(45a) Reducing Physical Aging of Microporous Polymer Membranes through Blending with Porous Polymer Networks

The Sixth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) highlighted that global average temperature increase relative to pre-industrial levels will likely exceed 1.5°C by 2050. To mitigate the effects of global warming, efficient and practical carbon capture technologies are urgently needed. Membrane techniques for CO₂ separation from gaseous streams, based on microporous polymers, would greatly enhance efficiency when compared to conventional thermal-based separations.

However, several issues arise when utilizing microporous glassy membranes, one being their rapid physical aging and instability under long-term industrial operations and harsh conditions. Hence, in this study, we evaluated the effect of relatively inexpensive (<$50/g) porous polymer networks (PPNs) composed of triptycene and isatin on the short- and long-term permeability and selectivity of poly(1-trimethylsilyl-1-propyne) (PTMSP), a model microporous polymer. The overarching hypothesis of this study is that polymer chains can interlock through the PPN porosity, which is expected to freeze chain dynamics and reduce the aging rate.

Samples of neat PTMSP, 5% wt. triptycene-isatin PPN in PTMSP (PTMSP-5PPN), and 20% wt triptycene-isatin PPN in PTMSP (PTMSP-20PPN) were fabricated. Membranes were characterized using ATR-FTIR and SEM to verify the material morphology and compatibility between PPN and PTMSP. Positron Annihilation Lifetime Spectroscopy (PALS) measurements were performed to systematically investigate the effect of PPN on the PTMSP pore size distribution and their long-term stability. NMR spin-lattice relaxation times (T₁) were measured on fresh and aged samples to investigate, at the molecular level, the physical interactions (e.g. rigidification, chain absorption) taking place between the PTMSP and the PPN. Furthermore, physical aging was tracked via N₂ permeability measurements in ~20 μm thick neat PTMSP, PTMSP-5PPN, and PTMSP-20PPN films at 35°C for one month, and the Struik Model was fit to extract a fundamental interpretation for the aging rate reduction. Permeability and gas uptake of 80 μm samples were measured as well to analyze the effect of PPN on gas permeability and selectivity. Blending with PPN was found to significantly reduce the PTMSP aging rate, even with only 5 wt.% loading. Various mechanisms of action for the observed behavior are assessed in terms of either a ‘threading’ effect in which PTMSP intercalates through the PPN or alternative mechanisms involving PTMSP adsorption to the PPN surface. Molecular simulations were run to support and rationalize the experimental findings.

At the best of pour knowledge, this is one of the most detailed analyses proposed so far to investigate the effect of PPN on the long-term stability of microporous glassy polymers.