Influence of cross-linking density on volume phase transition of liquid crystalline gels in a nematogenic solvent

The effect of cross-linking density (Cx) on volume phase transition of two liquid crystalline (LC) networks comprising dissimilar mesogens in a low molar mass LC has been investigated. Cross-linking density alters the modulus and nematicity of LC network each of which directly influence one of the three governing forces (elastic force, nematic interaction, isotropic mixing interaction) for the equilibrium swelling: An increment in Cx increases the network modulus, while it reduces the nematic-isotropic transition temperature of the pure (dry) LC network (TNIN). The LC networks in the nematogen are discontinuously transformed from the swollen state into the shrunken state, as a result of a single nematic phase formation inside the gel at TNIG. It has been found that as Cx increases, the magnitude of discontinuous volume change at TNIG decreases; TNIG shifts to higher temperatures, which is opposite to the Cx dependence of TNIN. As an extreme case of high cross-linking density, the LC networks with Cx=10 mol % in the nematic solvent exhibits the nematic–isotropic transition but do not undergo appreciable volume transition. The experimental swelling–temperature curves are compared with the prediction of a mean field theory for nematic gel. The theory successfully describes the effect of cross-linking density on the swelling and phase behavior observed.