(305b) Microporous Polymers to Microporous Carbons: Linking Structure to Guest Transport and Sorption

“Polymers of intrinsic microporosity” (PIMs) are an emerging class of microporous materials that have the unique capability to be solution processed into adsorbents, membranes, and other fluid-solid contactors. Moreover, it has been recently discovered that PIM materials can be pyrolytically converted into carbon molecular sieves (CMS) with well-defined micropores in the range of 5-8 Å, which is ideal for the separation of similarly-sized molecules. From an applications perspective, these materials harness the scalability of the polymer processing platform while providing flexibility in materials design for targeted mass transfer problems; i.e., will a microporous polymer or a microporous carbon be better suited for a particular task? A series of lingering scientific gaps include missing links between the chemical structures of the polymer and the resulting polymer-derived carbon as well as the linkage between these structures and the void space inherent in both materials. To address these gaps, the textural properties of the polymer precursor materials are typically characterized based on cryogenic sorption techniques developed for “rigid” microporous solids (e.g., zeolites). These approaches can lead to poor predictions of guest sorption and transport within the polymer, as the free volume regions present in PIMs are not persistent as in traditional rigid solids, but may change in time due to both chain motions and penetrant-induced dilation. However, the cryogenic sorption experiment still provides important information regarding the current free volume of the polymer and its ability to dilate. After pyrolysis of the PIM material, the textural properties of the rigid CMS are easily determined via cryogenic sorption techniques, but relating the void space microstructure to the structure of the carbon itself is more difficult. Neutron diffraction techniques provide this insight and an example will be given that compares the pore size from cryogenic sorption and diffraction. Finally, a relationship between void space microstructure and small molecule separation capabilities will be provided to begin creating a tentative design pathway from polymer structure to carbon-based separation device.