Elastic constants of nematic liquid crystals of uniaxial symmetry

We study, in detail, the influence of molecular interactions on the Frank elastic constants of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. This work is based on a weighted density functional formalism developed by us in a previous paper (Singh et al 1992 Phys. Rev. A 30 583). A brief summary of the status of theoretical development for the elastic constants of nematics is presented. Considering a pair potential having both repulsive and attractive parts, numerical calculations are reported for three nematics: MBBA, PAA and 8OCB. For these systems the length-to-width ratio x0 is estimated from the experimentally determined structure of the molecules. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution (HER) and the attractive potential is represented by the quadrupole and dispersion interactions. From the numerical results we observe that in the density range of nematics the contributions of the quadrupole and dispersion interactions are small as compared to the repulsive HER interaction. The inclusion of attractive interactions reduces the values of elastic constant ratios. The absolute values of elastic constants are sensitive to the values of potential parameters. The elastic constants and their ratios are in agreement with the experimental and computer simulation values. The temperature variation of elastic constants and their ratios are reported and compared with the experimental values. It is found that the calculated values are in agreement with the experimental data. The twist elastic constant has a weak temperature dependence but a pronounced influence is found on the bend elastic constants.

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