Effect of added salt on swelling dynamics of ultrathin films of strong polyelectrolytes

Abstract

Solvent mass uptake and swelling dynamics of spin coated poly(sodium-4-styrenesulfonate) (NaPSS) films containing different concentration of externally added monovalent salt ions have been performed in presence of water vapor to study the effect of low molecular weight salt ions on the swelling dynamics of charged polymeric chains in confined geometry. Diffusion coefficient of the water molecules within the NaPSS films, as obtained from the solvent mass uptake study, is found to be independent of salt concentration. Swelling dynamics of the chains in presence of water vapor has been studied in-situ using X-ray reflectivity technique. The actual salt concentration and the distribution of salt ions within the films were examined using X-ray Photoelectron Spectroscopy (XPS). The diffusion coefficients of the NaPSS chains show a salt concentration independent behavior and are of same order as that of salt free NaPSS chains. Whereas, the fraction of charged monomers, that determines the strength of the repulsive interaction between the polymer segments, increases with salt concentration when the later becomes comparable to that of already present counterions in the system. The Na and Cl ions show clear tendency to be redistributed within the films instead of forming NaCl salt. The negative Cl ions migrate towards the top surface whereas the positive sodium ions accumulate on the substrate. Counterion condensation process of the polyelectrolyte chains gets inhibited by the internal electric field produced between the top Cl− layer and the bottom Na+ layer which increases the effective charge of the polyelectrolyte chains.