NMR of Solutes in Liquid Crystalline Solvents: Mechanisms of Orientational Order

The exact nature of the physical interactions that lead to orientational order in liquid crystalline systems is not completely understood. Such understanding is of great importance in fields ranging from the industrial use of display devices to the understanding of structure and function in biological membranes. The liquid crystal molecules themselves are large and flexible and can exist in a number of different possible conformations. Because of the difficult statistical mechanical problem posed by the flexibility, it is usually ignored, and the liquid crystal molecules are described in terms of properties that are taken to be appropriate averages over all conformations. Most theories of orientational order assume the presence of some mean field, but the exact nature of the field is not specified. In spite of its simplicity, the mean field theory of Maier and Saupe [lj remains one of the most successful descriptions of orientational order in pure nematics. However, this theory says nothing of the exact nature of the intermolecular forces causing orientation, and the liquid crystal molecules are modelled as rigid rods where the different possible conformations are ignored.