Selective Swelling of Stretched Block Copolymer Hollow Fibers for Upgraded Membrane Performance

Block copolymers are showing great potential in the preparation of advanced membranes for precise separations, and there is a strong interest in the development of hollow-fiber membranes (HFMs) from block copolymers. Herein, we report polysulfone-block-poly(ethylene glycol) (PSF-b-PEG) HFMs with enhanced separation performance prepared by a convenient stretching strategy. Axial stretching is first applied to the melt-spun PSF-b-PEG hollow fibers, efficiently reducing the wall thickness and outer diameter of the hollow fibers. The stretched hollow fibers are then subjected to selective swelling, thus producing HFMs with interconnected nanoporosity. We find that the degree of stretching significantly influences the diameter, length, and separation performance as well as the crystallization behavior of the hollow fibers before and after selective swelling. With a stretching percentage of 527%, the HFMs exhibit simultaneously enhanced water permeance and rejection because of thinned fiber walls and reduced pore sizes. This work reveals the important role of axial stretching in upgrading the performance of block copolymer HFMs and demonstrates that the selective swelling coupled with melt spinning is promising in the cleaner preparation of HFMs.