Thermal Isomerization of Azobenzene Side Chain Copolymers

Irradiation of azobenzene induces isomerization between its trans- and cis forms. By introduction of azobenzene chromophores in polymers, it is possible to optically control a variety of polymer properties. The isomerization of azobenzene can be used to influence conformational, optical or surface properties of the poly mer material. The performance is dependent on the individual properties of the chromophore and the polymer, and on how the chromophore is attached to or otherwise dispersed in the polymer. The project has focused on how the polymer chain segments around the chromophore interacts or interferes with its isomerization reaction. It is well known that the isomerization of azobenzene chromophores in solid polymers is highly influenced by the free volume and mobility characteristics of the polymer environment. In order to study this, a number of poly(methyl methacrylate) copolymers with azobenzene chromophores in the side chain were synthe sized. The yield of the trans-cis photoisomerization and the reaction kinetics of the cis-trans thermal back- reaction of the copolymers were investigated. The copolymers differed with respect to the length of the side chain, the concentration of azobenzene side chains and the type of substituents on the azobenzene chromophores. In addition, the influence of physical ageing and external plasticization of the copolymers were investigated. The rate of the cis-trans thermal isomerization of the azo copolymers in solution was found to depend on the substituents on the azobenzene molecule. The isomerization kinetics in solid state was shown to be influenced by the glass transition temperature of the polymer, by the length of the azobenzene side chain and by the thermal history of the polymer. The isomerization proceeded with a distribution of rate constants that was shown to depend on the mechanical relaxation times of the surrounding polymer chain segments. Plasticization of the polymers resulted in narrower distribution of rate constants. The distribution of rate constants seemed to have a low-rate tail that was more pronounced for azobenzenes that were attached more closely to the polymer backbone. Physical ageing of the copolymers had only a small influence on the thermal isomerization that depended on the length of the azobenzene side chain