Effect of pressure on liquid crystal dimers

We report high pressure investigations on a homologous series of liquid crystalline dimeric molecules in which the terminal chain length is kept constant but the length of the methylene spacer connecting the two mesogenic units is varied. We find that unlike the nematic-isotropic transition temperature and associated entropy change, there is no alternation in the slope of the nematic-isotropic phase boundary in the pressure-temperature plane as a function of the length of the spacer group in the molecule. By applying the Clausius-Clapeyron equation we conclude that the volume change at the transition should exhibit a strong odd-even effect. Measurements on the shortest homologue of the series, which is non-mesomorphic, show the expected result that the application of pressure induces mesomorphism in non-mesomorphic compounds.