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[Ctilde]-SC2 and SAd -SA1 -S[Ctilde]-SA2 SC2 phase sequences. Major emphasis is given to the tilted ribbon smectic (S[Ctilde])-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[Ctilde] phase that can be described by an inverted Landau model. The latter transition is second order and exhibits a classical Landau step in C p, 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.