Chemistry Reference
In-Depth Information
homogeneous liquid crystalline alignment is caused by polarization of photo-chromophores at the
uppermost surfaces of the substrates due to repeated A/Z photo-isomerizations, similarly to
azobenzenes [
140
-
149
]. This was also shown to take place with stilbenes [
140
]. In addition, it was
demonstrated by them [
149
] that both photo-isomerization and photo-dimerization contribute to liquid
crystalline alignment. Photo-regulation in a polymethacrylate with
o
-cinnamate side chains displays
preferential formation of
-isomer. Dimerization, on the other hand, takes place more favorably in
other polymers, including poly(vinyl cinnamate) [
150
].
The liquid crystals alignment in films prepared from materials with cinnamate group after
irradiating the films with linearly polarized UV light is quite uniform. All the aggregate structures,
lamellar crystals, produced by the photocross-linking reaction were found to be square in shape [
151
].
This has application in flat panel liquid crystal displays. It led others to synthesize polymeric materials
that could be useful in photo-alignment. Lee et al. [
152
] synthesized a soluble photo-reactive
polyimide with cinnamate chromophore side groups. The polymer, poly(3,3
0
-bis(cinnamoyloxy)-
4,4
0
-biphenylene hexafluoroisopropylidene diphthalimide), has a reasonably high molecular weight
and forms good quality films through conventional solution spin-casting and drying.
Z
CF
3
CF
3
O
O
N
N
n
O
O
O
O
O
O
The polymer is thermally stable up to 340
C and positively birefringent. The photochemical reactions
of the polymer in solution and in films, as well as its molecular orientations, are induced by exposure to
linearly polarized ultraviolet light. As one might expect, the cinnamate chromophores undergo both
photo-isomerization and photo-dimerization. Also, exposure toUV light induces anisotropic orientations
of the polymer main chains and of the cinnamate side groups in the films. The irradiated films
homogeneously align nematic liquid crystal molecules along a direction at an angle of 107
with respect
to the polarization of the linearly polarized ultraviolet light. This coincides with the orientation direction
of the polyimide chains. Thus, the liquid crystal alignment process is principally governed in irradiated
polyimide films by the polymer main chains and the unreacted cinnamate side groups [
151
].
Nagata and Hizakae [
153
] reported preparation of a series of photocross-linkable biodegradable
polymers by condensation of dichlorides of 4,4
0
-(adipoyldioxy)dicinnamic acid and alkane diols of
various methylene lengths. They also used various poly(ethylene glycols) with molecular weights
ranging from 200 to 8,300.
Among other interesting polymers with cinnamate functional groups are high polymeric
phosphazenes that bear cinnamate groups [
154
]. A typical polymer synthesis is a follows:
