Dielectric studies of liquid crystals: the influence of molecular shape

Abstract

One of the first applications of the molecular field theory developed by Maier and Saupe (Maier W.; Saupe, A. Z. Naturforsch. 1958, 13a, 564–566) was to the static dielectric properties of a nematic liquid crystal by Maier and Meier (Maier, W.; Meier, G. Z. Naturforsch. 1961, 16a, 262). Then, in a paper published in 1966 with G. Meier, (Meier, G.; Saupe, A. Mol. Cryst. 1966, 1, 515) Saupe developed a simple theory to explain the low-frequency dielectric relaxation observed in the nematic phase around 1 MHz, but absent in the isotropic phase, and indeed absent in all other isotropic molecular fluids.

This paper reviews this work of Saupe and the subsequent development of the theory by Martin, Meier and Saupe (Martin, A.J.; Meier, G.; Saupe, A. Symp. Faraday Soc. 1971 , 5, 119), and then by others such as Nordio, Rigatti and Segre (Nordio, P.L.; Rigatti, G.; Segre, U. Mol. Phys. 1973, 25, 129). These theories described the effect of a nematic potential on the dipole relaxation times for a molecule, and provided a framework within which to analyse experimental measurements of dielectric relaxation in liquid crystals.

The purpose of this review is to examine the application of simple ideas from the theories of Saupe and others to a variety of experimental results for primarily nematic liquid crystals. A particular theme is developed which categorises liquid crystal-forming molecules (mesogens) in terms of their molecular shape. Experimental results of dielectric relaxation for examples of different shaped mesogens are then explained in terms of the theory. Recent work has extended the types of mesogen to include bent-core or V-shaped molecules and flexible-core mesogens, the latter of which, liquid crystal dimers are examples. New features have appeared in the experimental results for these materials, which have stimulated the development of a new theory of dielectric relaxation in flexible dimeric liquid crystals (Stocchero, M.; Ferrarini, A.; Moro, G.J.; Dunmur, D.A.; Luckhurst, G.R. J. Chem. Phys. 2004, 121, 8079). These results are reviewed.

Keywords:

• dielectric properties
• dielectric relaxation
• molecular shape
• liquid crystal dimers

Acknowledgements

The support of the School of Chemistry, University of Southampton, and of Hitachi Japan to DAD is gratefully acknowledged. We are also grateful for financial assistance from the European Union under a variety of Programs over a number of years. This support has encouraged many ongoing successful collaborations, such as that between the School of Chemistry, University of Southampton, UK, the Department of Chemistry, University of Padua, Italy, and the Department of Applied Physics, University of the Basque Country, Spain. It is this collaboration that has stimulated much of the work described in this paper. We are also grateful for support from the MICINN of Spain (project MAT2009-14636-C03-02,03 and MAT2008-01372), from the MEC (MAT2006-13571-C02-02), from the Gobierno del País Vasco (GI-C07-40-IT-484-07). SD acknowledges the recognition as an emergent research group (AGAUR-2009-SGR-1243) from the Generalitat de Catalunya Government