PHOTOINDUCED OPTICAL ANISOTROPY IN FILMS OF PHOTOCHROMIC LIQUID-CRYSTALLINE POLYMERS

The light-induced modification of the optical properties of photochrom ic liquid crystalline (side-group) polymers (LCPs) containing azobenze ne moieties was studied. Films of such polymers were irradiated with u npolarized and linearly polarized light. Unpolarized irradiation resul ts in a modification of the order parameter, whereas the director orie ntation remains constant. The light-induced disturbance of the supramo lecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline ph ases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. T his reorientation process is caused by an angular-dependent photoselec tion within the steady state of the photoisomerization of the azobenze ne units. The amount and kinetics of the reorientation differ consider ably as a function of certain structural features of the polymers, suc h as the content of photochromic moieties and the lengths of the space r chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photos election process may cause a reorientation of the non-photochromic moi eties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in o ptical data storage.