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, we postulate that the model is a good approximation to a conventional, nonnematic elastomer and fit our model to data from an isoprene rubber.