Small Angle X-Ray Studies of Liquid Crystal Phase Transitions II. Surface, Impurity and Electric Field Effects

Abstract

Phase transitions in nematic and cholesteric liquid forming materials have been investigated by small angle X-ray scattering. Measurements were performed on p-azoxyanisole in the isotropic and nematic liquid regions and on cholesteryl bromide in the isotropic and cholesteric liquid regions. For p-azoxyanisole premonitory phenomena have been observed in the isotropic liquid at temperatures considerably above the isotropic-nematic transition. These phenomena, which are indicative of ordering in the isotropic liquid, are strongly dependent upon sample purity, and are not evident in the purest materials. D.C. electric fields have also been found to induce ordering in the isotropic liquid. It was not possible to observe any similar ordering effects, or to detect the isotropic-cholesteric transition, in cholesteryl bromide by small angle X-ray scattering.

We have previously reported on results of a small angle X-ray study of the isotropic-nematic phase transition in p-azoxyanisole.⁽¹⁾ Before going into a discussion of the present experiments it is well to summarize our earlier findings and indicate some of the questions that required further investigation.