Role of network nematicity in swelling and phase equilibria of polymer networks in nematic solvents

Swelling and phase equilibria of polymer networks in a low molecular mass liquid crystal (LC) have been investigated as a function of network nematicity. LC networks with varying nematicity were prepared by copolymerization of mixtures of mesogenic monomers and non-mesomorphic (styrene) monomers with various compositions. Molar fraction of mesogenic monomer (x) in copolymer network strongly influences the swelling behavior as well as the nematic–isotropic (N–I) transitions in both dry and swollen states. The swollen networks of sufficiently high x with strong nematicity exhibit a sharp N–I transition and simultaneously undergo a discontinuous change in gel volume, i.e. volume phase transition. Meanwhile, the swollen networks of x≤0.8 with less nematicity show a broad N–I transition, and the resulting volume change proceeds continuously over a finite temperature range. When x decreases further down to less than 0.5, the nematicity of the dry networks vanishes. The nematic ordering in the swollen copolymer networks of x<0.5 occurs at the same temperature as the N–I transition temperature of the pure nematic solvent (TNIS), which yields the inflection in the swelling-temperature curve. A mean field theory considering network nematicity as a variable describes the effects of x on volume phase transition such as a shift of TNIG and a change in the magnitude of volume transition, apart from the emergence of continuous volume transition due to the broad N–I transition. The purely isotropic network of x=0 appreciably swells in the nematic solvent. The solvent inside the gel forms the nematic phase at the temperatures below TNIS*(=TNIS−2 °C), which conflicts with the classical theoretical prediction that nematic ordering of LC solvent in fully swollen isotropic network never occurs.