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-(methacryloyloxy)hexyloxy]-4-cyanobiphenyl were cast as thin films. When the polymer films with cis-azobenzenes were subjected to irradiation with linearly polarized Light at 436 nm, biaxiality of the azobenzene moieties was induced during the in-plane alignment process; when they were exposed to unpolarized light at 436 nm, three-dimensional manipulation of the polymer liquid crystals was brought about. The three-dimensional alignment behavior was examined by UV, FTIR, and polarizing optical microscopy techniques and by measuring the change in transmittance through crossed polarizers. It was found that the azobenzene moieties were aligned along the propagation direction of the irradiation light through repetition of trans-cis-trans isomerization. The induced anisotropy was large (an approximate order parameter as 0.36), reversible, and stable (more than 5 months at room temperature). Nonphotoactive mesogens, cyanobiphenyl, underwent three-dimensional reorientation efficiently together with the azobenzene moieties due to the cooperative motion.