Smart Solvent-Responsive Covalent Organic Framework Membranes with Self-regulating Pore Size

Covalent organic frameworks (COFs) are an emerging class of crystalline porous materials with low density, high porosity, and excellent physicochemical stability. Recently, smart COFs with special structures and functional groups have been widely concerned, which can respond to external stimuli such as pH, temperature, light, gas, etc. However, it is still a paramount challenge to fabricate smart COF membranes possessing the advantage of alternating the pore size to realize gradient separation of multiple organic pollutants. In this work, we fabricated a 2D amine-linked smart COF membrane coupling hardness with softness via interfacial polymerization under room temperature and atmospheric pressure conditions. Interestingly, these COF membranes show higher dye rejection performance in water than in acetone, and the pore size of smart COF membranes can be dynamically regulated by different solvents. Powder X-ray diffraction (PXRD) was used to identify the intrinsic structure change of this intriguing phenomenon. By serendipity, we found that the PXRD peak shifted to a lower degree when the membranes were immersed in acetone than in water, suggesting enlargement of the pore size due to flexible frameworks. The dynamic enlargement of the pore size was reversible when the solvent was switched from acetone to water. This study not only provides a frontier of smart COF membranes but also broadens potential applications of COF materials.