ELECTROOPTICAL PROPERTIES OF ELECTROOPTICAL PROPERTIES OF LC/POLYMER COMPOSITE MEDIALC/POLYMER COMPOSITE MEDIA

ELECTROOPTICAL PROPERTIES OF ELECTROOPTICAL PROPERTIES OF LC/POLYMER COMPOSITE MEDIALC/POLYMER COMPOSITE MEDIA

• phase separation in LC/polymer blends

• polymer dispersed liquid crystals (PDLC)• polymer-stabilized liquid crystals • phase separated composite films (PSCOF)

Phase separation Phase separation in liquid crystal/polymer blendsin liquid crystal/polymer blends

Phase separation Phase separation in liquid crystal/polymer blendsin liquid crystal/polymer blends

Formation of polymer matrix is the most important aspect of the composite devices.In most cases it bases on the polymerization induced phase separation process.

BABBAABmix TkF lnln

The free energy of mixing for a polymer/LC system is described by two terms:

BBAAABBTk

z 22

1) Flory-Huggins FE of liquid/liquid (de)mixing

For > 2 two minima appear in Fmix(A), which means that demixing takes place.

Demixing can be initiated by:• reducing the temperature of the mixture (TIPS)• increasing the unfavorable interaction energy between the costituents AB .

(for instance by initiating the polymerization process (PIPS)

or by solvent evaporation (SIPS)).

metastablemetastable

UCST

EFFECT OF ORIENTATIONAL ORDERINGEFFECT OF ORIENTATIONAL ORDERINGEFFECT OF ORIENTATIONAL ORDERINGEFFECT OF ORIENTATIONAL ORDERING2) Maier-Saupe FE describing orientational order of the LC constituents.

4ln)(ln)(2 dffkTuSF NNI

nematic order paramer

SN = <((3cos2)-1)/2>

For(u/kT) >4.5 a minimum of FI-N(S) appears at S > 0,

which means that transition to orientationally ordered (nematic) phase takes place.

angular distribution function

)(2

1)1()1ln()1(ln 22 STuSTkF B

POLYMER/LC DEMIXINGPOLYMER/LC DEMIXING POLYMER/LC DEMIXINGPOLYMER/LC DEMIXING

Volume fraction of LC Decrease of entropy due to the alignmentof LC molecules

Different combinations of coexisting phases are possible:• I-I liquid-liquid isotropic phases• I-I-N two isotropic liquids + nematic• I-N isotropic liquid + nematic

Phase diagram for uncured NOA65/E7 mixturesCloud point T versus for different curing times(W52LC/polyester resin mixtures, curing t1<t2<t3)

M. Mucha, Prog. Polym. Sci. 28, 837 (2003).

POLYMER/LC SOLUBILITYPOLYMER/LC SOLUBILITY POLYMER/LC SOLUBILITYPOLYMER/LC SOLUBILITY- Phase separation during polymer solidification is usually not complete, but some LC remains dissolved in the polymer matrix decreasing the polymer Tg. - On the other hand also some polymer constituents remain dissolved in the LC domains, which typically decreases the nematic-isotropic transition temperature TNI.

The amount of demixed LC can be determined by calorimetric methods, IR spectroscopy....

)(1)(

xPm

m

m

PSm

LC

p

LC

LC

)1(

)()(

xH

xHxP

NI

NI x = LC concentration (wt%) in the blend

mLC(PS) = mass of phase separated LCmLC = total mass of LC in the blend

P(x) is determined from calorimetric (enthalpy) measurements.

00

0

0

,1

)(

,0)(

xxx

xxxP

xxxP

Usually linear dependenceof P(x) is found by experiments.

(W52LC mixed with2 different polymers)

M. Mucha, Prog. Polym. Sci. 28, 837 (2003).

POLYMER/LC MORPHOLOGYPOLYMER/LC MORPHOLOGY POLYMER/LC MORPHOLOGYPOLYMER/LC MORPHOLOGY

Two nondestructive ways to study the morphology are optical microscopy, IR imaging,

The final polymer/LC morphology is determined by domain size coarsening process, which is at some point hindered by diffusion restrictions related to the polymer matrix.

Dynamical scaling law for domain size growth: R(t)(t)1/3 (Lifshitz-Slyozov law)

Photomicrographs showing growth of LC domains during TIPS (Photo by S. R. Challa, CWRU)

Infrared (IR) spectroscopy provides information on the relative concentration of various species in a sample and their spatial distribution

(http://plc.cwru.edu/tutorial/enhanced/files/pdlc/prep/prep.htm)

COMPOSITES WITH CROSS-LINKED POLYMERCOMPOSITES WITH CROSS-LINKED POLYMERCOMPOSITES WITH CROSS-LINKED POLYMERCOMPOSITES WITH CROSS-LINKED POLYMERIn case of cross-linked network formation, also the elastic free energy of the polymer network (network elasticity) contributes to the mixing/unmixing properties of the system.

pLCLCLCBmix TkF 0ln Entropy of mixing for cross-linked network = 0

ay

ax

222222

20

1112

3zzyyxx

B aaanL

TkF y ay

x ax

(a) (b) Comparison of theoreticalphase diagrams for(a) NLC/linear polymer and (b) NLC/cross-linked polymer sytem.

