Spatial reorientation of azobenzene side groups of a liquid crystalline polymer induced by linearly polarized light

The photoinduced 3D orientational structures in films of a liquid crystalli ne polyester, containing azobenzene side groups, are studied both experimen tally and theoretically. By using the null ellipsometry and the UV/Vis abso rption spectroscopy, the preferential in-plane alignment of the azobenzene fragments and in-plane reorientation under irradiation with linearly polari zed UV light are established. The uniaxial and biaxial orientational order of the azobenzene chromophores are detected. The biaxiality is observed in the intermediate stages of irradiation, whereas the uniaxial structure is m aintained in the photosaturated state of the photo-orientation process. The components of the order parameter tensor of the azobenzene fragments are e stimated for the initial state and after different doses of irradiation. Th e proposed theory takes into account biaxiality of the induced structures. Numerical analysis of the master equations for the order parameter tensor i s found to yield the results that are in good agreement with the experiment al dependencies of the order parameter components on the illumination time.