat the removal of these mines, Germans and
Italians developed anti-personnel (AP) landmines during the Second World War which consisted
of grenades and fuses to prevent allied forces from removing AT landmines.6 These mines were
much smaller than the AT landmines and were primarily designed to injure or kill military
soldiers, as opposed to merely disabling military vehicles. After the war, the employment of AP
landmines became more common and is still used in modern warfare. Today, landmines are often
deployed by terrorist organizations within civil conflicts and guerilla warfare, frequently creating
excessive human causalities.
1.2.2 Landmines: The Problem
The International Committee for the Red Cross estimated that 120 million unexploded
mines are buried in 70 countries around the world.7 In 1995, A United Nations (UN) estimate
indicated that at that time, only about 80,000 of the 120 million landmines were cleared and in
the same time period an additional 2.5 million mines were buried.7 From that estimate, with the
current technology at that time, it would take approximately 1,100 years to clear all mined areas
from the 1995 estimate.7 A more recent estimate indicated that there are 50 to 100 million
landmines buried in more than 80 countries with only 100,000 mines deactivated per year; this is
compared to the estimated two million landmines that are additionally laid annually.8 Table 1.2.1
shows countries that have ongoing issues with unexploded landmines. Due to the lifetime of
these mines and placement in public areas, civilians who were not associated with the original
conflict are often affected. One reason that many undetonated mines are still in place is the lack
8
of resources needed to remove them. The production of a landmine can cost as little as $3 U.S.,
but the removal cost can be as high as $1,000 U.S.8
1.2.3 Landmines: Human and Economic Costs
Landmines have affected both social and economic aspects of civilians’ lives in many
ways; from human causalities and injuries, to the limitation of the use of lands for farming or
schools, and general concerns of security are just a few of the problems faced due to hidden
mines. There are estimates that mines kill or injure a person every 20 minutes, 70 people a day,
or more than 20,000 people a year.8 Most of those killed or injured from these landmines are
noncombatants. Many countries dealing with landmine problems do not have adequate medical
resources to provide proper treatment and rehabilitation necessary for the injured victims.
Landmines are most detrimental to the economic development of countries by limiting
the use of lands for agriculture. It is estimated that landmines have limited access or degraded
some 900,000 km2 of land globally.9 The loss of productivity from agricultural lands due to
landmines has been linked to four factors: access denial, loss of biodiversity, chemical
contamination, and micro-relief disruption.9 This in turn affects the local economy, since many
of the countries depend on agriculture as a means of income, food, and goods for export.
9
Table 1.2.1. List of countries with estimated unexploded landmines.7
Countries
Unexploded Landmine Estimates
Afghanistan
10 million
Angola
15 million
Bosnia and Herzegovina
3 - 6 million
Cambodia
10 million
Croatia
3 million
Egypt
23 million
Iran
16 million
Iraq
10 million
Mozambique
2 million
10
1.3 Classification of Landmines
Landmines are classified into two categories: anti-personnel mines (AP) and anti-tank
mines (AT). These mines are encased in different types of materials; these include metal, plastic,
wood, or in some instances, contain no encasement at all. Modern landmines are comprised of
very few metal components, thereby preventing detection by metal detectors. The detonating
mechanism for landmines can be constructed from simple trip wires, pressure triggers, tilt rods,
acoustic or seismic fuses, and light or magnetic fuses. Landmines are utilized in a variety of
arrangements, some of which include, burial in fields with a mixture of other objects and clutter,
burial at various depths, scattered on the surface, planted in buildings, hidden using plant
vegetation, and placed near or under important roads.
1.3.1 Anti-Tank Landmines
Anti-tank (AT) mines have truncated square or cylinder shapes with lengths ranging from
150 to 300 mm and a thickness of 50 to 90 mm.10, 11 These mines are designed to be buried near
the surface or to depths greater than 150 mm below the surface. The AT mines contain
approximately 2 to 10 kg of explosives, and are activated by pressures of hundreds of kg.10, 11
These mines are typically buried under roads of critical importance for military use. The
limitations of roads affects transportation and economic development, but are considered less
damaging and destructive than AP mines.
1.3.2 Anti-Personnel Landmines
Anti-personnel (AP) mines are shaped in the form of disks or cylinders with diameters
from 20 to 150 mm and lengths from 50 to 100 mm.10, 11 These mines contain 10 to 250 g of
explosives, and are buried near the surface to a maximum depth of 50 mm, detonating under
pressures of 0.5 to 50 kg.10, 11 Locating and removing AP mines provides a challenge due to a
11
wide variety of mines; with more than 700 different types in existence, these mines are
constructed in different shapes, sizes, and materials. Table 1.3.2.1 provides an example of the
variety of AP mines based on their material, color, fuse, explosive charge, and weight. Many of
the injuries and causalities mentioned previously are caused by AP mines used in previous or
ongoing conflicts.
1.3.3 Chemical Signatures of Landmines
Common explosives used for the main charge in landmines shown in Figure 1.3.3.1 are
RDX, Composition B, tetryl, C-4, with the most common explosive material being TNT. Nearly
80% of the manufactured landmines in the world contain TNT.7 Even though military grade TNT
is the majority of the explosive material, other impurities were also found with higher vapor
concentrations than TNT itself, including 1,3-DNB, 2,4-DNT, 2,6-DNT, and 2,4-DNB (Figure
1.3.3.1).7, 12 The headspace above a military grade TNT filled landmine contained 0.35 to 9.7
pg/mL 1,3-DNB, 0.28 to 1.4 pg/ml of 2,4-DNT, and 0.070 to 0.078 pg/ml of TNT.12 Jenkins et
al. discovered that in many cases the most prevalent chemical signatures in the surface soil were
2,4-DNT, 2-ADNT, and 4-ADNT.12 The mines including plastic casings or those that were not
well-sealed allowed greater concentrations of the chemical signatures to escape, compared with
mines containing metal cases and those that were adequately sealed. Flux measurements were
recorded of 1,3-DNB, 2,4-DNT, and TNT from a few landmines at various temperatures.7 The
impurities, which accompanied TNT, showed an increase in vapor concentrations higher than
TNT itself. Since the impurities found with TNT exhibited higher vapor concentrations,
landmine and explosive detection systems could be directed to detect 1,3-DNB and 2,4-DNT
vapors as signatures for TNT based explosives.
12
Table 1.3.2.1. Examples of the variety of AP mines based on their material, color, fuse, explosive charge, and weight.13
Type Weight (kg)
Case Material Case Color Fuse Explosive
charge
Explosive Weight
(g)
Type 69 1.35 Cast Iron Olive drab Pressure or trip wire TNT 105
Type 72 0.125 Plastic Green Pressure TNT/RDX 75-100
M14 0.158 Plastic Olive drab Pressure Tetryl 29
M16A1 3.57 Steel Green Pressure or trip wire TNT 513
M18A1 1.58 Plastic Olive drab Command detonation C-4 682
Valmara 69 3.3 Plastic Green/Sand Trip wire or pressure Comp. B 597
VS-50 0.185 Plastic Olive drab /Sand Pressure RDX 43
PP-MI-SR 3.2 Steel/Plastic Olive drab Trip wire or pressure TNT 362
MON-200 25 Metal Olive drab Trip wire / Command detonation
TNT 12 kg
PMN 0.55 Bakelite Black Delay-armed/ pressure TNT/Tetryl 200
POMZ-2 2.3 Metal Olive drab Trip wire TNT 75
PMD-6 0.4 Wood Wood Pressure TNT 200
13
N+
O
-O
N+ O
-O
N+ O
-O
Tri-nitro-toluene(TNT)
HN
N+
O
-O
N+
O-O
N+
O
O-N+
O
-O
2,4,6-Trinitrophenylmethylnitramine(Tetryl)
N
N
N
N+ O
-O
N+
O
O-
N+
O
-O
1,3,5-Trinitro-1,3,5-triazacyclohexane(RDX)
C-4 (RDX Based)
Comp. B (RDX + TNT)N+
O
-ON+
O
O-
1,3-dinitrobenzene(1,3-DNB)
N+
O
-ON+
O
O-
2,4-dinitrotoluene(2,4-DNT)
N+
O
O-
N+
O
-O
2,6-dinitrotoluene(2,6-DNT)
N+
O
-O
2,4-dinitrobenzene(2,4-DNB)
NO O
Figure 1.3.3.1. Chemical structures of common explosives and TNT impurities found and used in landmines.
14
1.4 Landmine Detection Methods
The detection and removal of buried landmines is a significant concern in many countries
throughout the world. The development of AP mines has made detection very challenging, as
modern mines contain few metal components. They are commonly buried in areas containing
metallic debris to avoid detection, thus producing false alarms. A typical rate for locating and
eliminating mines is 80%, but for humanitarian demining, the U.N. requires a rate approaching
perfection, 99.6%.14 An optimal landmine detection system should have the ability to detect
landmines without reference to the type of explosive material the mine contains, shape, size, or
casing material. Additionally, the detection system should virtually produce no false alarms, be
operator friendly, exhibit reasonable operation speed, and be cost efficient. The following
sections will review currently employed and developing landmine detection systems used to
locate buried landmines.
1.4.1 Manual Detection Method
Prodders
The most common approach to humanitarian landmine detection is prodding. Typically, a
deminer will use a prod to enter the soil and scan a small grid. When the prod encounters an
object, the deminer will assess the contour of the object and cautiously remove the surrounding
soil until the object can be identified. This technique has a very successful detection rate, but is
very laborious and hazardous. The success of the deminer is achieved through experience
learning to recognize the physical characteristics of certain objects, and perceiving the difference
between a mine's casing and other various objects by sound. Even though this technique has a
high detection rate, the deminer is always at a constant risk of either stepping on a mine or
applying too much force during the prodding process, triggering detonation of the mine. Even
15
though this technique is applicable for humanitarian demining, alternative methods are being
investigated to lower the risk associated with demining.
1.4.2 Electromagnetic Detection Systems
Metal Detectors
Another common landmine detection system for humanitarian demining is hand held
metal detectors. The metal detector works by emitting a magnetic field to produce an eddy
current in a metallic object to generate a detectable magnetic field.6, 8, 11, 14 One disadvantage
associated with this detection system is that most modern mines contain very little metal
(primarily the firing pin) or contain no metal at all. The sensitivity of the metal detector can be
increased to detect smaller quantities of metal, but in return increases the possibility of false
alarms triggered by other metallic objects. Metal detectors primarily indicate that the buried
object contains metal, but cannot identify whether or not the object contains any explosive
materials. This detection system is relatively inexpensive when compared to other detection
systems, but is not time efficient and is a hazardous process for the operator.
Ground Penetrating Radar (GPR)
Ground penetrating radar (GPR) is used in many applications including civil engineering,
geology, and archeology for studying soil and detecting buried objects. GPR is capable of
detecting buried landmines by emitting radio waves into the ground, creating changes in the
reflected signal that arise from variations in the dielectric constants for objects in the soil.6, 8, 14, 15
The reflected signals create an image of a vertical slice of soil, which is then analyzed. Shorter
wavelengths provide greater soil penetration depth and better image resolution, but wide-band
frequencies provide better details and improve the signal to noise ratio.8, 15 GPR provides the
advantage of being able to detect landmines with a wide variety of casing materials due to the
16
different dielectric constants (compared to the soil) and is capable of generating an image of the
landmine.
A disadvantage of this detection system is that small objects require higher frequencies
(GHz), which have limited penetration depth.6, 8, 14 Higher frequencies also have increased image
clutter. Also, inhomogeneous subsoil can create false alarms. The system is also sensitive to
complex interactions produced by metal content, soil moisture, soil surface smoothness, and
radar frequencies.6 The high cost for this detection system precludes its application for
humanitarian demining.
Infrared (IR) and Hyper Spectral Methods
Infrared (IR) and hyper spectral detection systems have the ability to detect landmines by
observing variations in the electromagnetic radiation reflected or emitted by the mine or the soil
and vegetation above the mine compared to the surrounding environment.6, 8, 16 These detection
systems operate by two methods: thermal and non-thermal. Thermal detection systems observe
variations in temperatures between the soil and vegetation located near the mine compared to the
surrounding environment. During the day, mines absorb and release heat at a different rate than
their surrounding environment. Laser illumination or high-powered microwaves have been used
to produce these temperature profiles. Non-thermal detection systems observe the difference of
the reflected light (either natural or artificial) from areas surrounding a landmine. This system
can also detect the presence of a mine from the burying process, disrupting the soil's particle
distribution, and affecting the way the soil scatters light.6
These detection systems are advantageous, as they do not require physical contact and
can be used at a distance. The equipment is also lightweight, capable of analyzing a large area
(employed from an airplane), and offers fast image acquisition.
17
Although these systems possess some ideal characteristics for detecting landmines, there
are limitations that prevent this technology from being an ideal application for landmine
detection. The primary disadvantage is that the systems are dependent on environmental
conditions. The performance variability cannot always accurately locate and identify buried
landmines, which also prevent these methods from being a reliable detection system. The
wavelength frequencies employed by these system cannot penetrate the soil's surface and the
hyper spectral signatures produced from the landmine burying process can be eliminated by
weathering.6
Electrical Impedance Tomography (EIT)
Electrical impedance tomography (EIT) utilizes electrical currents to image the
conductivity distribution of the medium under investigation.6, 17 An array of electrodes is placed
on the ground to detect signals from the conductivity distribution, providing evidence of an
existing mine. This system is capable of detecting both metal and non-metal mines due to
anomalies produced in the conductivity distribution. EIT is well suited for detecting landmines
buried in wet soil environments, since moisture enhances the conductivity. The equipment is also
lightweight and relatively inexpensive compared to other systems.
A major disadvantage of this detection system is that it requires direct contact with the
area under investigation, thus increasing the possibility of detonating a mine. EIT does not work
in dry non-conductive or rocky surfaces, as the existence of conductivity potential would be
decreased. EIT is sensitive to electrical noise and performance deteriorates as the depth of the
object increases. This system could possibly be considered for the detection of shallow buried
mines located in wet environments.
18
X-Ray Backscatter (XBT)
X-Ray backscatter (XBT) employs the direct transmission of X-rays into the ground
which are backscattered to a detector from an irradiated object, producing an image of the object
due to X-rays passing through matter with an attenuation (absorbed or scattered).6, 8 This system
is advantageous, since landmines and soil have different mass densities and atomic numbers.
Since pass-through X-ray imaging is not achievable, the XBT system exploits the Compton
Principle due to the photons being captured from the irradiated object, allowing the detector and
emitter to be placed above the surface.6, 8 This system includes two techniques for capturing
images of buried landmines: collimate and un-collimated methods. Collimated methods utilize
focused beams and collimated detectors to produce an image, but this process increase the size
and weight of the instrument, while reducing the number of photons for imaging. Collimated
systems also require high power X-ray generators as sources, which in turn limit the systems for
humanitarian demining due to the power requirements, increase in size, and weight. Un-
collimated systems can be constructed at a smaller scale, made more lightweight, and illuminate
a large area with X-rays, making this method ideal for portable detection.
XBT systems are capable of distinguishing mines from soils using low energy photons in
the energy range of 60 to 200 keV.6, 8 The photons allow for larger cross sections compared to
other systems that employ nuclear reactions for sensing. Un-collimated systems can be made
smaller and more lightweight due to the reduced shielding thickness required to impede the low
energy photons.
A disadvantage of these systems is that the required energy range for XBT devices has
poor soil penetration depth, limiting the detection to shallow mines (less than 10 cm).6, 8 If source
strengths remain low for more secure operation, longer times are required for obtaining images.
19
These systems are sensitive to source/detector standoff variations and ground surface
fluctuations. In order to obtain images of small AP mines, high-resolution cameras
(approximately 1 cm) are needed, but image acquisition is difficult in the field.6, 8 Additionally,
this technology emits radiation, which would limit field use due to public concerns.
1.4.3 Acoustic and Seismic Detection Systems
Acoustic and seismic systems are capable of detecting landmines by exploiting the
vibrations of the mechanical properties of the mine's casing and components rather than the
electromagnetic properties.6, 8, 15 Acoustic and seismic systems work by emitting sound/seismic
waves from loud speakers above the ground. Some of the waves are reflected at the surface, but
the remaining waves propagate through the soil and are reflected (upward) toward the surface by
a buried mine; causing vibrations at the ground surface. Sensors located above the ground detect
these surface vibrations by differences in amplitude and frequency.
These systems are capable of detecting mechanical differences between soil and mines
and could complement GPR and EMI detection systems. One field study indicated that these
systems are proficient methods for the detection of AT mines due to a 95% detection rate with
lower occurrences of false alarms.6 These systems have the potential for low false alarm rates
from natural occurring clutter (rocks or scrap metal), but hollow items such as cans or bottles
may produce false alarms as the resonance signals are very similar to those of mines. A
disadvantage of these systems is the inability to detect deeply buried mines, as the resonant
signal decreases significantly with the depth. The scan speeds for these systems are also
considerably slow (in the range of 2 to 15 min per m2).6 These systems are not susceptible to
environmental conditions or weathering, but frozen soil and vegetation may affect the sensor's
capabilities.
20
1.4.4 Biological and Biomimetic Systems
These systems employ the use of mammals, insects, vegetation, and microorganisms for
the detection of landmines by sensing trace vapor chemical signatures released by landmines into
the soil and above the ground surface. Unlike the systems previously discussed which rely on
mechanical or electromagnetic properties for detection, these systems are classified by the ability
to locate buried landmines by the explosive materials incorporated in landmines. By focusing on
the explosive compounds, these methods have the potential to reduce false alarms. This section
will assess the main principles for the detection of landmines citing both the advantages and
disadvantages associated with each system.
Canines, Rats, and Pigs
Canines are commonly employed for humanitarian demining because of their keen sense
of smell and low false alarm rate. Their unique sense of smell can detect a wide range of
explosive materials and explosive vapors at very low concentrations. Canines are known to be
able to detect explosive vapor concentrations comparatively lower than any other chemical
sensor available. Canines are trained to sit when they sense explosive residues escaping from
buried landmines and are rewarded when they correctly identify the explosive vapor. After the
canine indicates the presence of a mine, a deminer will probe the area of interest. Another
technique known as remote explosive scent tracing mode allows the canines to smell samples
collected near suspected mine areas. If the canine indicates the presence of explosive vapors in a
sample from a certain area, the deminer will return to that area to locate the mine.6, 18 Canines
offer the ability to search a large area in a reasonable amount of time, work under many
environmental conditions, are easy to transport, and are highly reliable. The limitations
associated with using canines include cost and time of training, performance variations
21
depending on the canine, and the possibility distractions. The canines are also at a constant risk,
since they work directly in areas where the mines are located. Thus far, canines are expected to
be a mainstay in the detection of landmines for humanitarian demining.
African giant pouch rats provide an alternative option for the detection of buried
landmines, as they offer certain advantages over canines. These rats rely heavily on their sense of
smell due to poor eyesight, which makes them ideal for detection of explosive vapors. Like the
canines the rats are trained to associate the smell of explosives with a food reward.18 In the field,
rats indicate the presence of an explosive by stopping and scratching the area, allowing their
handler to mark the identified mine. Advantages of employing rats compared with canines are
lower cost and less training time. Training time and costs are much less compared with that of
canines, since fewer resources are needed to raise and maintain them. These rats are lightweight
and rarely detonate landmines when walking over them. A rat and its handler can search a
relatively large area (150 m2) in about half an hour and a larger number of them could be used in
an assigned area, further reducing the time to search the area.18 Some limitations associated with
the use of rats for landmine detection include their inability to indicate the type of explosive
material and the breed of rats must be considered with respect to climate and disease.18
Pigs are another source for landmine detection, as they are considered to have an
enhanced sense of smell.18 Pigs are considered very tranquil animals, and are not as easily
distracted when compared to canines. The pigs are trained by a four-stage process by which they
learn to locate mines by receiving food rewards, as with the training of dogs and rats. In the final
training stage, the pigs are taught to sit in order to indicate the presence of a mine; but this can
prove to be a difficult process, as sitting is not a natural response for pigs. The limitations
associated with pigs for detecting landmines include the challenging and time consuming
22
training process, difficulty of transporting, and effects of local climate and disease. Although
there are very few successful documented trials of pigs detecting landmines, they do provide an
additional option for landmine detection.
Insects
Honeybees offer a unique option for humanitarian landmine detection, as they are
capable of covering large areas in a short amount of time, have an acute sense of smell, and their
bodies act as portable samplers with the ability to collect contaminants in the gaseous, liquid, and
particle forms. Honey bees were trained to locate mines by teaching the bees to associate the
smell of a nitroaromatic with a possible food source.6, 18, 19 When trained, the bees would hover
over the explosive plume for a few seconds, indicating the presence of a mine. Researchers
developed an light detection and ranging system (LIDAR) capable of detecting flying bees that
were trained to locate buried mines.19 This remote standoff system utilizes a laser light emitted
over the area the where the bees fly; the light then strikes the bees and is scattered back and
collected by a receiver. The time between the outgoing laser pulse and the return signal is used to
measure the distance from the bees to the LIDAR, which provides both the range and coordinates
of the bees over the landmine. One advantage bees offer as landmine detectors is their ability to
detect buried landmines by explosive vapor plumes at low concentrations, limiting false alarms.
Bees are also capable of scanning large areas, require short training time, and are a means of a
more remote standoff detection system. While bees offer some advantages, there are also
challenges associated with using them. Bees are influenced by environmental conditions (climate
and weather) which limit their use based on the season. The main limitation with the LIDAR
system is distinguishing signals between bees and vegetation or other interfering objects. Bees
are also difficult to track and specialized equipment may be required to improve locating the
23
bees. Although bees provide a unique alternative for landmine detection, it appears that they
could be considered a less reliable and accurate detection system than other options.
Bacterial Biosensors
The general description for biosensors defines them as sensing devices that combine a
biological recognition element to a physiochemical transducer. When a specific interaction
occurs between the recognition element and targeted analyte, it produces a physiochemical
change, which is detected by the transducer. This provides a signal which is proportional to the
concentration of the targeted analyte.18 A reporter gene is incorporated into the biosensor, and
the biosensor will then fluoresce when the biosensor encounters the explosive material. Scientists
have engineered a strain of bacteria that fluoresces under laser light when it encounters TNT.
This system is known as the Microbial Mine Detection System (MMDS).6, 18 Generally, this
method requires spraying the engineered bacteria over a mine-affected area, likely by airplane.
The bacteria are allowed to grow for a few hours, allowing them the opportunity to absorb any
explosive residues present. A survey is then conducted in order to review the mine-affected area
by searching for fluorescent signals, indicating the location of the landmines.
The bacterial biosensors offer a system that can be engineered to detect specific explosive
materials such as TNT and other similar structure analogs, therefore minimizing the likelihood of
false alarms. The sensors can be applied to large areas in a short amount of time and allow
remote standoff detection. The cost of this method could be considered moderate, and even less
expensive depending on the search area.
The main limitation with the MMDS is that the bacteria are extremely sensitive to
environmental conditions. The bacteria cannot survive in extreme temperatures, and also cannot
be applied to areas with dry soil, since the soil would absorb the bacteria and eliminate a
24
detectable signal. The performance of the system is dependent upon the transport of explosive
residues in the soil, which could provide inaccurate locations of the mines. Public concerns
regarding the application of engineered bacteria could also limit this application.
1.4.5 Bulk Explosive Landmine Detection Systems
Bulk detection systems emphasize finding the bulk explosive contained within the mine
rather than trace explosive residues or vapors. The ability to detect bulk explosives helps reduce
the rate of false positives from clutter and other factors. A variety of these systems uses neutrons
to exploit their interaction with the 14N nuclei present in explosives.
Nuclear Quadrupole Resonance (NQR)
Nuclear quadrupole resonance (NQR) is a radio frequency based technique that exploits
the resonant response from the 14N nuclei present in explosive materials, therefore locating and
providing an estimated quantity and/or depth of the mine.6, 8, 14 NQR induces a radio frequency
pulse of an appropriate frequency in the subsurface by a coil suspended above the ground. The
radio frequency pulse causes the 14N nuclei in explosives to resonate and induce an electrical
potential in the receiver coil. The resonant signals' frequencies oscillate between 0.5 to 6 MHz
and are characteristic of a specific explosive.6, 8, 14
A significant aspect of NQR is its specificity to landmines due to signals only produced
when specific bulk explosive materials are present. NQR has a high detection rate and low
occurrences of false alarms; its alarm rate is driven by signal to noise ratio rather than ground
clutter, which affects other detection systems. Another progressive feature of NQR is that by
allowing sufficient inspection time, the detection rate is increased with a false alarm rate
approaching zero. NQR is also capable of success in diverse soil conditions and only detects the
presence of bulk explosive materials rather than simply detecting explosive residues.
25
One limitation of this system, however, is that the nuclear properties of TNT, the major
explosive material in many mines, has a substantially weak signal that may pose a considerable
signal to noise problem. NQR is also susceptible to radio frequency interferences, as the
frequencies required to detect TNT are in the AM frequency range.6, 14 Additionally, NQR is not
capable of detecting landmines with metal casings because the radio frequency is unable to
penetrate them or detect liquid explosives within a landmine.6 These restrictions are not
considered significant factors, since most modern mines are encased with plastics and are not
composed of liquid explosives. The detection rate is also susceptible to the distance between the
coil and bulk explosive; the coil is suspended very close to the surface, which may be difficult in
rough terrain or in areas where vegetation is present. Stationary detection is ideal for achieving
the best results, since motion detection reduces the signal to noise ratio and increases the time to
inspect an area. NQR has certain advantages over other detection systems, as it is capable of
detecting bulk explosives and is not affected by ground clutter or explosive residues. Conversely,
due to interferences from AM radio frequencies, this system's performance may be considered
unreliable for detecting landmines containing TNT.
Neutron Detection systems
Neutrons have the ability to pass easily through matter, interacting with atomic nuclei.
Neutron detection systems are capable of detecting mines by using gamma rays or neutrons. The
interactions between the neutrons and matter produce gamma rays or charged particles, which
provide a unique signature regarding the nucleus and chemical element with which they are
interacting. These provide elemental information to distinguish the mine from its surrounding
environment, since explosives are rich in hydrogen and nitrogen.6, 10 The observation of
variances in intensity, energy, and returning radiation allow identification of the bulk explosive
26
contained within the mine. There are many possible reactions involving neutrons or gamma rays,
but only three have shown potential to be used for landmine detection: thermal neutron analysis,
fast neutron analysis, and neutron moderation.6, 10 Thermal neutron analysis relies on the
emission of specific gamma rays from the nitrogen nuclei when thermal neutrons are captured.6,
10 This technique was used by the Canadian military as a means of detecting AT mines, but was
not as effective in detecting AP mines due to low amounts of explosive materials.20 Fast neutron
analysis employs the use of fast neutrons to excite the nuclei of the soil and mine by inelastic
scattering, causing the backscattered slow neutrons to be detected.6, 10 The potential advantage of
this system is that it could be used to detect explosives from soil by measuring the carbon,
hydrogen, and oxygen concentration ratios. Lastly, neutron moderation involves scanning an area
with neutrons from a low strength radiation source, and detecting the returning moderate and
slow neutrons.6, 10 This method observes anomalies produced from hydrogen nuclei, as
explosives contain two to three percent hydrogen by weight, compared to the soil which can
contain as little as zero percent to more than fifty percent hydrogen. The hydrogen density
anomaly could thereby be used to indicate the presence of a mine.
Neutron detection systems show promising potential to aid in the uncovering of
landmines by exploiting interactions between neutrons and nuclei present in the bulk explosive
material contained within the mine. Neutron moderation uses a low strength radiation source,
which reduces the shield requirements to protect deminers from radiation exposure. This allows
the potential to develop a hand held system in the future. By focusing on detecting the bulk
explosive material, a low false alarm rate can be achieved, since the system would not be
affected by ground clutter that affects other detection systems.
27
The main limiting factors for these systems are the large/heavy shielding requirements
and the possibility of public exposure to radiation. These systems are not capable of providing
information regarding the molecular structure present and are affected by ground surface
fluctuations and sensor height variations, increasing the potential of false alarms. These systems
are also sensitive to the nuclei of interest (C, N, O, and H), which are found in both the mine and
soil, making detection difficult and increasing the possibility of false alarms. Although these
systems do possess some ideal detection characteristics, the cost and concerns with working with
nuclear systems limit them from being applied for humanitarian demining.
1.4.6 Chemical Landmine Vapor Detection Systems
Chemical vapor detection systems are currently being researched and developed for
detecting landmines by identifying low concentrations of explosives in the air and soil. These
systems exploit the trace escaping vapors and residues from the bulk materials within the mine.
The development of these systems is based on the capability to distinguish a specific explosive
signature related to the particular nitroaromatic compounds of the explosive material. A variety
of vapor detection systems and techniques have been studied for landmine detection including:
electrochemical methods, fluorescence techniques, ion mobility spectrophotometers, polymer
coated sensors, and electronic nose systems.7, 13 The potential advantage these systems offer is
that they could be engineered to be lightweight, portable, small, inexpensive, and easy to operate.
Important criteria for these systems include low limits of detection (ppt), short response time,
and minimal false alarms. For many of these systems, the detection limit is not adequately
sensitive for trace vapor detection. Another limitation includes the ability to develop a sensor
with a low false alarm rate, as nitroaromatic residues other than explosives could potentially
28
produce a false alarm. These systems appear to be a promising option for humanitarian
demining, but further work is required to improve the detection limits and eliminate false alarms.
1.5 Explosive Detection Techniques
Similar to the detection of explosive materials in landmines, there is an ever-growing
concern for the detection of explosive materials being used by terrorist organizations in other
applications. Terrorists have been able to utilize explosives to attack civilian transportation
(airplanes, buses, etc.), federal buildings, and large public areas. As a result, security agencies
are focusing on establishing preventative measures to detect hidden explosives efficiently. Some
systems are already in use at airports and federal buildings, but like all systems, include
limitations that prevent the detection of all hidden explosives. The following section will review
current and developing explosive detection systems.
1.5.1 Bulk Explosive Detection Systems
These detection systems emphasize the identification of bulk explosive materials rather
than trace explosive residues or vapors. Bulk detection systems employ the use of penetrating
radiation that interacts with certain nuclei present in the explosive, which produces a specific
signal characteristic of the explosive. These systems provide a way to screen for hidden
explosives in an effective and non-intrusive manner. Even though these systems are considerably
large, many are already in use today at airports, docks, and government buildings.
X-Ray Detection Methods
X-ray detection systems are primarily used for identifying hidden explosives and
weapons, namely in luggage for airport security. X-rays offer the ability to provide information
about an object's density and effective atomic number.21 From the density and effective atomic
number, the materials of an object under investigation can be further identified. High-density
29
materials absorb more energy and appear darker in the image compared to low density materials,
which appear lighter at high energy levels. Low-density materials, such as explosives, rich in
nitrogen and oxygen, appear darker in the image at lower energy levels allowing low-density
materials to be identified by their effective atomic number. There is a variety of X-ray methods
used for airport security screening, with the most common being conventional transmission
imaging, dual energy X-ray imaging, scatter imaging, and 3-D imaging.13, 21-23
Dual Energy X-rays
Dual energy X-ray screening systems are an effective, non-intrusive system used to
screen for weapons and explosive materials.21 These systems employ both high energy and low
energy X-rays to image objects. Using high energy x-rays (>100 kV), high-density materials
appear darker in the image, since denser objects absorb more energy.22, 23 Conversely, low-
density materials appear lighter in the image. This allows for clearer image screening for metal
objects and potential weapons. An obstacle associated with X-rays is that an explosive material
could be concealed behind a high-density object, preventing the explosive from being identified.
