Viscoelastic Properties of Semidilute Polyelectrolyte Solutions

Zero-shear viscosity η° and steady-state complinance J_c of poly(N-methyl-2-vinylpyridinium chloride) solutions were measured in the absence and presence of added-salt over a wide range of polymer concentration. The polymer concentration dependence of η° in semidilute regions increases with increasing added-salt concentrations C_s and almost agrees with that for non-ionic polymers in good solvents at high added-salt concentrations. J_C in dilute regions depends on molecular weight M, polymer concentration C and C_s more strongly than that of non-ionic polymer solutions, as given by Jo ∝ M-2C_s. In entangled regions, on the other hand, J_c depends on polymer concentration only in the same manner as that of non-ionic polymer solutions, but its concentration dependence is weaker than that of non-ionic polymer solutions, as given by J_e∝C-1.3. Moreover, the polymer concentration dependence of the weight-average relaxation time t_w increases with increasing added-salt concentration in the semidilute region for η° and the entangled region for J_c. These viscoelastic properties of entangled-polyelectrolyte solutions can be well explained by the reptation model assuming that the correlation lengths related to entanglements are determined by the electrostatic interactions evaluated from the Donnan equilibrium.