Polymerization kinetics and monomer functionality effects in thiol-ene polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) are a class of electrooptic materials most often formed by polymer-induced phase separation, a one-step fabrication technique often based on photopolymerization of the commercial thiol-ene mixture NOA65. To allow further understanding regarding PDLC formation, this work systematically examines processing variables that influence thiol-ene-based PDLC morphology and subsequent performance, namely polymerization kinetics, polymer gel point, and liquid crystal (LC) phase separation. PDLC formulations containing a wide range of thiol and ene monomers were examined as a function of monomer (thiol and ene) functionality, thiol-ene stoichiometry, and ene monomer composition. Simultaneous examination of polymer evolution and LC phase separation by real-time infrared (RTIR) spectroscopy shows that both polymerization kinetics and the gel point of thiol-ene PDLC formulations are influential on the extent of LC phase separation. Increasing monomer functionality (both thiol and ene) reduces LC droplet size in PDLC morphology by reducing the gel point conversion of thiol-ene polymer. In addition to influencing gelation, increasing the functionality or the electron density of ene monomer increases the rate of polymerization which further reduces LC droplet size. The thiol-ene PDLC formulations studied exhibit decreased LC droplet size (from 1 mu m to 300 nm) that correlates directly to polymerization rate and gel point. Electrooptic switching behavior is also dependent on LC droplet size.