Atomic force microscopy studies of salt effects on polyelectrolyte multilayer film morphology

The morphology of multilayer films formed from polydiallyldimethylammonium chloride and polystyrene sulfonic acid deposited under a range of salt concentrations (from 10(-4) to 1.0 M) was investigated using atomic force microscopy (AFM). Ten-bilayer films that were deposited with less than 0.3 M added NaCl were flat and featureless, with similar characteristics to the underlying silica substrate. When formed at and above this salt concentration, a vermiculate morphology was observed. Thickness and roughness measurements were also carried out using the AFM and were found to increase with the concentration of added salt. The evolution of the vermiculate pattern was investigated by AFM studies of each layer that is deposited under high salt concentration (1.0 M NaCl). The first three bilayers were featureless and had a thickness of similar to6 nin/bilayer. A change in morphology was observed by the fourth bilayer, and the average thickness had increased to similar to 46 nm/bilayer. These results may be explained in terms of a transition from an extended conformation to a more compact form that polyelectrolytes undergo as a function of the ionic strength of a solution.