2018
Reg. No: 2011-NUST-TfrPhD-EM-E-88
This thesis is submitted as a partial fulfillment of the requirements for the
degree of
School of Chemical and Materials Engineering (SCME)
National University of Sciences and Technology (NUST)
H-12 Islamabad, Pakistan
i
Dedication
I dedicate this thesis to some special persons, my father, my late mother, wife and
daughter
ii
Acknowledgements
First, I am very thankful to Almighty Allah whose blessings and mercy made me to complete
this daunting and promising goal. It is with a profound sense of gratefulness to my honored
supervisor, Professor Dr. Habib Nasir, for trust, care, inspiration, valuable suggestions and his
help to the course to steer my research work effectively and proficiently. It was an honor for
me to do my research work under his esteemed supervision and learned from his personality,
enthusiasm, attitude, and immense knowledge. I am sincerely grateful to him for guidance
patience to complete my tasks.
I express my sincere thanks to my co-supervisor principal SCME, Professor Dr. Arshad
Hussain, as he always guided and encouraged me throughout my PhD research work. Many
thanks to HOD Chemical Engineering Department, Professor Dr. Bilal Khan Niazi for his
moral support at each step toward the completion of this thesis. Truthful thanks to my
honorable GEC members Dr. A.Q. Malik, Dr. Tayyaba Noor and Dr. Sofia Baig for valuable
discussions during all my research work. They always stimulated me to work hard for
completing my research goals.
I am also thankful to Dr. Muhammad Awais COMSATS Institute of Information Technology,
Islamabad, Dr. Sara Qaiser, Dr. Muhammad Arshad and Dr. Khalid Alamgheer from National
Center for Physics, for providing me characterizations facilities in their Institutes.
I am also thankful to all my class fellows especially Maj. Muhammad Farooq, Dr. Zaheer-u-
din Babar, Maj. Mudassar, Maj. Amir Mukhtar, Mr. Sajid Khan, Mr. Sajid Nawaz Malik and
Mr. Mukhtar Ahmad Gondal for their contribution to my research work. The technical
conversation with Dr. Muhammad Aftab Akram, Dr. Muhammad Ahsan, Dr. Sara Farukh, Dr.
Raheem Jan and Dr. Tariq Zaman, their guidance were always helpful to solve the problems.
Bundle of thanks to all SCME labs staff for their cooperation during this research work.
Furthermore, a lot of appreciation goes to my parents, my wife, sisters, brothers and last but
surely not the least my lovely daughter, who prayed for me from Allah and stimulated me
throughout all this study period. Whenever I got depressed during my research work and during
iii
my course work, their prayers and encouragements provided me new direction and energized
me to work with more passion.
I also sincerely acknowledge the support of National University of Sciences and Technology
through Mega S&T fund program.
(SYED SAJID ALI SHAH)
iv
Abstract
The graphene study has become the most interested topic owing to its exceptional properties.
Graphene brings dramatic changes in the possessions of composites, when it is used even at
very low concentration. Though, the heterogeneous distribution of graphene in the matrix is its
main drawback to be used as filler in the polymer composites. The strong intermolecular forces
of graphene result in agglomeration of graphene in the host the polymer matrix. The surface of
graphene is functionalized by different functional groups to reduce the π-π stacking to prevent
the agglomeration of graphene.
The main objective of this work was to develop an easy method for synthesizing the graphene-
based polymer nanocomposites. To get the graphene-based nanocomposites, first graphene
was exfoliated by using liquid-phase exfoliation in different organic solvents. Picric acid was
also used as a surfactant. The addition of picric acid has doubled the concentration in most of
solvents tested. Moreover, graphene oxide was produced by using Hummers’ methods,
modified Hummers’ method and Brodie method. The results indicate that modified Hummers’
method is the best one among the three methods.
In next step polyaniline was grafted on few layer graphene (FLG), and polyaniline grafted few-
layer graphene (PANI-g-FLG) samples were then blended with polyetherimide. The addition
of PANI-g-FLG introduced the conductive behavior in the insulative polymer. The maximum
dielectric constant (3.72 × 105) and AC conductivity (5.91 × 10−2 S⋅cm−1) were achieved at 1.5
wt.% of the FLG concentration. Moreover, polyaniline grafted few layer graphene was also
added in polyvinyl alcohol (PVA) by solution blending method. The tensile strength was
enhanced 62 % for the composites as compared to pure PVA. The dielectric constant was
enhanced from 3.29 of pure PVA to 5.14 × 1004 (at 1000Hz) for nanocomposites having 0.5
wt.% FLG loading. Similarly, AC conductivity increased from 9.05 × 10−9 for pure PVA to
2.17 × 10−2 S⋅cm−1 (at 1000Hz) for composites having 0.5 wt.% FLG loading.
The PANI-g-FLG was also combined with polystyrene to prepare nanocomposites with
varying concentration of FLG (0 to 2 wt.%). The tensile strength was doubled for the
composites as compared to pure polystyrene.
v
The graphene oxide produced by modified Hummers’ method was further functionalized and
incorporated into the diglycidyl ether of bisphenol A to prepare nanocomposites. The
concentration of PANI-g-GO were varied from 0 to 2.5 wt.% in the nanocomposites. The
tensile strength was enhanced 60% for the composites as compared to pure diglycidyl ether of
bisphenol A. The dielectric constant was improved from 1 to 8 and AC conductivity increased
from 1.05 × 10−8 S⋅cm−1 for pure to 6.15 × 10−8 S⋅cm−1 (at 1000Hz) for composites having 2.5
wt.% FLG loading.
These results provide a possibility of using flexible nanocomposites as a dielectric material to
be applied for energy storage devices such as embedded capacitors.
