High-Performance Carboxylated Polymers of Intrinsic Microporosity (PIMs) with Tunable Gas Transport Properties

Carboxylated polymers of intrinsic microporosity (carboxylated PIMs) are reported as potential high-performance materials for membrane-based gas separation. Carboxylated PIM membranes were prepared by in situ hydrolysis of the nitrile groups of PIM-1 films. Structural characterization was performed by Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR). The degree of hydrolysis was determined by carbon elemental analysis. The thermal properties were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Compared with PIM-1, carboxylated PIMs with different degrees of hydrolysis have similar thermal and mechanical properties but show higher selectivity for gas pairs such as O2/N2, CO2/N2, He/N2, and H2/N2 with a corresponding decrease in permeability. Selectivity coupled to high permeability combines to exceed the Robeson upper-bound line for the O2/N2 gas pair. This work demonstrates that significant improvements in gas separation properties may be obtained through postmodification of nitrile-based PIM membranes. The present work improves the understanding of the relationship of structure/permeation properties and also extends the PIM spectrum beyond those reported previously. In addition, the incorporation of carboxylic acid sites has the potential for further modification reactions such as grafting and cross-linking.