Eco-Friendly Solvent-Based Fabrication of Single-Layer and Polydopamine-Modified Bilayer PVDF-HFP Membranes

Polymeric membrane systems have emerged as an effective approach for water separations due to their high separation efficiency, simplicity, and adaptability to a wide range of water treatment applications. However, traditional membrane fabrication processes often rely on toxic organic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc), which pose environmental and health risks. Eco-friendly solvents have been investigated as an alternative to traditional toxic solvents. This study investigates the fabrication and performance of polymeric membranes using eco-friendly solvent systems, with a focus on bilayer membranes designed to improve separation performance over traditional single-layer membranes. Membranes were fabricated using eco-friendly solvents, Rhodiasolv© PolarClean and gamma-valerolactone in combination with polymers polysulfone (PSf) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Membranes were fabricated using nonsolvent-induced phase separation (NIPS) through doctor blade extrusion (DBE) and slot-die coating (SDC) methods in order to compare traditional laboratory-scale casting methods (DBE) with scalable fabrication techniques (SDC). Bilayer membranes were investigated to address limitations presented by single-layer membranes, with polydopamine (PDA) incorporated as an adhesion-promoting additive. Membrane characterization included scanning electron microscopy (SEM), contact angle measurements, and tensile testing, along with water permeability and solute rejection tests. Results showed that polymer concentration and casting method influenced permeability and solute rejection, with the SDC bilayers reaching the highest initial BSA rejection. Filtration used deionized water and model aqueous feeds containing 100 ppm NaCl, 100 ppm CaCl₂, and 100 ppm bovine serum albumin (BSA).