LIGHT-SCATTERING ON SEMIDILUTE POLYELECTROLYTE SOLUTIONS - MOLAR-MASSAND POLYELECTROLYTE CONCENTRATION-DEPENDENCE

Dynamic and static light scattering experiments on solutions of the fl exible highly charged polyelectrolyte sodium poly(styrenesulfonate) ha ve been performed in the semidilute concentration regime. Over a very wide polyelectrolyte concentration range double-exponential correlatio n functions were found. At the lowest polyelectrolyte concentrations t he 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 coeffic ient is both molar mass and polyelectrolyte concentration dependent at the lowest concentrations and becomes molar mass independent-at semid ilute 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 f ast diffusion coefficient follows the power law D-f proportional to c( 0.72), close to the theoretical prediction of Odijk's scaling theory f or a transient polyelectrolyte network. The apparent slow diffusion co efficient decreases with increasing molar mass and increasing polyelec trolyte concentration. The reciprocal normalized scattering intensity shows a strong angular dependence. The slow mode is interpreted as agg regates or domains that form in semidilute and concentrated solutions. The dimensions of these aggregates decrease with decreasing polyelect rolyte concentration and decreasing molar mass. The existence of aggre gates and the change in their dimensions are directly seen with conven tional light microscopy. The general trend of the apparent diffusion c oefficients of NaPSS in 0.01 M NaCl resembles earlier data on NaPSS wi thout added salt.