Azobenzene derivatives were among the first ten liquid crystalline compounds. But there have been no detailed investigations on azomesogens consisting of a heterocyclic moiety. Twelve different azo compounds have been condensed with nicotinic acid to evaluate the effect of variation of terminal group on the monosubstituted pyridine system. The relative thermal stabilities and the variation in the mesomorphic characteristics have also been studied.
A series of liquid-crystalline polystyrene derivatives with w-(4-(4-fluorophenylazo)phenoxy)alkoxy pendant groups have been synthesized and characterized. Homologues with four or more methylene units in the spacer chain exhibit smectic liquid-crystalline phases, while the homologue with three methylene units forms an amorphous glass. The smectic A-isotropic (SA-I) transition temperatures show a marked odd-even effect as a function of the parity of the spacer chain. This odd-even behaviour is also found in the variation of AWR at the S,-I transition across the series. A marked hysteresis between the transition temperatures determined on heating and the transition temperatures determined on cooling is observed. A molecular interpretation of these phenomena is proposed.
A number of binary systems have been investigated in which one component can form a nematic liquid-crystal phase and the other not. In all cases studied the liquid phase is apparently a homogeneous single phase which may or may not be anisotropic. There was no indication that two distinct liquid phases, one isotropic and the other anisotropic, could coexist over a range of temperature, for a binary mixture of a given composition. Thermodynamic arguments show, however, that such a situation is impossible for a normal first-order phase transition. Therefore, our results indicate either that the transition is not a first-order transition or that the two-phase region is extremely small. It seems likely that the same will be true in general for anisotropic liquids containing two components, such liquids being in general true solutions and not mixtures of phases.
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