Light scattering on semidilute polyelectrolyte solutions: Molar mass and polyelectrolyte concentration dependence

Dynamic and static light scattering experiments on solutions of the flexible highly charged polyelectrolyte sodium poly(styrenesulfonate) have been performed in the semidilute concentration regime. Over a very wide polyelectrolyte concentration range double-exponential correlation functions were found. At the lowest polyelectrolyte concentrations the correlation functions became Single exponential. Apparent fast and slow diffusion coefficients were determined for three molar masses at a fixed added salt concentration of 0.01 M. The fast diffusion coefficient is both molar mass and polyelectrolyte concentration dependent at the lowest concentrations and becomes molar mass independent-at semidilute concentrations. At even higher polyelectrolyte concentration the apparent fast diffusion coefficient is independent of both the molar mass and polyelectrolyte concentration. In the semidilute regime the fast diffusion coefficient follows the power law D-f proportional to c(0.72), close to the theoretical prediction of Odijk's scaling theory for a transient polyelectrolyte network. The apparent slow diffusion coefficient decreases with increasing molar mass and increasing polyelectrolyte concentration. The reciprocal normalized scattering intensity shows a strong angular dependence. The slow mode is interpreted as aggregates or domains that form in semidilute and concentrated solutions. The dimensions of these aggregates decrease with decreasing polyelectrolyte concentration and decreasing molar mass. The existence of aggregates and the change in their dimensions are directly seen with conventional light microscopy. The general trend of the apparent diffusion coefficients of NaPSS in 0.01 M NaCl resembles earlier data on NaPSS without added salt.