(491b) Highly Permeable and Selective Crosslinked Polybenzoxazole (PBO) Membranes for Gas Separation

Gas separation using polymer membranes has rapidly grown into an area of technological importance and usefulness, because of the progresses made by synthetic chemists in designing and synthesizing new sophisticated macromolecular structures. Unfortunately, most of the polymeric membranes suffer from plasticization induced by sorption of condensable gases (e.g., CO₂) and are not stable upon exposure to chemical contaminants. Chemical crosslinking has been demonstrated as an effective pathway to improve membranes’ resistance to plasticization and chemical contaminants in complex feed streams as well as physical aging. However, the much improved membrane stability resulted from crosslinking is always accompanied by significantly reduced gas permeability with little to no gain in selectivity. This is largely due to crosslinking-induced membrane densification that greatly reduces the free volume microporosity required for high gas permeability. Here we report a new series of crosslinked PBO membranes prepared via an approach involving concurrent thermal cyclodehydration and end-linking processes from telechelic oligomers with well-controlled molecular weight. This new macromolecular design of introducing bulky phenyl pendent groups at crosslink junctions upon thermal crosslinking led to disruption of chain packing counteracting the undesired densification effect of covalent crosslinking. As a result, even the most densely crosslinked membranes showed similarly high permeability compared to their linear counterparts along with expectedly improved selectivity and membrane stability. Moreover, further thermal treatment of the crosslinked membranes at 450 ^oC for 0.5 h resulted in significant increase in gas permeability while maintaining the same selectivity. For example, a thermally-treated, highly crosslinked PBO membranes (inter-crosslink chain length of 3000 g/mol) showed CO₂/CH₄ separation performance beyond the 2008 upper bound with CO₂ permeability of 563 barrer and an ideal CO₂/CH₄ selectivity of 29. The same membrane also displayed H₂/CH₄ and H₂/N₂ separation performance beyond the 2008 upper bounds and O2/N2 separation performance on the 2008 upper bound. In this talk, preparation and characterization of these new crosslinked PBO membranes will be presented. Discussions will focus on elucidating the fundamental relationship between microscopic structures and macroscopic transport properties for these innovative crosslinked polymer membranes.