Calorimetric study of the effect of bent-shaped dopant molecules on the critical behavior at the nematic-smectic-Ad phase transition

Abstract

We report results of calorimetric studies for the binary mixture of rodlike host n-alkyloxy-cyanobiphenyl (nOCB, n = 8,9) and bent-shaped guest 1,3-phenylene-bis[4-(3-methylbenzoyloxy)]-4 -n-dodecylbiphenyl-4 - carboxylate (BC12). The effect of bent-shaped dopant molecules on the critical behavior associated with the nematic-smectic-Ad phase transition has been studied in detail. The transition temperature for the nematicsmectic- Ad phase sharply decreases as the increase of the mole fraction of the dopant concentration (denoted X for the BC12/9OCB mixture and Y for the BC12/8OCB mixture). The dependence of the critical exponent α on X and Y is well explained in terms of the McMillan ratio. A nearly tricritical exponent has been obtained for the X = 0.01 mixture. X = 0.02 − 0.03 mixtures, pure 8OCB, and Y = 0.01 − 0.03 mixtures exhibit nonuniversal behaviorswith effective exponents lying between the 3D-XY and tricritical exponents. The heat capacity anomaly for Y = 0.05 has been well described with the 3D-XY exponent. The critical amplitude ratio A−/A+ is close to 1 and insensitive to the dopant concentration. No Fisher renormalization of the critical exponent has been observed even for nearly tricritical compositions, which indicates the smallness of the concentration plays a decisive role rather than the steepness of the N-SmAd phase boundary.