Thermodynamic investigation of the metastable phases of TBBA

An investigation was performed by differential scanning calorimetry on the metastable phases (VI and VII) of TBBA liquid crystal. In addition to the reversibility of the transitions smectic B- phase VI and phase VI-phase VII, a supercooling effect at this last transition and a large spread in the crystallization temperature were observed. Furthermore the transition entropies were determined together with a more correct phase diagram of TBBA. All these data refer to the first complete loop which was obtained by heating the virgin sample from room temperature to the smectic B phase and then cooling it again to room temperature. A different behaviour was obtained in the following complete loops. This behaviour, which cannot be explained at present, constitutes the basis for further experiments by other techniques

X-RAY STUDY OF PHASE TRANSITION INVOLVING ORDERED BIAXIAL PHASES

La technique de diffraction des rayons X a été utilisée pour étudier les transitions de phase suivantes qui concernent les phases ordonnées biaxiales du TBBA : solide-smectique Bc, smectique Bc-smectique VI, smectique VI-phase VII, phase VII-solide. La transition de phase solide-smectique Bc apparaît être dépendante de l'histoire thermique de l'échantillon, en accord avec des mesures récentes de microcalorimétrie. Une coexistence de phases a été observée aux transitions smectique VI-phase VII et solide-smectique Bc et un effet d'hystérésis à la transition smectique VI-phase VII.X-ray diffraction was used to investigate the following transitions involving ordered biaxial phases of TBBA : solid-smectic Bc, smectic Bc-smectic VI, smectic VI-phase VII, phase VII-solid. A dependence on the thermal history of the sample was observed at the solid-smectic Bc phase transition, in agreement with recent DSC data. A phase coexistence was observed at the smectic VI-phase VII and at the solid-smectic Bc phase transitions. An hysteresis effect was observed at the smectic VI-phase VII phase transition