X-ray and calorimetric study of smectic (C)tilde-smectic A2-smectic C2 transitions in a liquid crystal mixture

Abstract

The phase diagram has been established for binary mixtures of nonyloxy-biphenyl cyanobenzoate (9OBCB) and pentylphenylcyanobenzoyloxy benzoate (DB5CN). These frustrated liquid crystals exhibit a rich variety of polymorphic smectic A (SA) and smectic C (SC) phases. X-ray and calorimetric studies have been made of SAd-S(C)tilde-SC2 and SAd-SA1-S(C)tilde-SA2SC2 phase sequences. Major emphasis is given to the tilted ribbon smectic (S(C)tilde)-bilayer smectic A (SA2) transition and the SA2-SC2 transition. The former is first order, but there is substantial pretransitional heat capacity in the S(C)tilde phase that can be described by an inverted Landau model. The latter transition is second order and exhibits a classical Landau step in Cp, in contrast to the behaviour of SA-SC transitions in non-polar systems where an extended Landau model with a large sixth order term is required.