Study of Formation, Phase Behavior, and Microdroplet Size of a Polyurethane-based Polymer-dispersed Liquid Crystal

Abstract

Using calorimetry and scanning electron microscopy, we have investigated the formation process and phase behavior of a polyurethane-based polymer-dispersed liquid crystal system. We have measured the kinetics and energetics of the cure process during which liquid crystal microdroplets form by phase separation from the matrix as it cross-links. The greatest degree of cure occurs for sample cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linerly with increasing liquid crystal concentration due to a dilution effect. The time constant for the cure process decreases rapidly with increasing temperature but is much less sensitive to liquid crystal content. Samples cured below 375 K are apparently not fully phase separated, but subsequent treatment at higher temperatures evidently increases the degree of cure. The highest nematic-isotropic transition temperatures were achieved for liquid crystal concentrations above 40 volume percent. The nematic-isotropic transition enthalpy, △H NI , is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of △H NI with increasing liquid crystal concentration. Optimum microdroplet formation occurs at 375 K, but only for liquid crystal concentrations below about 53 volume percent. At higher concentrations a reversed phase ( polymer ball ) morphology is seen. For the lower concentration droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model