Cooperative Steric Modulation of Flexibility, Disorder, and Pore Size in Two-Dimensional Covalent Organic Framework Membranes for Enhanced Selective Ion Sieving

The inherent flexibility of two-dimensional covalent organic frameworks (2D COFs), stemming from weak interlayer interactions, often leads to structural disorder that significantly impacts their properties and functionality. However, this intrinsic, solvent-dependent flexibility and disorder pose considerable challenges for the fabrication, processing, and application of 2D COF membranes, hindering both the reproducibility of membrane synthesis and their long-term performance stability. In this work, we address these challenges by precisely controlling the number of methyl groups on the amine monomers, skillfully leveraging steric hindrance effects. Furthermore, we exploit the influence of solvent polarity on the solvent-mediated equilibria between stable and metastable states to synergistically tune the flexibility, disorder, and pore size of the 2D COF membranes. This approach simultaneously reduces the pore size and enhances the framework's solvent resistance, yielding methyl-functionalized 2D COF membranes that demonstrate preserved permeability homogeneity across different solvent treatments while enabling highly efficient U/Th ion separation.