Swelling behavior of polyelectrolyte multilayers in saturated water vapor

The swelling behavior of layer-by-layer assemblies of poly(styrenesulfonate) sodium salt (PSS) and poly(allylamine hydrochloride) (PAH) with various number of layers were investigated. The data presented in this paper suggest that swelling and deswelling are completely reversible and reproducible. At 99% relative humidity, a pronounced "odd-even effect" in the swollen thickness is observed depending on the type of polyelectrolyte in the outermost layer. Contact angle measurements confirmed that the PSS surface is more hydrophilic than the PAH surface. The "odd-even effect" indicates that water has to be pressed out of the multilayer when PAH is adsorbed onto the PSS surface of the multilayer and that again more water penetrates into the multilayer on adsorption of the next PSS layer. It was also found that the relative amount of swelling with respect to the total film thickness decreases with increasing number of layers. This is an indication that the loosely packed outer layers are more sensitive to environmental humidity and consequently swell in a significantly more pronounced way than the inner ones closer to the substrate.