Orientation of nematic elastomers and gels by electric fields

(Received 28 October 1993, received in final form 7 January 1994)

Abstract
Solid liquid crystals, formed by crosslinking polymeric nematics into elastomers, display novel and complex elasticity. The internal nematic direction experiences a barrier to its rotation which couples it to the deformations of standard elasticity. Electric fields acting on anisotropic chains induce orientational torques, which compete with rubber elastic effects. Outcome structures crucially depend on the mechanical constraints applied to the sample. In setups with no or few constraints, an electric field rotates the nematic director without resistance, inducing also a spontaneous shape change of a rubber or gel matrix. When certain strains are prevented in the sample by external constraints, the magnitude of the elastic barrier is much higher than the electric contribution and a very high electric field is required to create an observable director rotation. In weakly anisotropic elastomers, for instance conventional rubbers which have been strained during crosslinking, the characteristic field will be considerably lower. Experimental observations on nematic gels support our predictions.