Chemistry Reference
In-Depth Information
N
N
N
N
Also, when copolymers of polystyrene and 4-(methacryloyl-amino)azobenzene containing
2.2-6.5% of the latter are irradiated in a cyclohexane solution with 15 flashes of 347 nm of light.
The polymeric chains contract [
221
]. This occurs at a high rate per second as a consequence of
isomerization. At a later stage, several hundred seconds after the flash, there is evidence of polymer
aggregation and precipitation [
221
]. In addition, when azobenzene residues are introduced into the
main chain of poly(dimethylsiloxane), reversible solution viscosity changes can be obtained by
irradiation with ultraviolet light [
221
].
Isomerization from
groups, however, is not the only mechanism that
can affect photo-viscosity change in polymeric solutions. Thus, reversible solution viscosity changes
were also observed [
222
] in solutions of poly(dimethylacrylamide) with pendant triphenylmethane
leucohydroxide in methanol. This can be illustrated as follows:
cis
to
trans
and back of
azo
y
y
x
x
O
O
O
O
NH
2
NH
NH
2
NH
light
heat
N
C
OH
N
C
OH
2
2
So, as shown, the viscosity changes are due to positive charges that form on the pendant groups.
The solubility of a copolymer of styrene in cyclohexane was found to change reversibly upon
ultraviolet light irradiation when the copolymer contains small amounts (~2 mol%) of
spirobenzopyran among the pendant groups [
223
]. This is believed to be due to photo-isomerization
of the pendant spirobenzopyran groups to the polar merocyanine form with the resultant decrease in
polymer-solvent interaction and subsequent precipitation of the higher molecular weight fractions of
the polymer. A copolymer with a high content of spirobenzopyran groups (12.3 mol%) performs as a
negative photo-resist with high contrast [
223
].
10.6.4 Application to Optical Data Storage
Due to possible utilization of photo-induced orientation in polymeric films in optical data storage, this
phenomenon and the quadratic nonlinear optical effects were extensively investigated in the last few
years. It was reported, for instance, that to study photo-isomerization in a polymeric environment,
