Temperature-induced reversible distortion along director axis observed for monodomain nematic elastomer of cross-linked main-chain polyester

A nematic elastomer was prepared by cross-linking a main-chain liquid crystalline polyester in the isotropic liquid phase. A polydomain elastomer in the nematic phase 100 degrees C was stretched to it strain of 500% at a rapid rate of 100% min(-1), relaxed to 330% after the release from the grips of the stretcher and transformed to a monodomain sample with an orientation parameter of 0.48. The monodomain nematic elastomer decreased in length in the orientation direction by 45% on heating in the nematic temperature range from 80 to 150 degrees C and recovered on cooling to 80 degrees C without a change in the degree of orientational ordering. This temperature-induced reversible distortion of the nematic elastomer is well explained by the conformational change of the polymer chain including several hairpin foldings, the probability of which is proportional to the Boltzmann factor of hairpin energy.