Multistage Crosslinking of Nematic Networks

Abstract

Nematic polymer melts, cross-linked in their isotropic phase, typically form polydomain nematic rubbers and gels. Such elastomers, if strained and then further cross-linked, form nematic monodomain elastomers with interesting mechanical and thermal properties. We calculate the elastic free energy of a network which has undergone multiple cross-linking and obtain the surprising result that it is identical in form to that for a sample once cross-linked from an oriented nematic monodomain. Gaussian elasticity thus predicts that it is impossible to permanently imprint complex structure (including, for instance, biaxiality) into a nematic elastomer by imposing a complex sequence of strains interspersed by cross-linking stages. A multistage cross-linked network should therefore exhibit the same behavior as a single-stage system, including recently predicted and observed nematic transitions in response to imposed strains.