To address this issue, objects are scanned with low X-ray energies (< 80 kV), as the absorption is
dependent on the effective atomic number and thickness of the material.22, 23 Since most
chemical explosives are rich in nitrogen and oxygen, explosive materials concealed behind high-
density materials can be identified by appearing darker in the image and analysis of the material's
atomic number. Advantages associated with dual energy X-ray systems include the ability to
distinguish a material based on shape, whether or not it contains metal properties, its ability to
identify materials by effective atomic number, and cost efficiency. Limitations associated with
X-ray systems include the difficulty to distinguish objects from each other in an image, and they
30
cannot accurately determine an object's density in order to produce an estimated effective atomic
number.
Neutron Detection Systems
Neutron detection systems are similar to other methods used for landmine detection in
that they can determine elemental composition, have greater penetration depths, difficult to
shield materials from the probing radiation, and are capable of detecting nuclear materials.21
Common techniques for neutron analysis are thermal neutron analysis (TNA), fast neutron
analysis (FNA), pulsed fast neutron analysis (PFNA), pulsed fast thermal neutron analysis
(PFTNA), and nuclear resonance absorption (NRA).24 Table 1.5.1.1 provides a brief summary of
these techniques that includes the probing radiation, nuclear reactions, detected radiation, and
detected elements.
31
Table 1.5.1.1. Neutron analysis techniques.24
# Technique Probing Radiation Nuclear Reaction
Detected Radiation
Detected Elements
1 TNA Thermalized neutrons n, ɣ Neutron capture
ɣ- rays Cl, N, H, P, S,
nuclear materials
2 FNA Fast neutrons (14 MeV) n, n' ɣ
ɣ-rays produced from
inelastically scattered neutrons
O, C, N, H, Cl, P
3 PFNA Nanosecond pulses of fast neutrons n, n' ɣ
ɣ-rays produced from
inelastically scattered neutrons
O, C, N, Cl, H, Metals, Si, P, S, nuclear materials
4 PFTNA
Pulsed neutron source: fast neutrons
during the pulse, thermal neutrons between pulses
(n, n' ɣ) + (n, ɣ)
During pulse #2 + after pulse -
#1
N, Cl, H, C, O, P, S, nuclear
materials
5 NRA Nanosecond pulsed
fast neutron (0.5 - 4 MeV)
n, n
Elastically and resonantly scattered neutrons
H, O, C, N
32
1.5.2 Spectroscopic Explosive Detection Systems
Ion Mobility Spectrometry (IMS)
Ion Mobility Spectrometry (IMS) is used extensively in airports as a means of detecting
trace level concentrations of explosives on luggage. IMS is efficient in explosive detection
because it provides quantitative chemical information, structural information, low detection
limits, short analysis time, and low false alarm rates.21, 22, 25 IMS identifies explosives by their
mass/charge ratio and mobility. IMS instruments consist of an ion source (63Ni), ion gate, a drift
region, and a detector.21, 22, 25 IMS ionizes sample vapors at atmospheric pressure in the ion
source region. The ions are then injected into the drift tube where an electric field (≈ 200 V/cm)
is applied by the ion gate. Ions then travel in the electric field toward the detector, producing a
signal by collision neutralization. From the magnitude and position in time of the peak signals,
the sample vapor can be identified. Previous experiments have shown that IMS is able to detect
low concentrations of high vapor pressure explosives like TNT, but has difficultly detecting low
vapor pressure explosives such as RDX and HMX.25 False alarms can be produced in IMS from
nitrated compounds that have similar structures to explosive compounds, since IMS is not able to
differentiate nitrated compounds.26
Mass Spectrometry (MS)
Mass spectrometry (MS) has been extensively studied as an explosive detection system
due to its sensitivity and selectivity. MS separates ions based on their mass-to-charge ratio.27 The
sample vapor enters a high vacuum chamber where it is ionized by a variety of methods. Next,
the ions are accelerated into the spectrometer where they are separated by two approaches: time
separation or geometric separation. A variety of MS techniques has been successful in detecting
explosives with low limits of detection.21, 28-30 In recent years, issues with size, portability, and
33
power requirements for MS have been addressed and have led to the development of miniature
and mobile spectrometers.21 MS spectrometers have also been incorporated into personnel
screening portals at airports and federal buildings, allowing a wide variety of explosives to be
detected in a short amount of time.21
Terahertz Spectroscopy (THz)
Terahertz spectroscopy and imaging are emerging detection techniques that have the
ability to distinguish and identify hidden explosives, metallic weapons, and illegal drugs from
other materials. Three aspects generating interest in these systems are: (1) terahertz radiation is
able to transmit through non-metallic and non-polar mediums (2) hazardous materials
(explosives, biological agents, and chemical agents) have unique fingerprint characteristic THz
spectra that can be used to identify the materials, and (3) Terahertz radiation does not pose a
health risk to the individual being scanned or the system's operator.21 Terahertz radiation falls
between the microwave and infrared regions in the electromagnetic spectrum (0.1 to 10 THz).
THz spectroscopy exploits crystal lattice vibrations, hydrogen bond stretches, and intermolecular
vibrations of molecules in explosives. These compounds produce unique fingerprint spectral
signatures that allow these systems to identify explosives in both the pure, crystalline, and plastic
forms. THz spectroscopy and imaging have been able to identify common explosives (TNT,
RDX, HMX, and PETN) in various states using different techniques.31, 32 There are a few topics
that must be addressed regarding THz systems which include increasing the frame rate for real
time imaging, portability, hand held size detectors, cost, and power requirements.
Raman Spectroscopy
Raman spectroscopy is an analytical technique that examines molecular motions and
fingerprinting species through vibrational transitions after a molecule has experienced a laser
34
excitation.30, 33 During the instantaneous Raman process, some of the energy is lost to or gained
from the molecule under investigation. The returning scattered light is at a different wavelength
due to inelastic scattering. The energy difference correlates to the vibrational or rotational energy
of the molecule. By probing these vibrational modes and analyzing the resulting spectra, the
vibrational modes provide a fingerprint that allows identification of individual components of the
molecule. Raman spectroscopy offers several advantages as a detection system such as:
fingerprint identification, application to a variety of optically accessible samples, the ability of
solid, liquid, or gaseous samples to be analyzed, no sample preparation, non-invasive, detection
can be performed over a wide region of the spectrum (UV-NIR), detection can be performed
during the day or night, and construction of a portable detection system.34 A variety of Raman
spectroscopy techniques have shown the ability to detect explosive materials and the
development of portable Raman explosive detectors can detect explosives from a standoff
distance.21, 28, 33-35 Two principle concerns with Raman spectroscopy detection are that the
Raman signal has a weak intensity and fluorescence occurs when using near UV/Vis
wavelengths.
Laser-Induced Breakdown Spectroscopy (LIBS)
Laser-Induced Breakdown Spectroscopy (LIBS) is a relatively new technique that has
presented the ability to detect explosive materials optically.21, 28, 33, 36 LIBS is a spectroscopic
technique that relies on light emitted from a focused laser pulse to generate a micro plasma. The
plasma emits light with frequencies characteristic of the atomic, ionic, and molecular fragments
in the plasma plume. The line emissions collected by a spectrophotometer generate a spectrum
that allows elemental composition identification. LIBS as an explosive detection technique
presents several advantages, which include: no sample preparation, ideal sensitivity (ng to pg),
35
real time response, field portability, miniaturized components, and either point or standoff
detection.36 LIBS is capable of identifying energetic materials based on C:H:N:O elemental
ratios, since explosive materials contain more oxygen and nitrogen relative to carbon and
hydrogen. An obstacle associated with standoff explosive detection using LIBS is interference
from atmospheric oxygen and nitrogen. The atmospheric oxygen and nitrogen affect the N:O
ratio, complicating the identification of the explosive materials. This issue has been addressed
using pulsing techniques to reduce the effects from atmospheric oxygen and nitrogen. Even
though the focused laser pulses used to generate the micro plasma has not been reported to ignite
secondary explosives in laboratory experiments, there are concerns that the laser pulses might
possibly ignite bulk amounts of extremely sensitive explosives.
1.5.3 Olfactory Explosive Detection Systems
Animals
In section 1.4.4 Biological and Biomimetic Systems for Landmine Detection, the use of
animals, primarily canines, to detect explosive materials by their sense of smell was reviewed.3,
13, 21, 28, 37 Canines are renowned as the most commonly deployed detector for sensing explosives.
Several government and private agencies rely on the canines' sense of smell for detecting illegal
drugs, explosives, human remains, and human scent. The use of canines as a reliable explosive
detector is based on the sensitivity and specificity of the dog's sense of smell. A trained canine
has the capability of identifying a variety of explosive odor signatures. While canines represent
the fastest and most dependable explosive detector to date, the cost and time associated with
training and maintaining canines, behavioral variations, and workload performance are a few
disadvantages associated with the employment of canines.
36
Electronic Noses
The advantages associated with a canine's sense of smell created interest for researchers
to investigate the possibility of developing an artificial sensor that could mimic the canine's
selectivity and sensitivity without the obstacles presented by actually using canines. An
electronic nose typically consists of an array of sensing elements that are capable of interacting
with a vapor in a variety ways, coupled with a pattern recognition system.38 The interaction
between the vapor and sensing elements produces a signature response "fingerprint" that allows
the pattern recognition system to analyze the response, identifying the analyte. The array of
sensors provides sensitivity and selectivity to a wide range of analytes, component analysis, and
analyte identification. Electronic nose explosive detection sensors promote the development of
relatively inexpensive miniature sensors, which are ideal for the use for government and private
agencies. A variety of electronic nose sensors have been used for explosive detection such as:
fluorescent polymers, surface acoustic wave detectors, fiber optics and beads, polymeric thin
films, nanoparticle nanoclusters, microelectrochemical systems, and quartz crystal
microbalances.21, 29, 38-41 Many of these sensors have shown great potential for explosive
detection under laboratory conditions, but more work is needed for the development of these
sensors to be used in the field.
1.5.4 Chemical Sensors for Explosive Detection
Electrochemical Sensors
Electrochemical sensors are able to detect explosives by monitoring a signal that is
produced by changes in the electric current between the electrodes interacting with an explosive.
Electrochemical sensors are categorized into three groups: (1) potentiometric (measurement of
potential difference or voltage), (2) amperometric (measurement of current), and (3)
37
conductometric (measurement of conductivity).2, 28 Due to the redox properties of nitroaromatic
explosives, the ease of reducing the nitro groups are ideal for electrochemical detection. The
reduction processes of the nitro groups are dependent on pH, the number of nitro groups present,
position of the nitro groups, and the presence of substituents. Typically, the trinitroaromatic
species (i.e. TNT) are more easily reduced compared to dinitro or mononitroaromatic
compounds, nitramines, polynitrate esters, or peroxide base explosives. Advantages of using
electrochemical sensors include fast response times, cost efficiency, excellent sensitivity, low
detection limits, and miniature components for hand held devices. A wide variety of
electrochemical sensors and techniques have been used for detecting explosives such as:
electrode strips, on-line flow analysis, remote monitoring, gas phase detection, and lab on chip
detection.2, 10, 21, 28, 29, 42
Biosensors for Explosive Detection
Biosensors are analytical sensing devices that incorporate a biological recognition
element capable of producing a specific biological interaction with the targeted analyte and a
signal transducer. A variety of engineered biological recognition elements such as enzymes,
antibodies, peptides, single stranded DNA, etc. have presented the ability to detect explosives.2,
10, 28, 29, 43 Beneficial aspects of using biosensors include high specificity, mass production, long-
term storage, and commercially availability.43 Typically, biosensor methods for explosive
detection are based on solution phase detection from water or soil, but there are some examples
that have shown the ability to detect high vapor pressure explosives in the gaseous state. This
presents a limitation for biosensors for standoff detection for low vapor pressure explosives, as
there must be a direct interaction between receptor and explosive molecules. The time required
for the interaction between the recognition element and explosive molecule to produce a signal
38
may require an extended amount of time (on the order of sec., min., hrs., or days). This
uncertainty of the amount of time required for producing a signal presents problems for
expedited real time detection.
Polymer Sensors for Explosive Detection
Varieties of polymers have been synthesized for use in detecting explosives. Polymers
have demonstrated the ability to act as sensors, and be incorporated in detection systems as the
sensing elements for explosive detection. Polymers have been incorporated into such systems as
electronic noses, surface acoustic wave sensors, micro-cantilever sensors, fiber optic sensors,
fluorescence systems, luminescence sensors, etc.2, 21, 28, 29, 39-41 Polymers are capable of
interacting with explosive molecules by a wide range of interactions. Polymer sensors are very
advantageous components used in explosive detectors due to ease of synthesis, variety of
interactions with explosive vapors or particles, application to numerous systems, feasible
fabrication techniques, and cost efficiency. Polymer sensors can also achieve high specificity by
developing molecularly imprinted polymers with recognition sites for a specific target analyte.44
Many polymer sensors have shown adequate limits of detection in laboratory experiments, but
elimination of false positives in field experiments are still necessary to accurately determine the
reliability of polymer sensors for explosive detection.
1.5.5 Explosive Sensors Summary
The explosive sensors previously discussed focus on detecting trace vapors or residues
from explosive compounds. Table 1.5.5.1 provides information comparing the limit of detection
ranges for some explosive sensors currently in use and being developed. These sensors have
demonstrated adequate detection limits (in the ppt - ppm range), which could potentially detect
high explosives with low vapor pressures. These sensors provide the ability to detect explosives
39
in real time and do not require samples to be analyzed in the lab. Explosive sensors are
vulnerable to false positives, but intense efforts are being investigated to eliminate false
positives, increasing their reliability. Skepticism should be considered for the reported detection
limits, since there is not a universal testing standard for determining limits of detection, making
comparisons between sensors' sensitivity difficult. The cost, size, and operation associated with
these sensors should be considered, since these three factors would preclude some systems from
being utilized at security checkpoints.
Table 1.5.5.1. Explosive sensors limit of detection ranges.
1.6 Mach-Zehnder Interferometer Optical Waveguide Sensor
Mach-Zehnder interferometers (MZI) are extremely sensitive sensors of optical phase and
are capable of detecting very small changes in refractive index. A waveguide Mach-Zehnder
interferometer was designed to detect triazine at concentrations as low as 100 ng/L.45 A
waveguide sensor in the form of a Mach-Zehnder interferometer was capable of detecting optical
phase changes less than 2.2 π milliradians and was sensitive to refractive index changes of 10-6.46
The sensor could detect the difference in refractive index between pure water and a solution
containing 0.007% glucose (Δn ~ 10-5). A Mach-Zehnder interferometer gas sensor was capable
of detecting perchloroethylene with the limit of detection being 100 ppm using polysiloxane
Explosive Sensor Limit of Detection Range
Electrochemical ppb - ppm
IMS pg – ng
Electronic Nose ppt – ppm
Polymer ppt – ppm
Biosensor ng/L - pg/L
Canines ppt – ppm
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40
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41
1.7 Research Objectives
The main objective of this research project was to synthesize polymers, which could be
used as sensing materials for a MZI explosive detection sensor. Our objective was to synthesize
polymers containing electron rich aromatic monomers. Our interest particularly focused on
synthesizing random electron rich copolymers with the electron rich monomer, 2-vinyl-4,6-
diamino-1,3,5-triazine (VDAT). The VDAT electron rich copolymers could potentially have a
strong affinity for nitroaromatics due to hydrogen bonding between the amino group on the
polymer chain and the nitro groups or an electrostatic affinity toward the electron deficient
nitroaromatics by a complex formation.
To determine if the random electron rich copolymers had an affinity for nitroaromatics,
spin coated polymer films' refractive indices would be determined before and after exposure to a
nitroaromatic vapor using spectroscopic ellipsometry. An increase in the polymer film's
refractive index after exposure to a nitroaromatic vapor would confirm the polymer's affinity
toward the nitroaromatic. No change in the refractive index after exposure to a nitroaromatic
vapor would suggest that the polymer did not have an affinity for the nitroaromatic.
The last objective was to investigate new materials for sensing nitroaromatics by growing
co-crystals between electron rich donors and nitroaromatics. If a strong interaction was observed
between the electron rich donor and nitroaromatic, this would suggest a reagent to be used for
synthesizing an electron rich copolymer. Characterizing this strong interaction would also
provide some insight on the possible interaction between the random electron rich copolymers
and the nitroaromatics.
42
Chapter 2
Experimental
2.1 Sources of All Chemicals
2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) was purchased from Tokyo Kasei Kogyo Co.
Sodium persulfate, 1,4-dioxane 99+%, 1,3-dinitrobenzene 97%, methyl acrylate 99%,
2-vinylpyridine 97%, 2,2'- azobisisobutyronitrile 98%, acrylamide 97%, 1-vinylimidazole, 2,4-
diamino-6-methyl-1,3,5-triazine 98%, 10-methylphenothiazine 98%, 9-ethylcarbazole 97%, 9-
vinylcarbazole, phenothiazine 98+%, and styrene reagent plus ≥ 99%, were purchased from
Sigma Aldrich. Toluene HPLC grade and methyl ethyl ketone were purchased from Fisher
Scientific. Nitrobenzene ACS reagent grade, carbazole 96%, and methyl methacrylate were
purchased from ACROS. 2-Nitrotoluene 99+%, 3-nitrotoluene 99+%, benzoguanamine 99%,
Chloroform-d 99.8% were purchased from Alfa Aesar. Poly(4-Vinylpyridine) M.W. 300,000
was purchased from Polysciences Inc. Polyvinylimidazole M.W. 3,500 and polyvinylimidazole-
co-polyvinylaniline copolymer, M.W. 7,000, 5 mol % aniline were purchased from Selective
Technologies Inc. Acetonitrile was purchased from BDH. Methyl sulfoxide was purchased from
EMD. Dimethyl sulfoxide-d6 was purchased from Cambridge Isotope Laboratories Inc. Ethanol
and methanol was purchased from the chemistry stockroom. 2,2'-azobisisobutyronitrile 98%,
(AIBN) was recrystallized from methanol prior to using as a free radical initiator.
43
2.2 Polymer Syntheses
2.2.1 Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVDAT)
PVDAT was synthesized according to the method of Chen and Sun.49 For the PVDAT
synthesis, 4 mmols (0.55 g) of VDAT and 25 mL of D.I. H2O were transferred to a 250 mL three
neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a
condenser under a nitrogen atmosphere, equipped with a stir bar, and a thermometer. When the
reaction temperature inside the round bottom flask reached approximately 75 °C, 1.39 mmols
(0.33 g) of Na2S2O8 dissolved in 5 mL of D.I. H2O was added to initiate the free radical
polymerization shown in Scheme 2.2.1.1. The reaction mixture was heated to 95 °C and held for
two hours, dissolving the monomer and producing a colorless solution. After two hours, the free
radical polymerization was terminated by allowing the polymer solution to cool to room
temperature. After the solution cooled to room temperature, the polymer solution was transferred
to a 600 mL beaker. Excess MeOH (≈ 250 mL) was added, precipitating a white polymer, and
was stirred for several minutes. The polymer was collected by filtration and washed with copious
amounts of MeOH and D.I. H2O. The polymer was then dried under vacuum at 50 °C overnight.
The free radical polymerization produced an 87% yield (0.48 g) of PVDAT.
N
N
N
NH2H2N
N
N
N
NH2H2N
n
D.I. H2O at 95°CNa2S2O8
Scheme 2.2.1.1. Free radical polymerization of PVDAT.
44
2.2.2 Polystyrene-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PS-co-PVDAT)
PS-co-PVDAT copolymers were synthesized according to the method of Chen and Sun.49
Scheme 2.2.2.1 shows the free radical polymerization for the PS-co-PVDAT polymerization.
Styrene was purified by distillation under reduced pressure and VDAT was used as received.
DMSO was purified by distillation under reduced pressure from CaH2. For the PS-co-PVDAT
free radical polymerizations, the VDAT mol % concentrations were varied from 1, 5, 10, and 20
with styrene to equal 0.1 total mols. In the synthesis for the 20 mol % PS-co-PVDAT copolymer,
0.08 mols (8.33 g) of styrene and 0.02 mols (2.74 g) of VDAT were added to a 250 mL three
neck round bottom flask. The flask was placed in an oil bath on a hot plate fitted with a
condenser containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, equipped
with a stir bar, and a thermometer. Once the reaction temperature inside the round bottom flask
reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate
the free radical polymerization. The reaction temperature was increased to approximately 80 °C
and stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction
solution appeared colorless. During the reaction, the solution gradually changed from colorless to
yellow, indicating the reaction had come to completion. The polymer solution was allowed to
cool to room temperature and was then transferred to a 600 mL beaker. The copolymer was
precipitated in excess EtOH (approximately 250 mL), producing a white polymer, and was
stirred for several minutes. The polymer was collected by filtration and washed successively with
copious amounts of EtOH and D.I. H2O. The copolymer was then dried under vacuum at 50 °C
overnight. Washing the copolymer in excess EtOH typically removed all the unreacted
monomer. In some instances, Soxhlet extraction with EtOH or stirring the polymer in EtOH
under reflux in a one neck round bottom flask fitted with a condenser was performed to remove
45
any unreacted monomer. Polystyrene was synthesized by the same experimental procedure.
Table 2.2.2.1 lists the experimental amounts of VDAT, styrene, AIBN, DMSO, yields, and
temperature for PS-co-PVDAT copolymers and polystyrene polymerizations.
Table 2.2.2.1. Experimental amounts and conditions for PS-co-PVDAT polymerizations.
Polymer VDAT (mol %)
VDAT (mols)
VDAT (g)
Styrene (mol %)
Styrene (mols)
Styrene (g)
Yield (%)
Yield (g)
Temp. (°C)
PS-co-PVDAT 20 0.02 2.74 80 0.08 8.33 49 5.42 ≈ 80 °C
PS-co-PVDAT 10 0.01 1.37 90 0.09 9.37 34 3.64 ≈ 80 °C
PS-co-PVDAT 5 0.005 0.69 95 0.095 9.89 17 1.81 ≈ 80 °C
PS-co-PVDAT 1 0.001 0.14 99 0.099 10.31 11 1.17 ≈ 80 °C
PS 0 0 0 100 0.1 10.41 22 2.29 ≈ 80 °C *All polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO.
Scheme 2.2.2.1. Free radical polymerization for the synthesis of PS-co-PVDAT random copolymers.
2.2.3 Poly(methyl methacrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
(PMMA-co-PVDAT)
PMMA-co-PVDAT copolymers were synthesized according to the method of Chen and
Sun.49 Scheme 2.2.3.1 displays the free radical polymerization for the PMMA-co-PVDAT
copolymers. MMA was purified by distillation under reduced pressure and VDAT was used as
46
received. DMSO was purified by distillation under reduced pressure from CaH2. For the PMMA-
co-PVDAT free radical polymerizations, the VDAT mol % concentrations were varied from 1, 5,
10, and 20 with MMA to equal 0.1 total mols. In the synthesis for the 20 mol % PMMA-co-
PVDAT copolymer, 0.08 mols (8.01 g) of MMA and 0.02 mols (2.74 g) of VDAT were added to
a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted
with a condenser containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and
equipped with a stir bar and thermometer. Once the reaction temperature inside the round bottom
flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to
initiate the free radical polymerization. The reaction temperature was increased to approximately
80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMSO, the
reaction solution appeared a white-brown color. During the reaction, the solution gradually
changed from white-brown to yellow in color, indicating the reaction had come to completion.
The polymer solution was allowed to cool to room temperature and was then transferred to a 600
mL beaker. The copolymer was precipitated in excess MeOH (approximately 250 mL),
producing a white polymer with a faint white-brown tint and was stirred for several minutes. The
polymer was collected by filtration and washed successively with copious amounts of MeOH
and D.I. H2O. The copolymer was then dried under vacuum at 50 °C overnight. To remove
unreacted monomer from the copolymers, the copolymers were washed in MeOH under reflux
while being stirred in a one neck round bottom flask fitted with a condenser. PMMA was
synthesized by the same experimental procedure. Table 2.2.3.1 lists the experimental amounts of
VDAT, MMA, AIBN, DMSO, yields, and temperatures for the PMMA-co-PVDAT copolymers
and PMMA polymerizations.
47
Table 2.2.3.1. Experimental amounts for the PMMA-co-PVDAT copolymers and PMMA polymerizations.
Polymer VDAT (mol %)
VDAT (mols)
VDAT (g)
MMA (mol %)
MMA (mols)
MMA (g)
Yield (%)
Yield (g)
Temp. (°C)
PMMA-co-PVDAT 20 0.02 2.74 80 0.08 8.01 65 6.96 ≈ 80 °C
PMMA-co-PVDAT 10 0.01 1.37 90 0.09 9.01 40 4.18 ≈ 80 °C
PMMA-co-PVDAT 5 0.005 0.69 95 0.095 9.51 34 3.43 ≈ 80 °C
PMMA-co-PVDAT 1 0.001 0.14 99 0.099 9.91 47 4.72 ≈ 80 °C
PMMA 0 0 0 100 0.1 10.01 54 5.37 ≈ 80 °C * All polymers were synthesized using 2 mmols of AIBN (0.33 g) and 100 mL of DMSO.
Scheme 2.2.3.1. Free radical polymerization for the synthesis of PMMA-co-PVDAT random copolymers.
2.2.4 Poly(methyl acrylate)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PMA-co-PVDAT)
The PMA-co-PVDAT copolymer was synthesized according to the method of Chen and
Sun.49 Scheme 2.2.4.1 shows the free radical polymerization for the PMA-co-PVDAT
copolymer. MA was purified by distillation under reduced pressure and VDAT was used as
received. DMSO was purified by distillation under reduced pressure from CaH2. For the PMA-
co-PVDAT free radical polymerization, the VDAT concentration was 20 mol %. For the
synthesis of the 20 mol % PMA-co-PVDAT copolymer, 0.08 mols (6.89 g) of MA and 0.02 mols
(2.74 g) of VDAT were added to a 250 mL three neck round bottom flask. The flask was placed
48
in an oil bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled
DMSO under a nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the
reaction temperature inside the round bottom flask reached 60 °C, 2 mmols (0.33 g) of
recrystallized AIBN was added to the solution to initiate the free radical polymerization. The
reaction temperature was increased to approximately 80 °C and was stirred for five hours. Once
the monomers and initiator dissolved in DMSO, the reaction solution appeared a white-brown
color. During the reaction, the solution gradually changed from white-brown to a bright yellow
in color, indicating the reaction had come to completion. The polymer solution was allowed to
cool to room temperature and was transferred to a 600 mL beaker. The copolymer was
precipitated in excess MeOH (approximately 500 mL), producing a white-yellow tinted soft
polymer and was stirred for several minutes. The polymer was collected by filtration and washed
successively with copious amounts of MeOH and D.I. H2O. The copolymer was dried under
vacuum at 50 °C overnight. The copolymerization of PMA-co-PVDAT (20 mol % VDAT)
produced a 45% yield (4.33 g). PMA was synthesized by the same experimental procedure,
producing a soft transparent polymer with a 53% yield (4.55 g). The precipitated homopolymer
was allowed to sit overnight to allow the polymer to collect in the bottom of the beaker. After
sitting over night, the DMSO was decanted, and the polymer was washed with MeOH and D.I.
H2O. The polymer was dried overnight under vacuum at 50 °C.
Scheme 2.2.4.1. Free radical polymerization for the synthesis of a PMA-co-PVDAT random copolymer.
49
2.2.5 Poly(2-vinylpyridine)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (P2VP-co-PVDAT)
P2VP-co-PVDAT copolymers were synthesized according to the method of Chen and
Sun.49 Scheme 2.2.5.1 shows the free radical polymerization for the P2VP-co-PVDAT
copolymers. 2-VP was purified by distillation under reduced pressure and VDAT was used as
received. DMSO was purified by distillation under reduced pressure from CaH2. For the P2VP-
co-PVDAT free radical polymerizations, the VDAT mol % concentrations were varied from 20,
5, and 1 mol %. In the synthesis for the 20 mol % P2VP-co-PVDAT copolymer, 0.04 mols (4.21
g) of 2-VP and 10 mmols (1.37 g) of VDAT were added to a 250 mL three neck round bottom
flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser containing 50
mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a stir bar and
thermometer. Once the reaction temperature reached 60 °C, 1.04 mmols (0.17 g) of recrystallized
AIBN was added to the solution to initiate the free radical polymerization. The reaction
temperature was increased to approximately 80 °C and was stirred for five hours. Once the
monomers and initiator dissolved in DMSO, the reaction solution appeared colorless. During the
reaction, the solution gradually changed from colorless to a red-orange color, indicating the
reaction had come to completion. The polymer solution was allowed to cool to room temperature
and was transferred to a 600 mL beaker. The copolymer was precipitated in 10% (w.t.) NaCl D.I.
H2O solution (250 mL), producing an orange polymer. The polymer was collected by filtration
and washed successively with copious amounts of D.I. H2O. The copolymer was dried overnight
at 50 °C, producing a brittle polymer. P2VP was synthesized by the same experimental
procedure using hexane to precipitate a white-orange powder polymer. Table 2.2.5.1 lists the
experimental amounts of 2-VP, VDAT, yields, AIBN, DMSO, and temperature for the
polymerizations.
50
Table 2.2.5.1. Experimental amounts for P2VP-co-PVDAT copolymers and P2VP polymerizations.
Polymer VDAT (mol %)
VDAT (mmols)
VDAT (g)
2-VP (mol %)
2-VP (mols)
2-VP (g)
Yield (%)
Yield (g)
DMSO (mL)
P2VP-co-PVDATa 20 10 1.37 80 0.04 4.21 31 1.71 50
P2VP-co-PVDATb 5 1.25 0.1714 95 0.0237 2.4971 50 1.33 25
P2VP-co-PVDATb 1 0.025 0.0343 99 0.0247 2.6022 74 1.96 25
Poly(2-VP)c 0 0 0 100 0.1 10.51 65 6.86 100
a 1.04 mmols (0.17 g) of AIBN was used as the free radical initiator b 0.05 mmols (0.0821 g) of AIBN was used as the free radical initiator c 2 mmols (0.33 g) of AIBN was used as the free radical initiator * All polymerizations were performed at approximately 80 °C
Scheme 2.2.5.1. Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers.
2.2.6 Poly(acrylamide)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PAM-co-PVDAT)
PAM-co-PVDAT copolymers were synthesized according to the method of Chen and
Sun.49 Scheme 2.2.6.1 shows the free radical polymerization for PAM-co-PVDAT copolymers.
Acrylamide and VDAT were used as received. DMSO was purified by distillation under reduced
pressure from CaH2. For the PAM-co-PVDAT free radical polymerizations, the VDAT mol %
concentrations were varied from 1, 5, 10, and 20 with acrylamide. In the synthesis for the 20 mol
51
% PAM-co-PVDAT copolymer, 0.1 mols (7.18 g) of acrylamide and 0.025 mols (3.43 g) of
VDAT were added to a 250 mL three neck round bottom flask. The flask was placed in an oil
bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled DMSO under a
nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the reaction
temperature inside the round bottom flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized
AIBN was added to the solution to initiate the free radical polymerization. The reaction
temperature was increased to approximately 80 °C and stirred for five hours. Once the monomers
and initiator dissolved in DMSO, the reaction solution appeared an opaque white color. During
the reaction, the solution gradually changed from an opaque white to a light yellow tint,
indicating the reaction had come to completion. The polymer solution was allowed to cool to
room temperature and was transferred to a 600 mL beaker. The copolymers were precipitated in
excess MeOH (approximately 200 mL), producing a white polymer with a slight yellow tint and
was stirred for several minutes. The copolymer consisted of a fine powder with large clumps.
The polymer was collected by filtration and washed successively with copious amounts of
MeOH. The copolymer was dried under vacuum at 50 °C overnight. The copolymers were
purified by washing in MeOH under reflux by stirring in a one neck round bottom flask fitted
with a condenser. Table 2.2.6.1 lists the experimental amounts of VDAT, acrylamide, AIBN,
DMSO, yields, and temperature for the PAM-co-PVDAT copolymers and PAM polymerizations.
