Transcript No Slide Title
ELECTROOPTICAL PROPERTIES OF LC/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 in 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.
The free energy of mixing for a polymer/LC system is described by two terms: 1) Flory-Huggins FE of liquid/liquid (de)mixing
F mix
k B T
A
ln
A
B
ln
B
A
B
z
2
k B T
2
AB
AA
BB
UCST For > 2 two minima appear in F mix (
A
), which means that demixing takes place.
metastable
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 )
).
EFFECT OF ORIENTATIONAL ORDERING
2) Maier-Saupe FE describing orientational order of the LC constituents.
F I
N
uS
2
N
kT
f
( ) ln
f
( )
d
ln 4
nematic order paramer
S N
= < ((3cos 2 )
-
1)
/2
>
angular distribution function For (
u/kT
) >4.5 a minimum of
F I-N
(
S
) appears at S > 0, which means that transition to orientationally ordered (nematic) phase takes place.
POLYMER/LC DEMIXING
F
k B T
ln ( 1 ) ln( 1 ) ( 1 ) 1 2 2
uS
2
T
(
S
) Volume fraction of LC
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 Decrease of entropy due to the alignment of LC molecules Phase diagram for uncured NOA65/E7 mixtures Cloud point
T
versus for different curing times (W52LC/polyester resin mixtures, curing t 1 M. Mucha, Prog. Polym. Sci. 28, 837 (2003). - Phase separation during polymer solidification is usually not complete, but some LC remains dissolved in the polymer matrix decreasing the polymer T g . - On the other hand also some polymer constituents remain dissolved in the LC domains, which typically decreases the nematic-isotropic transition temperature T NI . The amount of demixed LC can be determined by calorimetric methods, IR spectroscopy.... m LC ( PS ) m LC 1 m p m LC P ( x ) P ( x ) H NI H NI ( x ( x ) 1 ) x P ( m m LC LC ( PS ) = mass of phase separated LC = total mass of LC in the blend = LC concentration (wt%) in the blend x ) is determined from calorimetric (enthalpy) measurements. P ( x ) 0 , x x 0 P ( x ) x 1 x x 0 0 , x x 0 (W52LC mixed with 2 different polymers) Usually linear dependence of P ( x ) is found by experiments. M. Mucha, Prog. Polym. Sci. 28, 837 (2003). 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) Two nondestructive ways to study the morphology are optical microscopy, IR imaging, 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) a y In 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. F mix k B T LC ln LC 0 LC p Entropy of mixing for cross-linked network = 0 a x x a x y a y F 3 k B T 2 nL 2 0 1 x 2 a x 2 1 y 2 a 2 y 1 z 2 a z 2 Elastic free energy of cross linked network ( entropic!! ) n = average number of polymer units extending between the two crosslinking points. (a) (b) Comparison of theoretical phase diagrams for (a) NLC/linear polymer and (b) NLC/cross-linked polymer sytem. D. Nwabunma and T. Kyu, Macromolecules 32, 664 (1999) 90% LC, 10% photo cross-linked polymer Optical polarization microscopy P A D. Nwabunma and T. Kyu, Macromolecules 32, 664 (1999) LC/P composite structures are clasified on the basis of the morphological properties. These are composite structures typically formed from mixtures with p > LC . They consist of spatially separated LC droplets embedded in a polymer matrix. U + Side view of a PDLC cell Discovered in 1985 by J.W. Doane Kent, USA 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, ... ) 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 T NI ) • Presence of external fields n ( r ) = nematic director field = ? F=F el +F surf +F field = min Boojum-type singularity Central defect Radial structure Homeotropic strong anchoring Axial structure Homeotropic weak anchoring Equatorial disclination line Bipolar structure Planar (tangential) strong anchoring optical polarization microscopy R. Ondris-Crawford et al., J. Appl. Phys. 69, 6381 (1991) Practically 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 ... ) f E s ( k , k' ) f 1 4 ( k e ikr , k' ) k' k' r , kr ε LC ( r' >>1 ) ε P V Rayleigh-Gans approximation scattered