D. Nwabunma and T. Kyu, Macromolecules 32, 664 (1999)

Elastic free energy of cross linked network (entropic!!)n = average number of polymer units extending between the two crosslinking points.

EXAMPLE: K21/NOA65 systemEXAMPLE: K21/NOA65 systemEXAMPLE: K21/NOA65 systemEXAMPLE: K21/NOA65 system

D. Nwabunma and T. Kyu, Macromolecules 32, 664 (1999)

90% LC, 10% photo cross-linked polymer

LC/P composite structures are clasified on the basis of the morphological properties.

P

A

Optical polarization microscopy

Polymer dispersed liquid crystals (PDLC)Polymer dispersed liquid crystals (PDLC)Polymer dispersed liquid crystals (PDLC)Polymer dispersed liquid crystals (PDLC)These are composite structures typically formed from mixtures with p>LC.They consist of spatially separated LC droplets embedded in a polymer matrix.

Typical droplet size ~ 1 m. The size and the shape of the droplets strongly depend on material composition and various parameters determining the phase separation process (phase composition of the initial state, curing temperature, curing rate, presence of external stress, EM fields, ...)

Discovered in 1985by J.W. DoaneKent, USA

+

-U

Side view of a PDLC cell

STRUCTURE OF NEMATIC DIRECTOR FIELDSTRUCTURE OF NEMATIC DIRECTOR FIELDSTRUCTURE OF NEMATIC DIRECTOR FIELDSTRUCTURE OF NEMATIC DIRECTOR FIELD

n(r) = nematic director field = ?^

LC orientational structure in the „cavity“depends on:• Anchoring properties at the LC/polymer interface• Size and shape of the cavity • Elastic constants of the LC• Temperature (with respect to the TNI)• Presence of external fields

Radial structureHomeotropic strong anchoring

Axial structureHomeotropic weak anchoring

Bipolar structurePlanar (tangential)strong anchoring

R. Ondris-Crawford et al., J. Appl. Phys. 69, 6381 (1991)

F=Fel+Fsurf+Ffield=min

Boojum-type singularity

Central defect

Equatorialdisclinationline

optical polarization microscopy

LIGHT SCATTERING FROM PDLCLIGHT SCATTERING FROM PDLCLIGHT SCATTERING FROM PDLCLIGHT SCATTERING FROM PDLCPractically all PDLC applications base on electric-field induced modification of light scattering properties (switchable windows, display devices, shutters...)

Light scattering from PDLC depends on:• Mismatch between the refractive index of the polymer and the LC (scattering intensity)• Size and shape of the LC domains (Rayleigh, Rayleigh-Gans, Mie scattering,...) • Configuration of nematic director field within the domains (Intradroplet interference)• Density of the LC domains (Interdroplet interference effects, multiple scattering...)

r

ef

ikr

),( k'kEs , kr>>1

k

k‘

dVr‘

scattered radiation in the far field

p

LC

'),( kr0Er'kE ieRayleigh-Gans approximation

Scattering from a single droplet S. Zumer and J. W. Doane, Phys. Rev. A 34, 3373 (1986)

Example: radial structureDifferential cross section

dVef rik

V

''),()(4

1),( r'kEεr'εk'k'k'k PLC

EXAMPLEEXAMPLE of SWITCHABLE STRUCTURE of SWITCHABLE STRUCTUREEXAMPLEEXAMPLE of SWITCHABLE STRUCTURE of SWITCHABLE STRUCTUREPDLC with bipolar droplets

(a) No Electric FieldRandom orientation of bipolar axesStrong scattering

(b) Applied External Electric FieldAligned configuration of bipolar axesLow scattering (especially if np = no,LC)

(http://plc.cwru.edu/tutorial/enhanced/files/pdlc/prep/prep.htm)

SwitchLite Privacy Glass™

Typical transmittance versus voltage dependenceE202/NOA 65 compositeR. Karapinar, 1998

Voltage (V)

Tra

nsm

itta

nce

(%)

SWITCHING FIELD and SWITCHING TIMESWITCHING FIELD and SWITCHING TIMESWITCHING FIELD and SWITCHING TIMESWITCHING FIELD and SWITCHING TIMEThe switching process is governed by competition between: a) elastic torque, b) field torque, c) viscous torqueand by boundary conditions at the LC/polymer interface

Basic parameters, which affect the switching properties of a single LC droplet:• dielectric anisotropy of the LC (10)• surface anchoring of at the LC-polymer interface (Ws10-6-10-4 Jm-2) boundary conditions!!