The homopolymer, polyacrylamide, produced a percent yield greater than 100% (149%). This
percent yield may be attributed to polyacrylamide absorbing water from the atmosphere or
water/DMSO trapped within the polymer matrix.
52
Table 2.2.6.1. Experimental amounts for PAM-co-PVDAT copolymers and PAM polymerizations.
Polymer VDAT (mol %)
VDAT (mols)
VDAT (g)
AM (mol %)
AM (mols)
AM (g)
Yield (%)
Yield (g)
Temp. (°C)
PAM-co-PVDAT 20 0.025 3.43 80 0.11 7.82 96 10.81 ≈ 80 °C
PAM-co-PVDAT 10 0.011 1.50 90 0.11 7.82 97 9.04 ≈ 80 °C
PAM-co-PVDAT 5 0.0055 0.75 95 0.11 7.82 95 8.14 ≈ 80 °C
PAM-co-PVDATa 1 2.5 E-4 0.0343 99 2.48 E-2 1.76 96 1.72 ≈ 80 °C
PAM 0 0 0 100 0.1 7.11 149 10.60 ≈ 80 °C a Polymer was synthesized using 0.05 mmols (0.0821 g) of AIBN and 25 mL of DMSO Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO
Scheme 2.2.6.1. Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers.
2.2.7 Poly(N-vinylcarbazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine)
(PVK-co-PVDAT)
PVK-co-PVDAT copolymers were synthesized according to the method of Chen and
Sun.49 Scheme 2.2.7.1 shows the free radical polymerization for the PVK-co-PVDAT
copolymers. N-vinylcarbazole and VDAT were used as received. DMSO was purified by
distillation under reduced pressure from CaH2. For the PVK-co-PVDAT free radical
polymerizations, the VDAT mol % concentrations were varied from 5, 10, and 20 with N-
vinylcarbazole. In the synthesis for the 20 mol % PVK-co-PVDAT copolymer, 0.08 mols (15.46
53
g) of N-vinylcarbazole and 0.02 mols (2.74 g) of VDAT were added to a 250 mL three neck
round bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser
containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a
stir bar and thermometer. Once the reaction temperature inside the round bottom flask reached
60 °C, 2 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate the free
radical polymerization. The reaction temperature was increased to approximately 80 °C and was
stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction solution
appeared colorless. During the reaction, the solution gradually changed from colorless to yellow
in color, indicating the reaction had come to completion. The polymer solution was allowed to
cool to room temperature and was then transferred to a 600 mL beaker. The copolymer was
precipitated in excess MeOH (approximately 200 mL), producing a white-brown polymer and
was stirred for several minutes. The polymer was collected by filtration and washed successively
with copious amounts of MeOH and D.I. H2O. The copolymer was dried under vacuum at 50 °C
overnight. The copolymer consisted of a fine powder with large clumps present. The copolymers
were purified by the previously stated purification step using MeOH under reflux while being
stirred in a one neck round bottom flask fitted with a condenser. The purification step did not
improve the percent yields. All of the copolymers' percent yields were greater than 100%. The
percent yields greater than 100% were most likely due to residual DMSO trapped within the
polymer matrix. The residual DMSO could not be removed due to its high boiling point. Table
2.2.7.1 lists the experimental amounts of VDAT, N-vinylcarbazole, AIBN, DMSO, DMF, yields,
and temperature for the PVK-co-PVDAT copolymers and PVK polymerizations.
54
Table 2.2.7.1. PVK-co-PVDAT copolymers and PVK experimental amounts for free radical polymerizations.
Polymer VDAT (mol %)
VDAT (mols)
VDAT (g)
VK (mol %)
VK (mols)
VK (g)
Yield (%)
Yield (g)
Temp. (°C)
PVK-co-PVDAT* 20 0.02 2.74 80 0.08 15.46 114 20.68 ≈ 80 °C
PVK-co-PVDAT* 10 0.01 1.37 90 0.09 17.39 158 29.71 ≈ 80 °C
PVK-co-PVDATa 5 0.0025 0.34 95 0.0475 9.18 103 9.77 ≈ 80 °C
PVKb 0 0 0 100 0.1 19.33 93 17.89 ≈ 80 °C
Scheme 2.2.7.1. Free radical polymerization for the synthesis of PVK-co-PVDAT random copolymers.
2.2.8 Polystyrene-co-Poly(N-vinylcarbazole) (PS-co-PVK)
PS-co-PVK copolymers were attempted to be synthesized according to the method of
Chen and Sun.49 Scheme 2.2.8.1 shows the free radical polymerization for the PS-co-PVK
copolymers. Styrene was purified by distillation under reduced pressure and N-vinylcarbazole
was used as received. DMF was purified by distillation under reduced pressure from CaH2. For
the PS-co-PVK free radical polymerizations, the N-vinylcarbazole mol % concentrations were
varied from 10, 15, and 20 with styrene. In the synthesis for the 20 mol % PS-co-PVK
copolymer, 0.08 mols (8.33 g) of styrene and 0.02 mols (3.87 g) of vinylcarbazole were added to
a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted
a 1 mmols (0.165 g) of AIBN was used as the free radical initiator b 100 mL of DMF was used as the reaction solvent *Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO
55
with a condenser containing 100 mL of freshly distilled DMF under a nitrogen atmosphere, and
equipped with a stir bar and thermometer. Once the reaction temperature inside the round bottom
flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to
initiate the free radical polymerization. The reaction temperature was increased to approximately
80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMF, the
reaction solution appeared colorless. During the reaction, the solution gradually changed from
colorless to a light yellow color, indicating the reaction had come to completion. The polymer
solution was allowed to cool to room temperature and then was transferred to a 600 mL beaker.
The copolymer was precipitated in excess MeOH (approximately 250 mL), producing a white
polymer, and was stirred for several minutes. The polymer was collected by filtration and
washed successively with copious amounts of MeOH and D.I. H2O. The copolymer was dried
under vacuum at 50 °C overnight. Table 2.2.8.1 lists the experimental amounts of styrene, N-
vinylcarbazole, AIBN, DMF, yields, and temperatures for the PVK-co-PVDAT copolymers and
PVK polymerizations.
Table 2.2.8.1. Experimental amounts for the PS-co-PVK polymerizations.
Polymer VK (mol %)
VK (mols)
VK (g)
Styrene (mol %)
Styrene (mols)
Styrene (g)
Yield (%)
Yield (g)
Temp. (°C)
PS-co-PVK 20 0.02 3.87 80 0.08 8.33 5 0.62 ≈ 80 °C
PS-co-PVK 15 0.015 2.90 85 0.09 8.85 21 2.52 ≈ 80 °C
PS-co-PVK 10 0.01 1.93 90 0.09 9.37 22 2.54 ≈ 80 °C
*Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMF
56
Scheme 2.2.8.1. Free radical polymerization for the synthesis of PS-co-PVK random copolymers.
2.2.9 Poly(methyl methacrylate)-co-Poly(N-vinylcarbazole) (PMMA-co-PVK)
PMMA-co-PVK copolymers were synthesized according to the method of Chen and
Sun.49 Scheme 2.2.9.1 shows the free radical polymerization for the PMMA-co-PVK
copolymers. MMA was purified by distillation under reduced pressure and N-vinylcarbazole was
used as received. DMF was purified by distillation under reduced pressure from CaH2. For the
PMMA-co-PVK free radical polymerizations, the N-vinylcarbazole mol % concentrations were
varied from 50 and 20 with MMA. In the synthesis for the 50 mol % PMMA-co-PVK
copolymer, 0.05 mols (5.01 g) of MMA and 0.05 mols (9.66 g) of vinylcarbazole were added to
a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted
with a condenser containing 100 mL of freshly distilled DMF under a nitrogen atmosphere, and
equipped with a stir bar and thermometer. Once the reaction temperature inside the round bottom
flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to
initiate the free radical polymerization. The reaction temperature was increased to approximately
80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMF, the
reaction solution appeared colorless. During the reaction, the solution gradually changed from
colorless to a light brown-yellow color, indicating the reaction had come to completion. The
polymer solution was allowed to cool to room temperature and then was transferred to a 600 mL
57
beaker. The copolymer was precipitated in excess MeOH (approximately 200 mL), producing a
white-brown polymer and was stirred for several minutes. The polymer was collected by
filtration and washed successively with copious amounts of MeOH and D.I. H2O. The copolymer
was dried under vacuum at 50 °C overnight. Table 2.2.9.1 lists the experimental amounts of
MMA, yields, N-vinylcarbazole, DMF, AIBN, and temperatures for the PMMA-co-PVK
copolymers.
Table 2.2.9.1. Experimental amounts for PMMA-co-PVK copolymer polymerizations.
Polymer VK (mol %)
VK (mols)
VK (g)
MMA (mol %)
MMA (mols)
MMA (g)
Yield (%)
Yield (g)
Temp. (°C)
PMMA-co-PVK 50 0.05 9.66 50 0.05 5.01 12 1.82 ≈ 80 °C
PMMA-co-PVK 20 0.02 3.87 80 0.08 8.01 80 9.49 ≈ 80 °C
*Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMF
Scheme 2.2.9.1. Free radical polymerization for the synthesis of PMMA-co-PVK random copolymers.
2.2.10 Poly(N-vinylimidazole)-co-Poly(2-vinyl-4,6-diamino-1,3,5-triazine) (PVI-co-PVDAT)
The PVI-co-PVDAT copolymer was synthesized according to the method of Chen and
Sun.49 Scheme 2.2.10.1 shows the free radical polymerization for the PVI-co-PVDAT
copolymer. VI was purified by distillation under reduced pressure and VDAT was used as
received. DMSO was purified by distillation under reduced pressure from CaH2. For the PVI-co-
58
PVDAT free radical polymerization, the VDAT mol % concentration was 20 with VI. In the
synthesis for the 20 mol % PVI-co-PVDAT copolymer, 0.08 mols (7.53 g) of VI and 0.02 mols
(2.74 g) of VDAT were added to a 250 mL three neck round bottom flask. The flask was placed
in an oil bath on a hot plate, fitted with a condenser containing 100 mL of freshly distilled
DMSO under a nitrogen atmosphere, and equipped with a stir bar and thermometer. Once the
reaction temperature inside the round bottom flask reached 60 °C, 0.002 mols (0.33 g) of
recrystallized AIBN was added to the solution to initiate the free radical polymerization. The
reaction temperature was increased to approximately 70 °C and was stirred for five hours. Once
the monomers and initiator dissolved in DMSO, the reaction solution appeared colorless. During
the reaction, the solution gradually changed from colorless to a light orange color, indicating the
reaction had come to completion. The polymer solution was allowed to cool to room temperature
and then was transferred to a 600 mL beaker. The copolymer was precipitated in excess MEK
(approximately 300 mL) producing a brittle orange polymer and was stirred for several minutes.
The polymer was collected by filtration and washed successively with copious amounts of
MeOH and D.I. H2O. The copolymer was dried under vacuum at 50 °C overnight. The
copolymerization produced a 71% yield (7.27 g) for PVI-co-PVDAT. Poly(N-vinylimidazole)
was synthesized by the same experimental procedure and conditions producing a 17% yield (1.62
g) light orange brittle polymer.
N
N
N
NH2 NH2
CH2 CH2
N
N
N
NH2 NH2
+ AIBN
DMSO, 70 °C
m n
N
N
N
N
Scheme 2.2.10.1. Free radical polymerization for the synthesis of a PVI-co-PVDAT random copolymer.
59
2.2.11 Polystyrene-co-Poly(N-vinylimidazole) (PS-co-PVI)
The PS-co-PVI copolymer was synthesized according to the method of Chen and Sun.49
Scheme 2.2.11.1 shows the free radical polymerization for the PS-co-PVI copolymer. Styrene
and VI were purified by distillation under reduced pressure. DMSO was purified by distillation
under reduced pressure from CaH2. For the PS-co-PVI free radical polymerization, the VI mol %
concentration was 20 with styrene. In the synthesis for the 20 mol % PS-co-PVI copolymer, 0.08
mols (8.33 g) of styrene and 0.02 mols (1.88 g) of VI were added to a 250 mL three neck round
bottom flask. The flask was placed in an oil bath on a hot plate, fitted with a condenser
containing 100 mL of freshly distilled DMSO under a nitrogen atmosphere, and equipped with a
stir bar and thermometer. Once the reaction temperature inside the round bottom flask reached
60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to initiate the free
radical polymerization. The reaction temperature was increased to approximately 70 °C and was
stirred for five hours. Once the monomers and initiator dissolved in DMSO, the reaction solution
appeared colorless. During the reaction, the solution gradually changed from colorless to a slight
orange color, indicating the reaction had come to completion. The polymer solution was allowed
to cool to room temperature and then was transferred to a 600 mL beaker. The copolymer was
precipitated in excess MeOH (approximately 250 mL) producing a brittle white-orange polymer
and was stirred for several minutes. The polymer was collected by filtration and washed
successively with copious amounts of MeOH. The copolymer was dried under vacuum at 50 °C
overnight. The copolymerization produced an 11% yield (1.13 g).
60
Scheme 2.2.11.1. Free radical polymerization for the synthesis of a PS-co-PVI random copolymer.
2.2.12 Poly(methyl methacrylate)-co-Poly(N-vinylimidazole) (PMMA-co-PVI)
The PMMA-co-PVI copolymer was synthesized according to the method of Chen and
Sun.49 Scheme 2.2.12.1 shows the free radical polymerization for the PMMA-co-PVI copolymer.
MMA and VI were purified by distillation under reduced pressure. DMSO was purified by
distillation under reduced pressure from CaH2. For the PMMA-co-PVI free radical
polymerization, the VI mol % concentration was 20 with MMA. In the synthesis for the 20 mol
% PMMA-co-PVI copolymer, 0.08 mols (8.01 g) of MMA and 0.02 mols (1.88 g) of VI were
added to a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot
plate, fitted with a condenser containing 100 mL of freshly distilled DMSO under a nitrogen
atmosphere, and equipped with a stir bar and thermometer. Once the reaction temperature inside
the round bottom flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to
the solution to initiate the free radical polymerization. The reaction temperature was increased to
approximately 70 °C and was stirred for five hours. Once the monomers and initiator dissolved
in DMSO, the reaction solution appeared colorless. During the reaction, the solution gradually
changed from colorless to a light orange color, indicating the reaction had come to completion.
The polymer solution was allowed to cool to room temperature and transferred to a 600 mL
beaker. The copolymer was precipitated in excess MeOH (approximately 300 mL), producing a
brittle white-orange polymer and was stirred for several minutes. The polymer was collected by
CH2 CH2
+ AIBN
DMSO, 70 °C
m n
N
N
N
N
61
filtration and washed successively with copious amounts of MeOH. The copolymer was dried
under vacuum at 50 °C overnight. The copolymerization produced a 64% yield (6.57 g).
CH2 CH2
+ AIBN
DMSO, 70 °C
m n
N
N
C
N
N
O
O
CH3
O
CH3
O
Scheme 2.2.12.1. Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer.
2.3 Co-Crystals with Nitroaromatics
2.3.1 General Co-Crystal Procedure with Nitroaromatics
1.0 mmol of the electron donor and electron acceptor were dissolved in separate test
tubes in the appropriate solvent by sonication or wrist action shaking. After the reagents
completely dissolved in the separate test tubes, they were transferred to a crystallization dish to
allow the solvent to evaporate at room temperature. After the solvent completely evaporated, the
crystals were collected from the crystallization dish. Table 2.3.1.1 lists the electron donors,
electron acceptors, and solvents used for growing co-crystals.
2.3.2 2,4-Diamino-6-methyl-1,3,5-triazine (MDAT) Co-Crystals with Nitroaromatics
MDAT co-crystals were attempted by the method of Xiao.50 5.0 mmols of MDAT and
5.0 mmols of a nitroaromatic reagent (2-NT, 3-NT, or 1,3-DNB) were added to a 250 mL three
neck round bottom flask in an oil bath on a hot plate. The flask was fitted with a condenser
containing 100 mL of EtOH and equipped with a stir bar and thermometer. The reaction
temperature was increased to approximately 50 °C and was stirred for three hours. During the
62
reaction, the nitroaromatic reagent dissolved, but MDAT was partially soluble in EtOH. After
three hours, the reaction solution was allowed to cool to room temperature and was then filtered.
The filtrate appeared colorless, but included small white particles. The filtrate was set aside for
one week and obtained white crystals with a faint yellow tint.
2.3.3 2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)
1.0 mmol of VDAT (0.14 g) was dissolved in 5 mL of DMSO by gently heating in a test
tube. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 15 mL of EtOH in a test tube by wrist
action shaking. The solutions were transferred to crystallization dish, allowing the formation of
crystals over six days. Light orange, needle like crystals formed and were removed from the
remaining DMSO.
2.3.4 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)
Three different ratios between 9-VC and 1,3-DNB were used to grow co-crystals. For the
1:1 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test tube by
sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 10
mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the two
solutions were combined in a crystallization dish, a bright yellow color solution rapidly formed.
The EtOH was allowed to evaporate at room temperature for two weeks. Yellow-orange clumps
formed in the crystallization dish after the EtOH evaporated.
For the 1:2 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test
tube by sonication, producing a colorless solution. 2.0 mmols (0.34 g) of 1,3-DNB was dissolved
in 15 mL of EtOH in a test tube by sonication, producing a light yellow colored solution. When
the two solutions were combined in a crystallization dish, a bright yellow color solution rapidly
63
formed. The EtOH was allowed to evaporate at room temperature for 3 weeks. Yellow-orange
needle like crystals formed in the crystallization dish after the EtOH evaporated.
For the 2:1 ratio, 2.0 mmols (0.38 g) of 9-VC was dissolved in 15 mL of EtOH in a test
tube by sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved
in 10 mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the
two solutions were combined in a crystallization dish, a bright yellow color solution rapidly
formed. The EtOH was allowed to evaporate at room temperature for four weeks. Yellow-orange
clumps formed in the crystallization after the EtOH evaporated.
64
Table 2.3.1.1. Co-crystals experimental reagents, solvents, and descriptions of crystals.
Electron Donor Electron Acceptor Solvent Description
Color Complex
Interaction 9-EC 2-NT EtOH White NO 9-EC 3-NT EtOH White NO 9-EC PNT EtOH White-Yellow NO 9-EC 1,3-DNB EtOH/Toluene Yellow-Orange YES 9-VC 2-NT EtOH White NO 9-VC NB EtOH White NO 9-VC 1,3-DNB EtOH/Toluene Yellow YES
Carbazole 2-NT EtOH Brown NO Carbazole NB EtOH Brown NO Carbazole 1,3-DNB EtOH/Toluene Brown YES
10-M 2-NT EtOH White NO 10-M 3-NT EtOH White NO 10-M PNT EtOH White NO 10-M 1,3-DNB EtOH Red-Purple YES
Phenothiazine 2-NT EtOH Brown NO Phenothiazine 3-NT EtOH Brown NO Phenothiazine PNT EtOH Brown NO Phenothiazine 1,3-DNB EtOH Brown YES
VDAT 2-NT H2O/EtOH (50:50) 65 °C White NO VDAT 3-NT H2O/EtOH (50:50) 65 °C White NO VDAT PNT H2O/EtOH (50:50) 65 °C White NO VDAT 1,3-DNB H2O/EtOH (50:50) 65 °C White NO MDAT 2-NT EtOH White NO MDAT 3-NT EtOH White NO MDAT PNT EtOH White NO MDAT 1,3-DNB EtOH White NO
Benzoguanamine 2-NT EtOH White NO Benzoguanamine 3-NT EtOH White NO Benzoguanamine PNT EtOH White NO Benzoguanamine 1,3-DNB EtOH White NO
2-VP 1,3-DNB EtOH White NO Vinylimidazole 1,3-DNB EtOH White NO
Acrylamide 1,3-DNB EtOH White NO
65
2.4 Instrumentation
FTIR spectra were recorded using a Jasco FT-IR 410 with the following parameters: 32
scans, 4 cm-1 resolution, % T, and a single background. 1 to 5 mg of a sample combined with
approximately 100 mg of dry KBr was ground into a fine powder. A pellet press was used to
produce a KBr pellet of the sample.
1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker 500 or 360
MHz spectrometers. For recording 13C NMR spectra, the following parameters were used: D1
(relaxation delay) was set to zero, TD (time domain) was set to 16,000 points, and NS (number
of scans) was set to 60,000 scans. For processing the 13C NMR spectra, a line broadening value
of 20 was used as a smoothing function.
The glass transition temperatures (Tg) of the copolymers were collected by using a TA Q-
200 DSC. A small polymer sample was ground into a fine powder. 5 to 10 mg of the polymer
sample was heated with an initial heating ramp rate of 20 °C min-1 to the appropriate temperature
for DSC use and was held for three minutes. The polymer was then cooled to -40 °C with a
cooling ramp rate of 10 °C min-1, and was re-heated with a ramp rate of 10 °C min-1. The Tg was
determined as the inflection point between the upper and lower points of the heat capacity
transition.
Thermogravimetric analysis was performed with a TA 2950 TGA. A small polymer
sample was ground into a fine powder. The heating ramp rate was 5 °C min-1, with the
temperature being held at 75 °C for one hour in order to remove any residual water. The
temperature was increased to 600 °C with a heating ramp rate of 5 °C min-1. The decomposition
temperature (Td) was determined when 10% weight loss occurred.
66
Size exclusion chromatography (SEC) was performed on a Tosoh EcoSEC system with a
refractive index detector and a TSKgel Super HZ4000 column. PS-co-PVDAT copolymers were
dissolved in HPLC grade tetrahydrofuran (THF) at a concentration of 1.0 mg mL-1 and a flow
rate of 0.35 mL min-1 was utilized for the system. The injection port, column, and RI detector
were all set at 40 ˚C and the system was calibrated with polystyrene standards of narrow
polydispersity.
Silicon wafers were etched to produce 1 in. x 1 in. wafers for spin coating. The wafers'
surfaces were cleaned with D.I. H2O by spin coating and a small amount of acetone was applied
to a kimwipe to remove any dust from the etching process. The wafers' surfaces were dried using
filtered nitrogen. The wafers were stored in petri dishes.
Thin polymer films were spin coated using a Laurell (Model WS-400B-6NPP/LITE). The
polymer solutions were spin coated by two techniques: static and dynamic. The static spin
coating technique involved flooding the surface of the silicon wafer before spin coating. The
dynamic technique required the polymer solution to be dispensed during the spin coating process
at low speeds. A silicon wafer (1 in. x 1 in.) surface was flooded with a polymer solution. The
rotation speeds and time were varied in order to control film thicknesses. After spin coating, the
films were dried in an oven at 60 °C for 2 hours to remove any residual solvent.
Refractive Index measurements were performed on a J.A. Woollam Co. variable angle
spectroscopic (VASE) ellipsometer. Each of the films ψ and Δ were determined from 300 to
1000 nm at angles of 60°, 65°, 70°, 75°, and 80°. The data was fit and constrained to a Cauchy
Model with a polymer layer on a silicon wafer. An estimated SiO2 thickness of 20 Å was used
for the Cauchy model. The optical constants (n and k) were fitted to the Cauchy model. The
67
refractive index (n) and extinction coefficient (k) of the polymer films were recorded before and
after exposure to a concentrated nitroaromatic vapor.
The polymer films were exposed to concentrated nitroaromatic vapors using a simple
exposure apparatus. A large jar approximately 4 in. x 3 in. contained a large amount of a
nitroaromatic compound. A smaller jar approximately 2 in. x 2 in. was placed inverted inside the
larger jar with the nitroaromatic compound, so the bottom of the smaller jar could be used as a
sample holder allowing the film to rest above the nitroaromatic compound. Once the film was
placed inside the exposure apparatus and the lid tighten, the exposure time would be recorded.
After the film was exposed to a nitroaromatic vapor for a determined amount of time, the film
would then be removed and the refractive index would be measured using ellipsometry. The
exposure experiments were performed at temperatures between 22 - 25 °C.
A DekTak II A profilometer was used to measure film thickness to confirm the film
thickness measurements of the ellipsometer. A 1 mm scan was performed using a slow scan
method to measure the thickness of the films from an etched made into the films and observe
surface roughness.
A Siemens CCD Smart (Area Detector) and Enraf-Nonius CAD-4 computer controlled
X-ray diffractometer was used to measure the crystal structure. Steven Kelley determined the
crystal structure.
The melting points of the co-crystals with nitroaromatics were determined using an
Electrothermal IA9100. Approximately 2 mg of the co-crystals were ground into a fine powder
and transferred to a capillary tube. The instrument was dried by increasing the temperature to
300 °C with the lens removed to allow water vapor to escape the instrument. The instrument was
allowed to cool and equilibrate at 30 °C for 24 hours. After equilibrating for 24 hours, three
68
samples were placed in the capillary tube holder and heated at a ramp rate of 1 °C/min. The
melting point was determined from the first sign of liquid formation until the formation of a
meniscus after the sample completely melted.
A Shimadzu UV-3600 (UV/Vis - NIR) spectrophotometer was used to record the diffuse
reflectance spectra of the nitroaromatic co-crystals. A crystal sample of 200 mg was ground into
a fine powder and transferred to the holder with two Teflon spacers. A microscope slide was
used to make the sample flush with the top of the holder. The scan range for diffuse reflectance
measurements were from 900 - 200 nm by 0.5 nm.
A Cary 5G UV-Vis-NIR spectrophotometer was used to record the electronic absorption
spectra of the nitroaromatic co-crystals. Scan Mode was used to record the electronic absorption
spectra with the following parameters: Ave. time (s) - 0.100, Data interval (nm) - 1.00, Scan rate
(nm/min) - 200, SBW - 2.00, Beam mode - Double, Slit height - Full. The electronic absorption
spectra were recorded from 800 to 200 nm using a Zero/Baseline correction. A stock solution
was prepared and dilutions were made until the solution's absorbance was below two.
69
Chapter 3
Polymer Characterization
A series of random copolymers were prepared containing 2-vinyl-4,6-diamino-1,3,5-
triazine (VDAT). The VDAT content was varied from 0 to 20 mole percent. Polymers with
higher VDAT content were insoluble and could not be used to cast films by spin coating. In this
dissertation, the VDAT content reported is the one based on the mole ratio of VDAT added to
the polymerization reaction.
3.1 PVDAT Characterization
The FTIR spectrum for PVDAT was recorded in KBr, as shown in Figure 3.1.1. The
broad peaks located at 3465, 3343, and 3215 cm-1 were assigned to the NH2 asymmetric and
symmetric stretching vibration modes. The broad peak's shoulder located at 2987 cm-1 was
assigned to the CH stretching vibrations. The peak located at 1635 cm-1 was assigned to the NH2
internal deformation vibrational mode. The peaks located at 1544 and 1449 cm-1 were assigned
as the triazine ring in-plane vibrations.51 The peak located at 1113 cm-1 was assigned as the (C-
N) bond of the amine group bound to the triazine ring carbon. The peak appearing at 822 cm-1
represented the out-of-plane triazine ring bending vibration.51 The triazine ring, in plane and out
of plane stretching vibrations, and amine stretching vibrations were used as a reference for
determining the presence of PVDAT within the copolymers.
70
Figure 3.1.1. The FTIR spectrum of PVDAT recorded in KBr.
4000 3500 3000 2500 2000 1500 1000 5000
20
40
60
80
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
3343
3215
1635 1544
14493465
1113
822
71
DSC and TGA were used to characterize PVDAT's thermal properties. Figure 3.1.2
shows the decomposition temperature (Td) of PVDAT by TGA under an air atmosphere. From
the graph, there was observed weight loss from 75 °C to ≈ 200 °C due to the presence of residual
water. The decomposition temperature was determined when 10% weight loss occurred from 201
°C (75.41%, 7.662 mg). The region near approximately 200 °C was chosen for calculating the Td
after residual water was removed from the sample. PVDAT exhibited a 10% weight loss at 371
°C (65.42%, 6.646 mg). The polymer completely decomposed at approximately 594 °C (0 mg).
The VDAT moieties improve the thermal stability of the polymer, since they are capable of
acting as free radical scavengers limiting chain scissions and depolymerization. 49
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (οC)
Figure 3.1.2. PVDAT TGA curve.
72
Figure 3.1.3 shows the DSC curve for PVDAT between 60 °C and 180 °C. The PVDAT
DSC curve did not exhibit a glass transition (Tg) or melting endotherm (Tm) due to strong
intermolecular interactions between the polymer's VDAT repeating units. The Tg may be at a
temperature above the thermal decomposition temperature. Since no Tg or Tm was observed, the
polymer was amorphous or possessed very little crystallinity.
60 80 100 120 140 160 180
-2.4
-2.2
-2.0
-1.8
-1.6
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (oC)
Figure 3.1.3. PVDAT DSC curve.
PVDAT was not soluble in any common solvents, due to strong intermolecular
interactions. This prevented the acquisition of 1H NMR, 13C NMR, and GPC data (Molecular
Weight).
73
3.2 PS-co-PVDAT Copolymers Characterization
The PS-co-PVDAT copolymers FTIR spectra were recorded in KBr, shown in Figure
3.2.1. The peaks located at 1601 and 1492 cm-1 were assigned the benzene ring (C=C) vibrations
for polystyrene. The NH2 asymmetric and symmetric stretching vibrational modes of PVDAT
appeared at 3470, 3403, and 3319 cm-1.52 The triazine ring in-plane vibrational modes were
observed at 1546 cm-1 and 1451 cm-1 with the out of plane bending vibrational mode appearing
at 822 cm-1. The appearance of the referenced PVDAT peaks provided evidence that PVDAT
was incorporated into the copolymers. As the VDAT concentration increased, a decrease in
transmittance was observed for the NH2 and the triazine ring vibrational modes.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
16011546
1492
1451
3470
3403 3319828
1 mol % VDAT
5 mol % VDAT
10 mol % VDAT
20 mol % VDAT
Figure 3.2.1. FTIR spectra for the PS-co-PVDAT copolymers.
74
1H and 13C NMR experiments were performed for copolymer structure characterization.
The 1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker 500 or 360
MHz spectrometer. To confirm the presence of VDAT incorporated into the copolymers, the
VDAT 1H NMR spectrum (Appendix Figure 1) was recorded and shown in the appendix to
determine the location of the vinyl and amine protons.
VDAT: 1H NMR (360 MHz, DMSO-d6, δ) 6.67 (br., 3.98 H, NH2), 6.42-6.26 (m, 2.02 H, CH),
5.62 (dd, 1.00 H, CH).
Figure 3.2.2 shows the PS-co-PVDAT 20 mol % VDAT 1H NMR spectrum. For the PS-co-
PVDAT 20 mol % VDAT 1H NMR spectrum, the VDAT vinyl protons were absent, indicating
that the starting monomer was completely incorporated into the copolymer. The two broad peaks
located between 2.0 - 0.8 ppm were assigned to the polymer's backbone methine and methylene
protons. The polystyrene phenyl protons appeared in the region from 7.50 - 6.50 ppm with the
PVDAT amino protons overlapping with the polystyrene phenyl protons. A trend observed in the
PS-co-PVDAT copolymers 1H NMR spectra was that as the VDAT concentration decreased, the
spectra appeared similar to the polystyrene 1H NMR spectrum. Less broadening was observed in
the polystyrene phenyl protons region with an increase in the number of peaks present. Some of
the PS-co-PVDAT copolymers spectra do show small peaks located in the vinyl protons region
for styrene and VDAT. These peaks were likely unreacted monomer from the free radical
polymerization, which were unable to be removed even after repeated washings with EtOH or
soxhlet extraction. The methine and methylene polymer backbone protons for polystyrene and
PS-co-PVDAT copolymers could not be differentiated due to both peaks being very broad and
overlapping.
75
Figure 3.2.2. PS-co-PVDAT 20 mol % 1H NMR (360 MHz, DMSO-d6) spectrum.
