Contribution of monomer functionality and additives to polymerization kinetics and liquid crystal phase separation in acrylate-based polymer-dispersed liquid crystals (PDLCs)

Fundamental control of the polymerization behaviour of polymer-dispersed liquid crystals (PDLCs) is critical to the formation of high-performance devices by polymer-induced phase separation (PIPS). Previous PDLC research has shown that monomer functionality and additives such as surfactants or reactive diluents can impart significant changes to the electro-optical behaviour of a system, especially in acrylate-based materials. The influence of monomer functionality and additives on the polymerization kinetics and LC phase separation were examined in the formation of acrylate-based PDLCs. Real-time infrared (RTIR) spectroscopy was utilized to simultaneously monitor polymerization rate, double bond conversion and LC phase separation. In the formation of PDLCs by PIPS, increasing acrylate monomer functionality reduces the polymerization rate, overall double bond conversion and the extent of LC phase separation. Interestingly, the additives octanoic acid and N-vinylpyrrolidone (NVP) increase the polymerization rate but suppress LC phase separation. During PDLC formation, both octanoic acid and NVP enhance the solubility of the LC in the growing polymer matrix, reducing the rate of liquid-gel demixing and decreasing nematic fraction in PDLCs. As a non-reactive component, octanoic acid increases the polymerization rate by plasticizing the crosslinked polymerization. NVP, a reactive diluent added to decrease viscosity, increases polymerization rate through favourable copolymerization with acrylate monomer.