radiation in the far field E ( k , r' ) e ik ' r ' dV Example : radial structure Differential cross section E ( k , r' ) E 0 e i kr ' LC k‘ k r‘ dV p Scattering from a single droplet S. Zumer and J. W. Doane, Phys. Rev. A 34 , 3373 (1986) PDLC with bipolar droplets (http://plc.cwru.edu/tutorial/enhanced/files/pdlc/prep/prep.htm) (a) No Electric Field Random orientation of bipolar axes Strong scattering (b) Applied External Electric Field Aligned configuration of bipolar axes Low scattering (especially if n p = n o,LC ) SwitchLite Privacy Glass™ Voltage (V) Typical transmittance versus voltage dependence E202/NOA 65 composite R. Karapinar, 1998 The switching process is governed by competition between: a) elastic torque, b) field torque, c) viscous torque and 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 ( • size and shape of the LC domain ( ellipsoid: a,b W s 10 -6 -10 -4 Jm -2 ) boundary conditions!! 500 nm, axial ratio I=(a/b) 1-3) • orientation of the LC within the domain (structure of n(r) ) • viscoelastic constants of the LC ( K 10 pN, 0.01 kgm -1 s -1 ) Specific example: ellipsoidal droplet with axial configuration and n 0 E : E a Threshold field E c 1 b 2 K 0 ( I 1 ) Switching-on time (E>>E c ) ~ 1 V/ m on ( ) E 2 0 Switching-off time off b 2 K ( I 2 1 ) B. G. Wu at al., Liquid Crystals 5, 1453 (1989) ~ 1 ms 100 Hz/8 V switching voltage Tranmittance of 2 m thick PDLC cell R. Karapinar, 1998 In reality the relaxation (switching-off) process takes place over several decades and exhibits complex (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). (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 PDLC 60 wt% polymer, 40 wt% LC M. Čopič and A. Mertelj, Phys. Rev. Lett. 80, 1449 (1998) SEM image of a typical polymer-filled LC 20 wt% polymer, 80 wt% LC M. Mucha, Prog. Polym. Sci. 28, 837 (2003). Polymer network provides mechanical stability of the LC structure !! Competition between surface constraints at substrates , polymer network/LC interface interaction and field induced torque. (a) (b) Two types of PSLC Structures: Polymer fibrils (a) and polymer walls (b) . LC acrylates + non-reactive LC RMS Voltage (V) RMS Voltage (V) 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). Electrically switchable mirrors, Fresnel lenses, ... Voltage Off Broad-band selective reflection from polymer stabilised cholesteric structure ( =p(n e -n o )) 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 LC Voltage On wavelength (nm) Selective reflection at different applied voltages. R. A. M. Hikmet and H. Kemperman, Nature 329, 1998 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. I ( z ) z Cell filled with mixture of LC/polymer precursor (T>>T For instance NOA 65 + E7 (60/40) U + NI ). Polymer Liquid crystal U + - Rubbed PI layer Photopolymerization takes place in the region of higher UV intensity. LC is accu mulated in the region of low UV intensity. The rate of phase separation is most important factor determining the final structure ! Q. Wang et al., 2004 Q. Wang, at al., 2004 (group of S. Kumar) f =1cm, 2 R ~200 m These kinds of lens-arrays can be addressed similar way as LCDs (promising for applications in robotics...) 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 )... • 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 ... Texture of LC/polymer mixture The number of different LC/P composite architectures is limited only by imagination!!POLYMER/LC SOLUBILITY
POLYMER/LC MORPHOLOGY
COMPOSITES WITH CROSS-LINKED POLYMER
EXAMPLE: K21/NOA65 system
Polymer dispersed liquid crystals (PDLC)
STRUCTURE OF NEMATIC DIRECTOR FIELD
LIGHT SCATTERING FROM PDLC
EXAMPLE of SWITCHABLE STRUCTURE
SWITCHING FIELD and SWITCHING TIME
REAL SWITCHING DYNAMICS
Polymer-stabilized liquid crystals
SWITCHABLE WINDOWS from PSLC
SOME OTHER APPLICATIONS
Phase separated composite films (PSCOF)
EXAMPLE OF A DEVICE based on PSCOF Microlens with E-switchable focus
SOME OTHER COMPOSITE STRUCTURES