Tailoring Polyelectrolyte Multilayer Nanofiltration Membranes by Aerosol-Assisted Printing: Insights into Membrane Formation Mechanisms

Polyelectrolyte multilayer (PEM) membranes, with advantageous features of versatile chemistry and structures, are driving the development of advanced nanofiltration (NF) membranes with exceptional performance. While developing a printing method holds great promise for the eventual mass production of these membranes, reports on the printing method and the underlying mechanisms of membrane formation are currently scarce. Herein, we develop an aerosol-assisted printing (AAP) system for fabricating PEM NF membranes with highly tunable separation characteristics. Our study unveils the three stages of membrane formation from assembly of polyethylenimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS): aerosol deposition, single PE layer formation, and PEM assembly. The droplet deposition is governed by inertial impaction, and the deposited PEs migrate/entangle to form a single PE layer. The thicknesses of the PE layer and PEM exhibit linear growth as the number of printing scan increases. Furthermore, PE interdigitation forms an effective polymeric network barrier, which increases the resistance to solute and water transport. By manipulating the PE deposition mass and layering, PEM membranes with tunable pore radii (0.40-0.56 nm) and water permeability (5-60 L·m^-2·h^-1·bar^-1) were obtained for various water treatment applications, ranging from micropollutant removal to humic acid filtration. Our study offers valuable mechanistic insights into the PEM formation and precise structural adjustment via printing, thus facilitating scalable manufacturing and widespread applications of the PEM NF membranes.