Hollow-fiber membranes of block copolymers by melt spinning and selective swelling

Abstract

Selective swelling of block copolymers has emerged as an efficient strategy to prepare ultrafiltration membranes with well-defined porosities. However, the application of this strategy is only limited to the preparation of flat-sheet membranes so far. Herein, we extend this selective swelling strategy, for the first time, to the preparation of hollow-fiber membranes (HFMs). A mechanically strong copolymer available at large scale, polysulfone-block-poly(ethylene glycol) (PSF-b-PEG), is employed as the membrane-forming polymer. The PSF-b-PEG melt without any diluters or additives exhibits good fluidity, enabling stable and continuous melt spinning of primary hollow fibers with dense fiber walls. Subsequent selective swelling in the mixture of n-propyl alcohol and acetone produces interconnected nanoporosities throughout the fiber walls, thus forming HFMs with PEG chains enriched on the membrane surface. The wall thicknesses as well as pore sizes, and thus the separation performances of the HFMs are tunable by changing the spinning and swelling parameters. In contrast to the strong hydrophobicity of HFMs prepared by cold stretching of polyolfins, our HFMs possess water-wettable surfaces due to the presence of PEG chains, and can be directly used in aqueous milieu. Because of the PSF-based skeleton and homogenous distribution of nanopores, the HFMs exhibit excellent tensile strengths up to 12.5 MPa, much higher than that of HFMs prepared by the commonly used phase inversion processes. Furthermore, the BSA rejection and water permeance of the HFMs currently can be tuned in the range of 29.9–88.2% and 13.7–119.3 L m-2·h-1·bar-1, respectively, by changing the spinning and/or swelling parameters. Their performances are expected to enhance by optimizing the hollow fiber geometries and pore structures. This strategy neither uses any solvents to fluidize the polymer nor produces wastewater, and therefore is a “cleaner” process. This work not only extends the usages of selective swelling, but also establishes a new process to manufacture HFMs.