Smectic C-Nematic Transition in the Mixtures 5_x6_1-x BABA

Abstract

We describe DSC investigations of the (S_C-N) transition in the mixtures 5_x6_1-xBABA, for 0 ≤ x < 1. This transition could only be observed for x ≤ 0.8. A marked difference was found in the chain length dependences of both the transition temperature, T _CN, and the transition entropy, ΔS_CN, between the pure and mixed compounds. This might be related to the difference between end hydrocarbon chain conformations of 5BABA and 6BABA. Specific heat measurements show (i) for x = 0, a first order transition with hardly any fluctuations, (ii) fort = 0.59, an almost continuous transition, but again with weak fluctuations. The fact that the fluctuations remain weak near T _CN even when ΔS_CN is very small, is attributed to large bare coherence lengths.