Polymer-based membranes for gas separations represent a great opportunity for reducing cost, energy usage, and CO<sub>2</sub> emissions in chemical separation processes. One major challenge of polymer gas separation membranes is physical aging, which occurs due to polymer chains relaxation to approach a thermodynamic equilibrium of chain packing. Aging polymers lose their free volume and becoming more densely packed over time causing a corresponding permeability drop. PIM-1 is a representative high-free-volume glassy polymer membrane material, showing ultrahigh permeability and good selectivity. However, PIM-1 has not seen widespread industrial usage due to its very rapid physical aging, e.g., permeability of large gases like methane dropping up to 30% within just one week. Existing methods that restrict chains segmental motion, such as crosslinking, effectively slow physical aging but typically cause large permeability losses. For example, a thermally crosslinked PIM-1 was reported to have an 80% decrease in methane permeability compared to the untreated film.  This research attempts to address this issue by exploring a macromolecular design of self semi-interpenetrating networks (ss-IPN), where linear PIM-1 chains penetrate through crosslinked unimodal networks of PIM-1 to mitigate crosslinking-induced permeability loss and prevent the free volume collapse via the rigid network ‘scaffold’. A series of PIM-1 ss-IPN membranes with systematically varied structures are prepared. Specifically, the crosslink density of the unimodal network is precisely controlled by varying the molecular weight of telechelic PIM-1 oligomers. Additionally, the effect of the molar ratio between linear and crosslinked content on transport properties and aging behavior is investigated. Results show a vastly improved aging resistance in PIM-1 ss-IPN membranes compared to linear PIM-1 control. For example, a PIM-1 ss-IPN membrane with 10:1 linear-to-crosslinked ratio and unimodal network of 4k intercrosslink chain length showed a methane permeability reduction of only 10% over two weeks while maintaining fresh permeability and selectivity seen in the pure linear PIM-1 films. Discussions will focus on the synthesis and characterization of these new PIM-1 ss-IPN membranes with an emphasis on the fundamental structure-property relationship.

Architecting self semi-interpenetrating network (ss-IPN) structures in PIM-1 membranes for enhanced physical aging resistance

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