Effects of Molecular Length on Nematic Mixtures

Abstract

The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...