Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side chain. 6. Biaxiality and three-dimensional reorientation

A liquid-crystalline polymer of 6-[4-(4-ethylphenyl)diazenylphenyloxy]hexyl methacrylate and copolymer of the azobenzene monomer with 4'-[6-(methacryl oyloxy)hexyloxy]-4-cyanobiphenyl were cast as thin films. When the polymer films with cis-azobenzenes were subjected to irradiation with linearly pola rized Light at 436 nm, biaxiality of the azobenzene moieties was induced du ring the in-plane alignment process; when they were exposed to unpolarized light at 436 nm, three-dimensional manipulation of the polymer liquid cryst als was brought about. The three-dimensional alignment behavior was examine d by UV, FTIR, and polarizing optical microscopy techniques and by measurin g the change in transmittance through crossed polarizers. It was found that the azobenzene moieties were aligned along the propagation direction of th e irradiation light through repetition of trans-cis-trans isomerization. Th e induced anisotropy was large (an approximate order parameter as 0.36), re versible, and stable (more than 5 months at room temperature). Nonphotoacti ve mesogens, cyanobiphenyl, underwent three-dimensional reorientation effic iently together with the azobenzene moieties due to the cooperative motion.