Nematic-smectic A-reentrant nematic transitions in 80CB : 60CB mixtures

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Abstract
We have carried out an extensive high resolution x-ray scattering study of the nematic-smectic A (N-S A) and reentrant nematic-smectic A (RN-SA) phase transition behaviour in mixtures of octyloxycyanobiphenyl (8OCB and hexyloxycyanobiphenyl (60CB). The smectic A phase boundary is found to be parabolic in the temperature-concentration plane with a median temperature TM = 38.06 °C and a critical concentration y0 = 0.427; here y is the 60CB :80CB molecular ratio. Detailed studies of the smectic fluctuations in the nematic phase are reported for y = 0.33, 0.413, 0.420, 0.429, 0.440 and 0.443. The first three concentrations exhibit N-SA and RN-SA transitions, whereas in the latter three samples with decreasing temperature the smectic correlation lengths and susceptibility exhibit maxima at TM and then decrease with a further decrease in temperature. The data are analysed using an extension of the Pershan-Prost optimal density theory. All of the data are well-described by the phenomenological theory; the critical exponents so-obtained are v ∥ = 0.76 ± 0.03, v⊥ = 0.62 ± 0.05 and γ = 1.49 ± 0.07. These agree quantitatively with values obtained in single layer materials with comparable nematic ranges; thus the N-SA and RN-SA transitions are identical in character to conventional N-SA transitions provided that one includes the crossover effects inherent in the parabolic phase boundary. Studies of the in-plane fluid structure factor in the N, SA and RN phases show that the mean molecular spacings and positional correlations are closely similar to those in single layer materials; in each phase the structure factor is well described by a circularly-averaged Lorentzian with a correlation length of 6.9 Å. These results argue strongly against pairing models for the reentrant behaviour. We conclude with a phenomenological discussion of the optimal density model which is so successful here and models based on competing order parameters which are required to describe the varied behaviour observed in other polar materials