Rheology of Solutions of Rodlike Polymers: Theory and Comparison with Experiments

The Doi theory (J. Polym. Sci., Polym. Phys. Ed. 1981,19, 229) of concentrated solutions of rodlike particles is compared with the recent treatment of Bahar and Erman (J . Polym. Sci., Polym. Phys. Ed. 1986,24,1361) of the lattice theory of rods in a potential flow field. The Doi theory is modified by introducing a flow term to its effective mean-field potential, similar to that of the lattice treatment. Results of calculations based on the modified Doi theory are in agreement with existing viscosity concentration data on a-helical poly(benzylg1utamate) in m-cresol. At relatively low shear rates, the experimentally observed sharp maximum in viscosity is found to be located in the biphasic region. In this region the orientational order parameter and viscosity are double-valued. The characteristic features of the biphasic regime predicted by the theory are discussed. The viscosity-concentration curves exhibit smoother maxima at higher shear rates, although no phase separation is predicted by the theory. As the flow rate is further increased, the maximum gradually disappears in agreement with experiments. Also, the experimentally observed Newtonian plateau in the plots of viscosity against shear rates is obtained by the theory. Quantitative agreement between theory and experiment fails at high shear rates.