The elastic anisotropy of nematic elastomers

We examine the robustness of order in nematic elastomers under mechanical s trains imposed along and perpendicularly to the director when director rota tion is prohibited. In contrast to electric and magnetic fields applied to conventional nematics, mechanical fields are shown theoretically and experi mentally to greatly affect the degree of nematic order and related quantiti es. Unlike in liquid nematics, one can impose fields perpendicular to the d irector, thereby inducing biaxial order which should be susceptible to expe rimental detection. Nematic elastomers with unchanging: director and degree of order should theoretically have the same elastic moduli for longitudina l and transverse extensions. This is violated when nematic order is permitt ed to relax in response to strains. Near the transition we predict the long itudinal modulus to be smaller than the transverse modulus, at lower temper atures the converse is true, with a crossover a few degrees below the trans ition. The differences are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain single-crystal nematic polymer networks, performed DSC, X-ray, birefringenc e, and thermo-mechanical characterisations, and then obtained linear moduli from stress-strain measurements.