THEORY OF SYSTEMS OF RODLIKE PARTICLES .2. THERMOTROPIC SYSTEMS WITH ORIENTATION-DEPENDENT INTERACTIONS

A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos**2 psi //i//j where psi //i//j is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T* measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T* and the axial ratio x as parameters. Steric effects of molecular shape asymmetry, embodied in x, are of foremost importance. Comparison of theory with experiments on those nematogens whose molecules can be approximated by rigid rods lends encouragement to the prospect of relating characteristics of the nematic-isotropic transition to molecular structure.