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 (6OPB8 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(∂lnτ /∂p)T, and activation enthalpy, Δ#H = R(∂lnτ /∂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.
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