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

We have investigated the relaxation of the polar order (P-1[cos theta]) in a poled styrene-maleic anhydride copolymer with side groups of modified dis persed red chromophores-using pyroelectric studies-as well as the relaxatio n of the orientational order (P-2[cos theta]) of the optical axes of the si de groups previously induced by polarized light-using holographic technique s. Isomerization cycles of the chromophores cause in the latter case the fo rmation of an optical grating. The relaxation processes were monitored with in the glassy state. In addition we investigated molecular relaxation proce sses, in particular the glass relaxation, using dielectric techniques. The observation was that the relaxations of the pyroelectric response and of th e holographic grating couple to the glass relaxation yet do not display ide ntical kinetics within the solid glassy state, showing that the optical gra ting is less stable. The difference arises obviously from the excess free v olume 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 conc lusion is that the stability of the grating can be considerably enhanced if chromophores are used characterized by a significantly lower equilibrium c oncentration of cis-species during irradiation.