Dielectric studies of intermolecular and intramolecular interactions in nematic liquid crystals

This paper discusses the role of dipole-dipole interactions in determining the dielectric response of nematic liquid crystals. The theory of dipole-dipole correlation in liquid crystals is reviewed and results of computer simulations of model dipolar mesogens are commented upon. Dielectric measurements are reported for a variety of nematic liquid crystals of differing molecular structures. It is found that dipole association is dependent on the molecular shape and the direction of the dipole moment within the molecule. Experimental results, theory and simulations are all in accord that rod-like molecules with longitudinal dipoles tend to self-organize with local anti-ferroelectric order. If the dipole direction is changed to be perpendicular to the long molecular axis, then there is local ferroelectric order. Disc-like molecules are predicted to exhibit local ferroelectric order if the dipole is along the axis of the disc, but may show anti-ferroelectric order if the dipole is in the plane of the disc. Introduction of flexibility into the molecular structure can have a large effect on the dielectric properties. This is illustrated for dimeric liquid crystals, and the results are interpreted in terms of a model of interconverting conformers having different molecular shapes and dipole moments.