13C NMR experiments provided further confirmation that VDAT was incorporated into
the copolymers. Figure 3.2.3 shows the PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum
recorded in DMSO-d6. The methine and methylene carbons were not observed in the spectrum
due to the overlapping DMSO-d6 solvent peak. The peaks located between 145 and 127 ppm
were assigned to the polystyrene phenyl carbons. The peak located at 166.91 ppm (C7) was
assigned to the two equivalent triazine carbons attached to the amino groups.49 A new peak
observed at 180.61 ppm was assigned as the triazine quaternary carbon (C8). The (C8) carbon
signal was only observed in the 20 mol % copolymer. The triazine (C7) carbon signal was
observed only in the 10 and 20 mol % copolymers. The weak intensities of the triazine carbons
peaks were due to the slow relaxation process and small VDAT concentrations.
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
H2 H1
TMS
DMSO-d6
H2O
H3, H6
H4, H5
76
Figure 3.2.3. PS-co-PVDAT 20 mol % VDAT 13C NMR spectrum (500 MHz, DMSO-d6).
Table 3.2.1. Polystyrene and PS-co-PVDAT 20 mol % VDAT copolymer 13C NMR peaks.
Carbon # Polystyrene (ppm) PS-co-PVDAT 20 mol % (ppm)1 40.62 N/A
2 46.06, 44.09 N/A
3 145.33 144.97
4 128.19 127.84, 127.27
5 128.19 127.84, 127.27
6 125.87 125.51
7 N/A 166.91
8 N/A 180.61
120125130135140145150155160165170175180185ppm
C8 C7 C3
C4, C5
C6
77
To confirm the triazine (C8) peak located at 180.61 ppm, the VDAT 13C NMR (Appendix
Figure 10) spectrum was recorded in DMSO-d6 and shown in the appendix.
VDAT: 13C NMR (500 MHz, DMSO-d6, δ) 169.90 (s, 1C), 167.72 (s, 2C), 136.75 (s, 1C),
123.75 (s, 1C).
The PS-co-PVDAT peak at 166.91 ppm (C7) was in agreement with the peak observed at 167.72
ppm for the VDAT two equivalent triazine carbons (C4) attached to the amino groups. The
VDAT triazine quaternary carbon (C3) attached to the vinyl group was observed at 169.90 ppm.
The VDAT triazine carbon (C3) signal confirmed the presence of the peak located at 180.61
ppm. The peak was shifted downfield to 180.61 ppm for the PS-co-PVDAT 20 mol % VDAT
copolymer due to polymer backbone shielding effects. The PS-co-PVDAT copolymers 13C NMR
spectra trend observed was that the triazine carbon signals were concentration dependent. One
triazine carbon signal (C7) was observed only in the 10 mol % copolymer and two triazine
carbon signals (C7 and C8) were observed in the 20 mol % VDAT copolymer. The 1 and 5 mol
% VDAT copolymers did not exhibit these peaks and appeared similar to the polystyrene 13C
NMR spectrum.
DSC and TGA characterized the PS-co-PVDAT copolymers and PS thermal properties.
TGA experiments were performed, characterizing the PS-co-PVDAT copolymers decomposition
temperatures (Td). The Td temperatures when 10% weight loss occurred are shown in Table
3.2.2. For calculating Td when 10% weight loss occurred, the starting temperature chosen was
100 °C to account for the removal of residual solvent or water present in the polymer matrix.
Figure 3.2.4 displays the TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer.
Polystyrene began to decompose near 300 °C compared to the copolymer that began showing
weight loss above 300 °C. Polystyrene completely decomposed at approximately 430 °C
78
compared to the copolymer that decomposed at approximately 594 °C. The noticeable curve
features in the copolymer's TGA curve resulted from temperatures above polystyrene's ceiling
temperature (Tc), leading to chain scissions and polystyrene depolymerization. Polystyrene has a
Tc at 310 °C.53 For vinyl polymers, there is an equilibrium between the free monomers and the
copolymer. The Tc is the temperature at which the propagation and depropagation rates are equal.
As the temperature increased above the Tc, the depropagation rate constant increased, leading to
depolymerization of the polymer back to the monomers. A temperature below the Tc, the
equilibrium favors the polymer formation. The curve features were present in the 10 and 20 mol
% VDAT copolymers. The lower VDAT concentration copolymers exhibited a TGA curve
similar to polystyrene with the exception of weight loss occurring at slightly higher temperatures
and complete decomposition at elevated temperatures. The VDAT moieties played an important
role in the thermal stability of the copolymers. The VDAT moieties were capable of acting as
free radical scavengers at high temperatures, slowing chain scission and depolymerization of the
styrene moieties and improving the thermal stability of the copolymers.49 The Td at 10% weight
loss began showing an increasing trend in the copolymers as the VDAT concentration increased,
except for the 10 and 20 mol % VDAT copolymers. The change in the trend for the two
copolymers may be attributed to the amount of polymer sample used during the experiment, the
depolymerization of the copolymer, and/or the loss of residual solvent or water trapped within
the polymer matrix increased weight loss. Even though the two copolymers do not follow the Td
trend, the thermal stability increase was observed in the TGA curves with complete
decomposition occurring at higher temperatures than polystyrene.
79
100 200 300 400 500 6000
20
40
60
80
100W
eigh
t Los
s (%
)
Temperature (οC)
PS 20 mol % VDAT
Figure 3.2.4. TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer.
Table 3.2.2. Thermal decomposition temperatures, Td (10% weight loss for PS and PS-co-PVDAT copolymers).
Polymer Td (°C)
PS 316
1 mol % 323
5 mol % 344
10 mol % 305
20 mol % 325
80
Figure 3.2.5 shows the DSC curves for PVDAT, PS-co-PVDAT copolymers, and PS. The
Tg temperatures for the PS-co-PVDAT copolymers and PS are listed in Table 3.2.3. In Figure
3.2.5, all polymers demonstrated a Tg, but not a Tm, which would indicate that the polymers were
amorphous, or possessed little crystallinity. A general trend observed was that as the VDAT
concentration increased, there was a gradual increase in Tg. To demonstrate this trend, PS
showed a Tg at 80 °C compared to the PS-co-PVDAT 20 mol % copolymer, which showed a Tg
at 144 °C. The Tg observed for polystyrene was lower than the literature reported Tg (100 °C).54
This decrease in Tg may be attributed to a low molecular weight homopolymer or unreacted
monomer trapped within the polymer matrix. The trend observed in the DSC curves
60 80 100 120 140 160 180
(W/g
)
End
othe
rm
Temperature (oC)
PS
1 mol%
5 mol %
10 mol %
20 mol %
PVDAT
Figure 3.2.5. PVDAT, PS-co-PVDAT copolymers, and PS DSC curves.
81
Table 3.2.3. Glass transition temperatures for PVDAT, PS-co-PVDAT copolymers, and PS.
Polymer Tg (°C) PVDAT N/A
20 mol % 144
10 mol % 121
5 mol % 107
1 mol % 86
PS 80
was attributed to an increase in strong intermolecular forces. A similar trend was observed by
Keo et al. for PS-co-PVDAT copolymers' DSC curves.52 By increasing the VDAT concentration,
more hydrogen bonding and dipole - dipole interactions were present, resulting in decreased
chain mobility and higher glass transition temperatures.
Dr. Todd Saylor performed size exclusion chromatography (SEC) on a Tosoh EcoSEC
system with a refractive index detector and a TSKgel Super HZ4000 column. The negative peak
at approximately eleven minutes was from the THF solvent, while the three peaks from
approximately nine to eleven minutes were from the THF leaching a plasticizer or a contaminant
present in the syringe filters. Table 3.2.4 lists the number average molecular weight (Mn), the
weight average molecular weight (Mw), the Z-average molecular weight (Mz), and the
polydispersity (Mw/ Mn). As the VDAT concentration increased in the copolymers, the number
average molecular weight (Mn) values showed a slight decrease. Mw and PDI followed a similar
trend as Mn, with the exception of the 5 mol % copolymer.
82
Table 3.2.4. PS-co-PVDAT copolymers GPC data.
Polymer Mn Mw Mz PDI (Mw/ Mn)
20 mol % 5379 7318 14703 1.36
10 mol % 5383 7552 11466 1.40
5 mol % 5453 10557 69666 1.94
1 mol % 6670 10279 15279 1.54
3.3 PMMA-co-PVDAT Copolymers Characterization
The FTIR spectra for the PMMA-co-PVDAT copolymers are shown in Figure 3.3.1. The
PVDAT NH2 asymmetric and symmetric stretching modes appeared at 3425 (broad) and 3228
(shoulder) cm-1. The CH stretching modes appeared between 2998 - 2843 cm-1. The methyl
methacrylate carbonyl peak was located at 1729 cm-1. The triazine ring in plane vibrations were
observed at 1638 and 1570 cm-1 which were in agreement with the reported literature
assignments.55 The triazine ring out of plane stretching vibration signal was located at 829 cm-1.
The triazine in plane stretching modes and NH2 stretching vibrations decreased in transmittance
as the VDAT concentration was increased. The methyl methacrylate carbonyl signal appearing at
1729 cm-1 showed little variation, indicating that the carbonyl did not undergo a chemical shift as
VDAT was incorporated into the copolymer. The observed PVDAT vibrational modes confirmed
the presence of PVDAT incorporated in the copolymers.
83
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
1 mol %
5 mol %
10 mol %
20 mol %
Figure 3.3.1. FTIR spectra for the PMMA-co-PVDAT copolymers.
To confirm the presence of VDAT in the copolymers, 1H and 13C NMR experiments were
performed on a Bruker 500 or 360 MHz NMR for copolymer structure characterization. Figure
3.3.2 shows the 1H NMR spectrum for the PMMA-co-PVDAT 20 mol % VDAT copolymer
recorded in DMSO-d6. The PMMA alpha methyl proton signals appeared between 1.25 to 0.72
ppm overlapping with the methylene PVDAT (H1V) proton signal.56 The PMMA polymer
backbone methylene proton signal (H1M) appeared between 2.04 - 1.31 ppm, overlapping with
the PVDAT methine proton (H2V) signal.56 The OCH3 (H4) proton signal was observed at 3.54
ppm, producing an intense peak. The VDAT vinyl protons (6.38 - 6.26 ppm and 5.63 - 5.60 ppm)
were absent in the PMMA-co-PVDAT 20 mol % VDAT copolymer spectrum, indicating
84
Figure 3.3.2. 1H NMR spectrum in DMSO-d6 for PMMA-co-PVDAT (20 mol %).
that the monomer was completely incorporated into the copolymer. The appearance of the broad
amine peak (6.55 ppm) in the copolymer spectrum was another indication that VDAT was
included in the copolymer. The broad amine peak was only present in the 20 mol % and only
slightly present in the 10 mol % copolymer spectra. The amine peak was observed in the 1 and 5
mol % copolymer spectra, but could only be observed when the spectra's intensities were
increased significantly. The PVDAT methylene and methine proton signals could not be
accurately identified. As the VDAT concentration increased in the copolymers, the PMMA alpha
methyl and methylene proton resonance signals became broader, suggesting that the PVDAT
-0.0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm
TMS
H5
H4
H1M, H2V H3, H1V
H3
85
polymer backbone proton signals were present. An observed trend in the copolymer 1H NMR
spectra was that as the VDAT concentration decreased in the copolymers, the 1H NMR spectrum
appeared more similar to the PMMA 1H NMR spectrum.
To provide further copolymer characterization, 13C NMR (500 MHz) experiments were
performed. The 13C NMR spectrum for the PMMA-co-PVDAT 20 mol % VDAT copolymer is
shown in Figure 3.3.3. In the copolymer's spectrum, two new peaks observed at 181.06 (C8) and
167.01 (C9) ppm provided evidence that VDAT was incorporated into the copolymer. The
PVDAT quaternary carbon (C8) signal was only observed in the 20 mol % copolymer. The
PVDAT (C9) carbon signal was observed in the 20, 10, and 5 mol % copolymers. PMMA carbon
signals in the copolymer spectrum did not exhibit any chemical shifts after the incorporation of
VDAT, compared with that of the PMMA 13C NMR spectrum. PVDAT's methine and methylene
carbon signals were not observed due to overlapping with the PMMA quaternary carbon (C3)
signal, PMMA methylene carbon signals (C1), and the DMSO-d6 solvent peak.56 An observed
13C NMR copolymer trend was that as the VDAT concentration decreased, the spectra appeared
similar to the PMMA 13C NMR spectrum.
86
Figure 3.3.3. 13C NMR spectrum in DMSO-d6 for the PMMA (80%)-co-PVDAT (20%) copolymer.
PMMA-co-PVDAT 20 mol % 13C NMR (500 MHz, DMSO-d6, δ): 181.06 (C8), 177.36-176.48
(C4), 167.01 (C9), 53.57-48.02 (C1), 51.60 (C5), 44.19-43.86 (C3), 20.68-16.29 (C2).
The thermal decomposition temperatures (Td) were determined for each copolymer using
TGA. The temperature at which the polymers' mass decreased to 10% of its original value was
reported as the Td. The Td for each polymer is listed in Table 3.3.1. The TGA curves for PMMA
and PMMA-co-PVDAT 10 mol % VDAT copolymer are shown in Figure 3.3.4. PMMA began
decomposing at approximately 225 °C and completely decomposed at approximately 385 °C.
The 10 mol % copolymer, on the other hand, began decomposing at approximately 280 °C and
completely decomposed at approximately 600 °C. The features observed in the 10 mol %
102030405060708090110130150170190ppm
C8C4
C9C2
C5
C3
C1
87
copolymer TGA curve occurred at temperatures above 350 °C. These features were the result of
chain scissions and PMMA depropagation to the free monomer due to temperatures higher than
PMMA's Tc (220 °C).53 These features were only observed in the 10 and 20 mol % copolymers.
For the four copolymers, as the VDAT concentration increased, Td increased, indicating a higher
thermal stability. The TGA curves for the 1 and 5 mol % copolymers appeared similar to the
PMMA TGA curve with the exception of weight loss and complete decomposition occurring at
elevated temperatures. The VDAT moieties played an important role in the thermal stability of
the copolymers. The VDAT moieties acted as free radical scavengers at high temperatures,
slowing chain scissions and depolymerization of the PMMA moieties improving the thermal
stability of the copolymers.49 The initial temperature used for calculating 10% weight loss was
76 °C, as the TGA curves did not exhibit significant weight loss after being heated for 1 hour at
75 °C.
88
100 200 300 400 500 6000
20
40
60
80
100W
eigh
t Los
s (%
)
Temperature (oC)
PMMA 10 mol %
Figure 3.3.4. TGA curves for PMMA and the copolymer containing 10 mol % VDAT.
Table 3.3.2. Thermal decomposition temperatures for PMMA and the copolymers of PMMA and PVDAT.
Polymer Td (°C)
PMMA 236
1 mol % 263
5 mol % 254
10 mol % 286
20 mol % 285
89
The DSC curves for the PMMA-co-PVDAT copolymers are shown in Figure 3.3.5, and
Table 3.3.3 lists the glass transition temperatures. The Tg for PMMA was observed at 103 °C,
which was lower than the reported literature Tg (105 °C).54 The 1 and 5 mol % copolymers, glass
transition temperatures were observed at 90 °C and 86 °C lower than the homopolymer. The
lower glass transition temperatures may be attributed to unreacted monomer within the
copolymer matrix. The 1H NMR spectrum with an increased intensity for the 1 mol % copolymer
(Appendix Figure 25) showed unreacted vinyl protons from either VDAT or MMA. The 10 and
20 mol % copolymers' glass transition temperatures were observed at 122 and 154 °C.
80 100 120 140 160
Hea
t Flo
w (W
/g)
End
othe
rm
Temperature (οC)
20 mol %
10 mol %
5 mol %
1 mol %
PMMA
Figure 3.3.5. PMMA and PMMA-co-PVDAT copolymers DSC curves.
90
Table 3.3.3. The glass transition temperatures for PMMA, PMMA-co-PVDAT copolymers, and PVDAT.
Polymer Tg (°C) PVDAT N/A
20 mol % 154
10 mol % 122
5 mol % 86
1 mol % 90
PMMA 105
The higher mol % copolymers follow a similar trend as the PS-co-PVDAT copolymers. As the
VDAT concentration increased, the Tg increased and became higher than the homopolymer's Tg.
Increasing the VDAT concentration increased hydrogen bonding and dipole - dipole interactions
within the copolymer matrix, limiting chain mobility and resulted in higher glass transition
temperatures.
GPC experiments were performed by Dr. Medhat Farahat on three PMMA-co-PVDAT
copolymer samples (20, 10, and 5 mol %) and Table 3.3.4 lists the Mn, Mw, Mz, and PDI. The
lower VDAT concentration copolymers (10 and 5 mol %) exhibited higher Mn, Mw, and Mz
compared to the 20 mol % copolymer. The PDI decreased as the VDAT concentration increased
in the copolymers.
Table 3.3.4. Molecular weights for the PMMA-co-PVDAT copolymers determined by GPC.
Polymer Mn Mw Mz PDI (Mw/ Mn)
20 mol % 6,100 7,870 9,350 1.27
10 mol % 16,100 24,100 32,600 1.49
5 mol % 14,100 23,100 31,100 1.64
91
3.4 PMA-co-PVDAT Copolymer Characterization
The FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer was recorded
in KBr shown in Figure 3.4.1. The broad peak and shoulder located at 3436 and 3224 cm-1 were
assigned to the NH2 asymmetric and symmetric vibrations overlapping with an OH stretching
vibration. The methyl acrylate carbonyl peak appeared at 1734 cm-1. The position of the carbonyl
(1734 cm-1) did not exhibit a chemical shift, indicating no interaction with VDAT during the
polymerization. The PVDAT in plane vibrations could not be accurately identified, but the out of
plane stretching vibration was observed at 829 cm-1. The copolymer spectrum was
4000 3500 3000 2500 2000 1500 1000 500
60
70
80
90
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
1734C=O
CH
NH2
829
Figure 3.4.1. FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer.
92
compared to the FTIR spectrum for Poly(methyl acrylate) recorded by Haken.57
The PMA and PMA-co-PVDAT copolymer structures were characterized by 1H and 13C
NMR. Figure 3.4.2 shows the PMA-co-PVDAT 20 mol % VDAT copolymer 1H NMR spectrum
(360 MHz, DMSO-d6). The PMA and PVDAT methine and methylene peaks overlapped
forming broad peaks located between 2.29 - 1.98 ppm (CH) and 1.83 - 1.15 ppm (CH2). The (O-
CH3) proton signal appeared at 3.51 ppm. The PVDAT NH2 proton signal was assigned to the
broad peak observed at 6.53 ppm. The VDAT and methyl acrylate vinyl proton signals (6.40 -
4.00 ppm) were not present in the spectrum, indicating that the monomers were completely
incorporated in the copolymer.
Figure 3.4.2. 1H NMR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer.
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
H4
H3
H2 H1
93
Figure 3.4.3 shows the 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-
PVDAT 20 mol % VDAT copolymer. First, the PMA 13C NMR spectrum (Appendix Figure 39)
was recorded and used as a reference for comparison. The PMA carbon signals were in
agreement with the reported literature assignments.58
PMA: 13C NMR (500 MHz, DMSO-d6, δ) 174.36 (C=O), 51.52 (O-CH3), 40.76 (CH), 34.27
(CH2).
Figure 3.4.3. The 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-PVDAT 20 mol % VDAT copolymer.
2030405060708090100110120130140150160170180ppm
C5C3
C6C4
C2 C1
94
The PMA carbon signals appeared at 174.72, 174.30 (C=O), 51.37 (O-CH3), 43.36 (CH), and
34.23 (CH2) ppm. The PVDAT carbon signals were observed at 178.77 (C5) and 166.94 (C6).
The PVDAT methine and methylene carbon signals could not be identified due to overlapping
with the PMA polymer backbone carbon signals and the DMSO-d6 solvent peak. The observed
broad peaks in the methine and methylene region suggested that the PVDAT methine and
methylene carbons were present in the polymer backbone. The appearance of the C5 and C6
carbon signals provided evidence that PVDAT was incorporated within the polymer matrix.
A DSC experiment was performed on the PMA-co-PVDAT 20 mol % VDAT copolymer
for characterizing the Tg (Figure 3.4.4). A DSC experiment was not performed on PMA due to
the soft properties of the homopolymer and low Tg. PMA has a reported Tg below room
Figure 3.4.4. DSC curve for the PMA-co-PVDAT 20 mol % VDAT copolymer.
95
temperature, at approximately 12.5 °C.59 The PMA-co-PVDAT copolymer exhibited a Tg at 60
°C. Introducing intermolecular forces (hydrogen bonding and dipole - dipole interactions)
increased the copolymer's Tg significantly, limiting the polymer chains' mobility. The increase in
Tg follows the trend observed in other PVDAT copolymers.
3.5 P2VP-co-PVDAT Copolymers Characterization
FTIR experiments were performed on the copolymers to determine if VDAT was
incorporated into the copolymers. The FTIR spectra for the P2VP-co-PVDAT copolymers were
recorded in KBr, shown in Figure 3.5.1. In the copolymers spectra, the PVDAT amine groups'
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
P2VP
1 mol %
5 mol %
20 mol %
Figure 3.5.1. FTIR spectra for P2VP and P2VP-co-PVDAT copolymers.
96
symmetric and asymmetric stretching vibrations appeared at 3198 and 3387 cm-1, overlapping
with a OH stretching vibration due to the presence of water in the sample. The amine vibrational
modes decreased in transmittance as the VDAT concentration increased. The copolymers' CH
stretching vibrations appeared between 2800 and 3000 cm-1. The triazine ring in plane vibrations
could not be accurately identified due to the peaks' broadness. The triazine ring out of plane
vibration was observed at 830 cm-1. The observed PVDAT amine and out of plane vibration
modes confirmed the presence of PVDAT in the copolymers.
1H and 13C NMR experiments were performed to provide structural characterization of
the copolymers. The P2VP 1H NMR spectrum (Appendix Figure 40) was recorded with the peak
positions used as a reference.60 Figure 3.5.2 shows the 1H NMR spectrum for the P2VP-co-
PVDAT 20 mol % VDAT copolymer. The copolymer's methine and methylene proton signals
were located between 2.4 - 0.7 ppm. The methine and methylene protons could not be
differentiated due to overlapping resonance signals. The PVDAT amine proton signal was
located between 6.75 - 6.05 ppm, overlapping with the P2VP H4 proton signal. The P2VP
aromatic proton signals were observed at 8.64 - 8.07 (H1), 7.80 - 7.21 (H3), and 6.98 - 6.56 (H2)
ppm. The VDAT vinyl protons were absent in the spectrum which indicated that the monomer
was completely incorporated in the copolymer. When compared to the P2VP 1H NMR spectrum,
peak intensities decreased and peak broadness increased as the VDAT concentration increased.
An observed 1H NMR spectra trend was that as the VDAT concentration decreased, the
copolymers' spectra appeared similar to the P2VP 1H NMR spectrum.
97
Figure 3.5.2. 1H NMR spectrum for the P2VP-co-PVDAT 20 mol % VDAT copolymer in DMSO-d6 using the 360 MHz spectrometer.
13C NMR experiments were performed to provide structure characterization. The 13C
NMR spectrum for the P2VP homopolymer (Appendix Figure 43) was recorded with the peak
assignments used as a reference.61 Figure 3.5.3 shows the P2VP-co-PVDAT 1 mol % VDAT
copolymer 13C NMR spectrum. The methine and methylene carbon signals appeared in the
region from 42 - 39 ppm, overlapping with the DMSO-d6 solvent peak. The peak located at
42.07 could not be assigned due to strong resonances from the P2VP methine and methylene
carbons.61 The P2VP aromatic carbon signals appeared at 164.10, 163.43 (C2), 148.79, 148.69
(C6), 135.74, 135.45 (C4), 122.52 (C3), and 120.79 (C5). The PVDAT quaternary carbon (C7)
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5ppm
H1 H3H2
H4, H5
98
signal was absent in the spectrum, but the PVDAT (C8) carbon signal was observed at 166.91
ppm. The PVDAT carbon (C8) signal was also observed in the 20 mol % copolymer, but not
observed in the 5 mol % VDAT copolymer.
Figure 3.5.3. The 13C NMR spectrum (500 MHz, DMSO-d6) for the P2VP-co-PVDAT 1 mol % VDAT copolymer.
DSC experiments were performed to determine the values of Tg for the copolymers.
Table 3.5.1 lists the homopolymer and copolymers' glass transition temperatures. The observed
Tg for P2VP was at 92 °C. The 20 mol % copolymer produced by the same experimental amounts
showed a Tg at 156 °C, significantly higher than the homopolymer. The 1 mol % copolymer
synthesized with smaller experimental amounts showed a slightly higher Tg than the
405060708090100110120130140150160170180ppm
C8
C2
C4C6
C3
C5
99
homopolymer (105 °C). The 1 mol % copolymer curve produced two endotherm features, but the
second feature at a higher temperature was chosen to be interpreted as the Tg. The 5 mol %
copolymer synthesized by smaller experimental amounts displayed a Tg much lower than the
homopolymer. This change in the Tg trend was likely due to experimental conditions and
amounts producing a copolymer with shorter polymer chains and low molecular weight. The 1
and 20 mol % copolymers follow the Tg trend observed in other PVDAT copolymers. By
increasing the VDAT concentration, strong intermolecular forces (hydrogen bonding and dipole -
dipole interactions) decreased the polymer's chains mobility, increasing the Tg.
20 40 60 80 100 120 140 160
(W/g
)
End
othe
rm
Temperature (οC)
P2VP
1 mol %
5 mol %
20 mol %
Figure 3.5.4. P2VP and P2VP-co-PVDAT copolymers DSC curves.
100
Table 3.5.1. P2VP and P2VP-co-PVDAT copolymers glass transition temperatures (°C).
Polymer Tg (°C) P2VP 92
1 mol % 105
5 mol % 60
20 mol % 156
3.6 PAM-co-PVDAT Copolymers Characterization
The FTIR spectra for the PAM and PAM-co-PVDAT copolymers are shown in Figure
3.6.1. The PVDAT and acrylamide amine asymmetric and symmetric stretching vibrations
appeared at 3350 and 3195 cm-1. The CH stretching vibration modes produced a broad peak at
2934 cm-1. The polyacrylamide carbonyl peak was observed at 1663 cm-1. The carbonyl peak's
position indicated that the copolymers possessed low average molecular weight, since ʋC=O
frequency increases as average molecular weight increases.62 One of the PVDAT triazine in
plane vibration modes was observed at 1545 cm-1 for the 20, 10, and 5 mol % copolymers. The
PVDAT triazine out of plane bending mode appeared at 829 cm-1 observed in the 20, 10, and 5
mol % copolymers. As the VDAT concentration increased in the copolymers, the triazine ring in
plane and out of plane bending modes became more apparent, suggesting that PVDAT was
incorporated into the copolymer.
101
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
20 mol %
10 mol %
5 mol %
1 mol %
PAM
Figure 3.6.1. PAM and PAM-co-PVDAT copolymers FTIR spectra.
1H and 13C NMR experiments were performed on the homopolymer and copolymers for
structure characterization. Figure 3.6.2 shows the 1H NMR spectrum for the PAM-co-PVDAT 20
mol % VDAT copolymer recorded in D2O. The copolymer's methine and methylene protons
signals appeared between 2.46 - 2.02 ppm and 1.92 - 1.03 ppm. The PAM methylene and
methine peak positions were in agreement with the peak positions reported in the literature.63
The PVDAT and PAM amine proton signals were not observed due to proton exchange with the
NMR solvent.
102
Figure 3.6.2. The 1H NMR spectrum (360 MHz, D2O) for the PAM-co-PVDAT 20 mol % VDAT copolymer.
The 13C NMR spectrum of the 20 mol % copolymer is shown in Figure 3.6.3. The
spectrum was referenced according to the carbonyl carbon signal observed at 180.00 ppm.63 The
polymer backbone methine and methylene carbon signals appeared at 42.28 ppm and 36.34 -
35.22 ppm. The triazine equivalent carbon signal (C5) was observed at 166.63 ppm. These
signals were observed in the 13C NMR spectra for the 20, 10, and 5 mol % copolymers. The
triazine quaternary carbon signal (C4) was not observed due to overlap with the acrylamide
carbonyl carbon signal (C3) located at 180.00 ppm.
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm
103
Figure 3.6.3. The 13C NMR spectrum (500 MHz) recorded in D2O for the PAM-co-PVDAT 20 mol % VDAT copolymer.
The DSC curves for the copolymers were recorded to characterize the glass transition
temperatures shown in Figure 3.6.4. Table 3.6.1 lists the glass transition temperatures for the
polymers. The Tg for the PAM homopolymer was 146 °C. All of the values of Tg for the
copolymers were higher than the homopolymer, following the trend observed for other PVDAT
copolymers. The 5, 10, and 20 mol % copolymers exhibited the trend that as the VDAT
concentration increased, the Tg increased due to strong intermolecular forces. The 1 mol % Tg
was observed at a higher temperature than the homopolymer and 5 mol % copolymer.
2030405060708090100110120130140150160170180190ppm
C3
C5
104
100 120 140 160 180 200 220 240 260
Hea
t Flo
w (W
/g)
End
othe
rm
Temperature (οC)
20 mol %
10 mol %
1 mol %
5 mol %
PAM
Figure 3.6.4. DSC curves for the PAM and PAM-co-PVDAT copolymers.
Table 3.6.1. PAM and PAM-co-PVDAT copolymers glass transition temperatures (°C).
Polymer Tg (°C) PAM 146
1 mol % 192
5 mol % 160
10 mol % 193
20 mol % 236
105
3.7 PVK-co-PVDAT Copolymers Characterization
The FTIR spectra for the PVK and PVK-co-PVDAT copolymers were recorded in KBr
shown in Figure 3.7.1. The discontinuity in the spectra eliminated the featureless region from
2800 to 2000 cm-1, thereby allowing a better display of the region from 500 to 2000 cm-1. The
VDAT amino groups asymmetric and symmetric stretching vibration modes appeared at 3396
and 3213 cm-1. The PVDAT triazine ring out of plane vibration mode was observed at 825 cm-1
and only one of the triazine ring in plane vibration modes was observed at 1543 cm-1. The CH
stretching vibration modes appeared between 3079 - 2919 cm-1.
4000 3500 3000 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVK
5 mol %
10 mol %
20 mol %
Figure 3.7.1. The FTIR spectra for PVK and PVK-co-PVDAT copolymers recorded in KBr pellets.
106
The PVK-co-PVDAT copolymers were insoluble in CDCl3 or DMSO-d6, preventing the
acquisition of 1H and 13C NMR spectra. The copolymers' insolubility suggested possible cross-
linking within the copolymers, since PVK was soluble in both the appropriate NMR solvents.
DSC experiments were performed to characterize the values of Tg for the homopolymer
and copolymers. The PVK and PVK-co-PVDAT DSC curves are shown in Figure 3.7.2 and
Table 3.7.1 lists the glass transition temperatures. A wide range of glass transition temperatures
from 150 to 248 °C have been reported for high molecular weight PVK.63 This range was
attributed to impurities, degradation products, or stereoregularity of the cationic polymerized
carbazole.64 The copolymers did not show an increasing trend in Tg observed by other PVDAT
copolymers. The Tg for the 20 and 5 mol % copolymers were slightly higher than the
homopolymer, which may be attributed to the molecular weight of the copolymers or non-
covalent interactions introduced by PVDAT. The 10 mol % copolymer Tg (188 °C) was
considerably lower than the other copolymers and homopolymer, suggesting a copolymer of
lower molecular weight or impurities present within the copolymer matrix.
Table 3.7.1. PVK and PVK-co-PVDAT copolymers glass transition temperatures (°C).
Polymer Tg (°C) PVK 216
5 mol % 223
10 mol % 188
20 mol % 218
107
100 120 140 160 180 200 220 240 260
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (οC)
PVK
5 mol %
10 mol %
20 mol %
Figure 3.7.2. DSC curves for PVK and PVK-co-PVDAT copolymers.
3.8 PS-co-PVK Copolymers Characterization
The 1H NMR spectra for the PS-co-PVK copolymers (Figure 3.8.1) were recorded in
CDCl3 for structure characterization. The PVK aromatic proton signals were not observed in the
copolymer spectra. The copolymers' spectra were identical to the polystyrene 1H NMR spectrum.
The reported calculated reactivity ratios found in the literature for styrene and vinylcarbazole in
DMF were rstyrene = 5.83 and rvinylcarbazole = 0.17.65 The reactivity ratio is defined as the ratio of
the rate constant of the propagating species, adding its own type of monomer to the rate constant
for the addition of the other monomer.53 The tendency for two monomers to copolymerize is
determined by r values between zero and unity. Since styrene's reactivity ratio is greater than
108
unity compared to vinylcarbazole reactivity ratio in DMF, the styrene monomer preferentially
adds the styrene monomer instead of the vinylcarbazole monomer. The 1H NMR spectra for the
copolymers confirmed that vinylcarbazole was not incorporated into the polymers.
Figure 3.8.1. The 1H NMR spectra for the PVK and PS-co-PVK copolymers recorded in CDCl3 (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
10 mol %
15 mol %
20 mol %
PVK
109
3.9 PMMA-co-PVK Copolymers Characterization
The FTIR spectra for the PVK homopolymer and the PMMA-co-PVK copolymers were
recorded by the KBr pellet method. The CH stretching modes appeared at 3051 - 2938 cm-1. The
PMMA carbonyl peak (C=O) was observed at 1730 cm-1. The PVK C=C vibrational bands were
assigned at 1618 and 1452 cm-1.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVK
20 mol %
50 mol %
Figure 3.9.1. The FTIR spectra for KBr pellets containing PVK and PMMA-co-PVK copolymers.
110
The 1H NMR spectrum for the PVK homopolymer was recorded in CDCl3, shown in
Figure 3.9.2, and assignments were made to the aromatic proton signals to be used as a reference.
Figure 3.9.2. The 1H NMR spectrum for the PVK homopolymer recorded in CDCl3 using the 360 MHz spectrometer.
The aromatic proton signals were assigned according to the reported literature assignments by
Karali et al.66 The polymer's backbone methine and methylene proton signals were located
between 3.60 - 2.47 ppm (H9) and 2.40 - 0.90 ppm (H10). The aromatic proton signals were
assigned as follows: (H1) 4.91 ppm, (H2) 6.19 ppm, (H3) 6.49 ppm, (H4) 7.55 ppm, (H5) 7.69
ppm, (H6) 7.02 ppm, (H7) 6.91 ppm, and (H8) 6.39 ppm. The 9-vinylcarbazole vinyl proton
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
DMF
DMF
H5 H4 H6H7
H3 H2H8
H1
H9 H10
111
signals (5.50 - 5.00 ppm) were not present, which indicated complete polymerization of the
monomer to the homopolymer.
After assigning the PVK proton signals, the 1H NMR spectra for the PMMA-co-PVK
copolymers were recorded and compared to the PVK spectrum. The spectra for the PMMA-co-
PVK 50 and 20 mol % copolymers are shown Figure 3.9.3. The vinylcarbazole vinyl proton
signals (5.51- 5.10 ppm) and the MMA vinyl proton signals (6.10 - 5.55 ppm) were not observed
in both spectra, indicating complete polymerization for both monomers. A noticeable difference
between the two spectra was that increasing the vinylcarbazole concentration led to
Figure 3.9.3. The 1H NMR spectra for PMMA-co-PVK 50 and 20 mol % copolymers recorded in CDCl3 (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
20 mol %
50 mol %
112
an increase in peak broadness, especially in the region for the methylene, methine, and α-methyl
group proton signals. This increase in peak broadness was attributed to the restricted rotation of
the bulky carbazole. For the PMMA-co-PVK 20 mol % spectrum, the methyl methacrylate
proton signals α-CH3 (1.18, 0.98, and 0.81 ppm), methylene (1.96 - 1.33 ppm), and OCH3 (3.57
and 3.55 ppm) were clearly observed, with some overlapping with the PVK methine and
methylene proton signals. The PVK aromatic proton signals were observed in the spectrum,
overlapping with the CDCl3 NMR solvent signal (7.24 ppm) and DMF solvent signal (8.02
ppm). On the other hand, the 50:50 (PMMA:PVK) copolymer spectrum's region between 3.00 -
0.00 ppm showed several broad peaks which could not be differentiated between the PVK's
methylene and methine protons signals or the PMMA α-methyl and methylene proton signals.
The PMMA OCH3 proton signal was observed at 3.52 and 3.34 ppm with the PVK aromatic
proton signals.
The 13C NMR spectra for PVK and the PMMA-co-PVK copolymers were recorded for
further characterization. The PVK homopolymer peak positions were used as a reference when
analyzing the copolymers' spectra. The PVK carbon assignments were in agreement with the
reported literature assignments.66 The 13C NMR spectrum for PVK and PMMA-co-PVK 50 mol
% vinylcarbazole are shown in Figure 3.9.4 and Figure 3.9.5. The PVK-co-PMMA copolymer
carbon assignments were in agreement with the reported carbon assignments found in the
literature.67 The PMMA carbon signals were observed at 177.18 - 175.50 ppm (C=O), 51.93 -
50.84 ppm (OCH3), 44.55 ppm (methyl methacrylate quaternary carbon (C (M)), and 19.13 -
16.53 ppm (α-CH3). The PVK aromatic carbon signals appeared between 140.34 - 109.79 ppm.
The PVK methine peak was observed at 48.20 ppm (CH (V)), overlapping with the methylene
carbon signal in the same region of the spectrum. An observable difference between the two 13C
113
NMR spectra was that as the vinylcarbazole concentration decreased, the PMMA carbon signals
were much clearer with less peak broadness.
Figure 3.9.4. The 13C NMR spectrum for PVK recorded in CDCl3 (500 MHz).
30405060708090100110120130140ppm
1a 8a
7,2
5
4, 6
3
5a 4a
8 19 10
114
Figure 3.9.5. The 13C NMR spectrum for the (50:50) PMMA-co-PVK copolymer (500 MHz, CDCl3).
The PMMA-co-PVK copolymers Tg values were characterized by DSC. The copolymers'
DSC curves are shown in Figure 3.9.6 from 80 °C to 150 °C. The glass transition temperatures
for the synthesized homopolymers PMMA (103 °C) and PVK (216 °C) have already been
reported. The PMMA-co-PVK copolymers containing 50 and 20 mol % vinylcarbazole glass
transition temperatures (Tg) were observed at 143 °C (50 mol %) and 132 °C (20 mol %). The
copolymers' glass transition temperatures increased, compared to the Tg for PMMA, but were
significantly less than the Tg for the PVK homopolymer. Increasing the vinylcarbazole
concentration increased the Tg to higher temperatures, which was attributed to non-covalent
interactions (hydrogen bonding), and the bulky carbazole units which restricted chain mobility.
102030405060708090100110120130140150160170180ppm
C=O 1a 8a
7,25
4,6,3
8 1
5a,4a
CH (V)
C (M)
(V+M)CH2
CH3
OCH3
115
80 90 100 110 120 130 140 150
Hea
t Flo
w (W
/g)
End
othe
rm
Temperature (οC)
50 mol %
20 mol %
Figure 3.9.6. The DSC curves for the PMMA-co-PVK copolymers.
3.10 PVI-co-PVDAT Copolymer Characterization
The FTIR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer were
recorded in KBr shown in Figure 3.10.1. The assignments for the vibrational modes for the PVI
homopolymer were in agreement with the structure assignments reported by Lippert and co-
workers.68 For the 20 mol % VDAT copolymer, the asymmetric and symmetric vibrational
modes for the primary amine appeared at 3310 and 3100 cm-1, overlapping with a broad OH
vibrational mode from the presence of water. Only one of the triazine ring in plane stretching
modes was observed at 1546 cm-1.
116
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVI
PVI-co-PVDAT
Figure 3.10.1. The FTIR spectra for PVI homopolymer (black) and PVI-co-PVDAT 20 mol % VDAT copolymer (red).
The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT 20 mol % VDAT
copolymer were recorded shown in Figure 3.10.2. The spectrum for the homopolymer showed
aromatic proton peaks located between 7.19 - 6.52 ppm. The PVI methine and methylene peaks
were observed between 3.83 - 2.51 ppm and 2.34 - 1.82 ppm. The PVI proton assignments were
in agreement with the reported literature assignments.69 The copolymer 1H NMR spectrum
showed very broad peak signals due to overlapping with the PVDAT proton signals. The
vinylimidazole and VDAT vinyl proton signals were not observed, suggesting complete
copolymerization. The PVI aromatic peaks were located between 7.55 - 6.38 ppm, overlapping
117
Figure 3.10.2. The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer.
with PVDAT's amine proton signal. The PVI methylene and methine proton signals were located
in similar regions compared to the PVI copolymer spectrum, but overlap from PVDAT's
methylene and methine proton signals lead to broad peaks observed between 3.92 - 1.25 ppm.
For a more thorough structure characterization, 13C NMR experiments were performed.
The PVI aromatic and methine carbon signals were observed at 136.26 (C1), 129.21 (C2),
116.59 (C3), and 50.70 ppm (PVI methine carbon signal). The observed PVI carbon peaks were
in agreement with reported literature assignments.70 The copolymer's methylene carbon signal
-0.0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm
PVI
PVI-co-PVDAT
ETOH
ETOH
CH
CH2
118
Figure 3.10.3. The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz).
was not observed due to the DMSO-d6 solvent signal overlapping with the methylene carbon
signal. The PVDAT aromatic carbon signals were observed at 166.91 (C4) and 178.82 (C5). The
observed PVDAT peaks indicated the copolymer was synthesized.
DSC experiments characterized the values of Tg for the homopolymer and copolymer.
The DSC curve for PVI (Appendix Figure 55) showed a Tg at 159 °C. The reported Tg value for
the PVI homopolymer is 182 °C.70 The synthesized homopolymer's Tg was lower than the
reported literature Tg. This decrease in Tg was attributed to impurities within the polymer matrix.
The copolymer Tg was not observed below 250 °C. This result suggested that the copolymer has
5060708090100110120130140150160170180ppm
119
a high thermal stability temperature. Introducing VDAT into the copolymer increased the Tg
significantly, which was attributed to an increase in hydrogen bonding and dipole - dipole
interactions, limiting chain mobility and resulting in a higher Tg.
3.11 PS-co-PVI Copolymer Characterization
The FTIR spectra for the PVI homopolymer and the PS-co-PVI 20 mol % vinylimidazole
copolymer were recorded in KBr shown in Figure 3.11.1. The copolymer spectrum did not
contain any strong PVI vibrational modes, which suggested that the copolymer was not
synthesized. The polystyrene vibrational modes were observed in the copolymer spectrum. The
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVI
PS-co-PVI
Figure 3.11.1. The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature.
120
polystyrene C=C overtones were located between 1937 - 1637 cm-1. The polystyrene C=C
aromatic vibrational modes were observed at 1600 and 1492 cm-1.
To confirm synthesis of the copolymer, 1H and 13C NMR experiments were performed to
characterize the copolymer's structure. The 1H NMR spectrum for the PS-co-PVI 20 mol %
vinylimidazole copolymer is shown in Figure 3.11.2 and compared to the polystyrene 1H NMR
spectrum. The two spectra were identical; suggesting the copolymer between styrene and
vinylimidazole was not synthesized. The 13C NMR spectra (Figure 3.11.3) also confirmed that
the copolymer was not synthesized. The polystyrene aromatic carbon peaks were observed
between 145.00 - 115.00 ppm and the polymer backbone carbon peaks (methine and methylene
carbons) were observed between 30.00 - 20.00 ppm. None of the PVI carbon signals were
observed in the copolymer's 13C NMR spectrum. A possible explanation for the un-synthesized
copolymer may be attributed the N-vinyl monomer. The N-vinylimidazole monomer produces a
highly reactive radical due to a lack of resonance stabilization in the propagation step of the
polymerization.71 The reactive propagating radical increases the possibility for chain transfer and
chain termination events, resulting in polymers with low molecular weights. In the literature, N-
vinylimidazole copolymers with styrene were synthesized by a controlled free radical
polymerizations.70, 71
121
Figure 3.11.2. The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
PS-co-PVI
Polystyrene
122
Figure 3.11.3. The 13C NMR spectra for the PS-co-PVI copolymer and the homopolymer (PVI).
3.12 PMMA-co-PVI Copolymer Characterization
The FTIR spectra for the PMMA-co-PVI 20 mol % vinylimidazole copolymer and the
PVI homopolymer are shown in Figure 3.12.1. The PMMA carbonyl (C=O) vibrational mode
was observed at 1728 cm-1. A broad OH vibrational mode was observed at 3435 cm-1 that
resulted from the presence of water in the FTIR sample. The peaks located at 1485, 1238, and
665 cm-1 were the observed imidazole ring stretching modes, which confirmed the presence of
PVI in the copolymer with methyl methacrylate.
2030405060708090100110120130140150160ppm
PS-co-PVI
PVI
123
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVI
PMMA-co-PVI
Figure 3.12.1. The FTIR spectra for the PMMA-co-PVI copolymer and the PVI homopolymer recorded in KBr pellets.
The 1H NMR spectra for the PMMA-co-PVI copolymer, the PMMA homopolymer, and
the PVI homopolymer were recorded and compared (Figure 3.12.2). For the copolymer
spectrum, the α-CH3 proton signals were observed at 1.26 - 0.44 ppm. The PMMA methylene
proton signals were located at 2.24 - 1.29 ppm, overlapping with the PVI methylene proton
signal. The PVI methine proton signal was observed at 3.38 - 3.15 ppm. The PMMA O-CH3
proton signals were located at 3.76 - 3.43 ppm. The PVI aromatic proton signals appeared
between 7.08 - 6.60 ppm. The vinyl proton signals for methyl methacrylate and 1-vinylimidazole
124
were not observed in the copolymer spectrum, suggesting that the monomers were
copolymerized.
Figure 3.12.2. The 1H NMR spectra for the homopolymers, PVI and PMMA, and the PMMA-co-PVI copolymer containing 20 mol % vinylimidazole.
The 13C NMR spectrum for the PMMA-co-PVI 20 mol % VI copolymer is shown in
Figure 3.12.3. The copolymer's carbon signals were in agreement with the assignments made by
Chiu and co-workers.72 The carbonyl carbon signals for PMMA were observed at 177.71 and
176.23 ppm. The PVI aromatic carbon signals were located at 136.91, 128.91, and 117.64 ppm.
The spectrum region between 55.00 - 48.30 ppm consisted of the following carbon signals: the
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
PVI
PMMA-co-PVI
PMMA
125
PVI methine, the PVI methylene, the PMMA OCH3, the PMMA quaternary carbon, and the
PMMA methylene carbon. The carbon signal located at 51.52 ppm corresponded to the PMMA
OCH3, overlapping with the PVI methine and PMMA methylene carbon signals. The PMMA
quaternary carbon signal was observed at 44.46 - 43.87 ppm, overlapping with the PVI
methylene carbon signals. The PMMA α-CH3 carbon signals were observed between 20.56 -
16.11 ppm.
Figure 3.12.3. The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz).
102030405060708090110130150170ppm
126
The DSC curves from 80 °C to 160 °C for the PMMA-co-PVI copolymer and PMMA
homopolymer are shown in Figure 3.12.4. The observed glass transition temperatures for the
copolymer and homopolymer were 132 °C (PMMA-co-PVI) and 103 °C (PMMA). The
copolymer exhibited a sharp endotherm transition compared to the homopolymer's gradual
endotherm transition. Incorporating PVI into the copolymer increased the Tg, compared to the
PMMA's Tg. This increase in Tg was attributed to an increase in hydrogen bonding, dipole -
dipole interactions, or the bulky vinylimidazole units limiting the polymer's chains mobility.
80 90 100 110 120 130 140 150
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (οC)
PMMA-co-PVI
PMMA
Figure 3.12.4. The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer.
127
Chapter 4
Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor In this chapter, a brief introduction to the fundamentals of ellipsometry will be discussed,
and the thin films' optical constants measured by ellipsometry will be presented. The first section
will discuss how ellipsometry is used to characterize the thickness and optical properties of thin
films. The next section will discuss the Cauchy model and data analysis. Lastly, the extracted
optical constants and thicknesses of the thin films after exposure to a nitroaromatic vapor will be
shown, providing supporting evidence that these films have the potential to be applied in the
development of a waveguide explosive sensor.
The polymers synthesized in Chapter 2 were used to make polymer solutions for spin
coating. Some of the polymers were not suitable for spin coating due to either insolubility in an
appropriate spin coating solvent or produced heterogeneous films (haziness or phase separation).
The data presented in this chapter is from the polymers that were used in spin coating of
homogeneous films without defects.
4.1 Ellipsometry Overview
Ellipsometry is a non-destructive technique that measures the change in the state of
polarization as light is reflected from the surface of a material.73 Ellipsometry is primarily used
to determine a material's thickness and optical constants, but also offers the ability to extract
128
properties such as composition, crystallinity, surface roughness, and other factors that are
dependent on the material's optical properties.
Ellipsometry utilizes elliptical polarized light, hence the technique's name. Light is an
electromagnetic wave traveling through space, consisting of an electric field vector and a
magnetic field vector. These vectors are mutually perpendicular to each other and perpendicular
to the propagation direction, allowing the wave to be described by its x and y components
traveling along the z-axis. Ellipsometry is concerned with the electric field vector (polarized
light). Elliptically polarized light is produced when two linearly polarized waves with the same
frequency are combined out of phase.73 If viewed (end-on) of the z-axis, the tips of the arrows
would appear to be moving on an ellipse shown in Figure 4.1.1.
Figure 4.1.1. Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013).
When an electromagnetic wave arrives at the interface between the air and film, the wave
can begin to slow, change direction, or be transmitted into the material (Figure 4.1.2). Not all of
the light enters the material, but some is reflected at the interface back into the air. The reflected
light from the material's surface allows the optical properties (n and k) to be characterized by the
complex index of refraction (Eq. 1), which describes how the light interacts with the material.
129
Figure 4.1.2. Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013).
The complex index of refraction can be described by a real and an imaginary number:
(Equation 1)
where n is the index of refraction, k is the extinction coefficient, and j is the √ 1. The index of
refraction (n) describes the inverse measure of the phase of velocity for light as it enters a
dielectric material compared to the speed of light expressed as:
(Equation 2)
where c is the speed of light and ʋ is the phase velocity. The extinction coefficient (k) related to
the absorption coefficient (α) describes the light's loss of intensity as it travels through the
material expressed as:
(Equation 3)
(Equation 4)
where d is the distance traveled into the material.
The incident light is reflected or transmitted at the air/material interface (Figure 4.1.2). It
is known that the angle between the incidence light and the material ( ) is equal to the angle of
130
reflection ( ). The refracted angle ( ) as light is transmitted into a dielectric material (k = 0)
can be described by Snell's law where all terms are real numbers:
(Equation 5)
As previously mentioned, ellipsometry measures the change in the state of polarization
as light is reflected or transmitted at the air/surface interface. The electric field vectors of linear
polarized light are projected in two orthogonal components shown in Figure 4.1.3. The electric
field vector parallel to the plane of incidence is referred to as Ep and the electric field vector that
is perpendicular to the plane of incidence as referred to as Es. Both the Ep and Es are independent
components and can interact differently when reflected from the material's surface. The Fresnel
reflection coefficients describe the ratio of the amplitude of the incidence wave compared to the
reflected wave denoted as rs (perpendicular wave to the plane of incidence) and rp (parallel wave
to the plane of incidence). The Fresnel reflection and transmission coefficients provide
information about the phase and amplitude ratio between the p-wave and s-wave given by:
(Equation 6.1)
(Equation 6.2)
(Equation 6.3)
(Equation 6.4)
131
Figure 4.1.3. Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013).
When a film is present on a substrate, the incident light will be reflected or transmitted at
the film/air interface. The resulting transmitted wave will propagate through the film, producing
multiple reflections and transmissions between the film/air interface and the film/substrate
interface, shown in Figure 4.1.4. The presence of multiple waves in the film introduces
interference, which is dependent on the amplitude and phase of the electric fields.
Figure 4.1.4. Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions.74
132
From the total reflection coefficients comparable to the Fresnel reflection coefficients, the phase
change in the wave as it propagates from the top to bottom through the film can be determined.
The film thickness can be determined by , film phase thickness, expressed as:
(Equation 7)
The p-waves and s-waves are not always in phase; after a reflection, there is a possibility of a
phase shift being produced, which can be different for both waves. The phase difference between
the p-wave and s-wave before the reflection and after the reflection can be described by the
parameter Δ, given by:
(Equation 8)
where is the phase difference before the reflection and is the phase difference after the
reflection. Similar to the phase shift, after the reflection a reduction in amplitude can be induced
for the p-wave and s-wave and the change in amplitude may not be the same for both waves. The
total reflection coefficients (the ratio of the amplitude for the reflected wave to the incidence
wave) for the p-wave (RP) and s-wave (RS) contain the magnitudes of the amplitude changes.
The tan Ψ is defined as the ratio of the magnitudes of the total reflection coefficients and is a real
number given by:
(Equation 9)
where Ψ is the angle whose tangent is the ratio of the magnitudes of the total reflection
coefficients.73 The complex number is defined as the complex ratio of the total reflection
coefficients that describe the change in polarization between the p-wave and s-wave expressed
as:
(Equation 10)
133
Ellipsometry addresses the phase and amplitude ratio between the p-wave and s-wave, which are
independent of each other. Using the fundamental equation of ellipsometry allows the two
independent parameters to be determined:
· (Equation 11)
where and Δ are measured quantities which characterize polarization effects of the surface
after the incident light has undergone a phase and amplitude change for the p-wave and s-wave.
These measured quantities are dependent on the wavelength, angle of incidence, optical
constants, and film morphology.
4.2 Data Analysis
Ellipsometry is able to determine the optical constants and film thickness by directly
measuring Δ and Ψ. After measuring ∆ and Ψ, a model is constructed to describe the sample's
response. The model includes layers with each optical constants and thickness of each layer
defined. If the optical constants and thickness are not known, an approximate value is given to
allow preliminary data calculations. The model and the Fresnel's equations provide the predicted
calculated response for ∆ and Ψ. The predicted values of ∆ and Ψ are then compared to the
experimental values of ∆ and Ψ. The optical properties and thickness of the unknown layer are
varied until the generated values of ∆ and Ψ are close to the experimental values. A fitting step is
performed to find the best fit between the generated data and experimental data. The fit between
the generated data and experimental data is evaluated by the mean square error (MSE). The MSE
quantifies the difference between the data curves, allowing parameters for the unknown material
layer to be adjusted until a minimum MSE is reached. When fitting the experimental and
generated data, the best fit is the assessment with the lowest value of MSE.
134
4.3 Cauchy Model
For many materials, there are electronic absorptions deep in the UV region of the
spectrum. This would lend to large values of k in this region. In the near infrared and through the
visible into the near UV there are no electronic absorptions and the value of k is zero. Through
the region from the near infrared to the near UV, the refractive index increases as the wavelength
decreases. This is optical dispersion. As the wavelength decreases into the UV region and
approaches the region where there are strong electronic absorptions, the refractive index
increases greatly. This is called anomalous dispersion.75
The Cauchy Dispersion model is an empirical model that is capable of describing the
wavelength dependence index of refraction for dielectric materials with little or no optical
absorption.76 The Cauchy model describes the relationship between the index of refraction and
wavelength given by:
(Equation 12)
where is the index of refraction, is the wavelength, and A, B, and C are Cauchy
parameters. The three parameters describe the index of refraction over a range of wavelengths.
The Cauchy model typically shows a decrease in refractive indices (n) as the wavelength
increases.
4.4 PS-co-PVDAT Films
Before spin coating thin films of the different copolymers, the copolymers solubility in an
ideal spin coating solvent (boiling point (b.p.) ≈ 100 to 130 °C) was determined. The ideal spin
coating solvent should possess suitable substrate wetting abilities, a b.p. that allows film
formation and does not quench the film in place (producing phase separation or haziness), and
also allows the films to be dried, removing the solvent leaving only the film. The lower VDAT
135
mol % copolymers (1 and 5 mol % VDAT) were soluble in toluene (b.p. 110 °C54), similar to
polystyrene. The 10 and 20 mol % VDAT copolymers were insoluble in toluene, but were
soluble in other polar organic solvents such as MEK (80 °C54), THF (66 °C54), DMF (153 °C54),
DMSO (192 °C54), pyridine (115 °C54), and 1,4-dioxane (101 °C54). The difference in solubility
between the lower mol % VDAT copolymers and higher VDAT mol % copolymers was
attributed to an increase in non-covalent interactions (hydrogen bonding and dipole - dipole
interactions). As the VDAT concentration increased, the solvents required a higher polar
solubility parameter. Copolymers containing concentrations greater than 20 mol % VDAT would
not likely be soluble in an appropriate spin coating solvents due to the insolubility of PVDAT.
To lower the b.p. of the higher b.p. polar organic solvents, attempts were made to include
lower b.p. solvents, which were miscible with the polar organic solvents such as EtOH, MeOH,
toluene, H2O, THF, or isopropanol. The amounts of the lower b.p. solvents were adjusted to a
maximum concentration in the higher b.p. solvents so that the copolymers remained in the
solution phase and did not precipitate. To increase the b.p. of the lower b.p. solvents, a similar
approach was used to include higher b.p. solvents. The adjusted b.p. attempts resulted in
heterogeneous films, which were not ideal for ellipsometry characterization.
Attempts were made to spin coat films of the PS-co-PVDAT 20 mol % VDAT copolymer
using THF, DMF, and 1,4-dioxane. The films spin coated from THF and DMF were very thin (≤
10 nm) and exhibited extreme surface roughness. It appeared that the solvents did not produce
viscous solutions, therefore creating thin films. Viscosity is a critical factor for spin coating thick
films. In addition, it was assumed that it would not be possible to remove DMF from the polymer
films due to the solvent's high b.p. and hygroscopic nature. The presence of DMF in the polymer
films may be potentially beneficial, acting as a plasticizing agent, allowing the films to become
136
more porous and polymer chains to be more mobile. 1,4-dioxane with low heat was capable of
dissolving the 20 mol % copolymer. After twenty-four hours, the 20 mol % VDAT copolymer
began to precipitate out of the 1,4-dioxane solution, but introducing heat allowed the copolymer
to dissolve back into the solution. Over time, the copolymer would precipitate out of 1,4-dioxane
and would not dissolve in the solvent.
The PS-co-PVDAT 10 mol % VDAT copolymer was soluble in MEK, 1,4-dioxane, THF,
and DMF. Similar to the 20 mol % VDAT copolymer, THF and DMF produced thin films with
rough surfaces. MEK produced films with transparent centers and haziness near the edges of the
silicon wafer. 1,4-dioxane produced homogenous films with no defects. The copolymer
precipitated out of the solvent over time.
For the PS-co-PVDAT copolymers, different film thicknesses were spin coated by
adjusting spin coating speeds and polymer solution concentrations. 0.3%, 1%, and 3% (w.t.)
copolymer solutions were prepared for spin coating, allowing variances in film thickness by
concentration. The 3% (w.t.) solutions produced films with a blue tint from the reflecting light,
indicating thick polymer films. The 1% and 0.3% (w.t.) solutions produced transparent films,
indicating thin copolymer films. The copolymer solutions were spin coated by both the static and
dynamic techniques. The static technique produced homogenous films with minimal surface
roughness, compared to the dynamic technique that produced homogeneous films with rough
surfaces. After determining appropriate spin coating parameters and concentrations for
producing quality films, the copolymer films were characterized by ellipsometry to determine
film thickness and optical constants (n and k) before and after exposure to a concentrated
nitroaromatic vapor.
137
The expected refractive index for the PS-co-PVDAT films were assumed to be similar to
the refractive index of polystyrene (n = 1.55 - 1.5977). The refractive index of PVDAT is not
known, but was estimated to possess a refractive index similar or higher than polystyrene. The
films were exposed to the concentrated high refractive index nitroaromatic vapors of PNT (n =
1.538), NB (n = 1.55654), and 1,3-DNB (n = 1.612) for a determined amount of time. The change
in film refractive index was attributed to the nitroaromatic vapor molecules with a higher
refractive index interacting with the PVDAT, by hydrogen bonding or electro-static interactions.
Figures 4.4.1 - 4.4.5 show a plot of the refractive index (n) as a function of wavelength for
polystyrene and the PS-co-PVDAT copolymers exposed to a concentrated nitroaromatic vapor.
Tables 4.4.1 - 4.4.5 provide the copolymers' Cauchy parameters fitted for the experimental data.
The ellipsometry curves shown in Figures 4.4.1 - 4.4.5 display expected refractive indices
for the Cauchy dispersion model. A decrease in refractive indices was observed as the
wavelength increased from the UV, through the visible, and into the near infrared. All of the
curves showed reasonable refractive indices that would be expected for polystyrene before
exposure to the nitroaromatic vapor. The extinction coefficient (k), describing the films' optical
absorption property, showed very little optical absorption (primarily in the UV region of the
spectrum from 380 - 300 nm) and did not affect the fit between the experimental and generated
data. The features observed between 700 - 1,000 nm were not expected, since the curves
typically lay flat in this region of the spectrum, due to no optical absorption. It was hypothesized
that these features may be attributed to surface roughness or a film defect, scattering or reflecting
the polarized light in a different manner than that of the bulk of the film. There was an observed
difference between the thicknesses measured by the profilometer and thicknesses determined by
the Cauchy model. The films' thicknesses measured by the profilometer were between 3 - 6 nm
138
less than the film thicknesses determined by the Cauchy model. These differences in thickness
occurred because of the etching process. It was likely that the etch made in the films did not
penetrate through the entire film to the substrate's surface. This would explain the slight
differences between the two measurements. The profilometer measurements were used to
confirm the ellipsometer's film thicknesses measurements, which were accurate. The low MSE
values for the spectra represented an effective comparison between the generated and
experimental data. The changes in refractive index for these spectra may not appear significant,
but MZI have shown the capability of detecting small changes in refractive index (10-6).
The refractive index curves for the polystyrene film exposed to PNT did not exhibit a
significant change in refractive index after exposure to the nitroaromatic. The PS-co-PVDAT
copolymers refractive index curves showed changes in the refractive index curves after the films
were exposed to a nitroaromatic vapor. This change in refractive index described the copolymer
films' affinity toward the nitroaromatics. The addition of PVDAT allowed nitroaromatic
molecules to form molecular complexes with the electron rich VDAT aromatic structure or the
nitro groups to form hydrogen bonds with the PVDAT amine groups.
The films exposed to concentrated nitroaromatic vapors showed varied results producing
large, minimal, or no change in refractive index. These changes may be attributed to the ability
of the nitroaromatic vapor molecules to enter the porous films and interact with PVDAT.
139
Figure 4.4.1. The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds.
1.54
1.56
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
1.74
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
140
Table 4.4.1. The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds.
Polymer: PS, 3% (w.t.) Toluene
Nitroaromatic: PNT 10 sec. exposure
Before After
MSE 5.233 5.327
Thickness (Å) 813.3 ± 0.7 813.3 ± 0.7
A 1.567 ± 2.31 E-3 1.566 ± 2.37 E-3
B 4.359 E-3 ± 8.23 E-4 4.33 E-3 ± 8.35 E-4
C 7.621 E-4 ± 7.61 E-5 7.741 E-4 ± 7.66 E-5
Δn 0.0006
Optical Constants MSE 4.586 4.725
Dektak (Å) 780
Spin Coating 5,000 rpm for 40 sec.
