Chemistry Reference
In-Depth Information
This polyimide is claimed to be thermally stable up to 340
C and has a glass transition temperature
of 181
C. Also, it was demonstrated that the cinnamate chromophores, upon irradiation with linearly
polarized ultraviolet light, undergo both photo-isomerization and dimerization. In addition, the light
exposure induces anisotropic orientation of the polymer main chains and of the cinnamate side groups
in the film. The irradiated films align homogeneously the nematic liquid crystal molecules along one
direction at an angle of 107
with respect to the polarization. The liquid crystalline alignment was
found to be thermally stable up to 200
C.
It was also reported [
225
] that photo-reactivity of side-chain liquid crystalline polymers can align
liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreac-
tion of the polymers. Presumably, this particular polymer can multiphoto-align the liquid crystal
pattern without a change of the direction of the linearly polarized UV light. The chemical structure of
such an aligning polymer is depicted as follows:
O
OO
O
6
n
O
p
n ΒΌ
where
2or6.
It was concluded [
226
], therefore, that the liquid crystals align both parallel and perpendicular to the
incident
direction on the photocross-linked polymer film by changing the degree of the reacted
cinnamoyl group. That can be controlled by irradiation time. A bias-tilt angle between the liquid
crystals director and the substrate is also realized by controlling the irradiation angle of the light [
226
].
Another approach to liquid crystalline alignment
E
azo
compounds in polymeric materials or as part of the polymer structure [
227
]. In recent years,
investigation of the use of azobenzene-containing polymers for liquid crystalline alignment became
quite thorough because of the potential application in holographic storage as well as optical and
photonic use [
228
-
230
]. The photo-alignment of liquid crystalline polymers containing azobenzene
groups has an advantage of local variation of the orientation order due to pixel-wise irradiation. This
is a process that is reported to take place via angular-dependent excitation, a series of
is based on photo-isomerization of
photo-
isomerization cycles, and rotational diffusion within the steady state of the photoreaction. This results
in the photochromic side group becoming oriented perpendicularly to the electric field vector of the
incident light and establishing an oblate order in the films.
Thus, studies of exposure of films of liquid crystalline polymers with azobenzene side chains to
linearly polarized light of 436 nm [
231
] show successive occurrences of uniaxial in-plane
orientations, followed by out-of-plane orientations of azobenzenes [
231
]. Two kinds of orientation
modes were observed. These are possibly extreme cases, when linearly polarized light with the
electric vector parallel to the
cis-trans
-direction. One is the uniaxial in-plane
orientation of the azobenzene with a dipole moment parallel to the
xz
-plane comes along the
z
x
x-
y
axis from the
to
-direction
z
and the other is out-of-plane (homeotropic) one toward the
-direction. Marked dependence of photo-
orientation processes on film temperatures was observed. In-plane orientation was generated in the
glassy state. Photo-orientation at higher temperatures, slightly below the transition temperature
between smectic and nematic phases, gives rise to distinct transformations from in-plane orientation
at the early state to successive out-of-plane reorientations [
232
]. These orientations can be illustrated
as follows:
