Growth of multilayer films of fixed and variable charge density polyelectrolytes: Effect of mutual charge and secondary interactions

We report on the effect of mutual polyelectrolyte charge and secondary interactions on the formation of polyelectrolyte multilayer films of a lowly charged copolymer of acrylamide and [3-(2-methylpropionamido)propyl]trimethylammonium chloride, AM-MAPTAC 10 (10 mol % of cationic monomers), and poly(acrylic acid) (PAA). These multilayer films were constructed on planar substrates by the sequential adsorption of AM-MAPTAC 10 and PAA at different pH conditions. Surface plasmon resonance spectroscopy (SPRS), quartz crystal microbalance (QCM), and atomic force microscopy (AFM) were employed to follow the film formation process. While AM-MAPTAC 10 is permanently charged, the ionization of PAA is adjusted by changing the pH of the adsorption solutions. In contrast with previous studies, it is shown that multilayer formation with one polyelectrolyte with a charge density of 10 mol % is possible. However, the multilayer growth follows a different pattern depending on the relative charge density of PAA. At low pH, where PAA is only lowly ionized, the films show an exponential-like growth, with a gradual increase of the thickness and adsorbed mass per layer with increasing layer number. At neutral to alkaline pH, where PAA approaches full ionization, multilayer growth proceeds via a series of adsorption-desorption steps, with immersion in PAA solutions leading to partial removal of the underlying AM-MAPTAC 10 layer. The obtained results are discussed in terms of an interplay between electrostatic and secondary interactions within the films and matching of the charge density of the polyelectrolytes employed to construct the films.