Nonlinear growth of multilayer films formed from weak polyelectrolytes

The layer-by-layer (LbL) adsorption technique allows construction of polyelectrolyte (PE) multilayer films with precisely defined properties and that opens a wide range of applications, where such materials can be used. One of the most important factors governing structure and interfacial properties of polyelectrolyte films is electrolyte concentration and pH of the PE solutions. In the present work we have focused our attention on multilayer films formed using combination of weak polyelectrolytes, PAH/PAA, whose charge can be tuned during the film preparation by pH of their solutions. PE films were formed by LbL technique on silicon wafers, Si/SiO2, and were characterized by ellipsometry, neutron reflectometry and AFM. Additionally quartz crystal microbalance (QCM) was used to determine mass of the films formed on the gold covered QCM crystals. We have found that the thickest films were formed when both polyelectrolytes were weakly charged, while the thinnest films were obtained for fully charged PAH and PAA. However, for the combination of weakly and strongly charged polyelectrolyte the non-monotonic increase of film thickness with a number of deposited PE layers was observed. We hypothesize, that it results from PE reorganization and formation of surface complexes during adsorption of polyelectrolyte layers.