Abstract
ABSTRACT X-ray diffraction and high-resolution adiabatic calorimetry have been used to study the polymorphic behaviour of 75OBC liquid crystal. On heating of 75OBC from the room temperature, the crystal phase (Cr-I) melts directly to smectic A (Sm-A) and then to isotropic phase. On cooling from isotropic phase the 75OBC undergoes successive transitions to Sm-A, hexatic B (Hex-B), crystalline (Cr-II) phases, and next to another crystal phase – Cr-III. The latter two crystalline phases were shown to relax slowly to Cr-I phase, which is stable at low temperatures. X-ray data indicate that the Cr-II phase possesses orthorhombic lattice with herringbone in-plane order of the molecules (Cr-E). The Cr-I and Cr-III phases belong to different low symmetry monoclinic lattices. Our results demonstrate a complex polymorphism of 75OBC compound: the thermodynamic pathway that involves monotropic Hex-B and orthorhombic Cr-E phases corresponds to a local thermodynamic minimum, while the other ones showing melting from the Cr-I to Sm-A phase belongs to the global minimum. The similar thermal behaviour was observed for other members of nmOBC homologous series, possessing a Sm-A – Hex-B – Cr-E phase sequence on cooling. The 75OBC polymorphic behaviour is discussed in terms of molecular packing and stability of Cr-E orthorhombic lattice that precedes the transition to the Hex-B phase.
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