Analysis of gas transport in molecularly-mixed composite membranes

Abstract

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 as concerns with two-phase processing systems are avoided. 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 gas permeation deviates significantly from predictions made with the Maxwell model for mixed matrix membranes. POCs were found to significantly alter membrane properties in the polymer phase because of intimate molecular interactions between the POC and polymer, violating one of the Maxwell model's underlying assumptions. This work provides preliminary information on the nature of guest transport in MMCMs to aid their future adaptation to industrially-relevant separation units.