vi
1.2 Background ………………………………………………………………….. 3
1.2.2 Graphene ………………………………………………………………. 4
1.4 Thesis outline ………………………………………………………………... 6
2.1 Graphene ……………………………………………………………………. 7
2.2.2.1 Liquid-phase exfoliation ………………………………………….. 10
2.2.4 Chemical vapor deposition method ………………………………........ 12
vii
2.4.1 Covalent functionalization of graphene ………………………………….. 14
2.4.2 Non-covalent functionalization of graphene …………………………...... 15
2.5 Graphene based polymer composites ………………………………............... 15
2.6 Synthesis of graphene based polymer composites …..………………………. 16
2.6.1 Solution casting method …………………………………………………. 16
2.6.2 Melt blending method …………………………………………................. 17
2.6.3 In-situ polymerization method ……………………………….................... 18
2.6.4 Electrodeposition method ………………………………………............... 19
3.1.1 Introduction ………………………………………………..……………. 21
3.1.2 Materials ………………………………………………….…................... 22
3.1.3 Exfoliation ………………………………………………..…………....... 22
3.1.4 Characterizations …………………………………………..………......... 23
3.2.1 Introduction ……………………………………………………...…….... 24
3.2.2 Materials ……………………………………………………...…………. 24
3.2.3.1 Oxidation of graphite by Hummers’ methods ………….................... 24
3.2.3.2 Oxidation of graphite by modified Hummers’ methods ..................... 25
3.2.3.3 Oxidation of graphite by Brodie methods ….……….…………….... 25
3.2.4 Exfoliation of graphene oxide …………………………………………... 25
3.2.5 Characterizations of graphene oxide ……………………………………. 25
3.3 Preparation of polyetherimide based composites ……………………............. 26
3.3.1 Introduction …………………………………………………………….... 26
3.3.2 Materials ………………………………………………………................. 27
3.3.4 Synthesis of PANI-g-FLG ……………………………………………….. 28
3.3.5 Preparation of PANI-g-FLG/PEI nanocomposites ………………………. 28
3.3.6 Characterizations …………………………………………………............ 28
3.4 Preparation of polyvinyl alcohol-based nanocomposites ……………………. 29
3.4.1 Introduction ………………………………………………………............ 29
3.4.2 Materials …………………………………………………………………. 30
3.4.4 Preparation of PANI-g-FLG/PVA nanocomposites ………………........... 30
3.5 Polystyrene based nanocomposites ………………………………………….. 31
3.5.1 Introduction ……………………………………………………………… 31
3.5.2 Materials ………………………………………………………………… 31
3.5.3 Preparation of nanocomposites …………………………………………... 31
3.6 Preparation of diglycidyl ether of bisphenol A based nanocomposites ……… 32
3.6.1 Introduction ……………………………………………………………… 32
3.6.2 Materials ………………………………………………………………… 33
3.6.4 Synthesis of Cl-g-GO ……………………………………………………. 33
3.6.5 Synthesis of amino-g-GO ………………………………………………... 34
3.6.6 Synthesis of PANI-g-GO …………………………………………............ 34
3.6.7 Preparation of PANI-g-GO/Epoxy nanocomposites ……………….......... 34
3.6.8 Characterizations ……………………………………………………….... 34
4.1.1 Introduction ………………………………………………………………. 35
4.1.5 SEM ……………………………………………………………………… 41
4.2 Exfoliation of graphene oxide ………………………………………….......... 47
4.2.1 Introduction ………………………………………………………............ 47
4.2.2 XRD …………………………………………………….…………........... 47
4.2.3 FTIR ………………………………………………….……………........... 48
4.3.1 Introduction ……………………………………………………………… 55
4.3.2 XRD ……………………………………………………………………… 55
4.3.3 FTIR …….………………………………………………………………... 56
4.3.5 Mechanical properties ……………………………………………………. 60
4.3.6 Dielectric properties ……………………………………………………... 61
4.3.7 AC conductivity ………………………………………………….............. 65
4.4 Polyvinyl alcohol-based nanocomposites ……………………………………. 71
4.4.1 Introduction …………………………………………………………….... 71
4.4.2 FTIR …….………………………………………………………............... 71
4.5.1 Introduction ……………………………………………………................ 86
4.5.2 XRD ……………………………………………………………………… 87
4.5.3 ATR-FTIR ……………………………………………………………….. 87
4.5.4 SEM ……………………………………………………………................ 88
4.6.1 Introduction ……………………………………………………................ 98
4.6.2 ATR-FTIR ……………………………………………………………….. 99
4.6.3 XRD ……………………………………………………………………… 100
4.6.4 SEM ……………………………………………………………………… 103
Future recommendations ………………………………………………................ 110
Figure 2.1. Layer structure of graphite .…………………………………........................... 7
Figure 2.2. Some common methods for the exfoliation of graphene …………………….. 9
Figure 3.1: Schematic flow diagram of preparation of graphene ……………………….... 23
Figure 4.1: XRD graphs of few layer graphene produced in different solvents with addition
of picric acid …………………………………………................... ……………………… 37
Figure 4.2: (A) The AFM analysis of graphene produced with addition of picric acid
(B) the AFM analysis of graphene produced without using the picric acid .……............... 38
Figure 4.3: FTIR Spectra of (A) graphite (B) graphene produce with addition of the picric
acid (C) graphene produced without using the picric acid …….………………………..... 40
Figure 4.4: Raman Spectra of (A) graphite (B) graphene produce with addition of picric
acid (C) graphene produced without using the picric acid ……………............................. 40
Figure 4.5: (a) SEM image of graphite (b) and (c) SEM image of few-layer graphene
produced by one step sonication, (d) SEM image of few-layer graphene produced by
sonication in addition of use of picric acid, (e) and (f) SEM image of few layer graphene
produced by one step sonication by using extra amount of picric acid
……………………………………..................................................................................... 44
Figure 4.6: Graph shows the linear relationship between absorbance per unit length (A/C)
and graphene concentration ……………………………………………………………… 45
Figure 4.7: Effect of concentration of picric acid on the production of graphene in
different solvents ……………………………………………………………………..….. 46
Figure 4.8: Graph shows the graphene concentration dispersed in different solvents with
and without addition of the picric acid …………………………………………………... 46
Figure 4.9: XRD Graph of graphene oxide prepared by different methods …………...... 48
Figure 4.10: FTIR spectra of graphene oxide prepared by different methods ................... 50
Figure 4.11: Raman spectra of graphite …………………………………………………. 51
Figure 4.12: Raman spectra of graphene oxide ………………………………………...... 51
Figure 4.13: SEM images of graphene oxide ……………………………………………. 53
Figure 4.14: AFM images of graphene oxide …………………………………………… 56
xii
Figure 4.15: XRD patterns of FLG, PANI-g-FLG and composites ……………………... 57
Figure 4.16: FTIR spectra of FLG, PANI-g-FLG, PANI, PEI and composites …………. 58
Figure 4.17. SEM images of composites with (a)1wt.% FLG (b) 1.5wt.% FLG (c) PANI-g-
FLG and (d) composite with 5wt.% of FLG …………………………………………….. 59
Figure 4.18. Bar graph of tensile strength of pure PEI, PEI/PANI and nanocomposites..... 61
Figure 4.19. Dielectric constant (a) at various frequencies (b) at 100 Hz and dielectric loss
temperature (c) at various frequencies (d) loss at 100 Hz of various samples …………... 63
Figure 4.20 (a) AC conductivity for neat PEI, PEI/PANI and nanocomposites of PANI-g-
FLG/ PEI with varying concentrations (b) AC conductivity for neat PEI, PEI/PANI and
nanocomposites of PANI-g-FLG/PEI with varying concentrations at the 100 Hz frequency
……………………………………………………………………..……………………... 67
Figure 4.21 (a) SET for neat PEI, PEI/PANI and nanocomposites of PANI-g-FLG/ PEI
with varying concentrations (b) SEA for neat PEI, PEI/PANI and composites of PANI-g-
FLG/PEI with varying concentrations (c) SET for neat PEI, PEI/PANI and nanocomposites
of PANI-g-FLG/ PEI with varying concentrations ……………………………………… 71
Figure 4.22. FTIR spectra of PANI, PANI-g-FLG, FLG, PANI-g-FLG/PVA composites and
PVA ………………………………………………………................................................ 72
Figure 4.23. XRD patterns of PANI-g-FLG, FLG and PANI-g-FLG/PVA composites
..…………………………………………………………………………………………... 73
Figure 4.24. SEM images of (a) PANI-g-FLG, PVA, Composite with 0.5wt.% FLG and
Composite with 5wt.% FLG loading …………………………….……………………… 75
Figure 4.25 (a) dielectric constant at 100 Hz and 1000 Hz ……………..……………….. 78
Figure 4.26. (a) dielectric loss at 100 Hz and 1000 Hz ..………………………………... 79
Figure 4.27. (a) AC conductivity for PVA and nanocomposites of PANI-g-FLG/PVA with
varying concentration of FLG (b) AC conductivity at room temperature PVA and
nanocomposites of PANI-g-FLG/PVA with varying concentration of FLG at the 100 Hz
and 1000 Hz frequency ………………. ……………………………………………..….. 81
Figure 4.28. Frequency dependence (a) SEA (b) SER (c) SET at room temperature for PVA
and various nanocomposite sample with different FLG loading .……………………..... 84
Figure 4.29. Tensile strength of neat PVA and PANI-g-FLG/PVA nanocomposites with
different loading of FLG ………………………………………………………………… 86
xiii
Figure 4.30. XRD spectra of FLG, PANI-g-FLG and PANI-g-FLG/PS nanocomposites
……………….……………………………………………………………........................ 88
Figure 4.31. FTIR spectra of PANI-g-FLG, PANI, FLG and PANI-g-FLG/PS
nanocomposites ………………………………………………………………………….. 90
Figure 4.32. (a) SEM image of PANI-g-FLG/PS nanocomposites with 1.5wt.% loading
and PANI-g-FLG/PS nanocomposites with 2wt.% loading ……………………………... 91
Figure 4.33. Dielectric constant PS and PANI-g-FLG/PS nanocomposites with different
FLG loadings……………………………………………………………………………... 92
FLG loadings ……………………………………………………...................................... 96
Figure 4.35. (a) SEA, (b) SER and (c) SET of pure PS and PANI-g-FLG/PS nanocomposites
with different FLG loadings ……………………………………………………………... 97
Figure 4.36. Tensile strength of pure PS and PANI-g-FLG/PS nanocomposites with
different FLG loadings …………………………………………………………………... 98
Figure 4.37. FTIR spectra of GO, Cl-g-GO, Amino-g-GO, PANI-g-GO and PANI-g-
GO/Epoxy nanocomposites with different filler loadings ………………………………. 99
Figure 4.38 XRD spectra of GO, Amino-g-GO, PANI-g-GO and PANI-g-GO/Epoxy
nanocomposites ………………………………………………………………………… 100
Figure 4.39 SEM image of (a) Epoxy (b) GO (c) PANI-g-GO and (d) PANI-g-GO/Epoxy
nanocomposites ………………………………………………………………………… 103
Figure 4.40 Tensile strength of pure epoxy and PANI-g-GO/Epoxy nanocomposites … 104
Figure 4.41 (a) Dielectric constant (b) dielectric loss (c) AC conductivity of pure epoxy
and PANI-g-GO/Epoxy nanocomposites with different filler loading ………………… 107
xiv
Journal Papers
1. Syed Sajid Ali Shah and Habib Nasir, Exfoliation of Graphene and its Application as
Filler in Reinforced Polymer Nanocomposites, Nano Hybrids and Composites Vol. 11
(7-21) 2016.
