Differential Thermal Analysis (DTA) under High Pressure VI: Phase Transitions of Some Liquid Crystals up to 3 kbar

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Abstract
With a high pressure DTA apparatus previously described the phase transition temperatures solid/nematic (melting temperatures) and nematic/liquid isotropic (clearing temperatures) of the liquid crystals MBBA, EBBA, PBBA, PEBAB, PAA, and PAP were measured as a function of pressure up to 3 kbar and in the temperature range 273 K to 550 K. For PBBA the pressure dependence of the transition temperature smectic/nematic was additionally determined.For all compounds rising pressure causes an increase of the transition temperatures as well as an extension of the nematic range (especially for PAA and PEBAB). According to the thermal pretreatment two different melting curves were found for MBBA, EBBA and PEBAB separating the nematic liquid from a stable and a metastable solid phase respectively. The melting curves diverge slightly for MBBA and EBBA: for PEBAB, however, an intersection point can be extrapolated at about 2600 bar and 450 K.From the peak areas at atmospheric pressure the transition enthalpies ΔHand from these the transition entropies ΔSwere determined. Using these values and the slopes of the experimentalT(p)transition curves the volume changes ΔVaccompanying the transitions solid/nematic, smectic/nematic and nematic/liquid isotropic respectively were calculated from the Clausius‐Clapeyron equation. The ΔH, ΔSand ΔVvalues thus obtained are discussed and compared with the literature data available.

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