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Light-Controlled Alignment of Cholesteric Liquid
Crystals on Photosensitive Materials
O. Kurochkin
E. Ouskova
Yu. Reznikov
Yu. Kurioz
O. Tereshchenko
R. Vovk
Institute of Physics, National Academy of Sciences of Ukraine,
Kyiv, Ukraine
D.-H. Kim
S.-K. Park
S.-B. Kwon
School of Display Engineering, Hoseo University, Asan City,
Chungnam, Korea
We report on investigation of photoalignment of cholesteric liquid crystal on phe-
nylone-based photosensitive material as aligning layer. We found that reflective
and electro-optical characteristics of cholesteric cells strongly depended on the
dose of irradiation of a photoaligning layer with polarized UV light. Irradiation
resulted in orientation of initially chaotically oriented planar domains in a direc-
tion determined with incident UV-light polarization and in narrowing of the
dependence of light scattering by a planar structure on the scattering angle. The
long enough exposure resulted in scattering and electro-optical characteristics
not worse than obtained with standard rubbing technology. The photosensitive
orientant possesses evident advantages of the effective control of cholesteric
textures.
Keywords: cholesteric liquid crystals; electro-optical switching; photoalignment
The authors are very thankful to V. Reshetnyak for useful discussions. The work was
partially by 21st Century Frontier R&D Program, ‘‘Next Generation Display Technology
Development’’ funded by the Ministry of Science and Technology of Korea.
Address correspondence to O. Kurochkin, Institute of Physics, National Academy of
Sciences of Ukraine, Pr. Nauki 46, Kyiv, 03028, Ukraine. E-mail: ouskova@iop.kiev.ua
Mol. Cryst. Liq. Cryst., Vol. 453, pp. 333–341, 2006
Copyright #Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600653878
333
1. INTRODUCTION
Uniform orientation and reproducible pretilt of the liquid crystal (LC)
director on aligning surfaces are required for application and oper-
ation of LC. Mechanical rubbing of the aligning polymer surfaces is
one of the traditional methods of LCs alignment [1]. Last decade,
Gibbon et al., Reznikov et al., and Schadt et al. proposed alternative,
photoaligning technique [2–5]. This technique uses polarized light to
induce anisotropy in a photosensitive aligning layer. The light-induced
anisotropy of the irradiated layer causes appearance of the easy orien-
tation axis ~
ee of LC director on a photosensitive surface.
The majority of studies of photoalignment of LCs and development of
photoalignment technology for mass production were concentrated
on alignment of nematic LCs. As concerns other LC phases, studies of
photoalignment of smectics was started just recently [6] and to our
knowledge, there were no systematic studies of photoalignment of
cholesteric LC (ChLC) carried out till now. At the same time, appli-
cation of photoalignment technology to ChLC looks extremely promis-
ing since a possibility to control the direction of the easy axis and the
anchoring energy on the surface allows, in turn, controlling scattering
and electro-optical characteristics of the cholesteric textures. Here we
report first investigation of photoalignment of commercial cholesteric
mixtures on photosensitive polymer materials.
2. EXPERIMENTS AND DISCUSSION
We investigated photoalignment of cholesteric liquid crystal mixture
BL-118 (clear point T
c
¼84C) from Merck on several photoaligning
materials (fluorinated polyvinyl-cinnmate, cellulose-cinnamate and
phenylone-based polymer). The polymer solution in appropriate solvent
was spin-coated on a glass substrate to produce polymer films. In parti-
cular, the phenylon-based polymer was dissolved in dimethylephorma-
mide (weight concentration was 15 g=l), and the solution was spin-coated
on a glass substrate covered with ITO at 7000rpm speed. After spin-
coating, films were cured at 120C for 1.5 hour to remove the solvent
and improve the mechanical properties of the films. The obtained films
were uniform and isotropic. The aligning films of fluorinated polyvinyl-
cinnamate and cellulose-cinnamate were produced analogously.
To induce anisotropy photosensitive films were exposed with line-
arly polarized UV light from a Hg-lamp at normal incidence to the film
surface. A water filter was applied to cut the IR-part of the lamp
irradiation. The intensity of UV IUV in the plane of the polymer film
was 110 mW=cm
2
.
334 O. Kurochkin et al.
The alignment of the cholesteric liquid crystal was tested in the
parallel combined cells consisted of a reference and a tested surfaces
and ChLC in between. The reference surface was coated with rubbed
polyimide layer. The tested surface was covered with the studied poly-
mer film and irradiated with different UV exposure dose. Calibrated
polymer spacers set the cell thickness at L¼5.5 mm. The cell was filled
at 85C and slowly cooled down to a room temperature.
The quality of the alignment, depending on the exposure dose, was
characterized by measuring the dependence of the intensity of scatter-
ing Iscat of the polarized beam of a YAG-laser (k¼532 nm) from the
tested cell on the scattering angle h(Fig. 1). For measurements the cell
faced by a tested surface was set perpendicular to the laser beam, and
the photodiode was rotated by a step-motor around the cell with a
radius 30 cm. To prevent a parasite scattering and reflection the back
reference substrate was covered by a black tape, and a lock-in modu-
lation technique was used to collect the data.
