CRITICAL HEAT-CAPACITY AT NEMATIC SMECTIC A1 AND SMECTIC-A1 SMECTIC-A2 TRANSITIONS IN DB5CN + TBBA

High-resolution heat capacity measurements have been made on mixtures of pentylphenylcyanobenzoyloxy benzoate (DB5CN) and terephthal-bis-butylaniline (TBBA). These mixtures exhibit a nematic (N)-smectic A1 (SA1)-smectic A2 (SA2) phase sequence. Detailed N-SA1 data were obtained on one mixture and are in good agreement with the 3D-XY model, which is known to provide an excellent description of the critical N-SA1 heat capacity in other systems. The SA1-SA2 heat capacity for all the investigated mixtures exhibits a finite cusp. The critical exponents for these heat capacity peaks are consistent with the Fisher-renormalized Ising value αR = -0.124, but the experimental situation may be more complex. The magnitude of the Cp(A1-A2) peaks decreases markedly as X increases. © 1990 Taylor & Francis Group, Ltd.