Negative Poisson's ratio and semi-soft elasticity of smectic-C liquid crystal elastomers

摘要

Models of smectic-C liquid crystal elastomers predict that it can displaysoft elasticity, in which the shape of the elastomer changes at no energy cost.The amplitude of the soft mode and the accompanying shears are dependent on theorientation of the layer normal and the director with respect to the stretchaxis. We demonstrate that in some geometries the director is forced to rotateperpendicular to the stretch axis, causing lateral expansion of the sample; anegative Poisson's ratio. Current models do not include the effect ofimperfections that must be present in the physical sample. We investigate theeffect of a simple model of these imperfections on the soft modes in monodomainsmectic-C elastomers in a variety of geometries. When stretching parallel tothe layer normal (with imposed strain) the elastomer has a negative stiffnessonce the director starts to rotate. We show that this is a result of thenegative Poisson's ratio in this geometry through a simple scalar model.