2. Syed Sajid Ali Shah and Habib Nasir, Liquid-Phase Exfoliation of Few-Layer
Graphene and Effect of Sonication Time on Concentration of Produced Few Layer
Graphene, Nano Hybrids and Composites Vol. 14 (17-24) 2017.
3. Syed Sajid Ali Shah, Habib Nasir and Abdul Saboor, " Improved Dielectric
Properties of Polyetherimide and Polyaniline Coated Few-Layer Graphene Based
Nanocomposites” Journal of material science, Vol.29, issue 1, (402-411), 2018.
4. Syed Sajid Ali Shah, Haris Mustafa, Habib Nasir and Arshad Hussain “Synthesis and
Characterizations of Epoxy/Graphene Oxide/Carbon Fiber Composites” Journal of
chemical society of Pakistan, Vol. 40, No. 03, 2018.
5. Syed Sajid Ali Shah and Habib Nasir, Synthesis of cyanate ester-based thermosets
resin by using copper(ii) oxalate as catalyst and its application in carbon fiber
composites Nano Hybrids and Composites, Nano Hybrids and Composites Vol. 22 (1-
9) 2018.
6. Syed Sajid Ali Shah and Habib Nasir, Liquid-Phase Exfoliation of Graphene in
Organic Solvents in Addition with Picric Acids” Journal of material science. (Under
Review)
7. Syed Sajid Ali Shah, Habib Nasir and Abdul Saboor, " Improved Dielectric Properties
of Graphene Based Polystyrene Nanocomposites” Journal of material chemistry and
physics. (Under Review)
Conference Presentations
1. Syed Sajid Ali Shah and Habib Nasir “Synthesis and characterization of three
components hybrid nano composite system for improved mechanical and dielectric
properties”
(Accepted in Conference) IUPAC-PKS 40, 2016, Jeju, Korea.
2. Syed Sajid Ali Shah and Habib Nasir, “Liquid-phase exfoliation of graphene of
graphene in organic solvents” ISAM-2015, Islamabad, Pakistan.
3. Syed Sajid Ali Shah and Habib Nasir, “Synthesis of polymer blends by using
copper (ii) oxalate as catalyst” APCOMS 2014, Islamabad, Pakistan.
4. Syed Sajid Ali Shah and Habib Nasir, “Synthesis and Characterizations of
Graphene Based Polymer Composites for Improved Dielectric Properties” ISAM-
2017, Islamabad, Pakistan.
5. Syed Sajid Ali Shah and Habib Nasir “Improved Electromagnetic Interference
Shielding Efficiency and Dielectric Properties of Graphene-based High-Density
Polyethylene Nanocomposites”
(Accepted in Conference) 3rd addition SMS EUROPE, 2017, Paris, France.
xvi
CNT carbon nanotube
DMF dimethylformamide
1.1.1 Composite material
The materials consisting of two or more than two components are known as composites
materials.
Composite materials have better physical and chemical properties than the materials from
which they are formed. In the composite material filler of high strength and modulus is
embedded in the matrix, where components retain their properties, yet they produce a
combination which has better characteristics than the individual one [1, 2]. During last few
decades, composite material has been the subject of attention owing to application in various
fields. The composites material has been the advantages of excellent fatigue properties, high
strength, low cost and corrosion resistance. Type of filler and matrix affect the final properties
of composite material [3].
1.1.2 Nanocomposites
Nanocomposite material is the material in which one constituent has less than 100 nm
dimensions. These materials include zero, one, two and three-dimensional materials.
Nanocomposites have replaced micro composites in most of the promising application,
because they have the advantage of elemental composition control at the nano scale and
therefore have produced best properties. The nanocomposite materials exhibit properties,
which are not commonly shown by the micro composites [4]. The type of nanofiller and matrix
affects the properties of nanocomposite. A nano-composite material has good mechanical,
thermal, catalytic, electrochemical and electrical properties. The nanocomposites are applied
to electronics industries, water purification systems, and as a catalyst, etc. [5].
1.1.3 Polymer nanocomposites
polymer matrix in single material. Recently, with the development of nanotechnology, the
reinforcement of nanofiller into the polymer and polymer blend have directed the novel
2
reinforced composite fillers are principally responsible for carrying the load and polymer
matrix is responsible for keeping the filler in desired location, orientation. The matrix also
separates the filler from each other to avoid agglomeration of filler in the composite [2]. The
polymer nanocomposite has added the valuable properties to the neat polymer, even at very
low filler loading, and with easy processability [6, 7]. The combination of nanofiller with
polymer brings drastic change in its thermal, electrical, mechanical and optical properties
[8].
Nanoscale materials have large surface area, therefore, they have exceptional physical
chemical properties [9]. The nanomaterial used as reinforcement in the polymer matrix to
prepare composite, is known as nanofiller. There are many nanofillers used by various research
groups, in which most promising are carbon nanofiller such as carbon nanotubes, graphene and
graphene oxide. Various factors which affect the ultimate properties of polymer
nanocomposites are surface area of the filler, the interaction between the polymer matrix and
filler, homogenous distribution of nanofiller in the polymer matrix [10, 11].
1.1.5 Graphene as nanofiller
The graphene nanosheets are novel carbon nanofillers with outstanding thermal, electrical and
mechanical properties. The graphene is very cost effective as compared to carbon nanotubes.
The uniform dispersion of graphene also lowers the friction coefficient. These features of
graphene have diverted the attention of researchers to use graphene as filler in polymer
nanocomposites [11, 12]. Different types of graphene-based nanofillers include mono-layer,
bi-layer, few-layer graphene (typically contains graphene layer between 2 and 10), graphene
nanoplates, graphene nanosheets and even graphite nano-plates. However, graphene with more
than 10 layers is of less interest because as their number of layers increases and at last the
graphite come into the play, thus ending the 2D chemistry of graphene [13].
The polymer nanocomposites containing graphene or graphene oxide as filler are known as
graphene-based nanocomposites. However, the key limitation of pristine graphene as filler is
its strong intermolecular interaction between the graphene layers, which result in poor
3
compatibility with polymers. The ultimate properties of graphene based polymer
nanocomposites depend upon the homogenous dispersion of graphene in the polymer matrix.
The aggregation of graphene in the polymer matrix is the main problem for the uniform
dispersion of graphene in the polymer matrix. To overcome this limitation, the graphene
surface can be modified to get the maximum uniform dispersion for the intended applications.
To modify the graphene surface usually functionalization of graphene is carried out via
covalent or non-covalent routes. The functionalized graphene has numerous advantages to be
used as an ideal filler for nanocomposites. The functionalized graphene has somewhat damage
carbon structure with all the physical properties of graphene. The functional group on the
surface of graphene enhanced the dispersion of graphene in polymers and improves the
interaction between the polymer and filler [14-18].
