Cross-Linkable Polyelectrolyte Multilayer Films of Tailored Charge Density

The layer-by-layer (LbL) deposition technique of anionic and cationic polyelectrolytes allows formation of thin and ultrathin nanostructured films with tunable properties and, therefore, functionality. Here, we report on the synthesis and characterization of new polyelectrolyte multilayer system consisting of anionic copolymer poly(sodium 3-(4-vinylbenzylsulfany1)-propane-1 sulfonate)-co-poly(2-hydroxyethyl-vinylbenzyl sulfoxide) with different charge densities (21, 36, 65, 81, and 100%) and cross-linkable cationic poly(4-methyl-1-(4-vinylbenzy1)-pyridinium chloride. We study the impact of salt concentration on film thickness and roughness for the different charge densities and discuss the observed trends with regard to charge effects. Furthermore we covalently cross-link the films by an aldol reaction, which preserves the number of charged groups while being easily followed via UV-vis spectroscopy because of the characteristic absorbance band of the coupling product. Cross-linking increases film stability markedly. Thus the new multilayer system allows tuning of both ionic and covalent cross-linking in a quantitative fashion.