Chemistry Reference
In-Depth Information
From the behavior of the mean absorbance, it was concluded that all the
azo
chromophores
revert to the
form on completion of a thermal back reaction. The observed increase in the
dichroic ratio over the first 25 h is believed to be due to the thermal back isomerization and not due
to the relaxation of the induced orientation [
225
]. Heating polymers at 170
C for 1 h fails to erase
the green light-induced dichroism in the samples. This dichroism is, however, completely erased on
heating the samples above their
trans
T
g
for 10 min. Irradiation of the films with incident light gives
holograms [
226
].
10.6.5 Liquid Crystalline Alignment
The relationship of glass transitions to mobility and isomerization in confined polymer systems
aroused much interest. It was influenced by need for alignment in liquid crystalline flat panel displays,
because in these displays films of polyimides are widely used. The surfaces are usually treated to
produce uniform alignment of the liquid crystals into suitable “pretilt” angles. The treatments
consisted of rubbing process with velvet fabrics. Search for new methods, however, led to develop-
ment of molecular structures that undergo alignment upon irradiation with linearly polarized UV light
[
222
,
223
]. Polymer-stabilized liquid crystals are low-molar-mass liquid crystal. Their bulk align-
ment or their texture is stabilized by a polymer network. Such polymer network is usually in low
concentration [
222
,
223
]. Several types of polarized-light-induced liquid crystalline aligning of
molecules were reported in the literature [
223
].
One photo-alignment material is poly(vinyl cinnamate). The polymer and its copolymers were
reported to have the ability to align in thin films in the direction perpendicular to the axis of the
linearly polarized ultraviolet light. This photo-alignment mechanism has not been fully elucidated at
present. A drawback to using poly(vinyl cinnamate) and its copolymers is a low glass transition
temperature. As a result, they remain mobile after treatment and chain orientation. Other materials
with higher
T
g
are, therefore, needed. Among the most promising ones are polyimides. They form
liquid crystal alignment layers in flat panel displays and possess good optical transparencies,
adhesion, heat resistance, and dimensional stability, and are good insulators.
There are various reports in the literature about preparations of soluble photo-reactive polyimides
with cinnamate chromophore side groups. Thus, it was reported by Lee et al. [
224
] that they prepared
a photo-reactive polyimide with cinnamate chromophores side groups:
F
3
C
CF
3
O
O
N
N
n
O
O
O
O
O
O