141
Figure 4.4.2. The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds.
1.5
1.52
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
142
Table 4.4.2. The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution.
Polymer: PS-co-PVDAT 20 mol % VDAT, 1% (w.t.) 1,4-dioxane
Nitroaromatic: NB 5 sec. exposure
Before After
MSE 2.96 2.944
Thickness (Å) 357.6 ± 0.7 358.2 ± 0.7
A 1.501 ± 4.00 E-3 1.511 ± 4.01 E-3
Δn 0.009
Optical Constants MSE 2.66 2.589
Dektak (Å) 337
Spin Coating 3,000 rpm for 40 sec.
143
Figure 4.4.3. The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
144
Table 4.4.3. The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film.
Polymer: PS-co-PVDAT 10 mol % VDAT, 1% (w.t.) MEK
Nitroaromatic: NB 5 sec. exposure
Before After
MSE 2.122 3.035
Thickness (Å) 508.7 ± 0.3 511.3 ± 0.3
A 1.555 ± 1.49 E-3 1.558 ± 2.09 E-3
Δn 0.003
Optical Constants MSE 2.106 2.962
Dektak (Å) 454
Spin Coating 3,000 rpm for 40 sec.
145
Figure 4.4.4. The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
146
Table 4.4.4. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film.
Polymer: PS-co-PVDAT 5 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: PNT 60 sec. exposure
Before After
MSE 3.402 4.151
Thickness (Å) 893.5 ± 0.3 894.2 ± 0.3
A 1.561 ± 1.05 E-3 1.562 ± 1.22 E-3
B 4.853 E-3 ± 3.49 E-4 4.967 E-3 ± 4.07 E-3
C 7.257 E-4 ± 2.76 E-5 7.346 E-4 ± 3.25 E-5
Δn .002
Optical Constants MSE 3.159 4.16
Dektak (Å) 861
Spin Coating 6,000 rpm for 1 min.
147
Figure 4.4.5. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
148
Table 4.4.5. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film.
Polymer: PS-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: NB 5 sec. exposure
Before After
MSE 2.985 3.285
Thickness (Å) 729.4 ± 0.4 732.5 ± 0.4
A 1.556 ± 1.47 E-3 1.558 ± 1.60 E-3
Δn 0.001
Optical Constants MSE 2.44 2.748
Dektak (Å) 703
Spin Coating 6,000 rpm for 40 sec.
149
Time exposure experiments were performed to determine the maximum concentration of
a nitroaromatic vapor that could be absorbed by the VDAT copolymer films to produce a
significant change in refractive index. PS-co-PVDAT 1 mol % VDAT copolymer films were
spin coated and exposed to PNT for five, twenty, and forty seconds to determine the maximum
change in refractive index over an extended exposure time. The ellipsometry curves shown in
Figures 4.4.6 - 4.4.8 show the change in refractive indices as a function of wavelength as the
exposure time to PNT was increased. Tables 4.4.6 - 4.4.8 provide the Cauchy parameters
determined before and after exposure to the PNT vapor.
The spectra's ranges were reduced to the region between 400 - 1,000 nm in order to
eliminate any optical absorption, which may have occurred in the UV region of the spectrum.
The ellipsometry curves displayed expected Cauchy dispersion model curves showing decreases
in refractive indices as the wavelength increased through the visible to the near infrared. The
refractive index curves before exposure to PNT were consistent with the refractive index for
polystyrene. All of the spectra displayed features in the region between 700 - 1,000 nm,
consistent with the previously shown spectra. The PS-co-PVDAT 1 mol % VDAT copolymer
film exposed to PNT for five seconds produced an average change in refractive index (Δn) of
0.002. When the exposure time was increased to twenty seconds, the average change in
refractive index (Δn) increased to 0.012. After exposure to PNT for forty seconds, there was no
observed increase in the average change in refractive index, suggesting the copolymer was
saturated with PNT.
The differences in film thickness measurements between the Cauchy model and
profilometer varied from 6 - 8 nm. These results were consistent with previously results and
confirmed the approximate thicknesses determined by the ellipsometer. The spin coated
150
copolymer film exposed to PNT for five seconds was approximately 12 nm thinner than the other
two copolymer films. The difference in film thickness could not be justified, but the profilometer
did confirm the film's approximate thickness.
Figure 4.4.6. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
1.63
400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
151
Table 4.4.6. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds.
Polymer: PS-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 2.043 4.136
Thickness (Å) 663.5 ± 0.3 662.5 ± 0.5
A 1.547 ± 1.82 E-3 1.559 ± 2.24 E-3
B 1.413 E-2 ± 9.51 E-4 7.829 E-3 ± 7.18 E-4
C -4.139 E-4 ± 1.16 E-4 3.329 E-4 ± 6.06 E-5
Δn 0.002
Optical Constants MSE 1.997 2.093
Dektak (Å) 580
Spin Coating 6,000 rpm for 1 min.
152
Figure 4.4.7. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
400 500 600 700 800 900 1000
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before
After
153
Table 4.4.7. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds.
Polymer: PS-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: PNT 20 sec. exposure
Before After
MSE 2.108 5.439
Thickness (Å) 784.4 ± 0.2 782.5 ± 0.5
A 1.550 ± 1.14 E-3 1.557 ± 2.24 E-3
B 1.031 E-2 ± 5.27 E-4 2.865 E-3 ± 7.03 E-4
C -3.689 E-4 ± 6.05 E-5 8.442 E-4 ± 6.17 E-5
Δn 0.012
Optical Constants MSE 2.203 5.571
Dektak (Å) 715
Spin Coating 6,000 rpm for 1 min.
154
Figure 4.4.8. The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
400 500 600 700 800 900 1000
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before
After
155
Table 4.4.8. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds.
Polymer: PS-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: PNT 40 sec. exposure
Before After
MSE 2.104 3.559
Thickness (Å) 788.0 ± 0.2 788.6 ± 0.3
A 1.550 ± 1.10 E-3 1.566 ± 1.35 E-3
B 1.064 E-2 ± 5.07 E-4 3.208 E-3 ± 4.94 E-4
C -2.906 E-5 ± 5.81 E-5 7.950 E-4 ± 3.921 E-5
Δn 0.012
Optical Constants MSE 2.069 3.044
Dektak (Å) 724
Spin Coating 6,000 rpm for 1 min.
156
4.5 PMMA-co-PVDAT Films
Similar to the PS-co-PVDAT copolymers, the solubility of the PMMA-co-PVDAT
copolymers in spin coating solvents was determined. The PMMA-co-PVDAT copolymers
exhibited the same trend as the PS-co-PVDAT copolymers. As the VDAT mol % increased in
the copolymers, the copolymers required more polar organic solvents. The lower mol % PMMA-
co-PVDAT copolymers were soluble in toluene and MEK, and produced homogeneous films
with no visible defects. Toluene was used as the spin coating due to its ideal b.p. The 10 and 20
mol % copolymers were soluble in polar organic solvents such as DMF, DMSO, and 1,4-
dioxane. 1,4-dioxane was used to prepare spin coated thin films. All of the polymer solutions
were filtered to remove any particles present in the polymer solutions. The polymer solutions'
concentrations and spin coating speeds were varied, allowing a variety of film thicknesses to be
prepared. After the films were spin coated, the polymer thin films were dried in an oven at 60 °C
for two hours to remove any residual solvent.
Initial attempts were performed to expose the PMMA-co-PVDAT copolymers thin films
to nitroaromatic vapors for short amounts of time (five, ten, twenty, and thirty seconds). The
films did not always produce changes in refractive indices after exposure to the nitroaromatic
vapors. In order to produce refractive index changes, the exposure times were increased
significantly to determine whether the films had an affinity for the nitroaromatic vapors.
Polymer films spin coated from the 20 and 10 mol % copolymers were exposed to PNT,
NB, and 1,3-DNB for two minutes. There was no observed change in the refractive indices after
exposure to the nitroaromatics. Significant surface roughness was observed for the 3% and 1%
(w.t.) polymer solutions confirmed by the profilometer. The surface roughness affected the
157
refractive index measurements, which produced curved features in the ellipsometry spectra not
representative of the polymer films' refractive indices.
Polymer films prepared from the 1 mol % and 5 mol % copolymers solutions allowed
spin coated films with less surface roughness that allowed refractive index measurements
representative of the copolymer thin films. Since no change in refractive index for the 10 and 20
mol % copolymer films were observed when exposed to a nitroaromatic for two minutes, the
exposure time was increased to determine whether a change in refractive index would occur.
Spin coated films from the 1 mol % and 5 mol % copolymers were exposed to NB, PNT, and
1,3-DNB for several minutes. Figures 4.5.1 - 4.5.4 show the ellipsometry curves plotted as the
wavelength dependence of the refractive index (n) for the copolymer films. Tables 4.5.1 - 4.5.4
list the Cauchy parameters, profilometer measured thicknesses, average change in refractive
index, exposure times, and spin coating parameters for the copolymer films.
The refractive indices observed for the ellipsometry curves for the PMMA, 1 mol %, and
5 mol % copolymer films were in agreement with the reported PMMA refractive index in the
literature (n = 1.491477). The ellipsometry curves displayed ideal refractive indices for the
Cauchy dispersion model (refractive index decreased as wavelength increased). The features
observed in the ellipsometry curves were attributed to the copolymer films' surface roughness,
reflecting light in a different manner compared to the bulk of the film. Surface roughness was not
accounted for during the refractive indices measurements. The PMMA film exposed to 1,3-DNB
for ten seconds showed no change in refractive index after exposure to the nitroaromatic vapor.
However, the PMMA-co-PVDAT copolymer films produced a change in refractive index after
exposure to a nitroaromatic vapor, similar to the PS-co-PVDAT. Again, the addition of PVDAT
in the copolymer films showed an affinity for nitroaromatics vapor molecules. It was observed
158
that the copolymer films exposed to NB produced larger refractive index changes compared to
the films exposed to 1,3-DNB. These results correlated to the vapor pressures of the
nitroaromatics. 1,3-DNB has a low vapor pressure (0.027 Pa at 20 °C) compared to the vapor
pressure of NB (24 Pa at 20 °C).78 The higher vapor pressure of NB allowed the vapor phase
molecules to enter the amorphous films more readily and interact with the PVDAT units, which
produced a large change in refractive index. There were observed differences for the film
thickness measurements between the Cauchy model and profilometer. These small differences
between the profilometer and Cauchy model were negligible, but did confirm the approximate
film thicknesses. The PMMA-co-PVDAT films demonstrated mixed results after being exposed
to a nitroaromatic vapor by producing small, large, or no change in refractive index. These mixed
results may be linked to the ability of the nitroaromatic vapor molecules to enter the amorphous
film and interact with the PVDAT moieties giving rise to a change in refractive index.
159
Figure 4.5.1. The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds.
1.46
1.47
1.48
1.49
1.5
1.51
1.52
1.53
1.54
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
160
Table 4.5.1. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds.
Polymer: PMMA, 3% (w.t.) Toluene
Nitroaromatic: 1,3-DNB 10 sec. exposure
Before After
MSE 3.143 3.098
Thickness (Å) 1,205.2 ± 0.4 1,205.5 ± 0.4
A 1.474 ± 7.60 E-4 1.474 ± 7.51 E-4
B 3.414 E-3 ± 3.2 E-4 3.13 E-3 ± 3.21 E-4
C 1.944 E-4 ± 3.12 E-5 2.212 E-4 ± 3.17 E-5
Δn 7.5 E-5
Optical Constants MSE 3.03 2.91
Dektak (Å) 1,192
Spin Coating 5,000 rpm for 40 sec.
161
Figure 4.5.2. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3-DNB for sixteen minutes.
1.46
1.47
1.48
1.49
1.5
1.51
1.52
1.53
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
162
Table 4.5.2. The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes.
Polymer: PMMA-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: 1,3-DNB 16 min. exposure
Before After
MSE 2.503 2.737
Thickness (Å) 770.2 ± 0.5 769.7 ± 0.5
A 1.460 ± 1.13 E-3 1.462 ± 1.22 E-3
B 7.303 E-3 ± 3.93 E-4 6.975 E-3 ± 4.17 E-4
C -1.395 E-4 ± 3.48 E-5 -1.246 E-3 ± 3.72 E-5
Δn 4.0 E-4
Optical Constants MSE 5.098 2.524
Dektak (Å) 738
Spin Coating 4,000 rpm for 40 sec.
163
Figure 4.5.3. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene solution exposed to NB for twenty-five minutes.
1.46
1.465
1.47
1.475
1.48
1.485
1.49
1.495
1.5
1.505
500 550 600 650 700 750 800 850 900 950 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
164
Table 4.5.3. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes.
Polymer: PMMA-co-PVDAT 1 mol % VDAT, 3% (w.t.) Toluene
Nitroaromatic: NB 25 min. exposure
Before After
MSE 2.631 3.399
Thickness (Å) 767.0 ± 0.4 832.8 ± 0.9
A 1.458 ± 1.46 E-3 1.454 ± 4.46 E-3
B 8.956 E-3 ± 8.82 E-4 1.854 E-2 ± 3.65 E-4
C -3.269 E-4 ± 1.23 E-4 -1.598 E-3 ± 6.93 E-4
Δn 0.009
Optical Constants MSE 1.877 3.645
Dektak (Å) 814
Spin Coating 4,000 rpm for 40 sec.
165
Figure 4.5.4. The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes.
1.4
1.42
1.44
1.46
1.48
1.5
1.52
1.54
1.56
1.58
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
166
Table 4.5.4. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes.
Polymer: PMMA-co-PVDAT 5 mol % VDAT, 1% (w.t.) Toluene
Nitroaromatic: NB 25 min. exposure
Before After
MSE 2.142 4.511
Thickness (Å) 333 ± 0.9 327 ± 2
A 1.432 ± 3.18 E-3 1.493 ± 7.77 E-3
B -3.672 E-3 ± 9.51 E-4 -4.648 E-3 ± 2.43 E-4
C 1.129 E-3 ± 9.80 E-5 9.555 E-4 ± 2.46 E-4
Δn 0.054
Optical Constants MSE 2.141 4.883
Dektak (Å) 324
Spin Coating 2,000 rpm for 40 sec.
167
Figure 4.5.5. The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes.
1.4
1.41
1.42
1.43
1.44
1.45
1.46
1.47
1.48
400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
168
Table 4.5.5. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes.
Polymer: PMMA-co-PVDAT 5 mol % VDAT, 1% (w.t.) Toluene
Nitroaromatic: 1,3-DNB 25 min. exposure
Before After
MSE 2.796 2.885
Thickness (Å) 283 ± 1 283 ± 1
A 1.410 ± 4.89 E-3 1.413 ± 5.02 E-3
B 3.756 E-3 ± 1.34 E-3 4.682 E-3 ± 1.38 E-3
C 5.461 E-1 ± 1.39 E-4 3.666 E-4 ± 1.43 E-4
Δn 0.003
Optical Constants MSE 2.856 2.994
Dektak (Å) 204
Spin Coating 4,000 rpm for 40 sec.
169
4.6 P2VP Polymer Film
The P2VP-co-PVDAT copolymers did not produce homogeneous films suitable for
ellipsometry characterization. However, the P2VP homopolymer did produce a homogeneous
film that was characterized by the spectroscopic ellipsometer to determine the film's optical
constants before and after exposure to a nitroaromatic vapor (Figure 4.6.1). Table 4.6.1 lists the
Cauchy parameters, average change in refractive index, and spin coating parameters. A 3% (w.t.)
toluene P2VP polymer solution was spin coated on a silicon wafer and exposed to PNT for five
seconds. The refractive index for the P2VP film before exposure was observed to be A ≈ 1.337
with a film thickness of ≈15 nm.* After the five-second exposure to PNT, the refractive index
increased to A ≈ 1.353 with a film thickness of ≈15 nm. The MSE values for the Cauchy
parameters and optical constants indicated a reasonable fit between the experimental and
generated data for the P2VP film. Exposure to the PNT vapor produced an average change in
refractive index (Δn) of 0.019 for the ellipsometry curves. The change in refractive index for the
P2VP film occurred due to the 2-vinylpyridine moieties hydrogen bonded with PNT nitro group.
Saloni et al. performed theoretical calculations for monomers incorporated in imprinted
polymers which described their ability to imprint with TNT in different solvents.79 2-
vinylpyridine theoretically was able to hydrogen bond to the TNT nitro functional groups for an
imprinted polymer. The features observed in the ellipsometry curves indicated a polymer film
with surface roughness. The film thickness was not measured by the profilometer to confirm the
ellipsometer's measured thickness. The change in refractive index for the P2VP film exposed to
* Since the refractive index for each film varied as a function of wavelength, it was decided to use the Cauchy parameter (A). The Cauchy parameter (A) is the refractive index at very long wavelengths when the refractive index does not change with wavelength. By using the parameter (A), we could compare the values for different films without the concern of the effect of dispersion.
170
PNT was greater than the required (Δn) of 0.003 to detect a change in refractive index for the
proposed MZI sensor.
Figure 4.6.1. The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds.
171
Table 4.6.1. The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds.
Polymer: P2VP, 3% (w.t.) Toluene
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 3.507 3.235
Thickness (Å) 150 ± 2 146 ± 2
A 1.337 ± 9.52 E-3 1.353 ± 9.14 E-3
B 4.786 E-3 ± 1.77 E-3 5.817 E-3 ± 1.77 E-3
C 1.038 E-3 ± 1.66 E-4 9.918 E-4 ± 1.65 E-4
Δn 0.019
Optical Constants MSE 3.365 3.12
Dektak (Å)
Spin Coating 6,000 rpm for 1 min.
172
4.7 Commercial Polymers Films
PVI, PVI-co-PVA, and P4VP (commercial polymers) were purchased to examine their
capabilities of producing changes in refractive index after being exposed to a concentrated
nitroaromatic vapor. These polymers were chosen due to the known ability of amine bases to
form complexes with nitroaromatic species in solution. These polymers were also found to be
soluble in EtOH. Since the polymers were soluble in EtOH (b.p. ≈ 78 °C54), slower spin coating
speeds were employed for producing homogeneous films.
A 3% (w.t.) solution of P4VP was prepared in EtOH by gently heating and placing the
vial containing the polymer solution in the wrist action shaker until the polymer completely
dissolved. Before spin coating, the polymer solution was filtered twice using 0.45 μm PTFE
filters to remove any undissolved particles in the solution. The polymer solution was spin coated
by the dynamic technique. Approximately 1 mL of the polymer solution was dropped constantly
(≈ 1 drop per sec.) at 1,500 rpm for thirty seconds. After thirty seconds, the film formation was
allowed to proceed and dry at 3,000 rpm for forty-five seconds. After spin coating the polymer
film, the film was placed in an oven at ≈ 60 °C for two hours. After drying, the film appeared a
yellow-blue tint without any visible defects. The P4VP polymer film was then characterized by
ellipsometry to determine the film's thickness and optical constants. The P4VP film was exposed
to a concentrated vapor of PNT.
The refractive index of the P4VP film was A ≈ 1.580, which was similar to the reported
literature refractive index (n = 1.57280). The refractive indices as a function of wavelength
determined by spectroscopic ellipsometry are shown in Figure 4.7.1 and Table 4.7.1 provides the
Cauchy parameters, profilometer measured thickness, and spin coating parameters. The
ellipsometry curves displayed reasonable refractive indices expected for the Cauchy dispersion
173
model (refractive indices decreased toward the visible spectrum). The features observed in the
ellipsometry curves were attributed to surface roughness, similar to previously presented spectra.
When fitting the data to the Cauchy model, surface roughness was not accounted for. The small
MSE values obtained from the fitted data indicated a reasonable fit between the experimental and
generated data. The thickness measured by the profilometer differed by 4.0 nm when compared
to the thickness measured by the ellipsometer. This result was consistent with the previous
results, confirming the approximate thickness of the polymer film. Exposure to the PNT vapor
produced an average change in refractive index (Δn) of 0.014. This change in refractive index
was attributed to two types of interactions, π- π stacking or charge transfer complexes, as P4VP
does not undergo hydrogen bonding with nitroaromatics. This result was consistent with the
results Tenhaeff et al. observed with the ability of P4VP to detect a nitroaromatic vapor (TNT).80
The observed ability of P4VP to detect PNT by producing a significant change in refractive
index proposed the polymer to be an ideal material to incorporate in the MZI.
174
Figure 4.7.1. The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds.
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
1.74
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
175
Table 4.7.1. The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds.
Polymer: Poly(4-VP), 3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 1.628 1.835
Thickness (Å) 166.5 ± 0.4 161.3 ± 0.4
A 1.580 ± 5.32 E-3 1.596 ± 6.42 E-3
B 2.795 E-3 ± 1.43 E-3 2.446 E-3 ± 1.74 E-3
C 9.30 E-4 ± 1.25 E-5 8.835 E-4 ± 1.50 E-5
Δn 0.014
Optical Constants MSE 2.31 3.16
Dektak (Å) 121
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
176
Similar to P4VP, the PVI homopolymer and PVI-co-PVA copolymer were soluble in
EtOH. Polymer solutions were prepared by dissolving the polymers in EtOH using the wrist
action shaker until the polymer completely dissolved. After the polymers completely dissolved,
the polymer solutions were filtered removing any undissolved polymer particles. The PVI and
PVI-co-PVA solutions were spin coated by the dynamic technique, producing thin polymer
films. After spin coating, the films were then placed in an oven at 60 °C for two hours to remove
any residual EtOH. After drying, the polymer thin films were characterized by spectroscopic
ellipsometry to determine the films' thicknesses and optical constants.
The PVI polymer film was exposed to a PNT concentrated vapor for five seconds. The
PVI refractive index curves were plotted as a function of wavelength shown in Figure 4.7.2 and
Table 4.7.2 provides the Cauchy parameters, average change in refractive index, and spin coating
parameters. The observed PVI refractive indices were consistent for the Cauchy model. Similar
to the P4VP films, the features observed in the curves were attributed to surface roughness. The
refractive index determined by the Cauchy model before exposure to PNT was approximately A
≈ 1.575. After exposure to the PNT vapor, the polymer film's refractive index increased to
approximately A ≈ 1.599. The five-second exposure to PNT produced an average change in
refractive index (Δn) of 0.014. The change in refractive index occurred due to two possible
interactions: non-covalent interactions (including hydrophilic/hydrophobic interactions,
electrostatic, hydrogen bonding, or van der Waals forces) or covalent interactions (Meisenheimer
complex). Kong et al. reported a possible Meisenheimer complex formed between a templated
cross-linked polymer containing imidazole units with TNT confirmed by 1H NMR titrations with
TNT.81 The MSE values suggested a good fit for the Cauchy model between the experimental
data and generated data. The thicknesses determined by the Cauchy model before and after
177
exposure to PNT were identical. The profilometer thickness measurement was not performed to
confirm the approximate film thickness determined by the ellipsometer.
Figure 4.7.2. The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT.
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
178
Table 4.7.2. The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for five seconds.
Polymer: PVI, 3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 1.951 1.377
Thickness (Å) 215.8 ± 0.5 215.4 ± 0.3
A 1.575 ± 4.85 E-3 1.599 ± 1.01 E-3
B 1.281 E-3 ± 1.38 E-3 -2.412 E-3 ± 1.01 E-3
C 8.313 E-4 ± 1.21 E-4 9.621 E-4 ± 8.78 E-5
Δn 0.014
Optical Constants MSE 3.26 4.61
Dektak
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
179
A very thin PVI-co-PVA copolymer film was characterized by spectroscopic
ellipsometry before and after exposure to PNT for five seconds (Figure 4.7.3). Table 4.7.3 lists
the Cauchy parameters, average change in refractive index, profilometer measured thickness, and
spin coating parameters. The initial refractive index measured by the ellipsometer before
exposure to the nitroaromatic vapor was observed at A ≈ 1.268. After the five-second exposure
to PNT, the film’s refractive index increased to A ≈ 1.275, which produced an average change in
refractive index (Δn) of 0.007. The features observed in the before and after ellipsometry curves
were attributed to the film's surface roughness. The MSE values suggested a good fit between the
experimental and generated data from the Cauchy model. The 0.3% (w.t) polymer solution
produced an expected very thin polymer film (≈ 16 nm). There was a 2 nm difference in film
thickness observed between the profilometer and Cauchy model due to the etching process, but
this small difference approximately confirmed the Cauchy model's predicted film thickness. The
refractive index measured for the copolymer was a low value for two monomers with higher
refractive indices. This observed low refractive index will be addressed later in this chapter.
180
Figure 4.7.3. The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds.
1.26
1.28
1.3
1.32
1.34
1.36
1.38
1.4
1.42
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
181
Table 4.7.3. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds.
Polymer: PVI-co-PVA, 0.3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 3.932 3.734
Thickness (Å) 167 ± 3 166 ± 3
A 1.268 ± 9.30 E-3 1.275 ± 8.91 E-3
B 5.961 E-3 ± 1.50 E-3 5.80 E-3 ± 1.45 E-3
C 4.78 E-4 ± 1.41 E-4 5.201 E-4 ± 3.16 E-4
Δn 0.007
Optical Constants MSE 3.586 3.444
Dektak (Å) 143
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
182
4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine
In Chapter 5, the growth of co-crystals between 10-methylphenothiazine (10-M) and 1,3-
DNB is discussed. The co-crystals appeared red-purple in color, suggesting a strong interaction
between the electron donor and the electron deficient nitroaromatic. This result prompted the
investigations of 10-M included in polystyrene thin films to determine if a charge transfer
complex would form and cause a change in refractive index when exposed to 1,3-DNB vapors.
Polystyrene was synthesized according to the Chen procedure.49 Polystyrene was purified
by washing the polymer in EtOH in a one-neck round bottom. After purifying the polymer, the
polymer was dried at 50 °C under vacuum overnight.
3% and 1% (w.t.) stock polystyrene solutions were prepared by dissolving the polymer in
toluene. The polymer solutions were then placed in a wrist action shaker until the polymer
completely dissolved. After the polymer dissolved, the polymer solutions were filtered once to
remove any particles present in the solution.
The first experiments performed were to determine the maximum 10-M concentration
that could be included in polystyrene solutions to allow homogeneous spin coated polymer films.
3% (w.t.) polystyrene/toluene solutions containing 0.1%, 0.3%, 0.7%, 1%, 3%, 6%, and 10%
(w.t.) 10-M were prepared by dissolving 10-M in the polystyrene/toluene solutions using the
wrist action shaker. After the 10-M dissolved in the polystyrene solutions, the solutions were
filtered again to remove any particles present in the solutions. Films were spin coated by the
static technique. The films were allowed to dry for twenty-four to forty-eight hours at room
temperature in a dark area, since 10-M was sensitive to the light. After drying, the polymer films
were inspected for any defects. The films spin coated from the polystyrene solutions containing
0.1% - 1% 10-M (w.t.) produced blue films with a slight purple tint. The film spin coated from
183
the 3% (w.t.) polystyrene solution containing 3% (w.t.) 10-M produced a blue-purple tint film
with a few white, needle-like crystals protruding from the film. The spin coated film from the
polystyrene solution containing 6% (w.t.) 10-M produced a blue-purple tint film with large areas
covered with white, needle-like crystals. The 10% (w.t.) 10-M spin coated film surprisingly did
not produce white, needle-like crystals, but rather produced a blue-purple tint film with a slight
green tint with cracks throughout, creating a mosaic pattern. The same experimental procedure
was performed for 1% (w.t.) polystyrene solutions which produced similar results with 1% (w.t.)
10-M being the maximum concentration included in polystyrene films that allowed
homogeneous spin coated films. Next, the polystyrene/10-M films' optical constants were
characterized by spectroscopic ellipsometry before and after exposure to 1,3-DNB vapors from
seconds to hours. Refractive index changes were unsuccessful for short exposure times. The
exposure times were increased until a change in refractive index was observed.
A polymer film spin coated from a 3% (w.t.) polystyrene solution containing 1% (w.t.)
10-M was exposed to 1,3-DNB for two hours (Figure 4.8.1). Table 4.8.1 lists the Cauchy
parameters, average change in refractive index, profilometer measured thickness, and spin
coating parameters. The refractive index curves displayed the trend observed for a Cauchy
dispersion model. The spectrum was fitted using the Cauchy model from 400 - 1,000 nm,
excluding any absorption, which may have occurred in the UV region. The initial refractive
index of the polystyrene/10-M film was A ≈ 1.567 and increased with decreasing wavelengths.
The 10-M/polystyrene film was exposed to 1,3-DNB for two hours. After the film was exposed
to 1,3-DNB, the film's refractive index increased to A ≈ 1.582. The average change in refractive
index from 400 - 1,000 nm was (Δn) ≈ 0.005. The MSE values for the Cauchy parameters and
optical constants suggested a reasonable fit between the experimental and generated data. The
184
profilometer’s measured thickness for the film was ≈ 95.9 nm, which was 7 nm thicker than the
thickness determined by the Cauchy model (≈ 89 nm). This difference in film thickness may be
due to the etching process, where the etch penetrated the silicon wafer's surface and created a
thicker film measurement. Even though there was a small difference between the film thickness
measurements, the profilometer measurement did approximately confirm the ellipsometer's
determined film thickness. The features observed in the ellipsometry curves follow the trend
observed for a film with some surface roughness.
185
400 500 600 700 800 900 1000
1.58
1.59
1.60
1.61
1.62
1.63
1.64
1.65
1.66
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.1. The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours.
186
Table 4.8.1. The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours.
Polymer: 3% (w.t.) PS, 1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 2 hrs. exposure
Before After
MSE 5.053 5.86
Thickness (Å) 888.3 ± 0.4 889.6 ± 0.4
A 1.567 ± 1.10 E-3 1.582 ± 2.99 E-3
B 1.651 E-2 ± 6.21 E-3 1.200 E-2 ± 1.52 E-3
C -7.346 E-4 ± 9.08 E-5 -2.279 E-4 ± 1.94 E-5
Δn 0.005
Optical Constants MSE 4.281 5.474
Dektak (Å) 959
Spin Coating 5,000 rpm for 40 sec.
187
A polystyrene/10-M film was spin coated from a 1% (w.t.) polystyrene solution
containing 0.5% (w.t.) 10-M. The polystyrene/10-M film thickness and optical constants were
characterized by ellipsometry before and after exposure to 1,3-DNB for three hours (Figure
4.8.2). Table 4.8.2 lists the Cauchy parameters, average change in refractive index, profilometer
measured thickness, and spin coating parameters for the film. The refractive index before
exposure to 1,3-DNB was A ≈ 1.470. After the film was exposed to 1,3-DNB for three hours, the
film's refractive index increased to A ≈ 1.480. The increase in refractive index was due to the 10-
M incorporated in the polymer film that hydrogen bonded with the nitro groups of 1,3-DNB. The
features observed in the refractive index curves were due to the presence of surface roughness,
which was not accounted for in the model. After exposure to the nitroaromatic, the film did not
change in color compared to the observed color change for the 10-M co-crystal with 1,3-DNB.
The before and after Cauchy MSE values described a good fit between the experimental and
generated data. The Cauchy model determined a film thickness of ≈ 27 nm, but the profilometer
measured a film thickness of ≈ 31 nm. The difference in film thickness was consistent with the
previous result where the profilometer’s measured thickness was slightly larger than the
thickness determined by the ellipsometer. Again, this difference resulted from the etching the
process. The minimal difference between both measurements did confirm the approximate film
thickness. This result led to the investigation to determine if a smaller concentration of 10-M in a
polystyrene film could produce a change in refractive index over an extended exposure period.
188
400 500 600 700 800 900 10001.46
1.48
1.50
1.52
1.54
1.56
1.58
1.60
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.2. Refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.
189
Table 4.8.2. The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.
