Adsorption mechanisms in layer-by-layer films

Adsorption processes in layer-by-layer films are discussed using poly(o-methoxyaniline) (POMA) as a case study and also comparing with results for other polymers from the literature. This follows a brief overview of the materials and characterization techniques employed for self-assembled films, including their possible applications. The original paradigm of the self-assembly method is associated with spontaneous adsorption of oppositely charged polymer layers. While this rationale has been successful in explaining adsorption mechanisms for some polyelectrolytes, for polyanilines other interactions must be included. For POMA, in particular, at least three types of interactions are identified, namely van der Waals forces, ionic interactions and H-bonding. Furthermore, H-bonding is responsible for a number of effects even for charged POMA where electrostatic attraction was expected to predominate. Such effects include POMA dedoping upon contact with a glass substrate at early stages of adsorption, and the non-linear increase in the adsorbed amount with the number of POMA/poly(ethenesulfonic acid) (PVS) bilayers deposited in a multilayer structure. Adsorption of a POMA layer on a glass substrate or on an already formed POMA/PVS film occurs in two steps: a fast, first-order kinetics process with a characteristic time of a few seconds and a slower process represented by a Johnson-Mehl-Avrami function with a characteristic time of hundreds of seconds. These correspond basically to nucleation and growth mechanisms which is corroborated by atomic force microscopy measurements. The amount of material adsorbed in any given layer depends on experimental parameters, especially polymer concentration and pH, owing to the different extents of H-bonding that may allow POMA to adsorb on itself or on PVS molecules