1.2 Background
1.2.1 History of graphene
The use of graphite was started six thousand years ago, in the Europe and was used to decorate
the pottery. However, the research on an isolated single-atom plane of graphite started in
1960s, when intercalation compounds of graphite were discovered, these compounds had high
basal-plane conductivity compared to original graphite [19, 20]. The researchers at that time
were excited that intercalation compound of graphite may lead to the lighter and cost-effective
substitute for existing metal conductors. Later, in 20th century the research was carried out to
expose the exceptional electrical properties of few layer graphite (graphene), however, the
finding the single and few layers of graphite was challenging goal from both experimental and
theoretical research aspects. The macromolecule was introduced between the graphitic atomic
planes in the intercalation system, producing isolated layers of graphite in a three-dimensional
matrix. The consequent elimination of the macromolecules resulted a blend of stacked
graphene layers without the control of the structure. At that time, a general believe was made
by both theoretical and experimental observation that 3D base is compulsory for the 2D
materials exist.
AB Initio calculations presented that a graphene layer was thermodynamically unstable if its
size was below 20 nm and it was considered to be stable only with macro size ( 24,000 carbon
atoms, as in graphite) [21].
4
Carbon, the basic element of the universe, an essential component of biomaterial, being the
sixth element in the periodic table with atomic number 6 and mass number 12, is recognized
to occurs in several allotropes owing to its valency. Till 1985 diamond and graphite were
known as allotropes of carbon. The discovery of fullerenes (Bucky balls) gives a new path to
the carbon research. The first Bucky balls discovered to consist of 60 carbon atoms (C-60),
later several other forms ranging from C20 to C80 were discovered. In 1991 the introduction
of carbon nanotubes was another breakthrough in this field [22].
The word graphite is derived from a Greek word “graphene” which means “to write”. The use
of graphite for writing on different materials started in 15th century. In 18th century it was
recognized as an allotrope of carbon. Graphite has a perfect structure with parallel layers’
planes joined together by sp2 bonding. Within each layer, each carbon atom is bonded to three
adjacent carbon atoms forming a two-dimensional continuous hexagon [23].
1.2.2 Graphene
Graphene is a single layer of carbon atoms bonded by sp2 hybridization in the hexagonal
structure. Graphene revealed exceptional mechanical, thermal and electrical properties due to
which it has been applied in various industrial applications. The exceptional properties of
graphene include high Young’s modulus about 1 TPa, the large specific surface area (2630 m2
g-1), outstanding thermal conductivity (3000–5000 W m-1 K-1) and exceptional optical
transparency (~97.7%) [20]. Graphene can be produced by a number of approaches, in which
chemical vapor deposition, liquid-phase exfoliation method, mechanical exfoliation and
oxidation-reduction methods are most commonly applied. However, the large-scale production
of graphene is still a challenge. Though, the chemical exfoliation method is the most feasible
method for the large-scale production of graphene [24].
1.2.3 Graphene oxide
Graphene oxide is usually derived from the exfoliation of graphite oxide. Graphene oxide is
mostly analogous to graphite oxide regarding the chemical structure, which has plenty of
oxygen-containing functional group on its carbon basal plane [25]. However, the physical
structure of graphene oxide is different from graphite oxide, as the graphite oxide retains the
stacked structure which is similar to graphite [26]. Generally, graphene oxide is usually
produced by exfoliation of graphite oxide through sonication in a solvent [27]. Consequently,
the product from this method has been given different names, usually recognized as reduced
5
graphene oxide (RGO). The reduced graphene oxide has different properties than pristine
graphene, the graphene oxide (GO) is hydrophilic, electrically insulate, due the disruption of
the sp2 bonding network in its carbon skeleton, where carbon base is surrounded by oxygen-
containing functional groups. Despite the different characteristics of reduced graphene oxide
from pristine graphene, the reduced graphene can be modified to use in several applications
[1].
1.2.4 Graphene-based polymer nanocomposites
The composite material in which one component is dispersed in nanometer range is known as
a nanocomposite. The nanocomposites have the advantages of reduced cost and outstanding
properties for different industrial applications. Many nanofillers have been used by the
researcher to make the nanocomposites. The nanocomposites in which graphene and its
derivatives are used as filler are called graphene-based nanocomposites [28-30].
The graphene-based polymer composites are prepared by using graphene oxide, pristine
graphene and modified graphene as starting material. The graphene oxide contains many
functional groups on its surface, these functional groups provide the sites for further
modification and functionalization [31]. Graphene has exceptional properties such as
enormously high mobility, high elasticity, high modulus and tunable band gap. Due to the
aforesaid excellent properties of graphene, it has applications in various industrial fields like
nanocomposites, batteries, super capacitors, biological sensors, nanoelectronics, fuel cells and
hydrogen storage [32].
1.3 Aims and Objectives
The key goal of this research work was to understand the reinforcing behavior of graphene and
graphene oxide in polymer-based nanocomposites. The thesis explores the effect of graphene
and graphene oxide on the reaction and properties of various polymers used in this work. Few-
layer graphene were produced by liquid phase exfoliation by using picric acid as a surfactant
in different solvents., moreover graphene oxide was produced by modified Hummers’ method.
In addition, graphene/ polymer and graphene oxide/ polymer nanocomposites were fabricated
and characterized by various techniques. Some of the specific aims of this study are listed
below:
• Exfoliation of graphite oxide into graphene oxide in different solvents.
6
• Exfoliation of graphite into graphene in different solvents (DMF, ethanol, etc.).
• Preparation of polymer blends from different polymers.
• Synthesis of graphene/polymer and GO/polymer nanocomposites synthesis of
graphene/polymer and GO/polymer nanocomposites with enhanced mechanical
and dielectric properties by using different blends of the polymer.
• Characterization of graphene/polymer nanocomposites by FTIR, TG/DTA, SEM
and XRD, etc.
1.4 Thesis Outline
The present study has been structured to present the detailed account on exfoliation of
graphene, functionalization of graphene and its application as fillers in the polymer
nanocomposites to improve their properties. The proposed research is planned in a successive
manner in the following chapters.
Chapter 1 is an introductory chapter, which introduces the objectives of this work and includes
the introduction of graphene and graphene-based polymer nanocomposites.
Chapter 2 comprises of the literature review on the production of graphene and graphene based
polymer composites.
Chapter 3 defines the strategy of experiments for the exfoliation of graphene liquid phase
exfoliation techniques, functionalization of graphene, exfoliation of graphene oxide,
functionalization of graphene oxide. The chapter also explains the approach for the application
of functionalized graphene as a filler in the polymer nanocomposite. Results and discussion of
various processes mentioned in chapter 3 are briefly explain in chapter 4. Since the study is
focused upon the graphene-based polymer nanocomposites, so the characterization of
nanocomposites is also discussed in Chapter 4.
Chapter 5 concludes the discussion on the exfoliation and functionalization of graphene and
graphene oxide; moreover, this chapter also concludes the properties of nanocomposites
prepared by using different strategies. In the end of chapter 5 future recommendations are
given.
Chapter 2
Literature Review
In this chapter, the background of exfoliation of graphene and its applications in polymer-
based nanocomposites is discussed.
2.1 Graphene
Graphene consist of graphitic layer in which each carbon atoms are in their lowest energy state
at ambient environmental conditions and these carbon atoms are bonded with sp2 hybridization
with other neighboring carbon atoms and form hexagonal ring. The three-dimensional crystal
structure of graphite is given in figure 2.1. The graphite is anisotropic because of the difference
between in-plane and out of plane carbon atoms bonded in a molecule. In plane, graphite has
higher modulus than in perpendicular to the plane. When graphene is in plane then it is stronger
than diamond. The graphene is thermally and electrically conductive due to presence of π
orbital in its structure [33].
Figure 2.1. Graphite structure indicating the layer structure having carbon bonded with sp2
hybridization forming hexagonal structure.
8
The methods used to produce carbon nanotubes were applied to exfoliate graphene. For
example chemical vapor deposition [34], and thermal decomposition on silicon carbide [35].
