Physical aging of defect-free asymmetric functionalized PIM hollow fiber membranes

Polymers of intrinsic microporosity (PIMs) have rigid and contorted backbones that result in inefficient packing and a correspondingly high fractional free volume, enabling these materials to overcome the permeability–selectivity trade-off inherent in membranes. However, as a result of their excess free volume, these polymers suffer from a phenomenon known as physical aging, which results in a decrease in free volume over time as the polymer chains relax toward their equilibrium state, thereby lowering a membrane's overall permeability. Therefore, it is important to understand this phenomenon in microporous materials, especially in industrially relevant form factors such as hollow fibers. In this work, high molecular weight PIM-1 was synthesized at a multi-decagram scale for use in hollow fiber spinning. The spun fibers then underwent solid-state functionalization to yield an amine-functionalized derivative of PIM-1 known as PIM-NH₂, providing moieties for hydrogen bonding and modifications to polymer chain mobility. These chemical and structural features can influence physical aging. As a result of their thinner selective layers, both PIM fiber compositions experienced accelerated aging and greater gains in selectivity when compared to thicker dense films. For PIM-NH₂ fibers, greater gains in selectivity were observed with aging, especially for CO₂-based separations when compared to PIM-1 fibers due to the higher sorption affinity that amines have towards CO₂. This work represents an in-depth aging study performed on hollow fiber membranes spun from microporous organic polymers, allowing for a better understanding of physical aging and the long-term performance of PIM-based materials in the hollow fiber geometry.