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

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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 samples cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linearly 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, ΔHNI, is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of ΔHNI 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 concentrations droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model.