Modified Polymer of Intrinsic Microporosity Membranes for Gas Separation

Energy consumed for chemical separations in the US is about 17,000 quadrillion Joules/year, accounting for 50% of the energy consumed by American industry and 15% of the US’s yearly energy consumption. More generally, 10% of world energy consumption, or 7.5 GJ per person every year, is devoted to chemical separations. Although membrane-based separations would improve the energy efficiency of existing technologies by 90%, some major roadblocks hinder further development in this area, such as selectivity-permeability trade-off and long-term instability exhibited by polymer membranes. Polymers of Intrinsic Microporosity, or PIMs, offer potential for membrane applications, but they suffer from long term instability due to plasticization and physical aging. The latter is caused by extremely high non-equilibrium fractional free volume, higher than 30%, trapped in these materials. Chemically and physically modified PIMs are being developed to enhance their long-term stability while maintaining or increasing their permeability and selectivity. Addition of fillers with polymers to form mixed matrix membranes is a common route to tune structural and transport properties, but many filler-polymer pairs contain interfacial defects when formed into membranes. We proposed a dual-modification of PIM-1 to address the challenges of aging and plasticization comprised of a thermal crosslinking approach and blending with hyper-crosslinked porous structures, called Porous Polymer Networks (PPNs). PIM-1 was functionalized by an acid-catalyzed hydrolysis of nitrile substituents to carboxylic acids, and the resulting acid groups were crosslinked in vacuo at temperatures as low as 180 ℃. Prior to the thermal-annealing, carboxy-PIM was blended with triptycene-isatin PPN. The rationale for doing so is that PPN can provide stable, aging-resistant configurational free volume in the form of triptycene while interacting with PIM-COOH via hydrogen bonding between the isatin lactam and PIM carboxyl moieties, thus eliminating interfacial defects by promoting intimate contact with the polymer matrix. Interfacial compatibility between PIM-COOH and the blended PPN was confirmed via scanning electron microscopy. We hypothesize that crosslinking of PPN with PIM-COOH occurs by a condensation between lactam-carboxyl and carboxyl-carboxyl pairs. To test this hypothesis, membranes were annealed in the presence of water to see if, through Le Chatelier’s Principle, the crosslinking was hindered. A mechanistic study by Fourier-transform infrared spectroscopy, thermogravimetric analysis, and fluorescence spectroscopy reveal that under dry conditions, water is evolved from the mixed systems, while heating in the presence of water significantly reduces the degree of crosslinking, which confirms that a reversible condensation is the first step in the crosslinking mechanism.