Side Chain Length Impact of Linear Polyamines on Filtration Properties in Polyelectrolyte Multilayer Membranes

This study explored the fundamentals of how altering the hydrophobicity through alkyl side chain length of linear polyamine derivatives (LPAs) affects nanofiltration (NF) membrane performance. Four LPAs, from methyl to butyl side chains, were synthesized via anionic ring-opening polymerization of activated aziridines and incorporated into symmetric and asymmetric polyelectrolyte multilayer (PEM) coatings on hollow fiber membranes. Poly(styrenesulfonate) (PSS) served as the polyanion, with polyethylenimine (PEI) as a reference polycation. Membranes were tested for water permeability, salt retention (MgSO₄ and NaCl), and molecular weight cutoff (MWCO). Symmetric LPA-based membranes showed similar MWCOs to PEI but lower permeability and salt retention. Interestingly, MWCO did not increase with longer alkyl chains, contrary to expectations based on hydrophobicity. PEI’s hyperbranched structure likely enhances pore closure more effectively than linear LPAs. Asymmetric membranes with a PEI/PSS base and LPA/PSS top layer offered a tunable balance between permeability and selectivity, with improved MWCO without sacrificing performance. These results underscore the potential of chemically tailoring LPAs to develop more efficient PEM-based membranes for micropollutant removal.