SIDE-CHAIN LIQUID-CRYSTAL COPOLYMERS AND ELASTOMERS WITH A NULL COUPLING BETWEEN THE POLYMER BACKBONE AND THE MESOGENIC GROUPS

The phase behaviour of a series of uncrosslinked and crosslinked side- chain liquid-crystal copolymers is reported. These materials show a re versible nematic-isotropic transition. The two parent homopolymers pre pared from the constituent monomers, which differ only in the length o f the coupling chain, exhibit, in one case, a preferential alignment o f the mesogenic units parallel to the polymer backbone (N(III)) and, i n the second case, an alignment of the mesogenic units perpendicular t o the backbone (N(I)). It is shown that it is possible to prepare a ra ndom copolymer, in which the competing influences of these two opposin g couplings lead to materials that exhibit no preferential alignment o f the mesogens with respect to the polymer chain. Such materials exhib it almost zero coupling between the mesogens and the polymer backbone (N0). At this 'null' composition for the elastomer, it is found that t he application of a mechanical field can lead to a transition between an N(III) nematic phase and an N(I) nematic phase. The coupling betwee n the mesogenic side groups and the polymer backbone can be resolved i nto 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 coupli ng, we show that the 'hinge effect is some 2.0 to 1.5 times as effecti ve as the 'nematic'-like interaction between the mesogens and the poly mer chain in determining the nature of the coupling.