Dipolar relaxation mechanisms in the vitreous state, in the glass transition region and in the mesophase, of a side chain polysiloxane liquid crystal

Abstract

The dipolar relaxation mechanisms in a side chain liquid crystalline polysiloxane have been studied by Thermally Stimulated Discharge Currents (t.s.d.c.) and by Dielectric Relaxation Spectroscopy (d.r.s.). The study was carried out in a wide temperature range covering the vitreous phase, the glass transition region and the liquid crystalline phase. Different discharges were observed in the t.s.d.c. spectrum of this polymer 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 transition and to motions involving reorientations of the components of the dipole 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 attributable to local motions along the side groups. It is emphasized that the comparison between the d.r.s. and the t.s.d.c. results is not straightforward and that more research work is needed in order to enable a clear attribution of the relaxation processes at the molecular level, and an unambiguous interpretation of the results obtained by the two techniques.