Chain Ordering in the Smectic C, Smectic A and Nematic Phases of Terephtal-bis-butyl-aniline (TBBA) and its Temperature Dependence

Abstract

The deuterium magnetic resonance (DMR) data of Deloche et at. (J. Physique, Coll., 36 (1975) C1-21) on the smectic C, smectic A and nematic phases of TBBA, concerning the butyl chains, are analyzed in terms of a model where the chain ordering is described by mean rotational potentials hindering the rotations around the C—C single bonds. These potentials are split into intra and intermolecular contributions. Using intramolecular potentials calculated with quantum chemistry methods, we estimate the shapes and the heights of the intermolecular contributions. We find that (i) the intermolecular potentials favor chain conformations which are aligned with the long molecular axis, (ii) their heights decrease as the temperature (or equivalently the molar volume) increases, (iii) the most probable conformation of the chains slightly changes with temperature, but is always very different from that of the isolated molecule. These results are additional support for the model we already proposed for the aromatic core of TBBA^14,15 and for PAA, to explain the temperature dependence of DMR splitting ratios in liquid crystals, in terms of changes of the most probable conformation (associated with changes of the free volume per molecule) rather than in terms of several. temperature dependent, molecular orientational order parameters.^5,7,8