J. Phys. I France
Volume 5, Numéro 12, December 1995
Page(s) 1667 - 1680
DOI: 10.1051/jp1:1995222
J. Phys. I France 5 (1995) 1667-1680

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

T.J. Phillips

Defence Research Agency, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, UK

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

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 $\Upsilon$, 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, $\bar \varepsilon$, when the system is cooled into the nematic phase.

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