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

Membranes based on microphase-separated copolymers offer an opportunity to address the need for resilient materials that can be used in organic solvent-based filtration. Specifically, copolymer repeat unit chemistries can be chosen to impart solvent compatibility, to tailor membrane nanostructure, and to enable postsynthetic modification. In this study, a poly(trifluoroethyl methacrylate-co-oligo(ethylene glycol) methyl ether methacrylate-co-glycidyl methacrylate) [P(TFEMA-OEGMA-GMA)] copolymer was synthesized and fabricated into flat sheet and hollow fiber membranes using a non-solvent-induced phase separation casting technique. The GMA repeat units possess epoxide groups that were used to cross-link the copolymer through a ring-opening reaction with diamines ranging from diaminoethane to diaminooctane. Transport experiments in water, methanol, ethanol, tetrahydrofuran, dimethylformamide, and toluene demonstrated that films reacted with longer diamines, such as diaminohexane, result in stable membranes. Conversely, the films reacted with shorter diamines degraded upon exposure to organic solvents. Because of their stability in organic solvents, transport through the diaminohexane-functionalized membranes was characterized in more detail using hydraulic permeability and neutral solute rejection experiments. The results of these experiments along with volumetric swelling and small-angle X-ray scattering (SAXS) analysis revealed that the solvent affinity for the constituent copolymer domains is critical in determining the permeation pathway. For P(TFEMA-OEGMA-GMA) membranes in protic solvents, such as ethanol, transport through the hydrophilic side chains of OEGMA was favored, while for membranes in aprotic solvents, such as toluene, transport through the hydrophobic matrix dominated. In neutral solute rejection experiments, 2000 g mol^-1 polypropylene glycol molecules (solvated size ∼2 nm) permeated through the hydrophobic domain unhindered but were fully rejected when permeation occurred through the hydrophilic region. These differences highlight the need to understand the interactions between the copolymer domains and solvent when solvent-resilient membranes are developed for organic solvent filtration.