Open and dense hollow fiber nanofiltration membranes through a streamlined polyelectrolyte-based spinning process

Polyelectrolyte composite hollow fiber membranes show a high potential to become applied in industrial nanofiltration applications such as water purification or downstream processing in several industries. This material platform allows multiple routes to achieve tailored selectivities and high permeances. Hollow fiber membranes promise a higher process efficiency due to higher packing densities and their backwash stability. However, the fabrication of composite polyelectrolyte hollow fiber nanofiltration membranes is more challenging compared to state of the art spiral wound modules. While the Layer-by-Layer deposition has proven versatile opportunities and is recently entering the commercial market, the multi-step post-treatment is elaborate. In this work, we streamline the fabrication of polyelectrolyte composite hollow fiber membranes. We extend our recently presented ”chemistry-in-a-spinneret” approach with a membrane modification in a subsequent coating bath. The additional modification can be incorporated in the composite hollow fiber membrane manufacturing process without additional process steps. Sulfonated polyethersulfone as a polyanionic additive is used in the polymer solution which complexates with the polycations polyethylenimine (PEI) or poly(diallyldimethylammonium chloride (PDADMAC) in the bore fluid during fiber spinning. The extruded fibers have a positively charged lumen surface which allows a subsequent modification with the polyanion poly(sodium 4-styrenesulfonate) (PSS) in the coating bath. The spun fibers reveal with 16-4 LMH/bar pure water permeance (PWP) and 2000-1100 Dalton (Da) molecular weight cut-off (MWCO) open nanofiltration characteristics, while the PEI+PSS modified fibers show dense nanofiltration characteristics (0.6 LMH/bar PWP and 360 Da MWCO). The fibers are backwash stable and the PDADMAC and PDADMAC+PSS fibers show a significantly higher sodium hypochlorite (NaOCl) stability compared to the PEI and PEI+PSS fibers. This new technique creates composite hollow fiber nanofiltration membranes with a polyelectrolyte bilayer without multi-step post-treatment. Therefore, it provides a promising alternative to the Layer-by-Layer post-modification, where two coating steps and one water washing post-treatment step are required to create a polyelectrolyte bilayer on a porous hollow fiber membrane support.