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.