Polymer: 1% (w.t.) PS, 0.5% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 3 hrs. exposure
Before After
MSE 2.41 2.283
Thickness (Å) 267.3 ± 0.8 270.1 ± 0.7
A 1.470 ± 5.22 E-3 1.480 ± 2.99 E-3
B 1.112 E-2 ± 2.50 E-3 1.133 E-2 ± 2.42 E-3
C 8.341 E-4 ± 3.34 E-4 8.565 E-4 ± 3.23 E-5
Δn 0.01
Optical Constants MSE 2.553 2.413
Dektak (Å) 307
Spin Coating 3,000 rpm for 40 sec.
190
A polystyrene/10-M film was spin coated on a silicon wafer from a 1% (w.t.)
polystyrene/toluene solution containing 0.1% (w.t) 10-M. The polystyrene/10-M solution
produced a blue colored film with a slight purple tint, which was characterized by ellipsometry
before and after exposure to 1,3-DNB to determine the film’s thickness and optical constants
(Figure. 4.8.3). Table 4.8.3 lists the Cauchy parameters, average change in refractive index,
profilometer measured thickness, and spin coating parameters for the polymer film. The
refractive index determined by the Cauchy model for the film before exposure to 1,3-DNB was
A ≈ 1.464. After the film was exposed to 1,3-DNB, the refractive index increased to A ≈ 1.469.
The small concentration of 10-M included in the polystyrene film showed the ability to produce
a small change in refractive index of Δn = 0.004 due to hydrogen bonding with the 1,3-DNB
nitro groups. The MSE values described a good fit between the experimental and generated data
for the Cauchy model. There was a minute difference in film thickness observed between the
Cauchy model's determined film thickness and the profilometer's measured film thickness. This
result was similar to previous film thickness measurements where the profilometer measured a
film thickness greater than the Cauchy model's determined film thickness. This minute difference
(≤ 1 nm) confirmed the approximate film thickness determined by the Cauchy model. The
features observed in the refractive index curves between 600 - 1,000 nm indicated surface
roughness for the film, since typically the refractive index curves lie flat in the near-infrared
region.
The inclusion of small concentrations of 10-M in polystyrene films exposed to 1,3-DNB
for an extended period of time showed the ability to produce changes in refractive index by non-
covalent interactions. These results provided evidence that a polymer containing 10-M moieties
in greater concentration might have a strong affinity for 1,3-DNB giving rise to a significant
191
change in refractive index with a shorter exposure time. These results suggested 10-M would be
an ideal material to be applied in the MZI sensor for detecting nitroaromatics.
400 500 600 700 800 900 1000
1.46
1.48
1.50
1.52
1.54
1.56
1.58
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.3. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.
192
Table 4.8.3. The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.
Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 3 hrs. exposure
Before After
MSE 2.215 2.119
Thickness (Å) 257.4 ± 0.8 257.7 ± 0.8
A 1.464 ± 4.86 E-3 1.469 ± 4.61 E-3
B 9.162 E-3 ± 2.26 E-3 9.333 E-3 ± 2.14 E-3
C 9.846 E-4 ± 3.03 E-4 9.830 E-4 ± 2.87 E-4
Δn 0.004
Optical Constants MSE 2.312 2.205
Dektak (Å) 262
Spin Coating 3,000 rpm for 40 sec.
193
During one experiment, a film spin coated from a 1% (w.t.) polystyrene/toluene solution
containing 0.1% (w.t.) 10-M produced a change in refractive index after a ten second exposure to
1,3-DNB (Figure 4.8.4). The refractive index before exposure to the 1,3-DNB vapors was A ≈
1.494. After the ten-second exposure, the refractive increased to A ≈ 1.499, which produced a
0.005 average refractive index change (Table 4.8.4). The MSE values for the Cauchy model
described an ideal fit between the experimental and generated data. The film thickness
determined by the Cauchy model and the profilometer measured thickness were in agreement,
which confirmed the thickness determined by the ellipsometer. Features appeared in the near-
infrared region of the refractive index curves because of the presence of surface roughness. This
result was consistent with the previous result for a film spin coated from a 1% (w.t.)
polystyrene/toluene solution containing 0.1% 10-M exposed to 1,3-DNB for three hours. This
result showed that in one instance longer exposure times were not required to produce a
detectable change in refractive index.
194
400 500 600 700 800 900 10001.48
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.4. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.
195
Table 4.8.4. The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.
Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 10 sec. exposure
Before After
MSE 2.698 3.421
Thickness (Å) 273.4 ± 0.8 271.6 ± 0.7
A 1.494 ± 5.54 E-3 1.499 ± 5.06 E-3
Δn 0.005
Optical Constants MSE 2.86 2.577
Dektak (Å) 272
Spin Coating 3,000 rpm for 40 sec.
196
Most of the refractive indices for the polymers reported in this chapter were in agreement
with the reported literature refractive indices. There were some instances where the observed
refractive indices for thin polymer films (≤ 100 nm) exhibited lower refractive indices values
than the reported literature values. The lower indices of refraction were due to a radially
symmetric segmental orientation of specific groups on the polymer chains acting as an optical
retarder, produced during the spin coating process. Schwab et al. measured the optical
birefringence of rubbed thin polystyrene films to investigate the relaxation processes of
molecules at the polymer/air interface that showed shifts in the index of refraction.82 Hu et al.
investigated anomalies of refractive index for spin coated thin polystyrene films.83 Hu found that
the index of refraction was a function of film thickness for polystyrene films less than 100 nm
and the bulk refractive index could be recovered by annealing the films above the Tg. Hu noted
that the observed change in refractive index was due to the symmetric segmental orientation
produced by the spin coating process, since the orientation induced during the spin coating
process is typically radially symmetric about the spin axis and uniformly either in or out of the
plane of the spin coated film.83
The polymer films in this chapter represented possible materials that could be
incorporated in the MZI sensor to detect nitroaromatic explosives. The PS-co-PVDAT
copolymers, P2VP, PVI, PVI-co-PVA, and polystyrene/10-M films exhibited changes in the
index of refraction after exposure to a concentrated nitroaromatic vapor, suggesting that these
materials should be considered as possible MZI sensing materials. The copolymers containing
low concentrations of the VDAT monomer produced homogeneous films with minimal surface
roughness, allowing the optical constant to be fully characterized by spectroscopic ellipsometry.
VDAT appeared to be a promising monomer to synthesize electron rich copolymers, but as the
197
concentration was increased in the copolymers, problems with solubility in an ideal spin coating
solvent and the ability to spin coat homogeneous films with minute surface roughness excluded
these polymers from possibly being used as a sensing material. These were the main
disadvantages associated with utilizing VDAT, which led to the investigations of other polymers'
abilities to sense nitroaromatics by refractive index changes. P2VP, P4VP, PVI, and PVI-co-
PVA all showed the ability to interact with the nitro groups of the nitroaromatics, which
produced changes in the refractive index curves. Most of the polymers were not soluble in an
ideal spin coating solvent, but using the static spin coating technique allowed homogeneous films
to be casted with some surface roughness that were characterized by ellipsometry.
The polystyrene films containing low concentrations of 10-M showed the unexpected
ability to interact with the 1,3-DNB vapors over long exposure periods. These initial results
provided evidence of further work needed to synthesize a polymer rich in 10-M moieties. A
polymer containing larger concentrations of 10-M may have the potential to produce significant
changes in the index of refraction after short exposure periods to 1,3-DNB.
It should be noted that some of the polymers synthesized in Chapter 3 did not show
changes in refractive index when exposed to a concentrated nitroaromatic vapor. When the
nitroaromatic (1,3-DNB) was added to the polymers, PVK and PMMA-co-PVK copolymers, in
the solution phase and dissolved, changes in color (colorless to yellow) were observed for the
polymer/nitroaromatic solutions, suggesting a strong interaction between the electron rich
polymers and 1,3-DNB. These polymers could still be used in the MZI sensor to assist in the
detection of the nitroaromatic (1,3-DNB) in the solution phase, as opposed to the vapor phase.
Lastly, a concerning problem observed for the polymers in this chapter was the variant
changes in the index of refraction after exposure to the concentrated nitroaromatic vapors.
198
Minimal, large, or no change in refractive indices after exposure to the nitroaromatic were
observed, limiting the reliability of these sensing materials. Still, there was enough data to
support the possibility of using these polymer films as nitroaromatic sensing materials for a MZI
sensor.
4.9 Polymer Thin Films Summary
Polymer thin films were investigated to determine their affinity for nitroaromatics by
measuring the change in refractive index after exposure to a nitroaromatic vapor using
ellipsometry. To demonstrate the polymer films affinities for nitroaromatics, Table 4.9.1 lists the
polymer films' average change in refractive index after a five-second exposure to a nitroaromatic
vapor. From the PVDAT copolymers, the copolymers consisting of polystyrene and PVDAT
showed the most promise as the sensing material for the MZI. The PS-co-PVDAT copolymers
allowed films to be casted from an ideal spin coating solvent with minimal surface roughness
and did exhibit an affinity for nitroaromatics. The PMMA-co-PVDAT copolymers appeared to
have a greater affinity for nitroaromatics compared to the PS-co-PVDAT copolymers, but
extreme surface roughness was observed for the PMMA-co-PVDAT copolymer films and a
change in refractive index was not observed for some of the PMMA-co-PVDAT copolymer
films after the five-second exposure to a nitroaromatic vapor. From the commercial polymers,
PVI and P4VP showed the most promise for detecting nitroaromatics due to the change in
refractive index after exposure to a nitroaromatic vapor. A change in refractive index was
observed for the PVI-co-PVA polymer film after being exposed to PNT for five seconds, but
extreme surface roughness was observed for the polymer film.
199
Table 4.9.1. Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor.
Polymer Nitroaromatic Exposure Time (sec.) ∆n
PVI-co-PVA PNT 5 0.010
P4VP PNT 5 0.014
PVI PNT 5 0.014
PS-co-PVDAT 1 mol % PNT 5 0.002
PS-co-PVDAT 1 mol % NB 5 0.013
PS-co-PVDAT 1 mol % 1,3-DNB 5 0.010
PS-co-PVDAT 5 mol % PNT 5 0.002
PS-co-PVDAT 5 mol % NB 5 0.005
PS-co-PVDAT 10 mol % NB 5 0.003
PS-co-PVDAT 10 mol % PNT 5 0.001
PS-co-PVDAT 10 mol % 1,3-DNB 5 0.003
PS-co-PVDAT 20 mol % NB 5 0.009
PS-co-PVDAT 20 mol % PNT 5 0.002
PMMA-co-PVDAT 5 mol % PNT 5 0.017
PMMA-co-PVDAT 20 mol % PNT 5 0.073
P2VP PNT 5 0.019
200
Chapter 5
Co-crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules Nitroaromatic molecules will form molecular complexes with electron-rich aromatic
molecules, producing intense color changes. 1,3-dinitrobenzene and 2,4-dinitrotoluene both
formed jet-black molecular complexes with benzidine (4,4’-diaminobiphenyl).84 Chloroform
solutions containing aniline and either 1,3-dinitrobenzene, 1,4-dinitrobenzene or 1,3,5-
trinitrobenzene exhibited new absorptions extending from the UV region into the visible region
with extinction coefficients of 102 to 103 M-1 cm-1.85 Mixtures of picryl chloride with
hexamethylbenzene or picric acid with naphthalene formed highly colored solutions in
chloroform.86, 87
The crystal structure packing observed in complexes between an electron rich donor and
nitroaromatic exhibit a general trend with alternating stacking in charge transfer complexes. The
crystal structure of a 1:1 molecular complex between 1,4,-dinitrobenzene and phenazine showed
alternating stacks of phenazine and 1,4-dinitrobenzene molecules with a 361 pm distance
between the center of the phenazine molecule and the center of the 1,4-dinitrobenzene
molecule.88 A 1:1 molecular complex of 2-aminobenzimidazole and 1,3,5-trinitrobenzene also
showed alternating stacks.89 In each case, the electron-poor nitro aromatic rings and the electron-
rich aromatic rings were stacked face-to-face. This pattern of alternating stacking of donor and
acceptor aromatic molecules face-to-face was also observed in the crystal structure of the
molecular complex of tetrathiafulvalene and 1,3-dinitrobenzene and in the molecular complex of
201
4-iodotetrathiafulvalene and 1,4-nitrobenzene.90, 91 These intense color changes observed when
an electron donor forms a molecular complex with various nitroaromatics provide evidence of
the ability to detect nitroaromatics by using an electron rich polymer. The polymer could form a
molecular complex with the electron deficient nitroaromatics by hydrogen bonding or pi→pi*
interactions. Varieties of electron donor and acceptor combinations that were attempted are
shown in Chapter 2. This chapter will focus on the electron donors and acceptors which
produced co-crystals confirmed by 1H NMR, FTIR, UV/Vis, diffuse reflectance, and X-ray
crystallography.
5.1 1,3-Dinitrobenzene Crystals (1,3-DNB)
1,3-DNB crystals were produced by dissolving 1,3-DNB in EtOH and allowing the EtOH
to evaporate at room temperature for two days. Needle-like crystals formed with a faint white-
yellow color shown in Figure 5.1.1.
Figure 5.1.1. Image of 1,3-DNB crystals.
202
The 1H NMR (360 MHz, CDCl3) spectrum of the 1,3-DNB crystals was recorded shown
in Figure 5.1.2. The 1,3-DNB proton signals were characterized by 1H NMR. The reported 1,3-
DNB proton signals were in agreement with reported literature values. The positions of the
proton signals were used as a reference to determine the presence of 1,3-DNB in the co-crystals.
Figure 5.1.2. 1H NMR spectrum of 1,3-DNB crystals (360 MHz, CDCl3).
1,3-DNB: 1H NMR (360 MHz, CDCl3, δ): 9.06 (t, 1.00H), 8.57 (dd, 2.04H), 7.80 (t, 1.05H).
The integration values and peak positions were used as a reference when comparing 1H NMR
spectra in order to determine the ratio between 1,3-DNB and the electron donors.
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
H1
H2
H3
H2O
CDCl3
203
The FTIR spectrum for the 1,3-DNB crystals is shown Figure 5.1.3. The peaks located at
1540 and 1347 cm-1 were assigned to the NO2 asymmetric and symmetric stretching vibrations.
The C-H stretching vibrations appeared at 2873, 3049, and 3108 cm-1. The benzene ring
stretching vibrations were observed at 1614 and 1602 cm-1 with the benzene ring overtones
appearing at 2873, 3049, and 3108 cm-1. The NO2 asymmetric and symmetric stretching
vibrations were used as a reference to determine if a complex formed between the electron
donor and electron acceptor, producing a shift in the vibrational bands.
4000 3500 3000 2500 2000 1500 1000 500
20
30
40
50
60
70
80
90
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
1540NO2 vas
1347NO2 vsym
CH stretchingvibrations
Benzene RingOvertones
Figure 5.1.3. The FTIR spectrum of the 1,3-dinitrobenzene crystals.
204
The electronic absorption spectrum for the 1,3-DNB crystals (Figure 5.1.4) was recorded
in acetonitrile at a concentration of 2.0 x 10-5 M. Acetonitrile was chosen as the applicable
solvent due to its UV/Vis solvent cut-off (≈ 190 nm for a 1 cm cuvette). The 1,3-DNB crystals'
electronic absorption spectrum displayed a λmax at 237 nm (ε = 1.80 x 104 M-1cm-1).
200 300 400 500 600 700 8000
5000
10000
15000
20000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals
Figure 5.1.4. Electronic absorption spectrum of 1,3-DNB crystals in acetonitrile.
205
The diffuse reflectance spectrum for the 1,3-DNB crystals (Figure 5.1.5) was recorded at
room temperature. The reflectance was more than 50% in the visible region from 450 - 800 nm.
The reflectance decreased to approximately 5% from 450 - 400 nm, which gave rise to the
yellow tint for the 1,3-DNB crystals.
200 300 400 500 600 700 8000
10
20
30
40
50
60
% R
efle
ctan
ce
Wavelength (nm)
Figure 5.1.5. Diffuse reflectance spectrum of the 1,3-DNB crystals.
The melting point for the 1,3-DNB crystals was measured for comparison with the
melting point for co-crystals containing 1,3-DNB and electron-rich aromatic molecules. The
melting range was defined at the temperature that liquid formation was visible until the crystals
completely melted and formed a meniscus. The observed melting range was 90.9 - 91.3 °C (lit.
206
89 °C54). The narrow melting range provided evidence of crystals with few inhomogeneities or
individual components.
5.2 9-Ethylcarbazole (9-EC) Co-Crystals with Nitroaromatics
Attempts to prepare 1:1 co-crystals with 9-EC and either 2-NT, 3-NT, and PNT were not
successful. The solutions evaporated, producing a mixture of crystals of the pure compounds.
However, when the EtOH solutions of 9-EC and 1,3-DNB were mixed, a yellow-orange color
rapidly appeared. The EtOH was allowed to evaporate for two days, producing yellow-orange
needle-like crystals shown in Figure 5.2.1. 9-EC crystals were prepared by the same procedure
producing, white-brown, needle-like crystals.
Figure 5.2.1. 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals.
The 1H NMR spectra for the 9-EC co-crystals, 9-EC crystals, and 1,3-DNB crystals
(Figure 5.2.2) were recorded in CDCl3 to determine the ratio between the electron donor and
acceptor. Table 5.2.1 lists the peak positions, splitting patterns, and integration values for the 1,3-
DNB crystals, 9-EC crystals, and 9-EC co-crystals. The peaks observed in the 9-EC co-crystals
207
1H NMR spectrum at 9.06, 8.56, and 7.79 ppm were assigned to the 1,3-DNB incorporated in the
co-crystals. The 9-EC proton signals were observed at 8.09, 7.45-7.39, 7.23-7.19, 4.36, and 1.42
ppm. Neither the 9-EC nor 1,3-DNB peaks exhibited a chemical shift in the spectrum. The
integration value for the 9-EC proton signal at 7.22-7.20 ppm in the 9-EC and 9-EC co-crystals
spectra was not an accurate integration due to the CDCl3 solvent peak overlapping. The
integration of the spectrum revealed an approximate 1:1 ratio between 9-EC and 1,3-DNB.
Figure 5.2.2. 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
1,3-DNB
9-EC
9-EC co-crystals
208
Table 5.2.1. 1H NMR peak positions, splitting patterns, and integration values of the 1,3-DNB crystals, 9-EC co-crystals, and 9-EC crystals.
1,3-DNB 9-EC co-crystals 9-EC δ S.P. Int. δ S.P. Int. δ S.P. Int.
9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 2.02
8.09 dt 1.86 8.10 dt 1.90 7.80 t 1.05 7.79 t 1.12
7.45-7.38 m 3.77 7.48-7.39 m 3.89 7.23-7.19 m 3.42 7.22-7.20 m 2.48 4.36 q 1.84 4.37 q 2.00 1.42 t 2.67 1.43 t 2.93
S.P. - Splitting Pattern Int. - Integration values
The FTIR spectra for the 9-EC crystals and co-crystals of 9-EC with 1,3-DNB are shown
in Figure 5.2.3. The symmetric and asymmetric stretching modes for the NO2 groups in the co-
crystal were shifted to lower energy, compared to those for the 1,3-DNB crystals. This shift
indicated intermolecular interactions between 9-EC and 1,3-DNB in the co-crystals. Table 5.2.2
shows the comparison between the NO2 asymmetric and symmetric stretching modes before and
after incorporation into the co-crystal.
209
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
3420
31183104
2981 1601
1536
1452
1345
30472978
2931
1594
14541377
1326
Free OH
CH stretching vibrations
9-EC
9-EC + 1,3-DNB
Figure 5.2.3. FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve).
Table 5.2.2. Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1, 3-DNB co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB white-yellow 1540 1347
9-EC white-brown N/A N/A
9-EC + 1,3-DNB yellow-orange 1536 1345
210
The electronic absorption spectrum of the 9-EC co-crystals with 1,3-DNB was recorded
in acetonitrile to determine if a charge complex formed in the dilute solutions. Figure 5.2.4
displays the electronic absorption spectra for the 1,3-DNB crystals, 9-EC crystals, and 9-EC co-
crystals with 1,3-DNB. There was no new absorption band that would be expected if a charge
transfer complex formed. There were no significant chemical shifts observed in the spectra
between 9-EC crystals and 9-EC co-crystals.
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals 9-EC crystals 9-EC co-crystals
Figure 5.2.4. Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile.
211
To determine if the spectrum for the 9-EC co-crystals was the result of a charge transfer
complex or just the combination of free 9-EC and 1,3-DNB molecules in solution, the spectra of
the 1,3-DNB crystals and 9-EC crystals were combined and compared against the 9-EC co-
crystals spectrum (Figure 5.2.5). The similarities between the 9-EC co-crystals electronic
absorption spectrum and the combined 1,3-DNB and 9-EC crystals electronic absorption
spectrum was the result of the independent 1,3-DNB and 9-EC molecules in solution rather than
the formation of a charge transfer complex. Clearly, 1,3-DNB and 9-EC did not form a charge
transfer complex in dilute ~10-5 M acetonitrile solutions.
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB + 9-EC 9-EC co-crystals
Figure 5.2.5. Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black).
212
The diffuse reflectance spectra were measured for the 9-EC crystals and 9-EC co-crystals
with 1,3-DNB (Figure 5.2.6). The reflectance for the 9-EC crystals was greater than 70%
throughout the visible region. Below 400 nm, the reflectance decreased to less than 40% as the
UV light was absorbed by the crystals. The reflectance spectrum for the co-crystals had a
reflectance of less than 40% in the near infrared and red region of the spectrum. The reflectance
decreased to 10% at wavelengths below 500 nm. The difference in absorption was expected,
since the 9-EC crystals were white-brown, compared to the co-crystals which were yellow-
orange in color.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
efle
ctan
ce
Wavelength (nm)
9-EC crystals 9-EC co-crystals
Figure 5.2.6. Diffuse reflectance spectra for 9-EC crystals (black) and the co-crystals of 9-EC and 1,3-DNB (red).
213
The melting points of the 9-EC crystals and 9-EC co-crystals were measured for
comparison. Table 5.2.3 lists the melting points for the 1,3-DNB crystals, 9-EC crystals, and 9-
EC co-crystals. The co-crystals with 1,3-DNB had a much lower melting range (48.4 - 50.1 °C)
compared to the 9-EC crystals (68 - 70 °C) and 1,3-DNB crystals (89 °C) melting ranges. This
lower, narrow melting point range indicated that there were few inhomogeneities or individual
components present. During the melt, the co-crystals produced a color change from a yellow-
orange to a red-orange color.
Table 5.2.3. Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3-DNB.
Crystals Melting Point (°C) Lit. Value (°C)
1,3-DNB 90.9 - 91.3 89 54
9-EC 71.0 - 71.8 68 - 70 92
9-EC co-crystal 48.4 - 50.1
The 9-EC co-crystal structure was analyzed by X-ray diffraction. A survey scan of a co-
crystal revealed that the crystal structure was 1,3-DNB. Ito et. al. reported similar results with
carbazole derivative co-crystals with 1,3-DNB.92 Ito made reference that the crystal adducts
might be too small for X-ray diffraction characterization.
5.3 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-DNB
9-VC did not form co-crystals with 2-NT or NB. It did form a co-crystal with 1,3-DNB,
which was confirmed by an intense color change. When the two solutions of 9-VC and 1,3-DNB
were combined in a crystallization dish, a bright yellow color rapidly appeared. The EtOH
evaporated at room temperature for two weeks, producing yellow crystals with spots as shown in
214
Figure 5.3.1. 9-VC crystals were prepared by the same procedure producing white needle-like
crystals.
Figure 5.3.1. Co-crystals of 9-VC and 1,3-DNB.
The 1H NMR spectra of the 9-VC co-crystals, 9-VC crystals, and 1,3-DNB crystals
(Figure 5.3.2) were recorded in CDCl3 to determine the ratio between the electron donor and
acceptor. The peak positions, multiplicities, and integrations are listed in Table 5.3.1. The 1H
NMR spectrum for the 9-VC co-crystals with 1,3-DNB showed peaks located at 9.07, 8.57, and
7.79 ppm assigned to 1,3-DNB incorporated in the co-crystal. The 9-VC proton signals were
observed at 8.06, 7.65, 7.46, 7.32-7.25, 5.54, and 5.15 ppm. Neither the 9-VC nor 1,3-DNB
peaks exhibited a chemical shift in the spectrum. The integration values for the 9-VC proton
signals located between 7.32-7.25 ppm in the 9-VC and 9-VC co-crystal spectra were not an
accurate integration due to the overlapping CDCl3 solvent peak. The integration of the spectrum
revealed an approximate 1:2 ratio between 9-VC and 1,3-DNB.
215
Figure 5.3.2. 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3).
Table 5.3.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals.
1,3-DNB 9-VC co-crystals 9-VC δ S.P. Int. δ S.P. Int. δ S.P. Int.
9.06 t 1.00 9.07 t 1.71 8.57 dd 1.98 8.57 dd 3.28
8.06 d 1.81 8.07 d 1.93 7.80 t 1.04 7.79 t 1.90
7.65 d 1.94 7.65 d 1.95 7.46 td 2.04 7.46 td 1.98 7.32-7.25 m 3.71 7.33-7.27 m 2.88 5.54 dd 1.02 5.55 dd 1.01 5.15 dd 1.00 5.16 dd 1.00
S.P. - Splitting Pattern Int. - Integration value
4.44.85.25.66.06.46.87.27.68.08.48.89.2ppm
1,3-DNB
9-VC co-crystals
9-VC
216
In Figure 5.3.3, the FTIR spectra for the 9-VC crystals (red) and the 9-VC co-crystals
with 1,3-DNB (black) are shown. The NO2 peaks at 1536 (asymmetric stretching mode) and
1345 cm-1 (symmetric stretching mode) were red shifted (4 and 2 nm) to lower energy compared
to the corresponding peaks in the FTIR spectrum for the 1,3-DNB crystals. This shift indicated
an intermolecular interaction between 9-VC and 1,3-DNB in the co-crystals. Table 5.3.2 lists the
NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 9-VC co-
crystals with 1,3-DNB.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
CH Bonding
1536 1345
9-VC co-crystals
9-VC
Figure 5.3.3. FTIR spectra for KBr pellets containing either 9-VC crystals (red) or the co-crystals of 9-VC and 1,3-DNB (black).
217
Table 5.3.2. NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and 9-VC co-crystals.
The co-crystals were not soluble in ethanol or acetonitrile; therefore, the electronic
absorption spectrum in solution was not obtained. As a result, we could not obtain co-crystals
suitable for single crystal X-ray diffraction. Ito et. al. reported similar results with co-crystals of
1,3-DNB and carbazole derivatives.92
The melting ranges were measured for the 9-VC crystals and 9-VC co-crystals with 1,3-
DNB (Table 5.3.3). The liquid formation temperature observed in the co-crystals (79.7 °C) was
higher than the liquid formation temperature for the 9-VC crystals (61.8 °C), but lower than the
liquid formation temperature for the 1,3-DNB crystals (90.9 °C). The co-crystals' observed
melting temperature (formation of meniscus) was higher compared to both the 1,3-DNB crystals
and 9-VC crystals. This broad melting range indicated the presence of inhomogeneities or
individual components. During the melt, the co-crystals produced color changes from yellow to
orange at approximately 80 °C before liquid formation.
Table 5.3.3. 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals melting points.
Crystals Melting Point (°C) Lit. Value (°C)
1,3-DNB 90.9 - 91.3 89 54
9-VC 61.8 - 62.9
9-VC co-crystal 79.7 - 116.9
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB White-yellow 1540 1347
9-VC White N/A N/A
9-VC + 1,3-DNB Yellow 1536 1345
218
5.4 Carbazole (CBZ) Co-Crystals with 1,3-DNB
Attempts to prepare co-crystals between CBZ and either 2-NT or NB were not successful.
When the solutions of CBZ and 1,3-DNB were combined in a crystallization dish, the solution
did not produce a color change. The color of the solution was similar to the CBZ solution (light
brown). The EtOH was allowed to evaporate for two days, producing small, needle-like crystals
with large flakes as shown in Figure 5.4.1. CBZ crystals were prepared by the same procedure,
producing small, light brown, needle-like crystals with brown flakes. Figure 5.4.1 shows images
of CBZ crystals (A) and CBZ co-crystals (B).
Figure 5.4.1. Crystals of carbazole (A) and co-crystals of carbazole and 1,3-DNB (B).
The 1H NMR spectra of the co-crystals, CBZ crystals, and 1,3-DNB crystals (Figure
5.4.2) were recorded in CDCl3 to determine the ratio between the electron donor and acceptor.
The peak positions, peak multiplicities, and peak integrations are listed in Table 5.4.1. The peaks
observed in the 1H NMR spectrum for the CBZ co-crystals with 1,3-DNB located at 9.06, 8.56,
and 7.79 ppm were assigned to 1,3-DNB incorporated in the co-crystals. The CBZ proton signals
were observed at 8.07-8.04, 7.43-7.38, and 7.24-7.20 ppm. Neither the CBZ nor 1,3-DNB peaks
(A) (B)
219
exhibited a chemical shift in the spectrum. The integration value for the CBZ proton signals
located between 7.24-7.20 ppm in the CBZ and CBZ co-crystal spectra were not an accurate
integration due to the overlapping CDCl3 solvent peak. The integration of the spectrum revealed
an approximate 1.2:1.0 ratio between CBZ and 1,3-DNB.
Figure 5.4.2. 1H NMR spectra of CBZ crystals, CBZ co-crystals with 1,3-DNB, and 1,3-DNB crystals (360 MHz, CDCl3).
6.36.77.17.57.98.38.79.1ppm
1,3-DNB
CBZ co-crystals
CBZ
220
Table 5.4.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, CBZ co-crystals, and CBZ crystals.
1,3-DNB CBZ co-crystals CBZ δ S.P. Int. δ S.P. Int. δ S.P. Int.
9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 1.94
8.07-8.04 m 3.49 8.07-8.03 m 2.94 7.80 t 1.05 7.79 t 1.07
7.43-7.38 m 4.80 7.43-7.38 m 4.00 7.24-7.20 m 4.50 7.23-7.21 m 3.85
S.P. - Splitting Pattern Int. - Integration values
The infrared spectra of the CBZ crystals and the co-crystals are shown in Figure 5.4.3
and Table 5.4.2 lists the NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB
crystals and CBZ co-crystals. The spectrum for the CBZ co-crystals showed that the NO2
asymmetric stretching mode (1537 cm-1) was red-shifted (3 nm) to lower energy, which indicated
that a weak complex occurred during the formation of the co-crystals. The NO2 symmetric
stretching mode (1347 cm-1) was at the same position as the symmetric stretch in the 1,3-DNB
crystals. Only one of the NO2 stretching vibrations exhibited a shift, suggesting a weak
intermolecular interaction between CBZ and 1,3-DNB.
221
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
Free OH
3419NH
1537
1347
CBZ co-crystals
CBZ crystals
Figure 5.4.3. FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co-crystals with 1,3-DNB (red).
Table 5.4.2. NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB White-yellow 1540 1347
CBZ Light brown N/A N/A
CBZ + 1,3-DNB Light brown 1537 1347
222
The electronic absorption spectrum of the CBZ co-crystals with 1,3-DNB was recorded
in acetonitrile to determine if a charge transfer complex formed within the co-crystals. Figure
5.4.4 shows the electronic absorption spectra for the 1,3-DNB crystals, CBZ crystals, and CBZ
co-crystals with 1,3-DNB. The expected weak broad absorption band for a charge transfer
complex was not observed in the spectrum, indicating that a charge transfer complex did not
form. None of the CBZ peaks in the co-crystal spectrum exhibited any significant chemical
shifts. To determine if the CBZ co-crystal spectrum was the result of a charge transfer complex
or just the interaction between CBZ and 1,3-DNB molecules in dilute solutions, the sum of the
1,3-DNB and CBZ electronic absorption spectra were compared with the CBZ co-crystals
spectrum (Figure 5.4.5). The combined spectra closely matched the spectrum for the CBZ co-
crystals. From this result, it was assumed that the spectrum for the co-crystals with a
concentration of 10-5 M was simply the sum of the spectra for free CBZ and free 1,3-DNB
molecules. There was no charge transfer complex formed at that concentration.
