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Abstract
In this paper, the current state of the art of molecular theories of nematic mesophases is reviewed. For this purpose, the many theories of nematogens which have been proposed are grouped into two classes: (1) the Maier-Saupe theory and its extensions and variants, which assume that nematic long-range order is primarily due to anisotropic intermolecular attractions and (2) the hard-rod and van der Waals theories, which assume that nematic order results primarily from excluded volume effects. These two types of theories are discussed in turn, with particular emphasis on the basic assumptions and approximations, the relative strengths and weaknesses, and the most fruitful applications of each of them. Finally, prospects for improving and extending current molecular theories are examined briefly.
