DTA and dielectric studies of a substance with the nematic, smectic A, and smectic C polymorphism at ambient and elevated pressures

Abstract

For the first time the low frequency relaxation process in two smectic phases (smectic A and smectic C) was studied at elevated pressures with the aid of DTA and dielectric spectroscopy. The substance studied, 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine (60PB8 in short) exhibits the nematic (N) - SA - SC phase sequence. The p-T phase diagram was established with DTA. However, the SA -SC transition was not observed in the DTA, but could be detected by dielectric relaxation measurements. The dielectric relaxation time measured as function of temperature and pressure, τ∥(p, T), enabled us to calculate the activation volume, Δ#V = RT(∂ln τ/∂p)T, and activation enthalpy, Δ#H = R(∂ln τ/∂T-1)p. It was found that Δ#V(SA) > Δ#V(SC) and Δ#H(N) ≫ Δ#H(SA) > Δ#H(SC), indicating that the molecular rotations around the short axes are more feasible in the tilted SC than in the orthogonal SA phase.