STRUCTURAL AND CONFORMATIONAL TRANSITIONS IN THERMOTROPIC POLYMERS WITH 3,3'-AZOBIPHENYL-CONTAINING MESOGENIC GROUPS IN THE MAIN-CHAIN

Conformational transitions of trans-cis and cis-trans type in 3,3'-sub stituted azobiphenyl units of mesogenic fragments incorporated into th e main chain of thermotropic polymers were studied using UV and IR spe ctroscopy. It was demonstrated that the fraction of cis isomer, which is formed when these polymers are UV-irradiated at a temperature below the glass transition, is much higher (in the photostationary state) t han in the polymers containing para-substituted azobiphenyl units in t he main chain. As compared to the isotropic melt, the activation energ y for thermal isomerization of azobiphenyl fragment in the LC state va ries in such a way that the equilibrium shifts towards higher content of the trans isomer, which is the most asymmetric. Specific features c ontrolling the formation of LC state in polymers with 3,3'-substituted azobiphenyl units in the main chain are discussed.