Page 187 - Photoreactive Organic Thin Films

MIKHAiLV. KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

We conclude this section with the remark that high-transparent, optically
isotropic, chiral and photosensitive IsoSm* copolymers represent novel and
versatile media for holographic grating recording, allowing UV sensibilization,
recording of both intensity and phase gratings, and holographic development
of hidden sensibilization patterns.

5.5 PHOTOINDUCED ALIGNMENT OF LOW MOLAR MASS LIQUID CRYSTALS

In recent years, many studies have been devoted to the photoinduced align-
ment of nematic liquid crystals by photosensitive polymers. Advantages of
this technique for optoelectronic technology are quite evident in comparison
with such traditional methods as mechanical buffing of an orienting polymer
layer: It is a "clean" technique that may be applied not only to flat substrates,
but also to curved ones or even those not accessible to mechanical contact at
all. In addition, buffing is irreversible, but photoinduced liquid crystal align-
ment may be changed optically. It may also be combined with buffing.
The most popular photoalignment technique is based on photo-crosslink-
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ing various poly(vinylcinnamates), PVC. ' The initially optically isotropic
PVC layer, which provides a degenerate planar alignment of a nematic liquid
crystal (NLC), becomes optically anisotropic due to a lack of chromophores
in a direction selected by polarized UV light and orients the NLC perpendicu-
lar to the light electric vector. The same alignment occurs when rod-like azo
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dyes are reoriented by light action. On the other hand, NLC alignment
parallel to the light electric vector may be achieved using coumarin dye
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layers. ' This case is more interesting because, for reasons of symmetry, it: is
possible to produce an orientation of nematics with an arbitrary tilt angle
using an oblique incidence of the exciting light. In both cases mentioned in
this paragraph, only two different orientations of a liquid crystal are possible:
degenerate planar (initial), and uniaxial either along or perpendicular to the
light vector.
We tested the azo dye IsoSm* copolymer, KW40, as a photo-orienting
layer for low molar mass nematic liquid crystals MBBA, 5CB, and E7. The
studies were done using hybrid liquid crystal cells made of 1 mm thick ITO
covered glass plates transparent in the range of X > 350nm. Hybrid cells are
the most convenient cells for anchoring energy determination because, in the
high field limit, the field-induced distortion is easily modelled by the semi-
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infinite medium. The homeotropic orientation was made using a surfactant
chromium distearyl chloride (chromolane). On the opposite plates, spin-
coated films of KW40 of different thicknesses (100 to 600 nm) were deposited
from a 1 to 2% cyclohexanon solution of the copolymer. The films were
irradiated by normally incident polarized UV or visible light before filling the
cell with a liquid crystal.
The initial texture of a hybrid cell filled with E7 is seen outside the dark
spot in Figure 5.23. It is a typical texture of a hybrid cell with degenerate
planar anchoring. Such texture was also observed for MBBA and 5CB. After
strong irradiation with UV light (either polarized or unpolarized; it does not
matter) corresponding to the end of the curve shown in Figure 5.1 OB, a