ORDER-DISORDER PHASE-TRANSITION IN LIQUID-CRYSTALS AS A FUNCTION OF MOLECULAR-STRUCTURE .1. ALKYL CYANOBIPHENYLS

The pretransitional behavior of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CN phi phi C//nH//2//n// plus //1) for n equals 5-12, was studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n equals 8-12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n equals 10 and 12 no nematic phase is observed. It is shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behavior, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than similar 3 degree C above the clearing temperature T//c where the systems exhibit a typical pretransitional behavior adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T//c. The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T//c where a strong divergence from the already critical behavior is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.