On Dipole Association and Short-Range Order in Nematic Discotic Liquid Crystals

(Received 30 March 1995, revised and accepted 8 August 1995)

Abstract
The Kirkwood dipole correlation factor g₁ of a nematic discotic liquid crystal is measured as a function of temperature. A large degree of anti-parallel dipole association is found, which persists into the isotropic phase. This is in conflict with previous mean-field models for dipole correlation, which predict parallel association in discogens. A cluster model of ordering, including short-range dipolar orientational order, quantified by a parameter , and including the experimental fact anti- parallel dipole association, is used to attempt a more reasonable treatment of dipole correlation in these systems. The theory is found to predict the dipole association rather well, with a great improvement over mean-field results, and shows other improvements over mean-field models, including a reduction in transition enthalpy and drop in the mean permittivity, , when the system is cooled into the nematic phase.