Photopolymerization-induced phase separation and morphology development in liquid crystal/polymer blends

In this research, a detailed experimental and theoretical investigation of photopolymerization-induced phase separation and morphology development in nematic liquid crystal (LC)/polymer blends has been undertaken. Two nematic liquid crystals were used: a single component namely 4-n-heptyl-4 '-cyanobiphenyl (designated K21) and a multi-component eutectic mixture (designated E44). The polymer was made in-situ by photopolymerizing thiolene-based optical adhesive (designated NOA65).;First, the phase diagrams of the K21/NOA65 and E44 NOA65 systems prior to photopolymerization were established. The phase diagrams were of the upper critical solution temperature (UCST)-type overlapping with the nematic-isotropic transition of the liquid crystals. The phase diagrams displayed isotropic liquid, isotropic liquid + isotropic liquid, isotropic liquid + nematic, and pure nematic coexistence regions.;The effect of temperature and monomer concentration on the photopolymerization behavior was studied. The maximum rate of photopolymerization was found to increase with temperature and concentration of monomer.;The effect of network elasticity on the phase behavior after photopolymerization was studied. Using the K21/NOA65 system as a model, the LC/crosslinked polymer phase diagram showed isotropic swollen network, isotropic network + isotropic solvent, and isotropic network + nematic solvent coexistence regions. The segment length between crosslinks exerted more influence on the phase diagram than the network functionality. A comparison of the phase behavior of LC/linear polymer and LC/crosslinked polymer showed that the elasticity of the network is the cause of the difference in the phase behavior of the two systems.;An experimental investigation of photopolymerization-induced phase separation dynamics and morphology development was conducted. The LC droplets were found to be unevenly distributed in the cured E44/NOA65 blends but uniformly distributed in the cured K21/NOA65 blends. The non-uniform distribution of the droplets in the E44/NOA65 system is believed to arise from the different affinities of the constituents LC in E44 to the NOA65 networks. The time evolution of morphology showed that the LC droplets separate initially as small droplets, which then grow via coalescence until fixed by network formation. The size of LC droplets was found to increase with LC concentration. The network morphology revealed interconnected domains seemingly running through the interstices of the LC droplets. Phase separation was found to occur via nucleation-initiated spinodal decomposition.;Finally, a theoretical simulation of phase separation dynamics and morphology development was undertaken for LC/linear polymer and LC/crosslinked polymer systems. The simulated morphology of LC/linear polymer was found to change from droplet for the off-critical blends to bicontinuous for the critical blend. Increasing the reaction temperature or the reaction constant leads to smaller domains. The simulated morphology of LC/crosslinked polymer showed that only the LC forms the droplets surrounded by seemingly interconnected network in agreement with experiment.