Reducing Physical Aging of Microporous Polymer Membranes through Porous Polymer Network Blending

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 the efficiency when compared to conventional, thermal-based separations. However, several issues arise when utilizing membrane materials, 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 using relatively inexpensive, under $50 per gram, hyper-crosslinked amorphous, microporous structures, known as porous polymer networks (PPNs), composed of triptycene and isatin. These networks were blended with a model microporous polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP) to properly understand the potential of PPN to decrease the transient free volume collapse of glassy polymer membrane materials. PPN, in particular, was chosen as it is compatible with organic polymers, has high surface areas, and tunable surface properties. Samples of neat PTMSP, 5% wt. triptycene-isatin PPN incorporated into PTMSP (PTMSP-5PPN), and 20% wt. triptycene-isatin PPN incorporated into 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 additionally performed on aged samples of neat PTMSP and PTMSP-5PPN to measure the pore size distribution changes. Furthermore, physical aging was tracked using N₂ permeability measurements of 20 μm neat PTMSP and PTMSP-20PPN films at 35°C for one month. Permeability and gas uptake of 80 μm samples were measured as well to analyze the effect of PPN on gas permeability and selectivity. Dilatometry measurements of neat PTMSP, PTMSP-5PPN, and PTMSP-20PPN were used to directly measure volumetric contraction of physical aging and make comparisons with permeability measurements. The potential for PPN to hinder the permeability decline due to physical aging is evaluated, and different mechanisms of action for the observed differences are assessed in terms of either a hindered excess free volume collapse by PPN or rearrangement of PTMSP’s free volume distribution and interconnectivity due to incorporation of PPN. This study is one of the first addressing, at a fundamental level, the molecular mechanism by which permeability decline due to physical aging can be reduced, opening the doors to a more rational design of membrane systems for large scale, energy-efficient separations.