Light-induced orientation of liquid crystalline terpolymers containing azobenzene and dye moieties

Films of liquid crystalline copoly(methacrylate)s with photochromic azobenzene and benzanilide side groups and related terpolymers with additional dye side groups were oriented by the irradiation with linearly polarized visible or, alternatively, UV light. The orientational behaviors of the terpolymers with benzoxazole, stilbene, and anthracene moieties were compared. The irradiation conditions for the photoorientation process were optimized concerning wavelength and dose with respect to the absorption of the dyes and their limited photostability. Caused by the cooperativity of the photoorientation process, the light-induced ordering of the azobenzene group is connected with the alignment of the nonphotochromic co-mesogenic and the dye side groups even below the glass transition temperature. The light-induced orientational order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures within the mesophase of the co- and terpolymers. Amplification factors up to 30 were found in this series of polymers. The photoinduction process and the subsequent amplification by thermotropic self-organization were investigated in dependence of the polymer composition, the irradiation dose, and the wavelength of the incident light. The required dose or the irradiation time was significantly reduced by the optimization of light-induced and thermal processing, respectively. In this way, dichroic polymer films containing different types of dyes were created.