Light induced structures in liquid crystalline side-chain polymers with azobenzene functional groups

We applied ellipsometry to study the distribution of azobenzene fragments i n the films of two types of comblike polymers with azobenzene moieties in t he side chains before and after ultraviolet (UV) light irradiation. The pol ymer with an alkyl chain at the end of the azobenzene fragment forms struct ures with preferred homeotropic alignment of the fragments. Irradiation of this polymer with nonpolarized UV light at normal incidence induces a homeo tropic alignment of azobenzene fragments. Oblique irradiation induces tilte d structures. Polarized UV light irradiation at normal beam incidence induc es biaxial structures with a fanlike distribution of azobenzene fragments a nd preferably out-of-plane alignment. The polymer with polar nitro group at the end of the azobenzene moiety shows preferential in-plane orientation. The degenerate in-plane alignment is retained for normal irradiation with n onpolarized UV light. Excitation with polarized light provides highly order ed in-plane alignment of the azobenzene fragments perpendicular to the UV l ight polarization. Re-orientation of the fragments under a second UV exposu re with orthogonal polarization involves out-of-plane rotation of the fragm ents. (C) 2001 American Institute of Physics.