These studied have produced few-layer graphene and were unable to produce monolayer
graphene. Many groups used chemical vapor deposition method for exfoliation of graphene.
The efforts continued, till 2004 Konstantin Novoselov and Andre Geim produce graphene by
mechanical exfoliation method. The graphene produced by this method has high quality and
size in hundreds of microns. After that the graphene research including deposition of
graphene on substrate, chemical modification of graphene and use of graphene in different
industrial application has dramatically increased since 2004 [21].
The IUPAC commission has recommended the word “graphene” instead of using older name
“graphite layers” which was inappropriate in study because three- dimensional stacking
structure is known as graphite. Nowadays graphene is the mono layer of sp2 hybridized carbon
atoms, also the basic component of graphite, carbon nanotube and fullerene [20]. Graphene
has superior electrical properties however its band gap limits its extra ordinary performance.
As results many research groups have focused on novel graphene derivatives, for example,
Fluro graphene, graphene oxide and hydrogenated graphene [27].
2.2 Exfoliation of graphene
After 2004, when first time graphene was exfoliated, an incredible reach has been carried out
on graphene in many fields and its application in various industries to improve the quality of
products. The graphene has excellent electrical, thermal, optical and mechanical properties;
therefore, it has wide range applications as a nanofiller. Different research groups have
produced graphene by various methods, such as mechanical exfoliation method, liquid-phase
exfoliation method, oxidation-reduction method and chemical vapor deposition method, etc.
These methods are illustrated in figure 2.2. These methods are broadly used for the exfoliation
of graphene and each method has some advantages besides some restrictions. The only
challenge which remains in the exfoliation of graphene is its large-scale production [36]. The
graphene production methods are listed below:
9
2.2.4 Intercalation compound method
Figure 2.2. Some common approaches for the exfoliation of graphene [37].
2.2.1 Mechanical exfoliation method
The graphene was first time exfoliated by Konstantin Novoselov, in 2004 [38], later on many
research groups prepared graphene by this method. This method applies micromechanical
cleavage for the exfoliation of graphene deprived of using any superior equipment. An
10
adhesive tape or AFM tip is used to peel off the single layer from graphite surface [19]. In
another study Wang Xueshen et al. used scotch for producing mono layer graphene, they
considered the thermal annealing effects on the tap residue and the surface of graphene, the
thermal annealing at 400oC under inert atmosphere completely remove the tape residue, but
under vacuum the graphene completely attach the surface of the substrate [39]. Graphene was
also produced by using shear-assisted supercritical CO2. The low viscosity and high diffusivity
of supercritical CO2, helps the gas to easily diffuse in between the interlayer spacing of
graphite, thus produce graphene sheets. High rotation speed of CO2 gas and high temperature
accelerates the exfoliation process, producing 90% graphene sheets with less than ten layers
[40].
The chemical exfoliation method applies two routes to produce graphene:
1. Liquid phase exfoliation
2. Oxidation reduction method
Incredible research work has been done to investigate liquid-phase exfoliation process, in
which graphene is produced by one step sonication of graphitic materials in solvents. The
strong intermolecular forces are present between the two-graphitic layer, an energy input of 2
eV nm-2 is required to overcome these forces. The ultrasound wave comprising of rarefaction
and compression cycles. As a result of rarefaction cavitation takes place, the negative acoustic
pressure creates the transient microbubbles, these microbubbles grow in very short time and
collapse with a sudden pressure up to 20 Mega pascals and up to 500 K temperature. These
microbubbles have very short heating and cooling rate, nearly up to 109 K s-1. The produced
shock waves apart the graphite layers and micro turbulence from bubbles collision leads to the
exfoliation process [41]. The ultrasonication time, power of ultrasonication instrument used
and solvent type are the major factors on which exfoliation process depends during
ultrasonication [42]. It was found that the best solvents for the exfoliation process are the
solvents with surface tension about 40 mJ/m2, because of the minimized interfacial tension
between the solvent and graphene.
11
Natural graphite is usually used as starting material for graphene produced by the liquid-phase
exfoliation. The nano-graphite and expandable graphite are also used as starting material, but
their use is comparatively less. Kozhemyakina et al. studied the liquid phase-exfoliation of
graphene and determine effect of initial material on the quality and quantity of graphene. They
used 22 different grade type of graphite families as well as carbon black were used as starting
material with different structure and size, the solvents used in this study were NMP and water
with surfactants. They reported that small size graphitic material are easily dispersed as
compared to the large grain size graphitic material [43].
2.2.2.2 Oxidation reduction method
The chemical oxidation of graphite has advantages as well disadvantages, as during the
oxidation process the basic sp2 structure of carbon atom is wrecked and monolayer graphene
has defects on its surface. The oxidation of graphite has also affected the electrical properties
of the produced graphene. The delocalized π-electrons are localized by the functional group,
as a result decreases the conductivity as well as mobility of electron in graphene oxide.
Therefore complete reduction of graphene oxide (GO) is carried out to get the optimum
properties of graphene [26]. The reduction process is basically done to get a graphitic material
which have no functional groups and be similar to graphene obtained by other exfoliation
methods [44]. Reduction of graphene oxide is us done by two ways, either by chemical process
or by thermal treatment [45].
2.2.3 Intercalation compound method
Graphite intercalation compound may preexist graphite oxide. The first graphite intercalation
compound was isolated in 1841 [19]. To produce graphite intercalation compound, a large
number of species can be introduced in the graphitic inter layers regions. An outstanding
engineering properties can be introduced into a material by using this technique [46]. This
technique is also applied for increasing the inter layer distance of graphite. For this purpose,
alkali metals, like lithium potassium and cesium are most suitable material for increasing the
interlayer distance of graphite. The alkali metals radially intercalate into the inter planner
distances of graphite and donate an electron to the graphite. Thus, the alkali ions intercalate
the graphite resulting in decrease in Vander Waals forces and the distance between graphite
layer increases. So, the graphite layer repels each other, resulting in separating of layer. As a
12
result, negatively charge graphene sheets are exfoliated from the surface of graphite, which are
strong reducing agents [25, 47]. The electrochemical intercalation of alkyl ammonium cation
into the pristine graphite produced Few-layer graphene [48]. Another group also produced few-
layer graphene by using this technique with the help of aqueous perchloric acid. By this
technique graphene with varying degree of oxidation can easily be produced [49]. The
graphene can also be prepared by ultrasonication of lauryl amine-intercalated graphite oxide
(GO) in the presence of hexane, and reduction by hydrazine. The lauryl amine increases the
spacing between the layers of GO by forming three types of bonding thus facilitating the
exfoliation of GO [50].
2.2.4 Chemical Vapor Deposition method
Chemical vapors deposition method is the generally used bottoms-up method to produce few-
layer graphene. Although some groups working on chemicals vapors deposition method
reported the monolayer graphene, however, the chemical synthesis of graphene with various
shapes and sizes have opened new paths for utilizing graphene in various applications. In 2006,
Somani and coworkers produced few-layer graphene by this approach, they apply camphor as
carbon in the CVD reactor and effectively exfoliated the graphene [51]. Ahmad N. Abbas et
al. in use chemical vapor transport technique to synthesize nano graphene molecule [52]. J.
Sánchez-Barriga et al. study the grown of graphene layer by chemical vapor deposition on
nickel and cobalt. They studied the ferromagnetism and spin orbit effects by magnetizing the
sample by spin- and angle-resolved photoelectron spectroscopy. The thin films were produce
on substrate and they reported that ferromagnetic polarization of graphene on Nickle and cobalt
were negligible [53].
2.3 Characterizations of Graphene
The characterization of graphene is very important to analyze the quality of graphene. The
properties of graphene based polymer composites depends upon the quality of graphene.
