Calorimetric study of smectic polymorphism in octyloxyphenyl-nitrobenzoyloxy benzoate + decyloxyphenyl-nitrobenzoyloxy benzoate mixtures

Abstract

Binary mixtures of octyloxyphenyl-nitrobenzoyloxy benzoate (DB8ONO2) and decycloxyphenyl-nitrobenzoyloxy benzoate (DB10ONO2) exhibit a rich variety of smectic polymorphism. An ac calorimetric study has been carried out on six mixtures with mole percent X of the decyl homolog between 51 and 57, a range that includes the Sm-Ad–Nr–Sm-A1 point at X≃56. The sample with X=57 exhibits a first-order direct Sm-Ad–Sm-A1 transition, while the sample with X=51.3 exhibits two transitions—a second-order Sm-Ad–Nr transition with a very small excess heat-capacity peak and a tricritical Nr–Sm-A1 transition with a large heat-capacity peak. For samples with intermediate compositions, only weakly first-order Nr–Sm-A1 heat-capacity peaks are observed in the vicinity of the Sm-Ad–Nr–Sm-A1 point. All six investigated samples exhibit the Sm-A1–Sm-C̃–Sm-C2 phase sequence at lower temperatures. The Sm-A1–Sm-C̃ transition is a weakly first-order phase transition with pretransitional excess heat capacity below the transition temperature. In contrast, the Sm-C̃–Sm-C2 transition is a weakly first-order inverted Landau transition with no pretransitional behavior in the Sm-C2 phase below the transition but a large mean-field excess heat capacity observed in the Sm-C̃ phase above the transition.