PROOF OF MULTILAYER STRUCTURAL ORGANIZATION IN SELF-ASSEMBLED POLYCATION POLYANION MOLECULAR FILMS

Multilayer organization of ultrathin polycation-polyanion self-assembled films is demonstrated using two approaches. (1) Fabrication of polyion superlattices with alternation of three different polyelectrolytes in (ABCB)n fashion, which gives rise to a Bragg peak in X-ray reflectivity. The spacing d = 93.4 angstrom corresponds to the repeat unit (ABCB)n. (2) Drying-induced manipulation of the film surface at regular intervals. Normally the layer-by-layer adsorption is carried out by keeping the film wet throughout all deposition cycles. Alternatively the film surface can be manipulated by gently drying the film in a stream of nitrogen or air after the adsorption of every layer. When the films are dried at regular intervals m during the alternating deposition of a polyanion and a polycation (AB)m (e.g. m = 3 for drying after deposition of every 6th layer), up to four orders of Bragg peaks are observed in X-ray reflectivity. In the case of poly(styrenesulphonate) alternated with poly(allylamine) the observed superlattice spacings d(m) were d2 = 104 angstrom, d3 = 150 angstrom and d4 = 188 angstrom. In both cases the observation of a superlattice structure probably results from the artificially enhanced variation in electron density along the layer normal on a length scale larger than the interfacial width of adjacent layers.