Atomic force microscopy (AFM), scanning electron microscope (SEM), and Transmission
electron microscopy (TEM) are used to study the morphology of graphene. The crystalline
structure is investigated by X-ray diffraction (XRD). The Fourier transform spectroscopy
(FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to study
the surface morphology. UV-visible spectrophotometry is used to determine the concentration
13
of graphene in the dispersion. SEM is the initial technique to characterize the graphene, SEM
gives the qualitative insight into the structure of graphene sheets. AFM is an effective
technique for the characterization of graphene, it gives the morphological information about
the produced graphene. The height of graphene sheets on substrate can be calculated by using
AFM. The AFM can also estimate the number of layer of graphene present in the exfoliated
graphene sheet. It is difficult to calculate the theoretical thickness of 0.34 nm because of the
difference of the attraction/repulsion between the substrate and graphene. The typical thickness
of graphene can be find out by AFM. The presence of solvents between the graphene sheets
and folded or wrinkled sheets also complicate the thickness finding of graphene by using AFM
[45, 54-57].
XPS and FTIR are used to investigate the type of functional groups attached to the surface of
graphene and amount of oxygen on the graphene surface.
Raman spectroscopy is the quick and effective technique to study the surface morphology and
to investigate the quality of graphene. It can measure the conversion of sp3-hybridized carbons
to sp2 by the removal of functional groups from the surface of graphene oxide. It also gives
information about the order and disorder of graphitic layer in the given sample. The conversion
of sp3-hybridized carbons to sp2 restores the electrical conductivity; hence, conductivity also
tells us the qualitative measure of the graphene oxide conversion to graphene [54, 55].
The crystal structure of parent graphite, graphene and graphene oxide is investigated by XRD.
For example, the graphite has a sharp peak appears at an angle of 26.3o while for graphene
oxide the peak appears at 10.5o, however, when graphene oxide is reduced then the peak
disappears. For graphene a broad and less intense peaks appears at 26.5o with increased d-
spacing than the graphite [45, 54, 58, 59].
The BET analysis used to find out the surface area of graphene by N2 adsorption. The
theoretical surface area of the disk like particles is inversely proportional to the thickness.
Therefore, surface area has been used to indicate the degree of exfoliation of graphene [54,
56].
The lateral size of graphene layer, morphology of graphene and number of layers of graphene
are determine by using TEM. Moreover, the electron diffraction patterns are also used to
14
differentiate between the single and bilayer graphene. HR-TEM can recognize the bonds on
functionalized sheets and atomic defects [60].
2.4 Functionalization of graphene
Prior to focus on graphene, an extensive research work was carried out on oxidation of
graphite, exfoliation of graphite oxide to produce of graphene oxide and characterizations of
both materials [26, 61, 62]. The graphene is differentiated from graphene oxide due to presence
of functional groups on the surface of graphene oxide. Graphene is hydrophobic whereas
graphene oxide is hydrophilic in nature. Moreover, graphene oxide contains both aliphatic (sp3)
and aromatic (sp2) domains, which act as sites for further attachment of chemical species on
its surface. The functional groups on the surface of graphene oxide can easily be removed to
produced reduced graphene oxide; however, the reduced graphene oxide is unsuitable for
mechanical reinforcement and electronic application in the polymers composites. Therefore,
usually functionalization of graphene and graphene oxide is carried out to get the desired
performance from the graphene and graphene oxide [63]. Functionalization of graphene is also
helpful for its dispersion in various solvents [64], and to increase its dispersion in polymer
matrix. The functionalization of graphene is carried out by different techniques, in which most
commonly method are given below [65, 66].
1. Covalent functionalization
2. Noncovalent functionalization
2.4.1 Covalent functionalization
Graphene have excellent electrical properties, however, significant disorder and defects on its
surface have adverse effects on its electrical properties. However, the defects on the surface
of graphene facilitates the functionalization process [67], these defects act as functionalization
centers [68] and thus covalent bond are created between the graphene and incoming functional
material bond [67]. The covalent functionalization of graphene results in the nonlinear
dispersion and the opening of a band gap at the Fermi points [66], and the choice of
functionalized material on the surface of graphene is an important factor for the application of
graphene based material in electronic industries [18].
15
The graphene oxide is also consider the modified form of graphene, it contains oxygen
containing functional groups [69]. To utilize graphene in conductive application these oxygen
functional group are reduced electrochemically [70], or chemically [71]. The conductivity of
the electrochemically reduced graphene oxide is higher than that of chemically reduced
graphene oxide [18].
The covalent functionalization of graphene results the strong adhesion between the polymer
matrix and graphene and produce strong mechanical properties [72]. Many groups have
improved the properties of graphene based polymer composites by covalent functionalization
of graphene, for example graphene/ polyethylene [73], graphene/ epoxy composites [74],
graphene/ polyurea [75], and graphene/ polyurethane [76].
2.4.2 Noncovalent functionalization
The noncovalent functionalization of graphene is carried out by creating the weak Vander
waals forces between the host species and graphene surface. The noncovalent
functionalization results in the unchanged band structure of graphene. In noncovalent
functionalization the charge transfer also occurs [66], the basic graphitic structure of graphene
remain unchanged by using this functionalization technique [77]. The van der Waals forces,
electrostatic interaction, π–π stacking interactions, hydrogen bonding, polymer wrapping and
coordination bonds are key forces for the noncovalent functionalization [78], and point defects
also facilitate this type of functionalization [67]. These defects are slightly found on graphene
surface prepared through standard sample growth processes, however, control defects creation
are possible by using chemical treatment and particle irradiation method [79]. The noncovalent
functionalization improves the reactivity. The noncovalent functionalization of graphene is
carried out by many groups to improve the electrical properties of the graphene based polymer
composites, for example graphene/ poly vinylidene fluoride composites [80].
2.5 Graphene based polymer composites
The composites materials in which graphene and graphene oxide are used as nanofiller, are
known as graphene-based polymer composites and the composites containing filler other than
graphene are known as polymer composites. The Polymer composites have extra ordinary
properties due which they have been used various industrial applications, such as thermal
16
insulators, electrical insulators and for load bearing applications. The polymer composites have
some restriction and introduction of graphene into the polymers defeated these restrictions.
The addition of graphene as a filler introduced new properties into the polymer matrix. For
example, introduction of graphene into conductive polymer not only increases the conductivity
of polymer but also enhances mechanical and thermal properties of polymer [81].
2.6 Synthesis of graphene-based polymer nanocomposites
Graphene-based polymers nanocomposites have promising properties, due to which research
on these materials have increased. Different approaches have been adopted to prepare
graphene-based polymer composites, in which most commonly used methods are discussed
below.
This method provides an easy approach for producing graphene-based polymer
nanocomposites with high efficiency [93]. The graphene/polyethylene succinate composites
were fabricated by this technique with different compositions, the thermal behavior of the
samples was investigated by differential scanning calorimetry (DSC). The crystallization
degree and glass transition temperature were studied. They used both isothermal and non-
isothermal modes and concluded that the graphene was incompatible with the polymer matrix,
however, the crystalline structure, overall, remained unchanged. The addition of graphene also
decreased the enthalpy of the polymer. For isothermal mode, the graphene has less affected the
enthalpy, thermodynamic stability against heating and recrystallization behavior [82]. Jung-
Tsai Chen, et al. prepared gas barrier film from polyvinyl alcohol (PVA) and graphene oxide
(GO). The PVA/GO films with only 0.07 vol. % GO showed maximum gas barrier
characteristics [83]. Dong Sheng Yu, et al. functionalized graphene by using aryl diazonium
salt, then aryl diazonium salt functionalized graphene (ADS-G) was polymerized with
polyvinyl alcohol (PVA) to prepare nanocomposites. They reported that ADS-G/PVA
composite have improved electrical, mechanical and thermal properties than PVA/r-GO
composites [84]. Another research group prepared the graphene/epoxy composites by
dispersing the graphite nanoplates in an acetone solution containing 15 wt. % epoxy resins.
