New Variant of Reentrant Polymorphism in Liquid Crystals

X-ray diffraction patterns and observations under the polarizing microscope made on binary mixtures of 4-cyano-4′-n-hexyloxybiphenyl (60CB) and 4-cyano-4′-n-octyloxy-biphenyl (80CB) to which the chiral material 4″-(2-methylbutylphenyl)-4′-(2-methyl-butyl)-4-biphenylcarboxylate (CE2) has been added, have revealed the occurrence of a new variant of liquid crystalline reentrant polymorphism: isotropic-cholesteric (twisted nematic)-smectic A-smectic E-cholesteric (twisted nematic)-solid. X-ray measurements of the layer spacings in the smectic A phase of such samples and similar ones where chiral CE2 has been replaced by chiral 4-cyano-4′-(2-methyl)butylphenyl (CB-15) are found to be consistent with the predictions of a recent microscopic model of the reentrant phenomena whereby the formation of molecular association pairs is an important factor determining the occurrence of the reentrant phase.