ELASTICITY OF NEMATIC NETWORKS AND NEMATIC EFFECTS IN CONVENTIONAL RUBBERS

Nematic rubbers are composed of cross-linked polymer chains with stiff rods either incorporated into their backbones or pendant as side chains. When nematic effects are strong, such rubbers exhibit discontinuous stress-strain relationships and spontaneous shape changes. We model such a rubber using Gaussian elasticity theory, including the nematic interaction via a mean field. Results are presented for the cases of uniaxial compression and biaxial extension, and unusual phase transitions are obtained. When nematic effects are very small (i.e., T much greater than T(c), where T(c) is the nematic-isotropic phase transition temperature of the rubber) we postulate that the model is a good approximation to a conventional, nonnematic elastomer and fit our model to data from an isoprene rubber.