High flux hydrophobic membranes for organic solvent nanofiltration (OSN)—Interfacial polymerization, surface modification and solvent activation

This paper describes the formation of a new generation of hydrophobic organic solvent nanofiltration (OSN) membranes: high flux hydrophobic thin film composite (TFC) membranes via interfacial polymerization. These are the first reported hydrophobic TFC membranes which are stable in DMF. They exhibit significantly higher permeabilities for nonpolar solvents, including toluene and ethyl acetate, than commercial OSN hydrophobic integrally skinned asymmetric and rubber coated membranes and yet have comparable or better selectivity. Solvent stable crosslinked polyimide ultrafiltration membranes were used as supports for the formation of these TFC membranes. For some membranes, a mixture of acyl chlorides (trimesoyl chloride blended with a monoacyl chloride containing fluorine) was used during the interfacial polymerization to manipulate molecular weight cut off and to make the membranes more hydrophobic. Measured by the rejection curves, the loosest membrane was prepared when the mixture of acyl chlorides was used, and the tightest when trimesoyl chloride was used alone. To increase nonpolar solvent flux the free acyl chloride groups on the TFC membrane surface were capped with different monomers containing hydrophobic groups. Comparison of TFC membranes formed with and without capping suggests that the chemistry of the membrane surface plays an important role in solvent permeation. As reported previously by our group, in order to “activate” solvent flux we post-treated the TFC membranes with DMF. Incorporation of F and Si to the polyamide top layer resulted in dramatically improved permeabilities for non-polar solvents. Such hydrophobic TFC membranes prepared via interfacial polymerization and treated with an activating solvent may lead to the next generation of high performance hydrophobic OSN membranes.