Liquid crystalline elastomers: the relationship between macroscopic behaviour and the level of backbone anisotropy

Nematic monodomain liquid crystalline elastomers have been prepared through in situ cross-linking of an acrylate based side-chain Liquid crystalline p olymer in a magnetic field. At the nematic-isotropic transition, the sample is found to undergo an anisotropic shape change. There is found to be an i ncrease in dimensions perpendicular - and a decrease parallel - to the dire ctor, this is consistent with alignment of the polymer backbone parallel to the direction of mesogen alignment in the nematic state. From a quantitati ve investigation of this behaviour, we estimate the level of backbone aniso tropy for the elastomer. As second measure of the backbone anisotropy, the monodomain sample was physically extended. We have investigated, in particu lar, the situation where a monodomain sample is deformed with the angle bet ween the director and the extension direction approaching 90 degrees. The b ehaviour on extension of these acrylate samples is related to alternative t heoretical interpretations and the backbone anisotropy determined. Comparis on of the chain anisotropy derived from these two approaches and the value obtained from previous small-angle neutron scattering measurements on deute rium labelled mixtures of the same polymer shows that some level of chain a nisotropy is retained in the isotropic or more strictly weakly paranematic state of the elastomer. The origin and implications of this behaviour are d iscussed. (C) 2001 Elsevier Science Ltd. All rights reserved.