ORIENTATION-DEPENDENT INTERACTIONS IN POLYMER SYSTEMS .4. CHAIN-LENGTH DEPENDENCE OF THE NEMATIC-ISOTROPIC TRANSITION BEHAVIOR OF THERMOTROPIC SEMIFLEXIBLE POLYMERS

The nematic-isotropic transition behavior of semiflexible polymers in the bulk was studied on the basis of three typical models of orientation-dependent interactions (the Onsager-Kimura-type mean- field model, the lattice version of the Onsager model, and the Maier-Saupe-type soft interaction model) and two polymer models (the wormlike chain and the freely jointed chain with randomly distributed joints). The critical value of x = q/D) required to stabilize the nematie phase was evaluated as a function of m = L/q for various combinations of the models, where L, q, and D are the contour length, the persistence length, and the diameter, respectively, of the chain. Even though x and x(infinity), the value of x at L --> infinity, strongly depended on the models, the predicted ln(x/x(infinity)) vs m relations were reasonably model-insensitive, offering a hopefully quantitative interpretation for the known dependence of the transition temperature T-i on chain length. (Note that q and hence x is a function of temperature.) Like T-i, the enthalpy change of transition was predicted to increase with L, approaching a constant value for large L. This behavior originates in the conformational change of semiflexible polymers and is not a characteristic of rigid rodlike molecules.