ELECTROOPTICAL%20PROPERTIES%20OF%20LC/POLYMER%20COMPOSITE%20MEDIA

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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)

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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
UCST

• For?? gt 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)).

metastable
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EFFECT OF ORIENTATIONAL ORDERING
2) Maier-Saupe FE describing orientational order
of the LC constituents.
angular distribution function
nematic order paramer SN lt((3cos2?)-1)/2gt
For?(u/kT) gt4.5 a minimum of ?FI-N(S) appears at
S gt 0, which means that transition to
orientationally ordered (nematic) phase takes
place.
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POLYMER/LC DEMIXING
Volume fraction of LC
Decrease of entropy due to the alignment of 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

Cloud point T versus ? for different curing
times (W52LC/polyester resin mixtures, curing
t1ltt2ltt3)
Phase diagram for uncured NOA65/E7 mixtures
M. Mucha, Prog. Polym. Sci. 28, 837 (2003).
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POLYMER/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....
mLC(PS) mass of phase separated LC mLC total
mass of LC in the blend
x LC concentration (wt) in the blend
P(x) is determined from calorimetric (enthalpy)
measurements.
(W52LC mixed with 2 different polymers)
Usually linear dependence of P(x) is found by
experiments.
M. Mucha, Prog. Polym. Sci. 28, 837 (2003).
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POLYMER/LC MORPHOLOGY
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)
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COMPOSITES WITH CROSS-LINKED POLYMER
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.
Entropy of mixing for cross-linked network 0
?y ay
ay
Elastic free energy of cross linked network
(entropic!!) n average number of polymer units
extending between the two crosslinking points.
ax
?x ax
(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)
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EXAMPLE K21/NOA65 system
Optical polarization microscopy
P
90 LC, 10 photo cross-linked polymer
A
D. Nwabunma and T. Kyu, Macromolecules 32, 664
(1999)
LC/P composite structures are clasified on the
basis of the morphological properties.
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Polymer dispersed liquid crystals (PDLC)
These are composite structures typically formed
from mixtures with ?pgt?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, ...)
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STRUCTURE OF NEMATIC DIRECTOR FIELD

• LC orientational structure in the cavitydepends
  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

n(r) nematic director field ?
FFelFsurfFfieldmin
Boojum-type singularity
Equatorial disclination line
Bipolar structure Planar (tangential) strong
anchoring
Central defect
Radial structure Homeotropic strong anchoring
Axial structure Homeotropic weak anchoring
R. Ondris-Crawford et al., J. Appl. Phys. 69,
6381 (1991)
optical polarization microscopy
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LIGHT SCATTERING FROM PDLC
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...)

, krgtgt1
scattered radiation in the far field
Example radial structure Differential cross
section
Rayleigh-Gans approximation
k
?LC
k
dV
r
?p
Scattering from a single droplet
S. Zumer and J. W. Doane, Phys. Rev. A 34, 3373
(1986)
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EXAMPLE of SWITCHABLE STRUCTURE
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 np no,LC)
Transmittance ()
Typical transmittance versus voltage
dependence E202/NOA 65 composite R. Karapinar,
1998
Voltage (V)
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SWITCHING FIELD and SWITCHING TIME
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
  (Ws?10-6-10-4 Jm-2) boundary conditions!!
• size and shape of the LC domain (ellipsoid
  a,b?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-1s-1)

Specific example ellipsoidal droplet with axial
configuration and n0??E
Threshold field
?
1 V/?m
?0
E
Switching-on time (EgtgtEc)
?
?0
b
?
Switching-off time
1 ms
a
B. G. Wu at al., Liquid Crystals 5, 1453 (1989)
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REAL SWITCHING DYNAMICS
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).
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Polymer-stabilized liquid crystals
(known also as polymer network LCs,
polymer-filled LCs, LC gels)
These are composite structures typically formed
from mixtures with ?LCgtgt?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. Copic 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 !!
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SWITCHABLE WINDOWS from PSLC
Competition between surface constraints at
substrates, polymer network/LC interface
interaction and field induced torque.
(b)
(a)
Two types of PSLC Structures Polymer fibrils
(a) and polymer walls (b).
LC acrylates non-reactive LC
Transmittance ()
Transmittance ()
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).
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SOME OTHER APPLICATIONS
Electrically 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).
Transmittance ()
wavelength (nm)
UV curable LC acrylates non-reactive LC
Selective reflection at different applied
voltages.
R. A. M. Hikmet and H. Kemperman, Nature 329, 1998
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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.
I(z)
Polymer

U

U
-
Liquid crystal
-
Rubbed PI layer
Photopolymerization takes place in the region of
higher UV intensity. LC is accu-mulated in the
region of low UV intensity.
Cell filled with mixture of LC/polymer precursor
(TgtgtTNI).
z
For instance NOA 65 E7 (60/40)
The rate of phase separation is most important
factor determining the final structure!
Q. Wang et al., 2004
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EXAMPLE OF A DEVICE based on PSCOF
Microlens with E-switchable focus
Q. Wang, at al., 2004 (group of S. Kumar)
f1cm, 2R200
?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)...
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SOME 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
• ...

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