Chemistry Reference
In-Depth Information
10.6.3 Changes in Viscosity and Solubility of Polymeric Solutions
Changes of viscosity in polymeric solutions that are associated with photo-induced conformational
changes of the macromolecules were observed by Lovrien [
214
]. He reported that solutions of a
copolymer of methacrylic acid and
-(2,2
0
-dimethoxyazobenzene)acrylamide exhibit an increase in
specific viscosity when irradiated with UV light. He also observed a decrease in the viscosity of a poly
(methacrylic acid) and chrysophenine solution in water under the influence of UV light [
214
]. This
was followed by various reports of photo-viscosity effects in solutions of azobenzene-based
polymers. Matejka and Dusek [
215
] studied a copolymer of styrene and maleic anhydride with
azobenzene in the side chains. UV light irradiation of a solution of this polymer in 1,4-dioxane
causes a decrease in specific viscosity between 24 and 30% and in tetrahydrofuran between 1 and 8%.
They also observed that this decrease in viscosity is reversible. The magnitude of the effect was found
to be related to the quantity of azobenzene linkages present in the polymer.
Hallensleben and Menzel [
218
] found that irradiation of poly(5-(4-phenylazobenzyl)-
L
-glutamate)
in 1,4-dioxane solution with UV light (
N
l >
470 nm) decreases the viscosity by 9%. Here too this
trans
cis
change in viscosity is accompanied by a
isomerization that was estimated to be 23%. With
additional irradiation by 360 nm UV light, the viscosity decreases an additional 9% and the
isomerization to
to
reaches 89%.
Irie et al. [
217
] synthesized a number of polyamides with azobenzene groups in the backbone. All
the polymers exhibit photo-viscosity effects. In solutions in
cis
N
0
-dimethylacetamide, a 60% reduc-
tion in specific viscosity can be achieved by UV light irradiation (410
N
,
> l >
350 nm). The initial
viscosity is regained by storage in the dark at room temperature for 30 h.
Changes in viscosity of solutions in dimethylsulfoxide of a range of polyureas with azobenzene
groups in the polymer backbone were reported [
216
]. The irradiations were carried out at 35
Cwith
UV light between 410 and 350 nm. It was observed that the intrinsic viscosity is about 40% lower
during UV irradiation than in the dark. Also, toluene solutions of polydimethylsiloxane with
azobenzene residues were shown to exhibit 20% lower viscosity under UV light irradiation than in
the dark [
219
]. This effect was attributed by them to conformational contraction of the polymer chains
due to dipole-dipole interaction between neighboring chromophores [
220
]. This conformational
change might possibly be illustrated as follows:
N
N
N
n
N
UV light
