Asymmetric polyelectrolyte multilayer membranes: Influence of bottom section polycation on layer growth and retention mechanisms

It has been previously demonstrated that asymmetric polyelectrolyte multilayer (PEM) membranes are highly promising for use in water treatment due to their ability to combine a high selectivity with a high permeability. This is achieved by first coating a more open (highly permeable) bottom PEM to fill the pores of the support membrane, and subsequently a more dense (highly selective) top PEM. However, fundamental understanding on the interaction between top and bottom PEM section and the effect of the bottom section on the resulting membrane properties is still lacking. In this study, symmetric membranes with different polycations are prepared and compared with asymmetric membranes which contain an additional poly (allylamine hydrochloride)/poly (acryl amide) (PAH/PAA) top section. PEM layer growth was first studied with optical reflectometry, demonstrating that the growth of the top PEM section is dependent on the previously deposited bottom PEM section. Membrane performance was assessed with cross-flow measurements, where permeability and molecular weight cutoff (MWCO) measurements showed distinct differences for the symmetric membranes. Although these properties converge upon coating of the top section, differences remain. This can be explained by a contribution of the bottom section to hydraulic resistance and by intermixing of the top and bottom section. The degree of intermixing is attributed to differences in mobility of the used polycations. Single salt retention measurements even show that changing the bottom section polycation allows for tuning of the salt retention mechanism of the resulting asymmetric membranes. Whereas membranes with PAH in the bottom section are dominated by dielectric exclusion, membranes with Poly (vinylbenzyltrimethylammonium chloride) (PVBTMAC) in the bottom section show a dominant Donnan exclusion. These observations were confirmed by varying both the salt concentration and the operational flux. Overall, our work demonstrates that asymmetric coating on different bottom PEM chemistries can be used as a promising tuning parameter in layer-by-layer coating. This allows for a more rational design of asymmetric PEM membranes, opening opportunities for tuning membranes towards specific applications.