Molecular Diffusion in the Nematic and Smectic C Phase of 4–4′-Di-n-Heptyloxyazoxybenzene (HOAB)

Abstract

Earlier measurements of the diffusion constant of the impurity molecule tetramethylsilane (TMS) when dissolved in a smectic A and smectic B liquid crystal have shown this quantity to be strongly anisotropic.¹ That is, the diffusion constant in a direction perpendicular to the long molecular axis, D_⊥, (diffusion within the smectic layers) is a much larger quantity than that in a direction parallel to the long molecular axis, D_||, with a typical value for the ratio D_||/D_⊥ ~ 0.1. In the earlier work it was not possible, however, to obtain a measure of this ratio in the nematic phase. In this paper we report such a measurement as well as a measurement of the anisotropy in the smectic C phase. Much to our surprise, the ratio D||/D_⊥ in the nematic phase varies strongly with temperature, exhibiting strong pretransition effects over most of the nematic temperature range.