Alkoxy-functionalized covalent organic polymer membranes for high-performance dye/salt separation

The trade-off between permeability and selectivity is a longstanding challenge in nanofiltration membranes that critically limits their practical performance in dye/salt separation. In this work, we present the rational design and fabrication of a series of alkoxy side chain–functionalized covalent organic polymer (COP) thin-film composite membranes (Bth-PEOx) on polyacrylonitrile (PAN) ultrafiltration supports via interfacial polymerization. The resulting membranes enable precise modulation of the nanoscale transport microenvironment—specifically, pore size distribution and surface hydrophilicity—through controlled incorporation of alkoxy side chains of varying length. The optimized Bth-PEO2 membrane achieves highly efficient dye/salt separation in Congo red (CR)/NaCl mixtures, delivering a high water permeance of 45.6 L m −2 h −1 bar −1 , >99% CR rejection, minimal NaCl rejection (6.23%), and an exceptional CR/NaCl separation factor of 117. In addition, the Bth-PEOx membranes exhibit outstanding chemical robustness across extreme pH conditions, superior mechanical resistance to hydraulic compaction, and stable performance during continuous filtration processes. This side chain engineering strategy overcomes the classic permeability–selectivity trade-off by enabling simultaneous improvement of both permeability and selectivity, thereby establishing a scalable, structure–property–guided design paradigm for energy-efficient dye/salt separation and advancing the practical treatment and resource recovery from high-salinity textile wastewater.