• size and shape of the LC domain (ellipsoid: a,b500 nm, axial ratio I=(a/b)1-3)• orientation of the LC within the domain (structure of n(r))• viscoelastic constants of the LC (K10 pN, 0.01 kgm-1s-1)

a

b

E

Threshold field

)1(1 2IK

bEc

Switching-on time (E>>Ec) 2)( Eon

Switching-off time)1( 2

2

IK

boff

Specific example: ellipsoidal droplet with axial configuration and n0E : 0

0

B. G. Wu at al., Liquid Crystals 5, 1453 (1989)

~ 1 V/m

~ 1 ms

REAL SWITCHING DYNAMICSREAL SWITCHING DYNAMICSREAL SWITCHING DYNAMICSREAL SWITCHING DYNAMICS

100 Hz/8 V switching voltage

Tranmittance of 2 m thick PDLC cell R. Karapinar, 1998

In reality the relaxation (switching-off) processtakes place over several decades and exhibitscomplex (glass-like) dynamic features.(reasons: shape anomaly, interface roughness, interconnected domains,

motion of ionic impurities, coupling between LC and elastic network,...)

I. Drevenšek-Olenik et al., to appear in J. Appl. Phys. (2006).

Polymer-stabilized liquid crystalsPolymer-stabilized liquid crystalsPolymer-stabilized liquid crystalsPolymer-stabilized liquid crystals(known also as: polymer network LCs, polymer-filled LCs, LC gels)

These are composite structures typically formed from mixtures with LC>>p.They consist of polymer voids forming a 3D network within the continuous LC phase.

SEM image of a typical PDLC60 wt% polymer, 40 wt% LCM. Čopič and A. Mertelj, Phys. Rev. Lett. 80, 1449 (1998) SEM image of a typical polymer-filled LC

20 wt% polymer, 80 wt% LCM. Mucha, Prog. Polym. Sci. 28, 837 (2003).

Polymer network provides mechanical stability of the LC structure !!

SWITCHABLE WINDOWS from PSWITCHABLE WINDOWS from PSSLCLCSWITCHABLE WINDOWS from PSWITCHABLE WINDOWS from PSSLCLCCompetition between surface constraints at substrates, polymer network/LC interface interaction and field induced torque.

Two types of PSLCStructures: Polymer fibrils (a) and polymer walls (b).

(a) (b)

RMS Voltage (V) RMS Voltage (V)

Tra

nsm

itta

nce

(%)

Tra

nsm

itta

nce

(%)

T(U) for 5 (squares), 7 (circles), and 10 (crosses) volume % fraction of the polymer, d = 6 m. R. A. M. Hikmet and H. M. J. Boots, Phys. Rev. E 51, 5824 (1995).

LC acrylates + non-reactive LC

SOME OTHER APPLICATIONSSOME OTHER APPLICATIONSSOME OTHER APPLICATIONSSOME OTHER APPLICATIONSElectrically switchable mirrors, Fresnel lenses, ...

Voltage Off Voltage On

Broad-band selective reflection from polymer stabilised cholesteric structure (=p(ne-no))

In usual cholesteric LC cells surface alignment layers are unable to restore the homogeneous helical configuration after the switching, so polymer network is used to increase the restoring force (in-situ photopolymerization = patterned gels).

UV curable LC acrylates + non-reactive LCSelective reflection at different applied voltages.

Tra

nsm

itta

nce

(%)

wavelength (nm)

R. A. M. Hikmet and H. Kemperman, Nature 329, 1998

Phase separated composite films (PSCOF) Phase separated composite films (PSCOF) Phase separated composite films (PSCOF) Phase separated composite films (PSCOF) These is a very thin layer of phase separated LC in contact with a polymer layer.They are obtain by photopolymerization using optical wavelengths in the region of absorption band of the liquid crystal, which produces intensity gradient along the z axis.

The rate of phase separation is most important factor determining the final structure!

+

-U

Cell filled with mixture of LC/polymer precursor (T>>TNI).z

I(z)

Polymer +

-U

Liquid crystal

Photopolymerization takes place in the region of higher UV intensity. LC is accu-mulated in the region of low UV intensity.

Rubbed PI layer

For instance NOA 65 + E7 (60/40)

Q. Wang et al., 2004

EXAMPLEEXAMPLE OF A DEVICE based on PSCOF OF A DEVICE based on PSCOFEXAMPLEEXAMPLE OF A DEVICE based on PSCOF OF A DEVICE based on PSCOF

General advantages of PSCOF: Easy and precise control of LC thickness (sub ms response time with sub micrometer films), mechanical ruggedness, flexibility, plastic substrates can be used, only one alignment layer is needed (convenient for TFT technoogy)...

Microlens with E-switchable focus

Q. Wang, at al., 2004 (group of S. Kumar)

f=1cm, 2R~200 m

These kinds of lens-arrays can be addressed similar way as LCDs(promising for applications in robotics...)

SOME OTHER COMPOSITE STRUCTURESSOME OTHER COMPOSITE STRUCTURESSOME OTHER COMPOSITE STRUCTURESSOME OTHER COMPOSITE STRUCTURES

• LCs in colloid templated polymer cavities• Liquid crystal/polymer membranes• Liquid crystal/polymer/azobenzene composites• Surface relief gratings with LCs• Liquid crystal/liquid crystal polymer blends...

The number of different LC/P composite architectures is limited only by imagination!!

Texture of LC/polymer mixture