Investigation of the influence of polyelectrolyte charge density on the growth of multilayer thin films prepared by the layer-by-layer technique

We examine the influence of the charge density along a polyelectrolyte chain on the buildup of multilayer films formed by the sequential adsorption of alternating layers of polyanions and polycations (the layer-by-layer technique). Model random copolymers of diallyldimethylammonium chloride (DADMAC) and N-methyl-N-vinylacetamide (NMVA) having matched molecular weights and varying percentages (24-100%) of the cationic DADMAC component are alternated with polyanionic polystyrene sulfonate (PSS). Multilayer buildup is monitored by UV-vis and FTIR spectroscopies, and with a quartz crystal microbalance (QCM) and is correlated with structural information obtained using atomic force microscopy. When films are deposited from polyelectrolyte. solutions containing added salt, a critical charge density limit (between 75% and 53%), below which no significant layer growth is possible, becomes apparent. Below the critical charge density, addition of the polyanion leads to almost complete removal of the previously deposited cationic copolymer and vice versa, and thus multilayers cannot grow. Above the critical charge density, the rate of film growth and the film morphology are strongly influenced by the solution structure of the adsorbing polyelectrolytes: thicker and rougher films are produced with increased salt concentration in the adsorption solutions.