Tricritical points of smectic A to nematic phase transitions for binary liquid crystal mixtures containing cyanobiphenyls

Abstract

The tricritical point of the smectic A (S_A) to nematic (N) phase transition in liquid crystals, at which the nature of the phase transition changes from first order to second order, is related to the change in the order parameter during the phase transition. In this study, ¹³C NMR was used to determine the order parameters of four binary liquid crystal systems as a function of temperature and composition in order to investigate the tricritical behaviour of their S_A-N transition. The first system was mixture of 4-n-pentyl-4′-cyanobiphenyl (5CB) and 4-n-pentylphenyl 4′-n-pentyloxybenzoate (5O5), which have an injected smectic A phase. The tricritical point of this system has a McMillan ratio of 0.949 ± 0.008. The three other systems were binary mixtures of 4-n-alkyl-4′-cyanobiphenyls (nCB). At the tricritical point, the McMillan ratio is 0.986 ± 0.007 for the 5CB/10CB mixtures, and 0.985 ± 0.008 for the 8CB/9CB mixtures, but a tricritical point was not observed for the 9CB/10CB mixtures. The results are compared with those of previous investigations.