High pressure studies on a nematogen with highly polar molecules: evidence for a nematic-nematic transition

Abstract

We have modified a high pressure optical set-up to enable measurements on the optical path difference of aligned liquid crystals. We present the phase diagram of a polar nematogen, viz. p-cyanophenyl p-n heptyl-benzoate which is shown to exhibit a nematic (N_d) to nematic (N₁) transition in the bulk at elevated pressures. We have also measured the temperature variations of the orientational order parameter (S) at various pres-sures to find that S has constant values at both the nematic to isotropic and N_d to N₁ transitions at different pressures.