Side-chain liquid-crystal copolymers and elastomers with a null coupling between the polymer backbone and the mesogenic groups

Abstract

The phase behaviour of a series of uncrosslinked and crosslinked side-chain liquid-crystal copolymers is reported. These materials show a reversible nematic-isotropic transition. The two parent homopolymers prepared from the constituent monomers, which differ only in the length of the coupling chain, exhibit, in one case, a preferential alignment of the mesogenic units parallel to the polymer backbone (N III) and, in the second case, an alignment of the mesogenic units perpendicular to the backbone (N I). It is shown that it is possible to prepare a random copolymer, in which the competing influences of these two opposing couplings lead to materials that exhibit no preferential alignment of the mesogens with respect to the polymer chain. Such materials exhibit almost zero coupling between the mesogens and the polymer backbone (N 0). At this ‘null’ composition for the elastomer, it is found that the application of a mechanical field can lead to a transition between an N III nematic phase and an N I nematic phase. The coupling between the mesogenic side groups and the polymer backbone can be resolved into the influence of the nematic field and a hinge effect arising from the detail of the chemical architecture of the coupling chain. Using these observations and the results of a mean-field model of the coupling, we show that the ‘hinge’ effect is some 2.0 to 1.5 times as effective as the ‘nematic’-like interaction between the mesogens and the polymer chain in determining the nature of the coupling.