Chemistry Reference
In-Depth Information
Table 10.2
Effectiveness of sensitizers on relative speed of cross-linking of poly(vinyl cinnamate)
Sensitizer
Relative speed
Sensitizer
Relative speed
(None)
1
4-Nitroaniline
100
Naphthalene
3
3-Nitrofluorene
113
Benzanthrone
7
4-Nitromethylaniline
137
Phenanthrene
14
4-Nitrobiphenyl
200
Crysene
18
Picramide
400
Benzophenone
20
4-Nitro-2,6-dichlorodimethylaniline
561
Anthrone
31
Michler's ketone
640
5-Nitroacenaphthene
84
N
-Acyl-4-nitro-1-nathylamine
1,100
From ref. [
131
] and other sources
It was also demonstrated that greater photosensitivity can be obtained by separating the cinnamic
group from the polymer backbone by introducing -CH
2
-CH
2
-O- spacers as follows [
129
,
130
]:
n
O
2
O
O
In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic
structures in the excited states of poly(vinyl cinnamate) [
131
,
132
]. They based their calculations
on the reaction of intermolecular concerted cycloaddition that take place according to the
Woodward-Hoffmann's rule. This means that the cyclobutane ring formation takes place if a nodal
plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest
occupied MO (HOMO) of the ground state cinnamoyloxy group. This is within the picture of Huckel
MO or Extended Huckel MO theory. The conclusion is that the concerted cycloadditions occur
favorably in the lowest triplet state T
1
and in the second excited singlet state S
2
[
132
].
The effectiveness of photosensitizers in accelerating the cross-linking reaction of poly(vinyl
cinnamate) is illustrated in Table
10.2
.
Some 4 decades after the original development of poly(vinyl cinnamate) into a useful photocross-
linkable polymer, a novel optical property of the polymer was observed. When the material is
irradiated with linearly polarized light, it exhibits polarization holography [
133
,
134
]. The exposure
of thin films of poly(vinyl cinnamate) to linearly polarized ultraviolet light causes uniaxial reorienta-
tion into liquid crystal layers [
135
-
142
]. Poly(vinyl cinnamate) and its derivatives have the ability to
align in thin films the liquid crystal moieties in the direction that is perpendicular to the polarization
axis of the linearly polarized ultraviolet light [
143
-
145
]. Schadt et al. [
136
] suggested that the surface-
settled homogeneous alignment of nematic liquid crystals results from photo-dimerizations of the
cinnamate moieties and formation of cyclobutane rings (as shown earlier) with an azimuthally oriented
order. This, he feels, determines the direction of the liquid crystal alignment [
140
]. Ichimura et al.
[
141
] suggested a different photo-alignment process. They claimed that
the photo-induced
