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 concentrati on on the structure of multilayers of weak polyelectrolytes. For poly(acryl ic acid) (PAA) and poly(allylamine) (PAH) multilayers assembled by the laye r-by-layer process in the presence of sodium chloride, washing with pure wa ter after deposition of each layer produces films with considerable surface roughness (root-mean-squared (rms) roughness similar to 17 nm for a 10 lay er film), as assessed by scanning force microscopy. In contrast, relatively smooth (rms roughness similar to1 nm) and homogeneous PAH/PAA multilayer f ilms are formed when the salt concentration is kept constant both during th e assembly process and in the washing steps. For such smooth films, subsequ ent exposure to pure water leads to the introduction of regular, discrete, nanometer-sized pores, thus providing a means of introducing lateral struct ure into the PAH/PAA multilayer films. Electrochemical measurements reveale d that the pores formed in less than 10 min. The sensitivity of the multila yer films to salt as well as the subsequent creation of nanopores potential ly makes them attractive candidates for use in controlled-release applicati ons where defined permeability characteristics are desired.