Polyelectrolyte Spin Assembly: Influence of Ionic Strength on the Growth of Multilayered Thin Films

ABSTRACT: Layer-by-layer (LbL) assembly of polymer electrolyte multilayers is now a well-established method for the fabrication of thin films by sequential adsorption of alter-nating layers of oppositely charged polyelectrolytes.Most commonly, such adsorptions have been from quiescent solutions of varying ionic strength and pH. Here, we report results on an alternative processing route for the achievement of polymeric multilayer assemblies of poly(sodium-4-styrene sulfonate) and poly(allylamine hydrochloride) that utilizes conven-tional spin coating. We investigated and describe herein the dependence of multilayer film buildup on solution ionic strength for comparison with similar dependence in quiescent adsorption. Using UV-Vis spectroscopy we monitored the growth of the multilayered films, while with Atomic Force Microscopy (AFM) we examined the surface features and mea-sured coating thicknesses at different salt concentrations. AFM and UV-Vis data reveal two regimes of behavior with increasing salt: strong salt-dependence at low salt contents, and weak salt-dependence for high salt contents. To explain this observation, we introduce the relevance of the dimensionless group De ̇, the local Deborah Number, to the problem. As ionic strength increases, increases so that spin-assembly flow influence