The Metabolemeter II-Detection of Mesogenic Polymorphism under Pressure Using Small Quantities

Every first order phase transition for mesogens can be detected by the continuous recording, as a function of temperature, of the pressure of the sample enclosed in a metallic cell. Studies of a compound (octylcyanobiphenyl) exibiting several transformations (crystal-smectic A, smectic A-nematic, nematic-liquid) show that this barometric method is convenient to detect polymorphism from one thermobarogram, and to draw the Clapeyron phase diagram; the smectic A-nematic transition involving small enthalpy and volume changes is clearly observable and this method is very sensitive. Moreover, the enthalpies and volumes of transition can be deduced from the pressure changes. Lastly, pressure and temperature are intensive data, and very little sample (1.3 mm³) may be used.