Asymmetric Poly(methylmethacrylate)-based polyelectrolyte multilayer membranes: The role of polyanion structure on separation properties

Nanofiltration (NF) membranes composed of polyelectrolyte multilayers (PEMs) are gaining popularity as tunable membranes for water purification. Multiple preparation parameters allow the variation of PEM properties and therefore their separation capabilities within a wide range. PEM membranes made from the well-studied weak polyelectrolytes poly (allylamine) (PAH) and poly (acrylic acid) (PAA) exhibit high selectivity for uncharged solutes. In this study, we explore the less studied polyelectrolyte poly (methacrylic acid) (PMAA) for use in PEM membranes. PMAA has an extra methyl group, affecting its pKa and hydrophobicity, resulting in different layer buildup and separation properties. Asymmetric ceramic membranes were fabricated with four bilayers of highly permeable PDADMAC and PSS as base layers and four bilayers of PAH/PAA or PAH/PMAA as selective top layers. Membranes with top layers composed of PAH/PMAA achieved a very low molecular weight cut-off (MWCO) of 143 Da while maintaining a higher pure water permeability (PWP). In contrast, the well-studied system PAH/PAA showed a much lower PWP at a similar MWCO. These performance differences are attributed to the different pKa, a stiffer steric structure and to the higher hydrophobicity of PMAA, resulting in different layer buildup, which finally influences the layer buildup on the membranes. This study highlights the significance of small molecular differences in monomers and their impact on the separation performance of a PEM-based membrane. It is also the first study to demonstrate the potential of PMAA for fabricating highly selective asymmetric PEM membranes.