Phase behavior of mixtures of low molar mass nematic liquid crystal and in situ photo-cross-linked polymer network

Phase equilibria and mesophase ordering behavior of mixtures of low molar mass nematic liquid crystal (LC) and photo-cross-linked polymeric network have been investigated theoretically and experimentally. The nematic LC used was a single component, viz., 4-n-heptyl-4'-cyanobiphenyl (K21). The polymer network was formed by photo-cross-linking multifunctional thiolene-based optical adhesive (NOA65). On the basis of a simple addition of the free energy densities of isotropic mixing and nematic ordering along with the elastic free energy density of the network, phase diagrams have been established by solving the total free energy equation self-consistently. The effects of average repeat units between cross-linked points, network functionality, and concentration dependence of network models on the phase diagrams of LC/polymer network were examined. The. calculated phase diagrams displayed isotropic network + nematic solvent (I + N), isotropic network + isotropic solvent (I + I), and isotropic swollen network(I) coexistence regions. Decreasing the segment length between cross-linked points or increasing the network functionality leads to a tight (dense) network that accommodates less LC solvent molecules. However, the segment length between the cross-links exerts a greater influence on the phase diagrams than the network functionality. A comparison has been made between the phase diagrams of LC/linear polymer and LC/cross-linked polymer systems. Of particular interest is that the coexistence curve of the LC/cross-linked polymer shows no critical point. Instead, it exhibits an upward turn in curvature as the LC volume fraction approaches unity. This behavior seems to be dominated by the contribution arising from the elastic free energy of the polymer network, which is entropic in origin. Light scattering and optical microscopic experiments have been carried out to test theoretical predictions. A good agreement between theory and experiment attests to the predictive capability of the theory.