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

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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 Mw=5000 to 1 200 000 at moderate concentrations in the interval c=10 – 45.6 g/L. Two diffusion coefficients, Df (fast) and Ds (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. Df is found to be independent of molecular weight over the broad range used in this study. On the other hand, Ds, 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 Ds and from the total scattered light intensity. The concentration dependence of Ds is in the form of a power law Ds∼c−ν, where the exponent ν is dependent on molecular weight in the form ν∼Mμw. This is in contrast to the isotropic model of polyelectrolyte solutions 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.