Behaviour of Sodium polystyrene sulfonate (NaPSS) in aqueous NaCl solutions*

For a polyelectrolyte system as heavily probed as NaPSS in water, the screening interactions in this system are relatively less well understood. In this study, we investigate the rheological behaviour of NaPSS with different molar masses (M_w ~ 30 - 2000 kg/mol) in aqueous salt solutions under a broad range of polymer and added salt concentrations and examine the applicability of the scaling theory for polyelectrolyte solutions to these measurements.

Although the measurements align well with the scaling laws at low polymer concentrations, we notice some significant deviations at higher concentrations, for instance, a weaker than expected dependence of the solution viscosity on added salt concentration. We observe an increase in the specific viscosity with increasing added NaCl concentration at high polymer and salt concentrations. We attempt to rationalise this upturn by associating it with the underscreening theory, but find no evidence of chain expansion at high added salt concentrations. Instead, SANS measurements of NaPSS/NaCl solutions show a steady decrease of the radius of gyration with increasing NaCl content over the entire salt concentration range studied. Another interesting finding is the shear-induced gelation observed for high-concentration high added salt samples.