Nanoporous thin films formed by salt-induced structural changes in multilayers of poly(acrylic acid) and poly(allylamine)

We report here on the influence of changes in the solution salt concentration on the structure of multilayers of weak polyelectrolytes. For poly(acrylic acid) (PAA) and poly(allylamine) (PAH) multilayers assembled by the layer-by-layer process in the presence of sodium chloride, washing with pure water after deposition of each layer produces films with considerable surface roughness (root-mean-squared (rms) roughness similar to 17 nm for a 10 layer film), as assessed by scanning force microscopy. In contrast, relatively smooth (rms roughness similar to1 nm) and homogeneous PAH/PAA multilayer films are formed when the salt concentration is kept constant both during the assembly process and in the washing steps. For such smooth films, subsequent exposure to pure water leads to the introduction of regular, discrete, nanometer-sized pores, thus providing a means of introducing lateral structure into the PAH/PAA multilayer films. Electrochemical measurements revealed that the pores formed in less than 10 min. The sensitivity of the multilayer films to salt as well as the subsequent creation of nanopores potentially makes them attractive candidates for use in controlled-release applications where defined permeability characteristics are desired.