Molecular orientational order in the mesophases of TBBA : Results of a consistent analysis of neutron, NQR and NMR data

Abstract

Incoherent neutron quasi-elastic scattering (NQES), 14N nuclear quadrupole resonance (NQR) and deuterium nuclear magnetic resonance (DMR) data in the smectic VI, smectic H, smectic C, smectic A and nematic phases of terephtal-bis-butyl-aniline (TBBA) are analysed consistently in terms of simple rotational models allowing both orientational order around the long molecular axis and fluctuations of this axis. It is found that : (i) In the SmVI phase, the molecules rotate around their long axis in a two fold periodic potential. The corresponding bipolar order parameter varies from ∼ 1 to ∼ 0.8 near the VI-H transition. (ii) In the SmH phase, the rotation is almost uniform, but a small bipolar ordering still exists, which varies from ∼ 0.15 to ∼ 0.06 near the H-C transition. In addition, the long axes fluctuate about their average positions, the average amplitude of these fluctuations varies from ∼ 0 to ∼ 22° near the H-C transition. (iii) In the SmC phase, the bipolar ordering is very weak (≤ 0.005). As the temperature increases the long axes tend to be perpendicular to the layers. The polar fluctuations of the axis remain practically constant (∼ 22°), but the azimuthal fluctuations increase rapidly and become uniform at the C-A transition. Values of corresponding order parameters are given. (iv) In the SmA and nematic phases, the polar fluctuations increase and are as large as ∼ 42° at the nematic-isotropic transition. The values of the nematic order parameter are given. In addition, in all these phases, except, possibly in the VI phase, the three phenyl rings reorient around their para axis, and in the C, A and nematic phases, the molecules undergo trans-trans isomerization on a time scale of 10-6-10 -7 s. All these results seem to rule out definitely the Meyer-MacMillan [1] and Meyer [2] theories for the tilted phases of TBBA