Effect of skewed cybotactic structure on the dielectric constants and conductivities of some binary mixtures exhibiting the nematic phase

Abstract

2-cyano-4-heptylphenyl-4'-pentyl-4-biphenyl carboxylate (7P(2CN)5BC), a nematogen with a cyano group making a large angle (~60°) with the long axis of the molecule, has a moderately strong negative dielectric anisotropy (Δε). A mixture of 85 mole per cent of 7P(2CN)5BC with 15% of 4-n-heptyl-4'-cyanobiphenyl (7CB) exhibits a reversal of the dielectric anisotropy Δε as the temperature is varied, becoming negative at higher temperatures. Δε of mixtures with higher concentrations of 7CB is positive throughout the nematic range. The conductivity anisotropy ΔS' of these systems has been investigated for a range of frequencies from 300 Hz to 30 kHz. In 7P(2CN)5BC, ΔS' changes sign becoming negative at higher temperatures for all frequencies investigated. The results are discussed in terms of the temperature variation of the tilt angle of the skewed cybotactic structure of 7P(2CN)5BC.