The suspension was mixed by stirring basketball milling machine instead of sonication. The
composites prepared by ball milling machine showed higher thermal conductivity than
17
composites prepared by using sonication [85]. The graphene was modified by epoxy resin
(Diglycidyl ether of bisphenol A) by Qingshi Meng, et al. then they introduced this modified
graphene into the epoxy resin and prepared the modified graphene-based composites by
solution casting method. They reported that the composites with modified graphene particles
have shown better mechanical and thermal properties than the composites with unmodified
graphene particle [86]. Bismarck Mensah, et al, prepared GO/acrylonitrile–butadiene rubber
nanocomposites and used dielectric spectroscopy test to indicate enhancement of about five
times in the real part of permittivity [87]. Ming Tian, et al, thermally expended the graphene
nanoplates (TGNPs) and then introduced them in polydimethylsiloxane (PDMS) to obtain
TGNP's/PDMS nanocomposites. The nanocomposites have shown improved dielectric
properties [88]. Another research group prepared reduced graphene oxide/polyimide
composite by solution casting method and reported that the nanocomposites with 30 vol.% GO
ratio have shown 282% increase in Young’s modulus compared with pure film, electrical
conductivity increased fourteen times and reduced the oxygen transmission rate as up to 93%
compared to pure polymer [89]. Lilong Yang, et al. thermally reduced the graphene oxide and
then prepared graphene/polyether ether ketone (PEEK) nanocomposites. They reported that
nanocomposites with thermally reduced graphene have good thermal and mechanical
properties as compared to nanocomposites with chemically reduced graphene and polyether
ether ketone [90].
This method provide an approach for preparing graphene based polymer nanocomposites at
commercial scale, as this is the more versatile and environment friendly than the other two in-
situ polymerization and solution casting method [91]. This method uses molten polymer at
high shear mixing rate and high temperature. The method is mostly used for the thermoplastics
polymers composites, some example of this methods are polyimide/ reduce graphene oxide
[92], poly ether ketone/ graphene [93] and ethylene vinyl-acetate copolymer/ reduced graphene
oxide [94]. Poor dispersion of filler in the matrix, breakage of basic graphitic structure and
utilization of high shear forces are the main limitation of this method [54, 95]. Poor dispersion
of filler result in inferior properties, implying high percolation thresholds thus lower the
conductivities [96].
Kim et al. used this technique to synthesize the graphene/polyurethane nanocomposites.
Thermally reduced graphene oxide was fed into a twin-screw extruder at 180oC and mixed
under dry nitrogen for 6 minutes. These samples were further processed into films (~0.1 mm)
by hot pressing at 180o C. The percolation threshold was achieved above 0.5 vol. % of graphene
loading, which is higher value than the same composites synthesized by the in-situ
polymerization technique [97].
Zhang et al. successfully dispersed thermally reduced graphene oxide in polyethylene
terephthalate at 285oC. The strong interaction between the functional groups on the surface of
graphene oxide and polymer results in uniform distribution of graphene in the polymer matrix.
They studied the dielectric properties of the nanocomposites and reported that the maximum
dielectric properties were attained at 2.5 vol. % of filler loading [98].
2.6.3 In-situ polymerization method
The graphene based polymer nanocomposites are most commonly synthesized by this method.
This method applies the polymerization of monomer in the presence of graphene sometime
initiated by heat or by catalyst [99]. In this method GO and polymer are mixed together by
using a suitable solvent, then the solvent is evaporated and the mixture is polymerized by using
certain condition required for the polymerization of monomer used [100]. Moreover, the
graphene based epoxy composites are widely produced by in situ polymerization [101]. Li-Bin
Zhang, et al. prepared graphene/polyimide nano composite by the reaction of GO and
polyimide. The isocyanate groups were introduced on the surface of graphene oxide. Then the
isocyanate modified graphene oxide was polymerized with polyimide monomer. The
incorporation of modified graphene oxide result in increased thermal stability and improved
tensile strength [102]. Beta-cyclodextrin functionalized graphene nano hybrids were prepared
by using classical covalent modification method at different temperatures conditions. Beta-
cyclodextrin has increased the thermal stability of graphene derivatives. Moreover, the
introduction of beta-cyclodextrin also improved the dispersibility of products in both
polar/protic and nonpolar/aprotic solvents. The introduction of 1 wt. % of the filler improved
thermal degradation temperatures polyvinyl alcohol [103]. Graphene was functionalized
through phenethyl alcohol by a diazonium reaction, specific functional groups were
successfully introduced, and polyurethane was attached to them by covalent bonding. The
19
nanocomposites, as compared to nanocomposites with reduced graphene oxide [104]. Weifei
Li research group chirally prepared the graphene/polyacetylene composite. They first
converted the GO into alkynyl-GO containing carbon atoms having the triple bond for
polymerization. Alkynyl-GO was polymerized with another chiral acetylenic monomer,
producing graphene oxide hybrid covalently bonded with chiral helical polyacetylene chains.
They used the GO derived hybrid as a chiral inducer for the crystallization of aniline
enantiomers, such as L-Alanine was induced to crystallize, forming rodlike crystals [105]. Min
Lian et al. functionalized graphene by Kevlar through π-π stacking and used this Kevlar
functionalized graphene in polymer/nanocomposites. They incorporated the Kevlar
functionalized graphene in polyvinyl chloride and polymethyl methacrylate and reported that
incorporation of Kevlar functionalized graphene has improved the mechanical properties of
polymers [106]. Sung Hun Ryu, et al. chemically grafted the Hexamethylene diamine on the
surface of graphene oxide. They introduced modified graphene oxide polypropylene to prepare
graphene based nanocomposites. The Hexamethylene diamine was grafted on the surface of
graphene oxide via two types of chemical reactions, first by addition reaction of amine groups
to the carboxylic acid sites of GO and second nucleophilic substitution reactions between an
amine and epoxied groups of graphene oxide. The nanocomposites having modified graphene
oxide showed better mechanical and electrical properties as compared to nanocomposites
reinforced with simple graphene oxide [107]. Sung Hun Ryu, et al. also modified graphene
oxide by chemically grafting long chain alkyl amines. The alkyl amines have also followed the
same reaction mechanism as discussed above. The nanocomposites reinforced with alkyl
amines modified graphene oxide have shown better mechanical and electrical properties than
the nanocomposites reinforced with pristine GO [108]. Some other example of polymer
graphene composites prepared by this method are polyvinyl alcohol /graphene [109],
polypyrrole /graphene oxide [110] and polyaniline / graphene [111]. Thus, many work has
been carried out to prepared nanocomposites by using in situ polymerization, because in this
method strong interaction is present between the filler and polymer, which results in uniform
dispersion of the filler [91]. This method leads to the high filler loading in the matrix; however,
the viscosity of the mixture increases by the polymerization process and further processing
become difficult and this ultimately restricts the filler loading friction [91].
20
2.6.4 Electrodeposition method
This method is mostly used for the synthesis of graphene/metal composite [48, 83-86],
however, it can also be applied for preparing graphene based polymer nanocomposites. Shanli
Yang, et al. synthesized graphene–chitosan by electrodeposition method. They reported this
method as environmentally friendly because of no further contamination, which is its main
advantage [112]. Anthraquinone/graphene nanocomposites were prepared by electrodeposition
and showed the electro catalytic performance for two-electron reduction of oxygen, thus this
composite is a possible metal free electro catalysts for oxygen reduction reaction [113]. Qixian
Zhang, et al. prepared graphene/polyallylamine-Au nanocomposites, in which they used
polyallylamine as a reducing agent in aqueous solution, and reported that the graphene has
played a role in supporting material for increasing the active area of Au particles. They also
reported that the nanocomposites can also use for the reduction of both H2O2 and O2 [114].
Xiao-Miao Feng, et al synthesized graphene based polyaniline nanocomposite film and
reported that the nanocomposite has large specific area, high electrical conductivity and good
biocompatibility. Graphene based polyaniline nanocomposite film has practical application in
H2O2 biosensor and supercapacitors [115]. Haihan Zhou, et al. prepared polypyrrole/graphene
oxide nanocomposites by electrodeposition method. The nanocomposites showed superior
capacitive behaviors, however, the thicker films have comparatively lower capacitance value
at high current density [116]. Limin Lu, et al. prepared graphene–poly 3,4-
ethylenedioxythiophene nanocomposites and reported that this method is useful for producing
the material having bio sensing and bio catalytic applications [117].
