Tailoring polyelectrolyte multilayer nanofiltration membranes through aerosol-assisted printing

Abstract

Polyelectrolyte multilayer (PEM) nanofiltration (NF) membranes, composed of alternatively charged polyelectrolytes (PE) assembled via layer-by-layer (LbL) deposition, have gained considerable attention owing to their highly tunable surface properties, excellent separation performance, and potential for multifunctional integration. The traditional LbL assembly method by dip coating typically involves sequential immersion of a substrate into oppositely charged PE solutions. However, this approach is inherently time-intensive due to the slow diffusion of PEs in aqueous media and requires large volumes of solutions, limiting its scalability for applications. This limitation presents a major bottleneck in the widespread application of PEM technology. Moreover, effectively addressing diverse water and wastewater challenges necessitates the development of PEM NF membranes with precisely controlled structures and performance characteristics tailored to specific treatment goals. To address the challenges of scalability, this work reports the first application of aerosol-assisted printing (AAP) to PEM NF membrane fabrication, and the overarching goal is to establish fabrication-structure-performance relationships to develop PEM NF membranes with tunable permeability and selectivity. In this thesis, an AAP system is built by modifying a commercial 3D printer with a Collison nebulizer. Different PEM membranes are fabricated using three PE pairs: polyethyleneimine (PEI)/poly(sodium 4-styrenesulfonate) (PSS), poly(allylamine hydrochloride) (PAH)/PSS, and poly(diallyldimethylammonium chloride) (PDDA)/PSS. A higher viscosity of the PE solution leads to larger droplets with increased momentum, thereby improving deposition efficiency and resulting in the formation of a thicker PE layer. The thicker and denser PEI/PSS membranes exhibit lower water permeability and higher rejection of neutral organic solutes than the other two membranes. The PAH/PSS membranes and PDDA/PSS membranes exhibit a loose structure. Among these membranes, the PEI/PSS membrane is chosen as the most promising PEM NF candidate because it can achieve NF-level separation with minimal fabrication time and material. The thicknesses of the PE layer and PEM exhibit linear growth as the printing scan number increases. Furthermore, PE interdigitation forms an effective polymeric network barrier, which increases resistance to solute and water transport. By manipulating the PE deposition mass and layering, PEM membranes with tunable pore radii and water permeability are obtained for various water treatment applications, ranging from humic acid retention to micropollutant removal. The drying process (water evaporation) is not merely a post-treatment step but an integral part of the assembly when using the AAP method. The drying of the PEM membrane proceeds through three sequential stages: compaction of the PEM, deformation of the substrate, and eventual tearing of the PEM due to interfacial stress. The compaction of the PEM layer improves selectivity, but the substrate deformation drives permeability loss. This difference results from the different elastic moduli and pore architectures of the PEM and substrates. Furthermore, incorporation of glycerol into either the substrate or the PEM can effectively preserve membrane structure during drying and maintain its filtration performance, due to the diffusion or convective mass transfer of glycerol between the PEM and substrate. This thesis demonstrates the AAP method as an effective and practical approach for fabricating PEM membranes at scale. The PEM membranes can be tailored from loose to dense NF to meet diverse water treatment needs. Overall, this thesis advances the fabrication, application, and storage of PEM membranes, providing key technical insights that support their broader application in sustainable water purification.