High pressure studies on 4'-n-alkyl-4-cyanobiphenyls

Abstract

The effect of pressure on the phase transitions in the pentyl to octyl homologues of 4'-n-alkyl-4-cyanobiphenyls has been studied. The slope dT/dP for the nematic-isotropic transition shows an alternation with the number of carbon atoms in the alkyl chain, while that for the solid- nematic (or smectic) transition does not show such a behaviour. The smectic A-nematic transition in 4'-n-octyl-4-cyanobiphenyl. which is first order at atmospheric pressure. becomes second order at high pressures the tricritical point occurring at 2.68 +/- 0.08 kbar. 92.5 +/- 1.5 degree C. The ratio of the smectic A-nematic transition temperature to the nematic-isotropic transition temperature at the tricritical point is 0.90, in reasonable agreement with the prediction of the mean field theory.