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DTA and Dielectric Studies of a Substance with Nematic, Smectic A and Smectic C Polymorphism at Ambient and Elevated Pressures

Stanislaw Urban ,J. Czub ,S. Pawlus ²,M. Sekula ²,A. Wuerflinger ³,R. Dabrowski ⁴
Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland Institute of Physics, Jagiellonian University, Krakow, Poland ²Institute of Physics, Silesian University, Katowice, Poland ³Faculty of Chemistry, Ruhr University of Bochum, Bochum, Germany ⁴Institute of Chemistry, Military University of Technology, Warsaw, Poland
For the first time the low frequency relaxation process in two liquid-like smectic phases (smectic A and smectic C) was studied at elevated pressures with the aid of DTA and dielectric spectroscopy methods. The substance studied, 2-(4-hexyloxyphenyl)-5-octyl-pyrimidine (6OPB8 in short) exhibits the nematic (N) – SmA – SmC phase sequence.The p-T phase diagram was established with DTA. However, the SmA – SmC transition was not observed in the DTA, but could be detected in the dielectric relaxation measurements. The dielectric relaxation time measured as function of temperature and pressure enabled us to calculate the activation volume and activation enthalpy. It was found that the molecular rotations around the short axes are more feasible in the tilted SmC than in the orthogonal SmA phase.

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