Effects of Molecular Length on Nematic Mixtures

Abstract

The properties of nematic mixtures of 4-alkoxyphenyl 4-alkylbenzoates are studied as a function of their average molecular length (L). The L's of the mixtures vary between 20.46 and 27.14 Å, but they all have clearpoints in the 51 to 58° C range. The flow viscosity increases linearly with L. The dielectric constant, refractive index, birefringence, and density decrease linearly with L, and the dielectric anisotropy becomes more negative. With tetrabutylammonium tetraphenylboride added as a salt dopant, the conductivity anisotropy (σ_∥/σ_τ) decreases with increasing L, and the dynamic-scattering (DS) threshold voltage increases correspondingly. The effect of L on the DS decay time (τD) is highly dependent on the surface alignment. In surface ⊥ cells, τD increases strongly with L, while in surface ∥ cells, τD decreases slightly with increasing L. The temperature dependence of σ_∥/σ_τ indicates that the longer L mixtures, with about ten or more total alkyl carbons from both end groups, have cybotactic nematic characteristics.