223
200 300 400 500 600 700 8000
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
55000
60000
65000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals CBZ crystals CBZ co-crystals
Figure 5.4.4. Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue).
224
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB + CBZ CBZ co-crystals
Figure 5.4.5. Comparison of the electronic absorption spectrum in acetonitrile for the co-crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for the 1,3-DNB crystals and the spectrum for the CBZ crystals (black).
225
The melting point ranges were measured for the CBZ crystals and the CBZ co-crystals
(Table 5.4.3). The co-crystals have a broad melting point range, compared with that of the 1,3-
DNB crystals and the CBZ crystals. The observed co-crystals' liquid formation temperature (82.4
°C) was lower than the liquid formation temperatures for the 1,3-DNB crystals (90.9 °C) and the
CBZ crystals (245.5 °C). The observed temperature when the co-crystals completely melted
forming a meniscus (209.3 °C) was higher compared to the 1,3-DNB crystals (91.3 °C), but
lower than the CBZ crystals (249.1°C). This broad melting range indicated the presence of
inhomogeneities or individual components within the co-crystals. During the melt, the CBZ co-
crystals changed from light brown to yellow in color before the first signs of the melt. Above 150
°C, the co-crystals changed color again from yellow to orange.
Table 5.4.3. 1,3-DNB crystals, CBZ crystals, and CBZ co-crystals melting points.
Crystals Melting Point (°C) Lit. Values (°C)
1,3-DNB 90.9 - 91.3 89 54
CBZ 245.5 - 249.1 245 54
CBZ co-crystal 82.4 - 209.3
226
5.5 Phenothiazine (PHZ) Co-Crystals with 1,3-DNB
Attempts to prepare (1:1) co-crystals of PHZ and either 2-NT, 3-NT, or PNT were
unsuccessful. When the two solutions of PHZ and 1,3-DNB were combined in a crystallization
dish, the solution did not produce a color change. The color of the solution was similar to the
PHZ solution (light brown). The EtOH was allowed to evaporate for two days, producing needle-
like crystals with large flakes as shown in Figure 5.5.1 (A). PHZ crystals (Figure 5.5.1 (B)) were
prepared by the same procedure producing small, white-brown, needle-like crystals with flakes.
Figure 5.5.1. Images of PHZ co-crystals (A) and PHZ crystals (B).
1H NMR spectra were recorded of the PHZ crystals and PHZ co-crystals containing 1,3-
DNB in order to determine the ratio between the electron donor and acceptor (Figure 5.5.2).
Table 5.5.1 lists the peak positions, peak multiplicities, and peak integrations. The 1,3-DNB
proton signals in the co-crystals' spectrum were observed at 9.07, 8.57, and 7.79 ppm. The PHZ
aromatic and NH proton signals were located at 7.00-6.95, 6.81, 6.54, and 5.78 ppm. The co-
crystals spectrum integration revealed an approximate 1.3:1.0 ratio between 1,3-DNB and PHZ.
(A) (B)
227
Figure 5.5.2. 1H NMR spectra (360 MHz, CDCl3) of the 1,3-DNB crystals, PHZ crystals, and the co-crystals made from PHZ and 1,3-DNB.
Table 5.5.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, PHZ co-crystals, and PHZ crystals.
1,3-DNB PHZ co-crystals PHZ δ S.P. Int. δ S.P. Int. δ S.P. Int.
9.06 t 1.00 9.07 t 1.33 8.57 dd 2.04 8.57 dd 2.67 7.80 t 1.05 7.79 t 1.42
7.00-6.95 m 3.87 6.99-6.96 m 3.96 6.81 td 2.05 6.81 s 1.96 6.54 dd 2.00 6.54 dd 2.00 5.78 s 0.95 5.77 s 1.14
S.P. - Splitting Pattern Int. - Integration values
5.56.06.57.07.58.08.59.0ppm
1,3-DNB
PHZ co-crystals
PHZ
228
Figure 5.5.3 shows the infrared spectra for the PHZ crystals and co-crystals made from
PHZ and 1,3-DNB. Table 5.5.2 lists the NO2 asymmetric and symmetric stretching modes for the
1,3-DNB crystals and the PHZ co-crystals. The NO2 asymmetric stretching mode (1539 cm-1)
was red shifted (1 nm) to lower energy in the co-crystal spectrum, which suggests a weak
intermolecular interaction between the electron donor and acceptor. The NO2 symmetric
stretching mode (1347 cm-1) was located at the same position for the 1,3-DNB crystals
symmetric stretching mode.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PHZ PHZ co-crystal
1539
13473340 NH
Figure 5.5.3. FTIR spectra of KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue).
229
Table 5.5.2. NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals.
To determine if a charge complex formed within the co-crystals, the electronic absorption
spectrum was recorded in acetonitrile. Figure 5.5.4 shows the electronic absorption spectra for
the 1,3-DNB crystals, PHZ crystals, and the co-crystals. There were no shifts in the peak
positions for the co-crystal spectrum, compared to the spectrum for the PHZ crystals. There was
no observable charge transfer band present in the PHZ co-crystals spectrum. To determine if a
charge complex formed within the co-crystals, the sum of the PHZ crystals and 1,3-DNB crystals
electronic absorption spectra were compared to the co-crystals electronic absorption spectrum
shown in Figure 5.5.5. The two spectra were identical, with no observable differences. The
spectra had similar results as the CBZ co-crystals, which suggested that the PHZ co-crystals
electronic absorption spectrum was primarily the result of free PHZ molecules and 1,3-DNB
molecules in dilute solutions.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB White-yellow 1540 1347
PHZ White brown N/A N/A
PHZ co-crystal Light brown 1539 1347
230
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals PHZ crystals PHZ co-crystals
Figure 5.5.4. Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue).
231
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε(M
-1 c
m-1)
Wavelength (nm)
Sum of 1,3-DNB and PHZ PHZ co-crystals
Figure 5.5.5. Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black).
232
The diffuse reflectance spectra for the PHZ crystals and the PHZ co-crystals containing
1,3-DNB are shown in Figure 5.5.6. The spectrum for the PHZ crystals showed a diffuse
reflectance of approximately 80% from the NIR (800 nm) to 500 nm. Below 500 nm, the
reflectance decreased to less than 10% below 400 nm. This was consistent with the light brown
color of the PHZ crystals. The diffuse reflectance for the co-crystals showed a decrease in the
reflectance from the NIR to 400 nm. This difference cannot be explained as simply due to the
innocent presence of 1,3-DNB. The diffuse reflectance spectrum for 1,3-DNB (Figure 5.1.5)
showed a high reflectance greater than 50% through 800 - 500 nm region. Here a new feature
was observed suggesting a strong intermolecular interaction between PHZ and 1,3-DNB.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
efle
ctan
ce
Wavelength (nm)
Phenothiazine crystals Phenothiazine co-crystals
Figure 5.5.6. Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red).
233
Table 5.5.3 shows the melting ranges for the 1,3-DNB crystals, PHZ crystals, and PHZ
co-crystals. The co-crystals had a broad melting point range, compared to those for the 1,3-DNB
and PHZ crystals. The co-crystals' observed liquid formation temperature (71.4 °C) was lower
than those for the 1,3-DNB crystals (90.9 °C) and the PHZ crystals (186.9 °C). The temperature
at which the co-crystals completely melted and formed a meniscus (145.7 °C) was higher than
the 1,3-DNB crystals' melting temperature (91.3 °C), but lower than the PHZ crystals' melting
temperature (189.4 °C). This broad melting range indicated the presence of inhomogeneities or
individual components within the co-crystals. During the melt, the PHZ co-crystals began
shrinking and changing color from light brown to a dark-red before the first signs of liquid
formation.
Table 5.5.3. Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals.
Crystals Melting Point (°C) Lit. Values (°C)
1,3-DNB 90.9 - 91.3 89 54
PHZ 186.9 - 189.4 184.9 54
PHZ co-crystal 71.4 - 145.7
234
5.6 10-Methylphenothiazine (10-M) Co-Crystals with 1,3-DNB
As with other electron rich aromatic molecules, attempts to prepare co-crystals with 10-
M and 2-NT, 3-NT, and PNT were not successful. However, when the two solutions of 10-M and
1,3-DNB were combined in the crystallization dish, a dark red color rapidly appeared. The EtOH
evaporated for two days, producing reddish-purple crystals as shown in Figure 5.6.1 (B). 10-M
crystals were prepared by the same experimental procedure, which produced white, needle-like
crystals as shown in Figure 5.6.1(A).
Figure 5.6.1. Images of 10-M crystals (A) and 10-M co-crystals with 1,3-DNB (B).
The 1H NMR spectra of the 10-M crystals and co-crystals containing 10-M and 1,3-DNB
were recorded in CDCl3 in order to determine the ratio between the electron donor and acceptor
(Figure 5.6.2). The peak positions, peak multiplicities, and peak integrations are listed in Table
5.6.1. The 1,3-DNB peaks were observed at 9.06, 8.56, and 7.79 ppm. The 10-M peaks were
located at 7.17-7.11, 6.91, 6.80, and 3.36 ppm. The co-crystal spectrum integration revealed an
approximate 1.0:1.1 ratio between 1,3-DNB and 10-M. Neither the 1,3-DNB nor the 10-M
proton signals in the co-crystals spectrum displayed a shift in peak positions.
(A) (B)
235
Figure 5.6.2. 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3).
Table 5.6.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals.
1,3-DNB 10-M co-crystals 10-M δ S.P. Int. δ S.P. Int. δ S.P. Int.
9.06 t 1.00 9.06 t 1.00 8.57 dd 2.04 8.56 dd 2.01 7.80 t 1.05 7.79 t 1.07
7.17-7.11 m 4.34 7.18-7.12 m 4.06 6.91 td 2.16 6.93 td 2.09 6.80 d 2.16 6.80 d 2.06 3.36 s 3.43 3.36 s 3.00
S.P. - Splitting Pattern Int. - Integration values
3.54.04.55.05.56.06.57.07.58.08.59.0ppm
1,3-DNB
10-M co-crystals
10-M
236
Figure 5.6.3 shows the FTIR spectra for 10-M and the co-crystals. Table 5.6.2 lists the
NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and the PHZ co-
crystals. The NO2 asymmetric and symmetric stretching modes were shifted 4 nm and 5 nm to
lower energy in the co-crystals, indicating a strong intermolecular interaction between the
electron donor and acceptor.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
1536 1342
10-M co-crystal
10-M
Figure 5.6.3. Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co-crystals containing 10-M and 1,3-DNB (red).
237
Table 5.6.2. NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB white-yellow 1540 1347
10-M White N/A N/A
10-M co-crystals red-purple 1536 1342
The electronic absorption spectra for the 1,3-DNB crystals, 10-M crystals, and the co-
crystals were recorded in acetonitrile (Figure 5.6.4). 10-M showed two peaks, an intense
absorption observed at 254 nm (ε=3.77 x 104 M-1 cm-1) and a weak absorption at 308 nm (ε=5.00
x 104 M-1 cm-1). 1,3-DNB had a single broad absorption at 237 nm (ε=1.80 x 104 M-1 cm-1). The
spectrum for the co-crystals made from 1,3-DNB and 10-M was simply the sum of the spectra
for 1,3-DNB and 10-M (Figure 5.6.5). This indicated that there was no intermolecular interaction
between 1,3-DNB and 10-M in the acetonitrile solution at 2.0 x 10-5 M concentration.
238
200 300 400 500 600 700 8000
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
55000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals 10-M crystals 10-M co-crystals
Figure 5.6.4. Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue).
239
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
ε (M
-1 c
m-1)
Wavelength (nm)
Sum of 1,3-DNB and 10-M 10-M co-crystals
Figure 5.6.5. Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black).
240
The diffuse reflectance spectrum (Figure 5.6.6) for the co-crystals containing 10-M and
1,3-DNB provided strong evidence for intermolecular interactions in the solid state. The
reflectance for the 10-M crystals was greater than 70% from 800 nm to 400 nm. Below 400 nm,
the reflectance dropped to less than 10%. This was consistent with the white color of the 10-M
crystals. The diffuse reflectance spectrum for the co-crystals was dramatically different. The
reflectance was slightly above 40% in the region from 800 nm to 650 nm. Below 650 nm, the
reflectance dropped below 10%. This was consistent with the dark red color of the co-crystals.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
efle
ctan
ce
Wavelength (nm)
10-M crystals
10-M co-crystals
Figure 5.6.6. Diffuse reflectance spectra for 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red).
241
This also indicated a strong intermolecular interaction between the 10-M and 1,3-DNB in the
solid state.
Table 5.6.3 lists the melting ranges measured for the 10-M crystals and 10-M co-crystals
with 1,3-DNB. The melting range for the co-crystals was significantly lower than the melting
range for the 10-M crystals and the 1,3-DNB crystals. The narrow melting range indicated the
existence of a co-crystal with few inhomogeneities or individual components. During the melt,
the co-crystals did not exhibit any color change.
Table 5.6.3. Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals.
Crystals Melting Point (°C) Lit. Values (°C)
1,3-DNB 90.9 - 91.3 89 54
10-M 101.7 - 104.1 99-100 93
10-M co-crystal 61.7 - 63.4
The co-crystals were suitable for single crystal X-ray diffraction in order to determine the
structure. Steven Kelley obtained and interpreted the X-ray diffraction data. The following is his
interpretation of the structure. The 1:1 co-crystal of 1,3-DNB and 10-M crystallized in the chiral,
orthorhombic space group P212121 with two symmetry-independent formula units (Z = 8). None
of the atoms or molecules reside on special positions. The 1,3-DNB molecules were planar,
except for the nitro groups, which are twisted slightly out-of-plane. There were no statistically
significant differences in bond lengths for the two 1,3-DNB molecules, and the nitro groups on
both molecules had approximately the same orientation relative to the ring. The 10-M molecules
had the typical geometry of phenothiazine and its derivatives, with both of the phenyl rings
joining at an acute angle. The corresponding bond distances and N- and S-centered bond angles
of both 10-M molecules were statistically equivalent to each other and very similar to those in
the reported crystal structure of 10-M.94
242
Figure 5.6.7. 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii.
The short contact environments around the symmetry-independent molecules were
different (Figure 5.6.7). Both 1,3-DNB molecules made short contacts to five 10-M molecules,
but no 1,3-DNB molecules. Both accepted hydrogen bonds through either nitrate group. This
interaction explained the decrease in the peak positions for the asymmetric and symmetric
vibrational modes for the nitro groups in the co-crystals. The major difference was that 1,3-DNB
molecule A formed π-π contacts with the end of a 10-M molecule, while molecule B formed
those contacts with the center.
243
Figure 5.6.8. Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance.
The 10-M molecules also had different short contact environments (Figures 5.6.8 and
5.6.9). 10-M (A) only interacted with 1,3-DNB molecules through hydrogen bonding to the nitro
groups or inter-ring π-π stacking. The nitrogen and sulfur atoms of 10-M (A) were not involved
in short contacts. Molecule (B) made short contacts to two 10-M molecules as well as four 1,3-
DNB molecules. The 10-M (B) molecules interacted with each other through herringbone-type
C-H---π interactions between the phenyl rings. 10-M (B) did not π-stack with any 1,3-DNB
molecules; instead, it donated hydrogen bonds to nitro groups on two 1,3-DNB molecules and
accepted hydrogen bonds from 1,3-DNB molecules at the N and S atoms.
244
Figure 5.6.9. Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts.
Infinite hydrogen bonded chains along b, formed by one of the 10-M phenyl rings
donating hydrogen bonds to 1,3-DNB molecules on either side of it, was a major structural
feature (Figure 5.6.10). Each of these chains only involved hydrogen bonds between 1,3-DNB
(A) and 10-M (A) or 1,3-DNB (B) and 10-M (B). The A and B chains were interdigitated with
each other, which allowed for extra hydrogen bonding between the chains as shown in Figure
5.6.10. The molecular recognition, which allowed co-crystallization, may stem from this
hydrogen bonding, as neither 1,3-DNB nor 10-M can form these chains without the other.
Figure 5.6.10. The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts.
245
Other supramolecular structures can be described in terms of how the “A” and “B”
chains, the hydrogen bonded chains formed between 1,3-DNB and 10-M (A) respectively,
interacted with each other. Two adjacent A chains formed a dimer through π-π stacking. These
chain dimers interacted with each other through π- π stacking as well forming a 3-D network.
The interactions between these dimers are shown in Figure 5.6.11. The B chains were
interwoven into the A chain network through π- π stacking between 1,3-DNB (B) molecules and
10-M (A) molecules as well as hydrogen bonding of 1,3-DNB (A) molecules to the sulfur atom
on 10-M (B) molecules. The B chains also formed a network with each other through the 10-M-
10-M C-H π contact and hydrogen bonding between 1,3-DNB (B) molecules and the nitrogen
atom on 10-M. Figure 5.6.12 shows the network of A chain dimers network and the packing
down the b axis.
Figure. 5.6.11. View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts.
246
Figure 5.6.12. Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue.
247
Chapter 6
Conclusions and Future Works
Random copolymers of styrene or methyl methacrylate and the VDAT monomer showed
the potential to sense nitroaromatics by changes produced in the index of refraction after
exposure to a concentrated nitroaromatic vapor. The electron rich structure of VDAT presented a
problem for the solubility of copolymers in a suitable spin coating solvent. This solubility
dilemma limited the synthesis of copolymers with larger concentrations of VDAT due to the
insolubility of PVDAT. Copolymers rich in PVDAT moieties may be capable of producing
larger changes in the index of refraction after exposure to a nitroaromatic vapor, but a polar
solvent with an ideal boiling point with the ability to dissolve these electron rich copolymers
needs to be identified in order to allow the spin coating of homogeneous films. A disadvantage
of VDAT observed when synthesizing copolymers with other electron rich monomers was a
cross-linking effect, making the copolymers insoluble in solvents at or near room temperature.
VDAT appeared to have the ideal structure for sensing nitroaromatics due to its electron rich ring
containing amino functional groups, but these characteristics might be the monomer's downfall
as it limits the synthesis and solubility of polymers that would allow the production of thin films
for the MZI sensor.
The problems associated with the use of the PVDAT copolymers led to the investigations
of other polymers to determine their potential to detect nitroaromatics by changes in the
248
refractive index. Pyridine and imidazole based polymers (P4VP, PVI, and PVI-co-PVA) showed
the ability to sense nitroaromatics by changes in the refractive index after exposure to a
nitroaromatic vapor. These polymers were not soluble in an ideal spin coating solvent, but
homogeneous films were casted by adjusting the spin coating parameters for the dynamic
technique. There was surface roughness observed for these polymer films, which was expected
due to the low boiling point of EtOH. The change in the refractive index after exposure to a
nitroaromatic results attracted interest to synthesize copolymers with pyridine or imidazole
monomers with VDAT. These investigations resulted in brittle hard polymers, which had limited
solubility in spin coating solvents. Due to limited solubility, full characterization of the optical
constants was not possible; however, if suitable solvents were found for casting films of these
copolymers, these films would potentially have an affinity for nitroaromatics based on previous
results.
The interactions between the electron rich polymers and electron deficient nitroaromatics
led to research and the production of co-crystals between electron rich reagents and a
nitroaromatic. Attempts to produce co-crystals between VDAT and nitroaromatics were
unsuccessful due to VDAT solubility in polar organic solvents with high boiling points or H2O at
elevated temperatures. These unsuccessful attempts led to studies using other electron rich
reagents. Co-crystals were produced between 1,3-DNB with 9-VC, 9-EC, and 10-M. The color
changes associated with the formation of the complexes with 1,3-DNB suggested a strong
interaction between the reagents, which was confirmed by FTIR. The nitro groups' asymmetric
and symmetric stretching modes were red shifted to lower energy. The electronic absorption
spectra did not confirm a charge transfer complex in dilute solutions, but rather showed the
interaction between the electron donor molecules and the nitroaromatic molecules in the solution
249
phase. Only the 10-M co-crystal with 1,3-DNB was able to characterized by X-ray diffraction
allowing a crystal structure to be determined. The 10-M molecules interacted with the 1,3-DNB
molecules through hydrogen bonding and π-π contacts. At this time, no crystal structures have
been determined between the carbazole derivatives with 1,3-DNB. This obstacle may be due to
co-crystals' size produced during the slow evaporation process. The strong affinity the electron
donors had for 1,3-DNB suggested they should be considered as sensing materials.
The results for the 10-M co-crystals with 1,3-DNB led to the development of polystyrene
films containing small concentrations of 10-M to determine if the 10-M would interact with the
1,3-DNB vapors to produce a change in the index of refraction for the films. After the maximum
concentration of 10-M that could be included in a polystyrene film was determined, the optical
constants of the polystyrene/10-M film were characterized after long exposure times to 1,3-DNB.
Unexpectedly, the low concentrations of 10-M were capable of producing a change in refractive
index. This result confirmed that 10-M would be an ideal sensing material.
Future investigations should still focus on synthesizing electron rich copolymers with an
affinity for nitroaromatics. Polymers containing carbazole or phenothiazine derivatives should be
considered, since previous results showed the strong affinity these reagents had for 1,3-DNB.
One would expect these polymers to show a significant change in the index of refraction after
exposure to 1,3-DNB if the electron donating properties of reagent are not altered significantly.
These types of polymers should also be investigated as a solution phase nitroaromatic sensor due
to the color changes observed during the growing of the co-crystals.
Another important part of this project that must be considered is the development of
imprinted polymers from the homopolymers and copolymers that exhibited an affinity for
nitroaromatics. The polymer would be imprinted with a specific nitroaromatic, creating a specific
250
cavity for the targeted analyte in the polymer. After the targeted analyte is removed from the
polymer, a specific imprint site will be left where only the targeted analyte can enter the site and
interact at the recognition site. The development of imprinted polymer films would allow the
detection of a specific nitroaromatic and eliminate any false positives. A foreseeable problem
that must be considered is the solubility of VDAT in polar organic solvents. Removing the
majority of the solvent could be problematic for producing a thin imprinted polymer film.
For the last part of this research project, the copolymer films should be applied to a MZI
to determine the sensor's sensitivity. From experimental results in the literature, this will be an
exciting part of the project to see how sensitive the MZI is to changes in the index of refraction
after exposure to a concentrated nitroaromatic vapor. To determine the MZI sensitivity and limit
of detection, a vapor generator will need to be constructed to control the amount of a
nitroaromatic vapor required to determine a certain change in refractive index.
An additional side note for this project might be to evaluate the thermal properties of the
copolymers for heat resistant materials. The TGA characterization of the PS-co-PVDAT
copolymers and PMMA-co-PVDAT copolymers showed a significant increase in the
decomposition temperatures for the copolymers. Even though some of the copolymers were not
characterized by TGA to determine their decomposition temperatures, they may possess high
decomposition temperatures similar to liquid crystal polymers.
251
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259
APPENDIX
Appendix Figure 1. VDAT 1H NMR spectrum (360 MHz, DMSO-d6).
3.23.64.04.44.85.25.66.06.46.87.27.68.0ppm
5.60
5.61
5.63
5.63
6.26
6.29
6.31
6.33
6.37
6.38
6.42
6.42
6.67
D
C, B
A
260
Appendix Figure 2. PS-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
CDCl3
DMSO
ETOH
H2O
AcetoneETOHSi grease
CH2 CH CH2 CH
N
N
N
H2N NH2
m n
261
Appendix Figure 3. PS-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
CDCl3
DMSO
Acetone
H2O
Si greaseETOH
ETOH
CH2 CH CH2 CH
N
N
N
H2N NH2
m n
262
Appendix Figure 4. PS-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
CDCl3
Acetone
DMSO
H2O
Si grease
CH2 CH CH2 CH
N
N
N
H2N NH2
m n
263
Appendix Figure 5. Polystyrene 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
CDCl3
DMSO
Acetone
H2O
Si grease
CH2CH
n
264
Appendix Figure 6. Polystyrene 13C NMR spectrum (500 MHz, CDCl3).
30405060708090100110120130140150160170180ppm
C2
C1
C3
C4, C5
C6
265
Appendix Figure 7. PS-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3).
30405060708090100110120130140150160170180ppm
C3
C4, C5
C6
C2
C1
266
Appendix Figure 8. PS-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, CDCl3).
30405060708090100110120130140150160170180ppm
C3
C4, C5
C6
C2
C1
267
Appendix Figure 9. PS-co-PVDAT 10 mol % VDAT 13C NMR spectrum (500 MHz, CDCl3).
30405060708090100110120130140150160170180190ppm
C1
C2
C6
C4, C5
C3C7
268
Appendix Figure 10. VDAT 13C NMR spectrum (500 MHz, DMSO-d6).
405060708090100110120130140150160170ppm
C1C2C3
C4
269
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(% )
Temperature (oC)
Appendix Figure 11. Polystyrene TGA curve.
270
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (oC)
Appendix Figure 12. PS-co-PVDAT 1 mol % VDAT copolymer TGA curve.
271
100 200 300 400 500 600
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (οC)
Appendix Figure 13. PS-co-PVDAT 5 mol % VDAT copolymer TGA curve.
272
100 200 300 400 500 600
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (οC)
Appendix Figure 14. The PS-co-PVDAT 10 mol % VDAT copolymer TGA curve.
273
Appendix Figure 15. The PS-co-PVDAT 1 mol % VDAT copolymer GPC data.
Appendix Figure 16. The PS-co-PVDAT 5 mol % VDAT copolymer GPC data.
274
Appendix Figure 17. The PS-co-PVDAT 10 mol % VDAT copolymer GPC data.
Appendix Figure 18. The PS-co-PVDAT 20 mol % VDAT copolymer GPC data.
275
4000 3500 3000 2500 2000 1500 1000 5000
20
40
60
80
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
3425 - 3228 NH2
2998 - 2843 CH
1637, 1570 in-plane
1733C=O
1544
C=N
Appendix Figure 19. FTIR spectrum of the PMMA-co-PVDAT 20 mol % VDAT copolymer.
276
4000 3500 3000 2500 2000 1500 1000 5000
10
20
30
40
50
60
70
80
90
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
3425 - 3228 NH2
2997 - 2842 CH
1728C=O
1636, 1570
in-plane 1545
C=N
Appendix Figure 20. FTIR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer.
277
4000 3500 3000 2500 2000 1500 1000 5000
10
20
30
40
50
60
70
80
90
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
3420 NH2
2999 - 2842 CH
1727C=O
1638, 1570in - plane 1543
C=N
Appendix Figure 21. FTIR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer.
278
4000 3500 3000 2500 2000 1500 1000 5000
20
40
60
80
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
NH
3378, 3439
2995 - 2842
CH
1730
C=O
1609, 1569
in - plane
1548
C=N
Appendix Figure 22. FTIR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer.
279
Appendix Figure 23. The 1H NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer.
-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.5ppm
TMS
H3
H1
H2
280
Appendix Figure 24. The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm
H4
H3
H1M
DMSO
281
Appendix Figure 25. The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix.
4.44.85.25.66.06.46.8ppm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
282
Appendix Figure 26. The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm
DMSO
H4
H3H1M
283
Appendix Figure 27. The 1H NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
0.01.02.03.04.05.06.07.08.09.0ppm
H5
H3, H1V
DMS0
H4
H1M, H2V
284
Appendix Figure 28. The 13C NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer.
102030405060708090100110120130140150160170180190ppm
C4 C1
C5
C3C2
DMSO-d6
285
Appendix Figure 29. The 13C NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
102030405060708090100110120130140150160170180ppm
C4C9
C1
C5
C3C2
286
Appendix Figure 30. The 13C NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
102030405060708090100110120130140150160170180190ppm
C4C9
C2C3
C1
C5
287
Appendix Figure 31. The 13C NMR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer in DMSO-d6 using the 500 MHz spectrometer.
102030405060708090100110120130140150160170180ppm
C4
C2
C5
C1
C3
288
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (oC)
Appendix Figure 32. TGA curve for the PMMA-co-PVDAT 1 mol % VDAT copolymer.
289
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (oC)
Appendix Figure 33. The TGA curve for the PMMA-co-PVDAT 5 mol % VDAT copolymer.
290
100 200 300 400 500 6000
20
40
60
80
100
Wei
ght L
oss
(%)
Temperature (oC)
Appendix Figure 34. The TGA curve for the PMMA-co-PVDAT 20 mol % VDAT copolymer.
291
Appendix Figure 35. GPC curve and data for the PMMA-co-PVDAT 20 mol % VDAT copolymer.
292
Appendix Figure 36. The GPC curve and data for the PMMA-co-PVDAT 10 mol % VDAT copolymer.
293
Appendix Figure 37. The GPC curve and data for the PMMA-co-PVDAT 5 mol % VDAT copolymer.
294
Appendix Figure 38. The PMA 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0ppm
H3
DMSO
H2
H1
295
Appendix Figure 39. The PMA 13C NMR spectrum (500 MHz, DMSO-d6).
2030405060708090100110120130140150160170180ppm
34.2
7
40.7
6
51.5
2
174.
36
C3
C4C2
C1
296
Appendix Figure 40. The P2VP 1H NMR spectrum (360 MHz, CDCl3).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0ppm
H1H3
H2H4
DMS0
CH2CH
N
n
H4
H3
H2
H1
297
Appendix Figure 41. The P2VP-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5ppm
H1 H3H2
H4, H5
298
Appendix Figure 42. The P2VP-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum (360 MHz, DMSO-d6).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5ppm
H1 H3H2
H4, H5
299
Appendix Figure 43. The P2VP 13C NMR spectrum (500 MHz, DMSO-d6).
2030405060708090100110120130140150160170180190ppm
C2C6
C4
C3
C5
CH2CH
N
n
C3
C4
C5
C6
C2
300
Appendix Figure 44. The P2VP-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 180 - 110 ppm.
115120125130135140145150155160165170175ppm
C2C6
C4 C3 C5
301
Appendix Figure 45. The P2VP-co-PVDAT 20 mol % VDAT copolymer 13C NMR spectrum (500 MHz, DMSO-d6) from 190 - 110 ppm.
110115120125130135140145150155160165170175180185ppm
C8
C2
C6C4
C3
C5
302
Appendix Figure 46. The PAM polymer 1H NMR spectrum (360 MHz, D2O).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm
H2
H1
DMSO
303
Appendix Figure 47. The PAM-co-PVDAT 1 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.5ppm
304
Appendix Figure 48. The PAM-co-PVDAT 5 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm
305
Appendix Figure 49. The PAM-co-PVDAT 10 mol % VDAT copolymer 1H NMR spectrum recorded in D2O (360 MHz).
0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm
306
Appendix Figure 50. The Poly(acrylamide) (PAM) 13C NMR spectrum (500 MHz, D2O).
2030405060708090100110120130140150160170180ppm
C3 C2C1
DMSO
307
Appendix Figure 51. The PAM-co-PVDAT 1 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).
2030405060708090100110120130140150160170180ppm
308
Appendix Figure 52. The PAM-co-PVDAT 5 mol % VDAT copolymer 13C NMR spectrum (500 MHz, D2O).
2030405060708090100110120130140150160170180ppm
309
Appendix Figure 53. The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer.
160166172178184190ppm
C3
C7
310
Appendix Figure 54. The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz).
102030405060708090100110120130140150160170180ppm
20 mol % PVK
C=O1a 8a
7,25
4,6,3
5a 4a
8 1
(V+M)
10
9
CH (V)
C(M)
311
100 120 140 160 180-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (οC)
Appendix Figure 55. The DSC curve for the PVI homopolymer.