Abstract
Abstract High-resolution ac calorimetric studies have been carried out on heptyloxybenzoyloxy cyanostilbene (T7), octyloxybenzoyloxy cyanostilbene (T8), and a mixture of a hex-ylphenyl cyanobenzoyloxy benzoate (DB6) and terephtal-bisbutyi aniline (TBBA). The heat capacity Cp . exhibits a sharp, asymmetric peak near the second-order nematic (N)-smectic-A1(SmA1) transition in these systems, all of which possess a nematic phase that is stable over a very wide temperature range. For a DB6+ 15% TBBA mixture, there is also a SmA1, = SmA2 transition at which Cp exhibits a distinct cusp. The critical exponent a is 0.05 (very close to a log singularity) for the SmA 1-SmA 2 transition and -0.14 for the SmA 1 -SmA 2 transition. These values are in good agreement with critical x-ray studies but do not conform with current theoretical predictions.
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