Influence of Solvent Affinity on Transport through Cross-Linked Copolymer Membranes for Organic Solvent Nanofiltration

Membranes based on microphase-separated copolymers offeran opportunityto address the need for resilient materials that can be used in organicsolvent-based filtration. Specifically, copolymer repeat unit chemistriescan be chosen to impart solvent compatibility, to tailor membranenanostructure, and to enable postsynthetic modification. In this study,a poly(trifluoroethyl methacrylate-co-oligo(ethyleneglycol) methyl ether methacrylate-co-glycidyl methacrylate)[P(TFEMA-OEGMA-GMA)] copolymer was synthesized and fabricated intoflat sheet and hollow fiber membranes using a non-solvent-inducedphase separation casting technique. The GMA repeat units possess epoxidegroups that were used to cross-link the copolymer through a ring-openingreaction with diamines ranging from diaminoethane to diaminooctane.Transport experiments in water, methanol, ethanol, tetrahydrofuran,dimethylformamide, and toluene demonstrated that films reacted withlonger diamines, such as diaminohexane, result in stable membranes.Conversely, the films reacted with shorter diamines degraded uponexposure to organic solvents. Because of their stability in organicsolvents, transport through the diaminohexane-functionalized membraneswas characterized in more detail using hydraulic permeability andneutral solute rejection experiments. The results of these experimentsalong with volumetric swelling and small-angle X-ray scattering (SAXS)analysis revealed that the solvent affinity for the constituent copolymerdomains is critical in determining the permeation pathway. For P(TFEMA-OEGMA-GMA)membranes in protic solvents, such as ethanol, transport through thehydrophilic side chains of OEGMA was favored, while for membranesin aprotic solvents, such as toluene, transport through the hydrophobicmatrix dominated. In neutral solute rejection experiments, 2000 gmol(-1) polypropylene glycol molecules (solvated size & SIM;2 nm) permeated through the hydrophobic domain unhinderedbut were fully rejected when permeation occurred through the hydrophilicregion. These differences highlight the need to understand the interactionsbetween the copolymer domains and solvent when solvent-resilientmembranes are developed for organic solvent filtration.