Linear and Exponential Growth Regimes of Multi layers of Weak Polyelectrolytes in Dependence on pH

Multilayers of poly(allyl amine hydrochloride) (PAH) and poly(acrylic acid) (PAA) are formed by layer-by-layer self-assembly at defined pH values and investigated by dissipative quartz-crystal microbalance (QCM-D) and ellipsometry. Five distinct pH regimes with different growth mechanisms and growth laws are identified. At intermediate pH values, a central regime is characterized by linear growth, accompanied by very thin and very rigid films. Adjacent regimes toward higher and lower pH show exponential layer growth, and very soft films are formed, Toward very low and very high pH, again a sharp change of mechanism occurs and the growth law becomes linear again, while very asymmetric and soft layers are formed. The symmetry of the regimes and the respective film properties are attributed to charge dilution of the polyanion or the polycation chain, respectively. In either of the live regimes the mechanism of layer formation is explained by the charge densities along the chains, as supported by ATR-FTIR experiments, and varying interchain interactions.