Morphological phase behaviour of a homologous series of thermotropic cubic mesogens under pressure

Abstract

The phase behaviour of a homologous series of thermotropic cubic mesogens of 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (BABH-n, where n is the number of carbon atoms in both terminal alkoxy chains, ranging from n = 11 to n = 18) was studied under hydrostatic pressure using a polarising optical microscope (POM) equipped with a high-pressure optical cell. Pressure-temperature phase diagrams were constructed for all the samples. All of the samples showed the crystal (Cr)–cubic (Cub)–isotropic liquid (I) phase sequence under atmospheric and lower pressures, but the Cub phase was replaced completely by a high-pressure smectic C (SmC) phase; SmC(hp), under higher pressures. There is an intermediate-pressure region as a boundary in which the Cr–SmC(hp)–Cub–I phase sequence was recognised. The SmC(hp)–Cub transition lines showed positive slopes with pressure and shifted to higher pressures with increasing alkoxy chain length in BABH-n. What should noted here is the inversion of the Cub and SmC phases in the phase sequence, in comparison to those (Cr–Cub–SmC–I) for BABH-8 and -10. In BABH-14, -16 and -18, a solid state with a featureless texture and stronger intensities of transmitted POM light appeared just below the SmC(hp) phase on cooling under intermediate and high pressures. This phenomenon is not due to the formation of a monotropic Cub phase but is an induction period for spherulitic crystallisation from the SmC(hp) phase.