DIPOLAR RELAXATION MECHANISMS IN THE VITREOUS STATE, IN THE GLASS-TRANSITION REGION AND IN THE MESOPHASE, OF A SIDE-CHAIN POLYSILOXANE LIQUID-CRYSTAL

The dipolar relaxation mechanisms in a side chain liquid crystalline p olysiloxane have been studied by Thermally Stimulated Discharge Curren ts (t.s.d.c.) and by Dielectric Relaxation Spectroscopy (d.r.s.). The study was carried out in a wide temperature range covering the vitreou s phase, the glass transition region and the liquid crystalline phase. Different discharges were observed in the t.s.d.c. spectrum of this p olymer which were attributed, in the order of increasing temperature, to local non-cooperative motions probably involving internal rotations in the spacer and in the alkyl group of the mesogenic moiety, to the Brownian motions of the main chain associated with the glass transitio n and to motions involving reorientations of the components of the dip ole moment of the mesogenic side group in the liquid crystalline phase . The dielectric relaxation spectrum, on the other hand, is dominated by two relaxation processes both of which are above the measured glass transition temperature and shows also a much broader and less intense relaxation below the glass transition temperature which is attributab le to local motions along the side groups. It is emphasized that the c omparison between the d.r.s. and the t.s.d.c. results is not straightf orward and that more research work is needed in order to enable a clea r attribution of the relaxation processes at the molecular level, and an unambiguous interpretation of the results obtained by the two techn iques.