Facile metal-ion infiltration into polyimide membranes with coordination crosslinking for efficient gas separation

We report a facile metal-ion infiltration method for engineering the properties of 6FDA-DAM:DABA (6FDA: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, DAM: 2,4,6-trimethyl-1,3-diaminobenzene, DABA: 3,5-diaminobenzoic acid; referred to as 6FDD) polyimide membranes. The 6FDD polyimide membrane samples were simply immersed in metal ion (copper, zinc, and cobalt) solutions of various concentrations (1 and 10 wt%) to obtain metal-ion-infiltrated 6FDD membranes. The metal ions and the carboxylate group of the 6FDD polyimide form a coordination bond, which can increase the d-spacing between the inter-polymer chain distances and reduce polymer chain mobility. The coordination between the metal ion and the carboxylated polymer chain enhances the mechanical properties of the polyimide membranes. The metal ion coordination provides not only increased stiffness but also high resistance to the breaking point under elongation. The metal ions were distributed uniformly across the entire 6FDD membrane without any noticeable defects, which indicated the effectiveness and simplicity of the proposed method, which can be applied to versatile membrane module types and conformations. The metal-ion infiltrated 6FDD membrane exhibited improved gas permeability compared to that of the pure 6FDD membrane because of the enhanced diffusivity coefficient resulting from the increased d-spacing. Particularly, the metal-ion-infiltrated 6FDD membranes exhibited significantly higher plasticization resistance because of a decrease in the normalized CO2 permeability at 750 psi compared to that of the pure 6FDD membrane. For example, the normalized CO2 permeability of the 10 wt% copper-ion-infiltrated 6FDD membrane increased by 120% but that of pure 6FDD membrane increased by 315%. This was because the metal–carboxylate group coordination strongly prevented relaxation of the polymer chain when it was exposed to plasticizing gases at high gas feed pressures.