We have studied the polar alignment produced in a range of comb polymers with a methacrylate backbone and oxynitrostilbene mesogenic groups attached to the backbone at variable densities by a 3 or 6 carbon spacer. These polymers, when poled, display second order non-linear optical activity, piezoelectricity and pyroelectricity. The magnitude of these effects depends upon the degree of polar ordering of the side groups and we have therefore investigated the efficiency of the poling process using the pyroelectric coefficient and thermally stimulated currents as primary diagnostic techniques. For constant poling conditions it is found that the polarization and pyroelectric response are not linearly related to the volume fraction of mesogen, i.e. samples of high mesogen concentration display lower polarization and pyroelectric activity than might be expected from extrapolation of low concentration data. Indeed, for the 6 carbon spacer material there would appear to be an optimum mesogen concentration around 30 mol%. Also, though the 3 and 6 carbon spacer materials are similar at low mesogen concentration, the latter material gives a lower response when it forms a liquid crystalline phase at high concentrations. We therefore suggest that the polar mesogens tend to pack anti-parallel at higher concentrations and that this interaction is not readily overcome by the applied field. Though these polymers are brittle at high mesogen concentration, ductile thin films can be cast if the density of side chains is reduced to l0 mol%. Despite the reduction in mesogen concentration, the pyroelectric activity is not proportionally reduced, implying more effective poling due to reduced interaction between the mesogens.
Посилання на статтю:
The alignment of polar mesogens in electroactive polymers / P. L. Carr, G. R, Davies and I. M. Ward* // Polymer. – 1993. – Vol 34. – P. 05/08/2017 01:00:00.