Planar and Homeotropic Alignment of LC Polymers by the Combination of Photoorientation and Self-Organization

Upon irradiation with linearly polarized light a photoorientation process occurs in spin-coated films of polymethacrylates with 4-hexyloxyazobenzene side groups containing para methoxy and trifluoromethoxy tail groups. It results in the induction of an oblate orientational distribution perpendicular to the electric field vector causing an optical in-plane anisotropy up to an order parameter of about 0.52. The annealing of the photoreoriented films above T g results in a prolate homeotropic alignment in the center of the irradiated spot up to a degree of order of 0.78, whereas an amplification of the photoinduced in-plane anisotropy is observed in the interim region to the nonirradiated film area. The development of the in-plane and the out-of-plane components is compared for the photoorientation and the subsequent photoreorientation process in this series of polymers. In both cases, the photogenerated order in the glassy state acts as an initializing force for the thermotropic self-organization resulting in a significant narrowing of an uniaxial orientational distribution. Thus, a photoinduced “command” effect in the bulk of the LC polymers is caused by the combination of both principles of ordering the photoorientation and the liquid crystallinity.