Specific Heat Study of First Order Smectic A to Smectic B (Hexatic) Transitions in n-Alkyl-4'n-Pentanoyloxy-Biphenyl-4-Carboxylates
CONNECTICUT UNIV STORRS LIQUID CRYSTALLINE POLYMER RESEARCH CENTER
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A recently synthesized and characterized homologous series of liquid crystal compounds n4C00BC is shown by high resolution a.c. microcalorimetry experiments on the n3, 6 homologues to most likely exhibit Smectic A to Smectic B Hexatic SmBH phase transitions exclusively. This is in partial contrast to earlier tentative assignments, based on low resolution calorimetry, x-ray and microscopy studies, which had crystal smectic B SmBX for n4-8 and SmBH for n3 and 9. The observed SmA-SmBH thermal anomalies exhibit strong fluctuation contributions but are clearly first order. Experimental results are compared with the extensively characterized n-alkyl-4-alkoxy-biphenyl-4- carboxylates nmobes and discussed in the context of Goodbys empirical molecular structure rules for the existence of the SmBH, and in the context of a generic phase diagram SmA,BX,BH recently predicted by Aharony et al. The latter illustrates the physical content of our earlier suggestion that short range hexagonal positional order is responsible for the ubiquitous tricriticality of the SmA-SmBH phase transition.
- Organic Chemistry