Boosting the permeation of ultrafiltration membranes by covalent organic frameworks nanofillers: Nanofibers doing better than nanoparticles

Mixed matrix membranes (MMMs) have attracted everlasting attentions, as their large permeance and high selectivity can be accessible synchronously. Herein, we show that covalent organic framework (COF) nanofibers serving as the nanofillers are doing better than COF nanoparticles in boosting the separation performance of MMMs. The nanofibers and the nanoparticles are synthesized and doped into polysulfone matrices to prepare MMMs. The high porosity and relative hydrophilicity of TpPa nanofibers and nanoparticles enable rapid exchange of solvent and nonsolvent during the process of nonsolvent induced phase separation, thus leading to instantaneous demixing. Furthermore, compared with TpPa nanoparticles possessing small and irregular shapes, the one-dimensional structure of TpPa nanofibers weakens the entanglement of the surrounding polysulfone linear chains, resulting in much pronounced instantaneous demixing. In contrast with the nanoparticle-incorporated membranes, the porosity of nanofiber-incorporated membranes is significantly improved. The resultant nanofiber-incorporated membrane demonstrates a high water permeance of 424.4 L m⁻² h⁻¹ bar⁻¹, along with a BSA rejection rate of 94.3%. This work clearly reveals that porous one-dimensional structures such as COF nanofibers are highly promising in the preparation of high-performance MMMs.