Rational Design of Highly Selective and Plasticization Resistant Polymers of Intrinsic Microporosity (PIMs) Inspired By Competitive Sorption

Diffusion-selective polymers of intrinsic microporosity (PIMs) have shown excellent pure-gas separation performance due to their rigid backbones, inefficient packing, and high free volume. Their out-of-equilibrium packing morphologies, however, make PIMs susceptible to physical aging and their intrachain rigidity alone has shown insufficient to mitigate plasticization, where selectivity is significantly reduced for gas mixtures at high-pressure. A number of studies on the mixed-gas transport of PIMs have shown the effects of competitive sorption on separation performance, where gases with high polymer affinity (e.g., CO₂) can reduce the sorption of co-penetrants in a mixture (e.g., CH₄, N₂) and increase sorption selectivity. However, due to CO₂-induced plasticization at high pressures, decreases in diffusion selectivity can outweigh beneficial competition effects. This trade-off in performance is especially detrimental for PIMs with little CO₂ affinity and poor plasticization resistance, as they rely primarily on diffusion-selective transport that may be significantly reduced at high pressure.

Herein, we report on mixed-gas and high-pressure transport properties for six PIMs with identical benzodioxane backbones and a diverse set of backbone functionalities. Low-pressure mixed-gas tests indicate a relationship between CO₂ sorption affinity and enhancements in CO₂/CH₄ mixed-gas selectivity compared to pure-gas for all PIMs considered. The best results are reported for amine-functionalized PIM-1 (PIM-NH₂), which shows an unprecedented 140% and 250% increase in CO₂/CH₄ and CO₂/N₂ mixed-gas selectivity, respectively, compared to that of pure-gas tests at 2 atm. Moreover, PIM-NH₂ films retain high mixed-gas selectivity (>20) up to a total mixed-gas pressure of 26 atm in 50/50 CO₂/CH₄ mixtures, demonstrating strong plasticization resistance. Pure-gas sorption and mixed-gas permeation performance for the six PIMs were compared across a range of reported microporous polymers, elucidating structure/property relationships that can enable rational design of polymers capable to perform well in industrially relevant scenarios. Results demonstrate the promise of primary amine functionalization for developing highly sorption-selective and plasticization-resistant membranes for gas separations.