Dynamics of poly (styrenesulfonate) sodium salt in aqueous solution

The diffusion of poly(styrenesulfonate) sodium salt (NaPSS) was investigated using dialysis dynamic light scattering (DLS) and dialysis fluorescence photobleaching recovery (FPR). Never-dried or "virgin" NaPSS was synthesized directly from 4-styrenesulfonic sodium salt to achieve 100% sulfonation. Upon reducing the ionic strength directly in the DLS cell by dialysis, a clean sample developed clearly distinct fast and slow modes that were first identified as an extraordinary phase in low-salt solutions of poly-L-lysine by Lin. Lee. and Schurr [Biopolymers 1978, 17, 1041]. This result complements published polyelectrolyte investigations in a hi.-h-dielectric constant organic solvent, and also studies where the degree of ionization was tuned, which confirms that hydrophobic patches along the polymer chain are not required for the extraordinary behavior. The fast mode dominated even at low ionic strength, with scattering amplitude exceeding 70% of the total. For the virgin Na-PSS sample in the dialysis cell, there is no convincing evidence of a slow mode at high salt ( >= 200 mM NaCl). The appearance of distinct slow and fast modes proved reversible upon removing and adding salt by dialysis, without any other perturbation save restoration of the concentration by dialysis centrifugation. This suggests that the behavior represents a thermodynamically equilibrated state. Dialysis FPR measurements of aqueous solutions of a commercial NaPSS that was labeled with fluoresceinamine (LNaPSS) showed no obvious long-range ordering. A reversible decrease in the optical tracer self-diffusion coefficient of LNaPSS as salt is dialyzed out of the solution is instead attributed to chain expansion. Comparison of FPR and DLS on a mixed LNaPSS/NaPSS sample suggests that the residence time of a chain in temporal aggregates [Schmitz et al. Biopolymers 1984, 23 1637] that are thought to be responsible for the DLS slow mode is shorter than the FPR time scale.