Synthesis and physical properties of liquid-crystalline copolymers with azobenzene segments

To investigate the effects of photoisomerizable azobenzene segments on the liquid-crystalline characteristics and thermal properties of polymers, a series of liquid-crystalline homopolymers and copolymers with azobenzene segments was synthesized. The azobenzene contents of the copolymers were estimated with elemental analysis. The photoisomerization of the azobenzene derivatives was studied with ultraviolet-visible (UV-vis) spectroscopy. The UV-vis absorption of the copolymers was found to be parallel with the content of the azobenzene segments. UV irradiation was found to cause a decrease in the copolymer transmittance around 355 nm due to the photoinduced isomerization from entgegen (E) to zusammen (Z). The phase-transition temperatures and molecular weights of the polymers were investigated with differential scanning calorimetry and gel permeation chromatography, respectively. The variation in the phase-transition temperature of the homopolymers before and after UV (365 nm) irradiation was investigated. The bended Z structure was found to disturb the order of the orientation of liquid crystals and to lower the phase-transition temperature. The appearance of the polymer film was changed from opaque to clear after sufficient UV irradiation. The image recording of the polymer films was achieved after UV irradiation through a mask with pictures. The stability and reliability of the Nematic-Isotropic phase transition of the homopolymers was evaluated with repeated cycles of 365-nm UV irradiation and heating at 130°C. After the recycle phase transition was repeated nine times, no significant decay in the response and transmittance could be found.