Individual molecules with intrinsic porosity, such as porous organic cages (POCs), have significant potential to improve the performance of a variety of separations media. An exemplar application is the blending of POCs with polymers to make molecularly mixed composite membranes (MMCMs). The intimate interaction between individual cage molecules and polymer chains results in a “solid-solution” that avoids longstanding interfacial issues associated with mixed matrix membranes. Moreover, as the cages are soluble in polymer solutions, the processing of these composites can be easily adapted to established polymer-based technologies. MMCMs are still a relatively new development, and underlying transport processes within the membrane are not well understood. Here, we offer a detailed interpretation of guest transport through these solid solutions. We demonstrate how the presence of cage molecules affects polymer chain motions that can impact guest transport through the polymer phase. We also show how cage loading affects membrane free volume We find that permeation increases steadily up to an inflection point where permeation then increases much more rapidly. We hypothesize this is due to the creation of additional free volume in the bulk polymer. This work provides preliminary information on the nature of guest transport in MMCMs to aid their future adaptation to industrially-relevant separation units.

Analysis of the transport of guest molecules in molecularly mixed composite membranes containing porous organic cages

ACS Fall 2021

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