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 N-(2,2-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 (l > 470 nm) decreases the viscosity by 9%. Here too this change in viscosity is accompanied by a trans to cis 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 cis 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 N,N0-dimethylacetamide, a 60% reduction in specific viscosity can be achieved by UV light irradiation (410 > λ > 350 nm). The initial viscosity is regained by storage in the dark at room temperature for 30 h. Changes in viscosity of solutions in dimethyl sulfoxide of a range of polyureas with azobenzene groups in the polymer backbone were reported [216]. The irradiations were carried out at 35Cwith 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:

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 cis to trans and back of azo 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:

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].

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مواضيع ذات صلة

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams

Polymers with Pendant Azide Groups

Polymers with Pendant Abietate and Dibenzazepine Groups

Polymers with Pendant Maleimide Groups

Polymers with Pendant Furan Groups

Polymers with Functional Groups Similar to Cinnamates

Polymers with Functional Chalcone Groups