21
3.1.1 Introduction
Graphene consists of a honeycomb-like structure of a single layer of carbon bonded with sp2
hybridization [118]. Natural Graphite contains graphene sheets as its basic structure [119],
therefore graphite is usually used as starting material to produce graphene. Graphene was first
exfoliated in 2004 [38], however, the intercalation compounds of graphite were reported in
1840 [120]. Graphene is the toughest material [121] and has excellent mechanical, electrical
and thermal properties. An incredible research work is being done on graphene now a day
[122]. Low-cost mass production of graphene has become the need of day due to widespread
applications of graphene in electronics, energy, composites and biotechnology [123].
The graphene has been exfoliated by several methods, the techniques which are mostly adopted
for the production of graphene are mechanical exfoliation [124], chemical exfoliation
technique [125], and chemical vapor deposition method [126]. In all these methods, the
chemical exfoliation method is most feasible and is further divided into two methods, first
oxidation-reduction method [127] and second is liquid-phase exfoliation method [122]. The
oxidation-reduction method has three steps, in first step graphite is oxidized, second step is the
exfoliation of graphene oxide from oxidized graphite and at the end reduced graphene is
produced [128, 129], by using various methods such as heat treatment [130]. However,
graphene produced by oxidation-reduction method has significant defects on its surface [131]
and the strong oxidizing agent leads to the environmental pollution [132]. Furthermore, the
electrical and structural properties of graphene are not completely restored causing significant
difficulties in many applications [133]. Therefore, synthesis of graphene by liquid-phase
exfoliation has got many considerations in the recent few years [134]. Liquid-phase exfoliation
method involves the ultrasonication of graphite in suitable solvents and then removal of solvent
to get pristine graphene [135]. Many research groups have exfoliated the graphene through
organic solvents [136]. However, the ultrasonication was first time used by Hernands et al. for
22
exfoliation. The ultrasonic energy drives the solvent molecules into the graphite layer, thus
producing the graphene by exfoliating graphite layers [137]. This method has the limitation of
low graphene yield production. To exploit the full advantage associated with the dispersions
of pristine graphene in solvents by adopting liquid-phase exfoliation method, it is essential to
acquire the supreme obtainable concentration of few-layer graphene while maintaining the
pristine graphitic lattice of the graphene nanosheets. The choice of solvents in liquid phase
exfoliation is a very important factor. Usually, the solvents having surface tension value range
from 40 to 50 mN/m are used to produce graphene by liquid-phase exfoliation [138]. Water,
ethanol and methanol are not suitable for exfoliation of graphene by liquid-phase exfoliation
method because they have surface tensions 72.8 mN/m, 22.3mN/m and 22.5mN/m respectively
[120]. The pristine graphene is enormously hydrophobic therefore, water as a solvent without
the assistance of surfactants for the dispersion of graphene is unusable.
In this work, graphene was exfoliated by liquid-phase exfoliation in different solvents.
Moreover, picric acid was used to facilitate the exfoliation process. It was noted that the
production of graphene was increased by the addition of picric acid. The picric acid enables
the solvent to overcome the van der waals forces present between the adjacent layers of
graphite and considerably increases the production yields of graphene.
3.1.2 Materials
The natural graphite powder (450 Um) was obtained from Nacional de Graphite, Brazil and all
the solvents used in this work and picric acid were obtained from Sigma- Aldrich.
3.1.3 Exfoliation
In a typical method 0.5g of graphite powder was dispersed in 100 ml of solvents. In addition,
the same amount of graphite powder was also dispersed in each solvent containing picric acid
with different concentration. These dispersions were sonicated for 10 hours. For removal of
solvents, the resultant dispersions were centrifuged for 1 hours at 4000 rpm. After
centrifugation, the solvents were removed and the graphene samples were dried. The
temperature for drying 60oC under vacuum for period of 36 hours. Figure 1 shows the
schematic diagram of graphene exfoliation.
23
3.1.4 Characterizations
Atomic force microscopy (AFM) JSM-5200 (JOEl) with an operating frequency of 174.504
KHz on SiO2 substrate was used to observe the exfoliation of graphene. Scanning electron
microscope (SEM) JSM-490A (JOEL) were used to study the morphology of products. The
micrographs were recorded in gentle beam mode without metallic coating. X-ray diffraction
(XRD) Theta-Theta (STOE) were used to study the crystalline structure of products. UV-
visible spectrophotometer (UV-2800 UV-Vis spectrophotometer) were used to find the
concentration of graphene in various samples. The sample were placed in a quartz cell with
one-centimeter optical path over the wave length range of 200 nm to 800 nm. By measuring
the absorbance of graphene, the concentration of exfoliated graphene was calculated by using
the Beer-Lambert law. The FTIR spectra of graphene was taken by using FTIR (KBr disk
method; Perkin Elmer 2000 spectrometer, USA), at frequency in the range of 400–4000 cm-1.
24
3.2.1 Introduction
In spite of the novelty of graphene and its application in various fields, the study of graphene
oxide extends back many decades to the study of the chemistry of graphite. In 1859 B.C. Brodie
(British chemist) oxidized the graphite by using the potassium chlorate in fuming nitric acid.
The successive oxidation further increased the oxygen ratio. The optimum graphite oxide was
obtained by performing four successive reactions. The graphite oxide produced by Brodie was
easily dispersible in water, but the dispersion acidic media was difficult. Moreover, the
graphite oxide lost the oxygen contents on heating [139]. After 40 years Brodie work was
improved by L. Staudenmaier, he added the potassium chlorate in acidic medium containing
sulfuric acid and nitric acid. The small change in procedure resulted in overcome the repetition
of reaction, a single step reaction gives the same results as Brodie’s multiple reaction results
[140] After the 60 years of L. Staudenmaier’ work, Hummers and Offeman established an
alternative oxidation technique, they used potassium permanganate and sulfuric acid and
achieved the similar level of oxidation [141]. Although many other methods have been used to
get graphite oxide, however, the above three discussed methods are the primary approaches
for the oxidation of graphite [26].
3.2.2 Materials
The natural graphite powder (450 Um) was obtained from Nacional de Graphite, Brazil.
Potassium permanganate (KMnO4) (analytical grade), hydrogen peroxide (30%), sodium
nitrate (NaNO3) (analytical grade), sulfuric acid and HCl (36%) were bought from Sigma-
Aldrich.
3.2.3 Oxidation of graphite
Graphite was oxidized by three different methods, which are mentioned below.
3.2.3.1 Oxidation of graphite by Hummers method
In this method graphite was oxidized by using modified Hummers’ method. Briefly 1-gram
graphite was added to the 23-ml concentrated sulfuric acid (H2SO4) and stirred for five
minutes. 0.5-gram NaNO3 was slowly added to the mixture. Then the mixture was cool down
to 0oC and 3-gram KMnO4 was slowly introduced to the reaction mixture and stirred for 4
25
hours. 35 ml of 35% hydrogen peroxide H2O2 was added to the solution to dissolve the
unreactive KMnO4 followed by 30 minutes of stirring. Finally, the mixture was centrifuge at
6000 rpm for 45 minutes to get graphite oxide. The obtained graphite oxide was washed with
ultrapure water until the pH was neutral.
3.2.3.2 Preparation of graphene oxide by modified Hummers method
In this method graphite was oxidized by using Hummers’ method, in briefed 1-gram graphite
powder was added to the 23-ml concentrated sulfuric acid (H2SO4) and stirred for five minutes.
Then the mixture was cool down to 0oC and 3-gram potassium permanganate was gradually
introduced to reaction mixture. Then the reaction was continued for 4 hours by magnetic
stirring at 35oC to fully oxide the graphite. 35 ml of 35% H2O2 was added to the solution to
dissolve the unreactive KMnO4 followed by 30 minutes stirring. Finally, the mixture was
centrifuge at 6000 rpm for 45 minutes to get graphite oxide. The obtained graphite oxide was
washed with ultrapure water until the pH was neutral.
3.2.3.3 Oxidation of graphite by Brodie method
An acidic mixture