Effect of added salt on morphology of ultrathin polyelectrolyte films

Morphological evolution of polyelectrolyte ultrathin films as a function of externally added monovalent salt concentration and its correlation with the solution state morphology have been studied using dynamic light scattering (DLS) and tapping mode AFM. DLS data shows the presence of two different dynamical modes of relaxation in the solution state. Films are prepared from aqueous solution of poly(sodium-4-styrenesulfonate) (NaPSS) with different concentration of added NaCl on the hydrophilic silicon substrate using spin coating technique. Smooth and featureless surface morphology for salt free NaPSS films become rough with addition of salt and beyond a critical salt concentration DLA like fractal structures are formed on the films. Comparison of the surface morphology with slow mode dynamics in DLS establishes the presence of multichain polyelectrolyte clusters in the DLA pattern formed on the surface of the films. Compositional analysis of the films shows an accumulation of excess NaCl on the DLA structures with the increase of salt concentration. Our results show that the DLA patterns are primarily formed by the excess salt ions on their solidification during spin coating and multichain polyelectrolyte clusters nucleate on the preformed nuclei leading to the resultant surface morphology.