(144i) Reducing Physical Aging of Microporous Polymer Membranes through Blending with Porous Polymer Networks: Experimental Analysis and Molecular Simulations

Although polymer membranes could help reduce the energy burden of conventional thermally based separations, their practical usability is limited by a number of unsolved issues, such as the lack of adequate long-term stability and durability. Microporous glassy polymers, which have emerged as a promising material platform for membrane manufacturing, suffer from poor long-term stability, as they rapidly age, even when fabricated as bulky films. Hence, to help address this roadblock in the field, we evaluated the effect of porous polymer networks (PPNs), which are relatively inexpensive (<$50/g), hyper-crosslinked microporous materials, on the aging propensity of a model microporous polymer, namely PTMSP. PPN was chosen as it is compatible with organic polymers, has high internal surface areas, and adjustable surface properties, making it a platform for tuning the interaction with various polymeric candidates.

Samples of neat PTMSP, 5% wt. triptycene-isatin PPN in PTMSP, and 20% wt triptycene-isatin PPN in PTMSP were fabricated and tested as gas separation membranes. Samples were characterized using ATR-FTIR and SEM to investigate their morphology. Positron Annihilation Lifetime Spectroscopy (PALS) measurements were performed to investigate the effect of PPN on the PTMSP free volume architecture, distribution and stability, showing that partial PTMSP adsorption in the PPN surface porosity breaks the free volume connectivity. NMR spin-lattice relaxation times (T1) were measured on fresh and aged samples to identify the interactions taking place. Furthermore, physical aging was tracked using N2 permeability measurements of ~20 μm thick films for one month, and the Struik Model was fit to extract an interpretation for the aging rate reduction. PPN incorporation was found to significantly reduce the rate of physical aging, even with only 5 wt.% loading. MD simulations helped elucidate the molecular mechanism by which PPN affect the PTMSP aging propensity and transport properties.