Photoresponsive polymers: An investigation of their photoinduced temperature changes during photoviscosity measurements

Photoresponsive polyelectrolyte of polyacrylic acid with spiropyran in the side chain was synthesized by reacting polyacryloyl chloride with 1′-(2-hydroxyethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] and by subsequent hydrolysis of acid chloride. The conformation of this material was photomanipulated and the associated changes in viscosity were studied by using a custom-modified rheometer. It was observed that the specific viscosity of the spiropyran-based polymer solution increased by about 12% upon UV irradiation and decreased due to visible irradiation. The viscosity changes were found to be reversible. The photoinduced heating effects during in situ photoviscosity measurements on two classes of materials, the polyelectrolyte and a series of azobenzene-based polymers, were also measured using a modified rheometer cone. The origin and magnitude of the heating effect were identified, and the associated viscosity changes of the solutions were decoupled to obtain a more accurate view of the photoviscosity behaviour. The magnitude of the photoinduced heating effect was found to be linearly dependent on chromophore molecules joined to the polymer chains. The photoinduced heating effect accounted for varying amounts of the observed change in specific viscosity ranging from as little as 2.5% for a highly photoresponsive solution to all of the observed specific viscosity change measured for non-photoresponsive systems.