Methods for Determination of Solubility Limits of Polymer-Dispersed Liquid Crystals

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Abstract
Measurement of the solubility limit, A, of a liquid crystal (LC) in a polymer matrix is an important consideration in optimizing polymer-dispersed liquid crystal (PDLC) films for a variety of light-control applications. By reducing A, we can improve the efficiency of LC usage. We describe four methods–differential scanning calorimetry (DSC), threshold for light scattering (TLS), refractive index (RI) measurement, and scanning electron microscopy (SEM)–for determining A-values. We discuss the advantages and drawbacks of the various methods and present results to illustrate their use. Solubility limits can take on a wide range of values depending on the PDLC components, their cure temperatures, and the measurement temperature. For example, DSC measurements show that solubility increases with increasing measurement temperature. Also, thermally cured PDLCs studied so far have tended to have lower A-values than UV-cured ones. For thermally cured systems, A-values ranged from 8 to 36 volume percent; for UV-cured PDLCs, the range was broader: 9 to 53 volume percent. An important goal of future research would be the development of liquid crystal/polymer combinations for which phase separation is maximized (low A-values).