Concentration and molecular weight regime diagram of salt-free polyelectrolyte solutions as studied by light scattering

Abstract

Static and dynamic light-scattering measurements were carried out on salt-free aqueous sodium poly(styrene sulfonate) solutions. The concentration dependence of the solution behavior was investigated in the range c=0.01– 45.6 g/L for three samples with Mw=5000, 100 000, and 1 200 000. A critical concentration ccr≂0.5 g/L was experimentally established. Above ccr, two diffusion coefficients Df (fast) and Ds (slow) were measured. Df, corresponding to the coupled diffusion of polyions and counterions, is independent of concentration. Ds, corresponding to the presence of multichain domains in solution, increases upon dilution. The angular dependence of the scattering intensity as it reflects the size of domains becomes less pronounced upon dilution and, at c=ccr, the scattering intensity is independent of angle. The concentration dependence of the excess scattering intensity is stronger than linear (the reduced intensity I/c increases with concentration). Below ccr, Df decreases upon dilution and finally merges with Ds. The scattering intensity is first independent of angle, then upon dilution a broad maximum appears. The concentration dependence of the excess scattering intensity is weaker than linear (the reduced intensity I/c decreases with concentration). These experimental data indicate that, for c<ccr, repulsive Coulombic interactions screened by the presence of counterions dominate. For c>ccr, the solution structure and dynamics are predominantly influenced by coupled polyion-counterion dynamics and the presence of attractive forces giving rise to domains. The critical concentration ccr is independent of molecular weight. A concentration molecular weight regime diagram is constructed combining the present data with previous results on the molecular weight dependence and with literature data on very dilute solutions.