Qualitative observations of the cholesteric structures in polarized
microscope shown that irradiation of the polymer with UV light
resulted in orientation of initially chaotically planar-oriented domains
in a direction determined with the incident UV light polarization.
Increase of the exposure results in improvement of the domain order-
ing. For phenylone-based polymer the irradiation with the exposure
t
exp
>20 min resulted in a high-quality mirror reflection in a green
part of visible spectrum. Irradiation of the other studied polymers
brought similar results but the quality of the photoalignment was
FIGURE 1 Dependence of the intensity of scattering Iscat from the planar
cholesteric texture on the scattering angle hat irradiation of phenylone-based
polymer surface with different exposure times t
exp
.0
-angle corresponds to
the scattering being normal to the cell. The experimental points around
0-angle are not showed because of a large scale of the plot.
Cholesteric Liquid Crystals on Photosensitive Materials 335
not so impressive. Therefore, below we focus attention on the results
obtained for phenylone-based material.
For quantitative characterization of photoalignment on phenylone-
based surface we introduced angular selectivity of the planar texture
determined as a width of the angular distribution Iscat (Fig. 1) at
1=40 of the distribution maximum. The dependence of the angular sel-
ectivity of the planar photoaligned cholesteric structure on the
exposure is depicted in Figure 2.
The data for standard rubbed polyimide cell are also presented in
the figure. One can see that increase of the exposure time t
exp
, that
is equivalent to increase of the irradiation dose D¼It, results in an
essential contraction of the angular selectivity approaching to one of
the texture on the rubbed polyimide surface at t
exp
>20 min.
The quality of the alignment of the planar ChCL cell is mainly
determined with an angular distribution of LC director on the aligning
surface. It is reasonable to suggest that orientation ability of a ChLC
mixture consisted of a nematic matrix and a chiral dopant is close to
the orientation ability of nematic matrix. Therefore, we investigated
alignment of nematic LC, which was a matrix of ChLC BL-118, on
UV-exposed phenylone-based polymer surface, non-irradiated polymer
surface, and rubbed polyimide surface. The experiments wereperformed
with light transmission polarimetry method, which is based on analysis
of the polarization characteristics of light passed through the cell.
FIGURE 2 The angular selectivity hdependence of cholesteric texture
aligned with phenylone-based polymer surface on the UV light irradiation
dose D. Dashed line — angular selectivity of the cholesteric planar structure
aligned with rubbed polyimide surface. The planar texture angular selectivity
was determined as a width of the angular distribution Iscat (Fig. 1) at 1=40 of
the distribution maximum.
336 O. Kurochkin et al.
The experimental set-up consisted of consecutive elements: He-Ne
laser (k¼633 nm), polarizer, collimating lenses, tested liquid crystal
cell, quarter-wave plate, analyzer, zooming lens, CCD camera, and a
computer. Linear polarized laser beam passed through the tested pla-
nar symmetrical cell, which both substrates were covered with the
same aligning layers, (cell thickness –50 mm). The polarization vector
was set parallel to the director on the aligning surfaces. The light
passed through the cell was detected with CCD camera and digitized
at different orientation of polarizers and quarter-wave plate. Measure-
ments of the Stokes parameters S
i
[7,8] of the transmitted light in
each pixel of the CCD camera finally allowed obtaining a spatial distri-
bution of orientation of a big axis of elliptically polarized transmitted
light, and an ellipticity:
w¼0:5 arc tan S2
S2
;
b
a¼tan 0:5arc sin S3
S0
;
where wis the azimuth angle between the big axis of the polarization
ellipse of transmitted light and polarization vector of incident light, b–
small axis of polarization ellipse, and a– big axis of polarization ellipse.
By this way we obtained a ‘‘map’’ of polarization state of the light
after the cell. The spatial resolution of the map, 4 mm, was determined
by a CCD resolution and the software. The accuracy of the measure-
ments of azimuth angle wwas 0.7, and the ellipticity b=awas determ-
ined with the accuracy 0.008).
Spatial distribution of the azimuth angle and ellipticity of light polar-
ization are showed on Figure 3. Standard deviation rof polarization
parameters of transmitted light from the mean values wðdÞand b=aðdÞ
characterizes a quality of the LC orientation on the studied surfaces.
The values rw
ph ¼0:047, rb=a
ph ¼0:1 were obtained for non-irradiated
phenylone-based material, and rw
ph;UV ¼0:017, rb=a
ph;UV ¼0:011 we
determined for UV-irradiated phenylone-based polymer. For compari-
son, the rubbed polyimide surface gave the values rw
PI ¼0:014,
rb=a
PI ¼0:022. One can see that the spatial standard deviation for non-
irradiated surface is much bigger than for UV-irradiated surface, which
standard deviation value approaches to one of rubbed polyimide surface.
To clear up an affect of photoalignment on the electro-optical
switching of the ChLC cell we studied the dependencies of planar-
homeotropic transition of cholesteric mixture on the exposure time.
