Effects of Molecular Length on Nematic Mixtures. III. Anisotropic Properties of 4-Alkylphenyl 4-Alkoxybenzoate Mixtures

Abstract

Properties of multicomponent nematic mixtures of 4-alkylphenyl 4-alkoxybenzoates are studied as a function of temperature and their average molecular length (L). The L's vary between 21.47 and 27.67 Å for mixtures with clearpoints in the 43 to 60°C range. The refractive indices, density, and the ε₁ component of dielectric permitivity all decrease approximately linearly as L increases. There are much larger changes in flow viscosity (η), conductivity anisotropy ([sgrave]|/[sgrave]|), and dielectric anisotropy (Δε). At 25°C as L increases: The is relatively high and increases sharply (from 47 to 78 cP), the [sgrave]|/[sgrave]| decreases and drops below unity (from 1.35 to 0.58), and the Δε changes from positive to negative (from +0.27 to -0.26). The temperature dependence of [sgrave]|/[sgrave]|, with tetrabutyl-ammonium tetraphenylboride as a dopant, indicates cybotactic nematic characteristics are present for the longer L mixtures in which there are an average of about nine or more alkyl carbons from both end groups. The large decrease in Δε at higher L is attributed to increased molecular association effects in the cybotactic nematic mixtures.