UV-tunable upper critical solution temperature behavior of azobenzene containing poly(methyl methacrylate) in aqueous ethanol

A series of azobenzene containing copolymers were synthesized by post-modification of poly(methyl methacrylate-co-pentafluorophenyl methacrylate) with an amine-functionalized azobenzene. Light- and thermo-responsive behavior of these copolymers was investigated in ethanol-water solvent mixtures with various amounts of ethanol. The upper critical solution temperature (UCST) of the polymer solutions, resulting from the poly(methyl methacrylate), was found to be highly tunable by the substitution degree of the copolymers as well as the ethanol content of the solvent mixture. In addition, the copolymers are light responsive based on the cis-trans isomerization of the azobenzene group under UV irradiation. The cloud point temperatures (T_CP) of the polymer solutions decreased after UV-irradiation due to the higher dipole moment of the cis-isomer of the azobenzene moiety leading to better solubility in ethanol-water solvent mixtures. Furthermore, the UV responsiveness was found to strongly depend on the solvent composition, revealing a higher decrease in cloud point after UV-irradiation in ethanol-water solvent mixture with higher water content. To the best of our knowledge, this is the first reported dual responsive polymer that combines light responsiveness with UCST behavior in aqueous solvent mixtures.