Polymerization-induced phase separation III. Morphologies and contrast ratios of polymer dispersed liquid crystals

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Abstract
Polymerization induced phase separation in mixtures of liquid crystals (LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixing, in most cases of practical interest. At high LC content or low temperature of polymerization liquid-liquid separation cannot be excluded. Depending on the elasticity and homogeneity of the polymer network at the onset of phase separation, spherical or non-spherical LC domains are observed; non-spherical domains reflect an inhomogeneous gel structure. The change from spherical to non-spherical occurs in a very narrow range of LC concentrations and curing temperatures. The transition between these two morphologies can be explained using conversion phase diagrams obtained from the Flory-Huggins-Dusek theory. The contrast ratio of PDLCs made from the Merck mixture passes through a maximum when the droplet shape at the onset of phase separation changes from spherical to non-spherical. Lowering the LC content or increasing the temperature leads to smaller LC domains which scatter less efficiently. The reverse changes lead to early phase separation and large LC domains which also scatter inefficiently. It is speculated that the maximum of the contrast ratio is related to secondary phase separation, leading to subdomains of an appropriate size.

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