Dynamics of moderately concentrated salt-free polyelectrolyte solutions: Molecular weight dependence

Abstract

Detailed static and dynamic light‐scattering experiments were performed on salt‐free solutions of sodium poly(styrene sulfonate) with 13 different molecular weights ranging from M w =5000 to 1 200 000 at moderate concentrations in the interval c=10 – 45.6 g/L. Two diffusion coefficients, D f (fast) and D s (slow), were characterized as is typical for such a system. The fast‐diffusive mode, corresponding to the coupled diffusion of polyions and counterions, is completely dominated by the counterion’s influence. D f is found to be independent of molecular weight over the broad range used in this study. On the other hand, D s , which corresponds to the dynamics of large multichain domains in solution, is strongly dependent on molecular weight. The apparent dimensions of the domains are calculated from the angular dependencies of D s and from the total scattered light intensity. The concentration dependence of D s is in the form of a power law D s ∼c −ν, where the exponent ν is dependent on molecular weight in the form ν∼M μ w . This is in contrast to the isotropic model of polyelectrolytesolutions proposed by de Gennes and Odijk, which predicts that the behavior of a semidilute solution does not depend on molecular weight. It appears that interactions on a much larger scale than the correlation length of the entangled network must be taken into account.