Ac-voltage (50 Hz) of different values Uwas applied to the planar cell
during 1 s. After 3 s the voltage was switched off and the reflection
Cholesteric Liquid Crystals on Photosensitive Materials 337
intensity Iref was measured. Application of 45 V for 1 s recovers the initial
planar structure of ChLC cell and the next measurement cycle could be
carried out. We found that at t
exp
>10 min the transitions virtually did
not differ from the characteristics of the rubbed-aligned planar struc-
tures. Shorter exposure times resulted in decrease of the textures’ reflec-
tivity but the driving voltages characteristics of planar – focal-conic –
homeotropic texture transition remained constant (Fig. 4).
We found that irradiation of phenylone-based layer with unpolar-
ized UV light also influenced on characteristics of cholesteric textures.
FIGURE 3 Spatial dependences of azimuth angle w(a) and ellipticity b=a(b)
of light transmitted through symmetrical planar cell filled with nematic
matrix of BL-118. Aligning layers of the cells were rubbed polyimide (), irra-
diated (&) and non-irradiated (þ) phenylone-based polymer. d– location in the
laser spot.
FIGURE 4 Dependencies of the intensity of the light reflection Iref from the
cholesteric structure aligned with phenylone-based polymer surface on applied
voltage U.
338 O. Kurochkin et al.
The polarizing microscopy images of planar textures of ChLC cell,
which different areas of aligning layer were non-irradiated and
irradiated with polarized and unpolarized light during t
exp
¼30 min
at IUV ¼110 mW=cm
2
, are presented in Figure 5. One can see that
the area irradiated with unpolarized light looks brighter than non-
irradiated area and polarization of UV irradiation improves the reflec-
tivity of the textures drastically.
The characteristics of the light scattering from the non-irradiated
textures and textures produced by irradiation with polarized and
unpolarized UV light are presented in Figure 6. One can see that
FIGURE 5 Planar cholesteric textures on phenylone-based polymer aligning
surface: (1) – non-irradiated area; (2) – area irradiated with polarized light,
t
exp
¼30 min, IUV ¼110 mW=cm
2
; (3) – non-irradiated area; (4) – irradiated
with unpolarized light area, t
exp
¼30 min, IUV ¼110 mW=cm
2
. Width of the
photos is 0.9 mm.
FIGURE 6 Dependencies of light scattering intensity Iscat from the cholesteric
texture aligned with phenylone-based polymer surface on the scattering angle h.
Cholesteric Liquid Crystals on Photosensitive Materials 339
the angle dependencies of reflection for non-irradiated surface and
for the surface irradiated with unpolarized light are virtually the same
at the reflection angle h>2, and the angle range of the reflection
are much wider then for polarized irradiation in both cases. At the
same time, the scattering at the small angles 2<h<2are different
for unpolarized irradiation and ‘‘no’’ -irradiation cases. For instance, at
h¼1:5the ratio of the scattering intensity for different treatment
I1:I2:I3¼1:1:8:122 (where I1;I2;I3are the scattering intensity in the
case of ‘‘no’’ -irradiation, irradiation with unpolarized light and
irradiation with polarized light, correspondingly).
Angle dependence of scattering in a planar texture of ChLC is
determined mostly by the angular dependence of the axes of choles-
teric spirals in domains and by the size distribution of cholesteric
domains. We did not found essential changes in the domains sizes
on the polymer surface after irradiation. Therefore, the difference in
the scattering is determined mostly by the difference in the angular
distribution of the orientation of the cholesteric spirals. Decrease of
the pretilt angle of the director on the polymer surface after
irradiation might cause the observed constriction of the angular
dependence of scattering, but the pretilt angle of the nematic compo-
nent of cholesteric mixture was found to be zero for both surfaces.
We believe that the difference in angular scattering dependence is
caused by changes in efficiency of adsorption of LC molecules after
irradiation of the polymer surface. It is a layer of adsorbed LC mole-
cules that plays a role of the aligning layer stabilizing the distribution
of cholesteric spirals in a LC cell. Irradiation of the polymer is sup-
posed to encourage a formation of this adsorbed layer, which promotes
a planar alignment of the director. It should result in increase of the
anchoring energy of ChLC that, in turn, encourages planar orientation
of the spirals and constriction of the angular scattering dependence.
Obtained results showed that photoalignment technology can be
successfully applied to align cholesteric liquid crystals. Strong depen-
dence of the width of the angular selectivity of the planar textures on
the exposure dose allows controlling the reflectivity of the cholesteric
cells effectively. Low-temperature photoalignment process makes this
technology very promising for plastic LCD applications.
3. CONCLUSIONS
Our results showed that photoalignment technology can be success-
fully applied to align cholesteric liquid crystals. Reflective and electro-
optical characteristics of cholesteric cells strongly depended on UV
exposure dose that allowed controlling the cholesteric cells reflectivity
340 O. Kurochkin et al.
effectively. The optimized UV polymer treatment allowed obtaining the
same quality of alignment as the one provided by standard polyimide
orientants. At the same time, the photosensitive orientant possesses
evident advantage of the effective control of cholesteric textures.
Low-temperature photoalignment process makes it very promising for
plastic LCD applications. A possibility of fabrication of multi-domain
cholesteric structures revealing improved scattering characteristics
was demonstrated.
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