Relaxation behaviour of electrically induced polar orientation and of optically induced non-polar orientation in an azo-chromophore side group polymer

Abstract

We have investigated the relaxation of the polar order (P1cos) in a poled styrene-maleic anhydride copolymer with side groups of modified dispersed red chromophores - using pyroelectric studies - as well as the relaxation of the orientational order (P2cos) of the optical axes of the side groups previously induced by polarized light - using holographic techniques. Isomerization cycles of the chromophores cause in the latter case the formation of an optical grating. The relaxation processes were monitored within the glassy state. In addition we investigated molecular relaxation processes, in particular the glass relaxation, using dielectric techniques. The observation was that the relaxations of the pyroelectric response and of the holographic grating couple to the glass relaxation yet do not display identical kinetics within the solid glassy state, showing that the optical grating is less stable. The difference arises obviously from the excess free volume induced by the isomerization cycle in the optical storage experiment. The formation of this excess free volume is apparent from the formation of a surface relief pattern in the storage experiment. A straightforward conclusion is that the stability of the grating can be considerably enhanced if chromophores are used characterized by a significantly lower equilibrium concentration of cis-species during irradiation.