Relation between the orientational ordering and the tricritical behavior for smectic-A to smectic-C phase transition

Abstract

The smectic-A to smectic-C transition for two binary mixtures of liquid crystals has been studied. These systems are mixtures of a 4′-(2-chloroalkoxy)-4-heptyloxybiphenyl, designated B7 or C7, with 4-butyloxyphenyl-4′-decyloxybenzoate (10O4̄). Pure B7 and C7, in their racemic forms, display characteristics of a first-order smectic-A to smectic-C transition. With increasing mole fraction of 10O4̄, the behavior of the phase transition changes from first order to second order. The order parameter of the mesogenic core of B7 and C7 in these systems was determined by the use of 13C nuclear magnetic resonance. It was found that the order parameter of the liquid crystal systems at the transition temperature is discontinuous for a first-order smectic-A to smectic-C transition, and continuous for a second-order transition. The composition of the tricritical mixture (XTCP, where X is the mole fraction of 10O4̄) was determined to be XTCP=0.12±0.01 for the B 7/10O4̄ system and XTCP=0.13±0.01 for the C7/10O4̄ system. The former has not been reported before, and the latter agrees reasonably well with previous studies.