(98a) Increasing Diffusion Selectivity Via Free Volume Manipulation (FVM) and in Situ Thermal Oxidative Crosslinks of Amine-Functionalized PIM-1 Membrane

Polymers of intrinsic microporosity (PIMs) show promise as membrane materials because of their high free volume and diffusion selectivity from their rigid backbone structures. PIMs can be functionalized with polar moieties like amines to enhance CO₂ sorption; however, the introduction of polar groups that form hydrogen bonds also reduces free volume, making it challenging to increase CO₂ sorption without compromising diffusivity. To overcome the loss of free volume induced by polar moieties, a solid-state deprotection approach called free volume manipulation (FVM) has been developed to reconstruct physical packing structures of polymers without significantly changing the polymer chemistry. FVM protects polar moieties with labile and bulky groups to disrupt chain packing and widen intersegmental distances. When the labile groups are deprotected from the solid-state film using heat treatments, the polymer recovers its original polar moiety, increases free volume, and gives access to new diffusion pathways for gas penetrants. FVM was demonstrated to be highly effective for amine-functionalized PIM-1 (PIM-NH₂) using labile tert-butoxycarbonyl (tBOC) group and resulted in a simultaneous boost in permeability and selectivity, which is a difficult enhancement for polymer membranes due to the trade-off relationship between permeability and selectivity. During FVM, light urea crosslinks form and play a significant role in preserving the increased free volume, narrowing the free volume element distribution, and providing high energetic barriers for the diffusion of larger penetrant molecules, thereby increasing the diffusion selectivity of the membrane.

In this study, the diffusion selectivity of PIM-NH₂ is further increased by systematically dosing in a small amount of oxygen during the deprotection of tBOC groups to induce thermal oxidative crosslinks. The oxidative crosslinks were found to occur starting at approximately 170 °C in the presence of oxygen, and chemical characterizations indicated the formation of isocyanate and new oxidative crosslink species like amides. The pure-gas permeation and sorption results significantly changed with oxygen dose concentrations, indicating that the new oxidative crosslinks impacted the gas transport properties. For example, dosing in 2 mol % O₂ during FVM yielded a 6-fold increase in CO₂/CH₄ selectivity and a 76% increase in CO₂ permeability while dosing in 3 mol % O₂ yielded a 10-fold increase in CO₂/CH₄ selectivity and 30% decrease in CO₂ permeability compared to PIM-NH₂. In addition, the increased density of crosslinks that covalently bond polymer segments together provided a high resistance to CO₂ plasticization and physical aging. This study demonstrates a low-temperature crosslinking strategy using labile pendant groups that tunes the diffusion properties of amine-functionalized polymers and yields stable long-term membrane performance.