Influence of Molecular Structure on Liquid Crystalline Properties and Phase Transitions in Mixed Liquid Crystals

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Abstract
A number of binary systems comprising a liquid crystalline Schiff's base, viz. p-acetoxybenzal-p-phenetidine and a non-liquid crystalline Schiff's base are studied. In all these cases, the liquid phase, be it anisotropic or not, is a single homogeneous phase. There is no indication of two distinct liquid phases, one anisotropic and the other isotropic, coexisting side by side over a range of temperature, for a binary system of a given composition. The shape of the transition lines in the phase diagrams of the binary mixtures is dependent on the type of the admixed non-mesomorphic component. In the case of systems comprising isomorphous components, the transition lines are almost linear. If the molecules of the two components differ in shape and size, owing to the difficulty in packing themselves together the transition lines of such binary systems exhibit a concavity. When the admixed component is altogether an odd molecule in shape, size and structure, compared to the mesomorphic component, the transition lines of such binary systems would be very steep, indicating the extremely little tendency of the odd non-mesomorphic component towards mixed mesomorphism. The systems investigated here generally comprise isomorphous components. The slope of the transition line in these cases stands as a measure of the tendency of the non-liquid crystalline component to form mixed liquid crystals. The group slope values obtained from the slope of the transition lines are found to be additive and agree well with those obtained by Dave and Dewar4 and Dave and Lohar5 showing that the effect of the terminal groups is specific. The order of efficiency of the groups for mixed liquid crystal formation, in the present investigation, is as under:

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