Pretransitional behaviour in nematogens composed of flexible molecules

Abstract

Certain properties of the isotropic phases of nematogenic liquid crystals are observed to diverge as the transition to the nematic phase is approached. Here we develop a molecular field theory for the divergence or bifurcation temperature, T*, of nematogens composed of flexible molecules. The theory is based on an extension of the Marcelja model and results in a relatively simple expression for the divergence temperature because of the low orientational order. The molecular factors which influence T* are considered in detail. The theory is used to predict the variation in the divergence temperature along two homologous series. In one, the 4-n-alkyloxy-4′-cyanobiphenyls, there is a single flexible alkyl chain attached to a mesogenic group while in the other series, the α-ω-bis(4,4′-cyanobiphenyloxy)alkanes, the alkyl chain links two mesogenic groups. The theory predicts a significantly larger alternation in T* and T*/T _NI for the series of dimers than for the conventional nematogens. These predictions are in agreement with results for a main chain liquid-crystalline polymer and have subsequently been confirmed for the dimers.