(593a) Using Green Solvents for the Fabrication of Adsorptive Membranes Via Surface Segregation and Vapor Induced Phase Separation Method

The demand for clean water is increasing due to population growth, climate change, and emerging pollutants. Due to their rapid mass transfer characteristics, adsorptive membranes with high porosities and binding site densities show promise for removing trace-level contaminants in a cost-effective and energy-efficient manner. However, traditional fabrication methods based on nonsolvent or vapor induced phase separation (NIPS or VIPS) processes rely on organic solvents, such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and dimethylacetamide (DMAc), that have negative environmental and health impacts. This study investigated the use of Cyrene, a bio-derived and eco-responsible solvent, in the surface-segregation-vapor-induced phase separation (SVIPS) method to create polysulfone-based composite membranes with functionalized pore walls.

In NIPS and VIPS techniques, casting solutions containing a single homopolymer are often used to fabricate membranes with high porosities. In comparison, the SVIPS process uses a casting solution that contains both a homopolymer and an amphiphilic copolymer. When this mixed casting solution is exposed to water vapor, the more hydrophilic block in the copolymer moves towards to the surface of the pore walls while the hydrophobic blocks remain embedded in the homopolymer matrix. Subsequently, the hydrophilic block lining the pore walls can be used directly as binding sites or as functional handles to further tailor the sorbent chemistry.

In this study, polysulfone/polystyrene-block-poly(acrylic acid) (Psf/PS-PAA) membranes were fabricated using the SVIPS process. Casting solutions containing 8-12 wt% polysulfone (Psf) in Cyrene were first examined, and a 10 wt% solution was found to avoid large shrinkage in the cast membrane area and led to highly porous cellular structures. While Cyrene has good solubility towards Psf, its solubility towards copolymers is limited. To overcome this, ethyl acetate (EA) or methyl ethyl ketone (MEK) were added to Cyrene at different ratio to fully dissolve the copolymer and create homogeneous casting. The casting times and nonsolvent selections were optimized to achieve highly-porous membranes that have a Psf matrix and high density of PAA brushes on the surface of the pore walls. Membrane properties such as permeabilities and binding capacities towards copper before and after modification with polyethylenimine (PEI) moieties were also characterized and compared with Psf/PS-PAA membranes generated using SVIPS with traditional organic solvents. The results of these experiments demonstrated that green solvents can be used to create high-quality polymer membranes with comparable performance, which enables sustainable strategies for the fabrication of next-generation sorbents.