Phase diagram and photopolymerization behavior of mixtures of UV-curable multifunctional monomer and low molar mass nematic liquid crystal

A miscibility phase diagram of mixtures of UV-curable multifunctional thiolene-based optical adhesive (NOA65) and low molar mass 4-n-heptyl-4-cyanobiphenyl nematic liquid crystal (K21) has been established using light scattering, optical microscopy, and differential scanning calorimetry. The emergence of phase morphology was investigated during the heating and cooling cycles to identify various coexistence regions. A teapot phase diagram was obtained in which an upper critical solution temperature (UCST) overlapped with the nematic-isotropic transition of the liquid crystal. The phase diagram exhibited Liquid + liquid, Liquid + nematic, and pure nematic coexistence regions. Photopolymerization was carried out on various mixtures of NOA65 and K21 to fabricate polymer/liquid crystal composites by initiation in the one-phase and two-phase regions of the phase diagram. The rate constants for propagation and termination reactions (k(p) and k(t)) were determined as functions of temperature and conversion. On the basis of the observed k(p) and k(t), the conversion rate was calculated by solving the rate equations for the photopolymerization process. It was found that the maximum conversion rate increased with increasing temperature and initial monomer concentration. The experimental results were in good accord with the calculated results in terms of both the order of magnitude and the trend. Of particular interest was that the domain morphology was uniformly distributed for the curing in the single phase, whereas the curing in the two-phase region resulted in nonuniformity of domain sizes.