In Situ Growth of Cationic Covalent Organic Frameworks (COFs) for Mixed Matrix Membranes with Enhanced Performances

Covalent organic frameworks (COFs) are increasingly utilized as doping agents for the design of advanced ultrafiltration mixed matrix membranes, thanks to their prominent nanoporosity and excellent polymer compatibility. However, current strategies are largely limited in the complicated postaddition of neutral COF particulates. Herein, cationic COFs, namely, TpEB, with sizes down to ∼39 nm are in situ synthesized in polyacrylonitrile (PAN) solution as crystalline fillers for the production of highly permeable TpEB–PAN ultrafiltration membranes. After the condensation of monomer pairs, the growth of cationic TpEB crystallites is restrained due to the electrostatic interaction with negatively charged PAN chains, leading to the formation of a homogeneous TpEB-incorporated casting solution. During the subsequent nonsolvent-induced phase separation process, TpEB crystallites facilitate exchange between the solvent and the nonsolvent because of their hydrophilic and nanoporous nature, accelerating the rate of phase inversion to form a highly porous membrane surface. Thus-prepared TpEB–PAN membranes deliver a tight rejection of BSA with water permeance of up to 380 L m–2 h–1 bar–1, which is 35.6% higher than that of the original PAN membranes prepared without TpEB. The TpEB–PAN membranes also exhibit enhanced operation stabilities and fouling resistances. This in situ growth strategy suggests a new avenue for the preparation of advanced mixed matrix membranes.