Pretransitional behaviour in liquid crystals. The roles of nuclear magnetic resonance spectroscopy and molecular field theory

Abstract

The liquid crystal–isotropic phase transition is weak, and so the angular correlations, characteristic of the ordered phase, grow within the isotropic phase as the transition to the liquid crystal is approached. This pretransitional behaviour has been well studied through the orientational order induced by a magnetic field, via the resulting optical birefringence. This is observed to diverge in the vicinity of the phase transition and such behaviour may be understood, at the molecular level, by theories based on the molecular field approximation. However, both the experimental and the theoretical analyses are oversimplified because they assume that the constituent molecules of a liquid crystal are rigid and cylindrically symmetric, whereas in reality the molecules are of low symmetry and flexible. To characterise the field-induced orientational order of such complicated molecules in detail it is necessary to use a technique with a far greater information content than the single number obtained from the birefringence. Deuterium n.m.r. spectroscopy has proved to be particularly valuable for investigating the details of the orientational order of liquid-crystalline phases themselves, and is now being applied, with equal success, to the study of pretransitional behaviour in the preceding isotropic phase. Here such new investigations are surveyed for pure liquid crystals, both nematic and smectic, and for liquid-crystal mixtures. The results of these unique experiments may be rationalised rather well by extending the molecular field theory to include molecular flexibility and multicomponent mixtures.