Theory of polymer gels with liquid crystal solvents

Abstract

We consider networks made of flexible chains (having N monomers between crosslinks) swollen in a nematogenic solvent (a solvent which may become nematic below a certain temperature Tc). 1) Above T c a uniaxial stress t, applied to the network, induces a nematic order Si inside the solvent. This leads to a large increase Δ n of the orientational birefringence. The ratio Δn/ t is independent of N, in agreement with stress-optical measurements performed by Gebhard and Rehage [1]. Δn/t is predicted to increase as T approaches Tco. 2) Below T c, we predict that the gel collapses into a dense state for all values of N. In this state, the volume fraction ϕ of monomers is very large (ϕ ≳ 0.1) and is independent of N. Inside the collapsed gel, the N-I transition temperature is strongly reduced : the nematic phase should often disappear completely. On the other hand, a swollen gel with a nematic solvent could be maintained : a) by suddenly cooling a dilute network (N large) ; b) is the polymer chains themselves are also nematogenic