(640d) Synthesis, Characterization, and Gas Transport Properties of Two Isomeric Thermally Rearranged (TR) Polymers for Gas Separation Membranes

Thermally rearranged (TR) polymers, which are typically polybenzoxazoles with a rigid-rod structure, have recently gained interest in gas separation application due to their excellent gas transport and separation performance, particularly for CO₂/CH₄ mixtures.  In fact, for these separations, TR polymers have permeabilities and selectivities that are above the Robeson upper bound.  Another attractive feature is their apparent strong resistance to plasticization.  In this study, the permeabilities of H₂, N₂, O₂, CH₄, and CO_2­ were measured over pressure ranging from 3 atm to 17 atm at 35^oC for aromatic polyimides based on 3,3’-diamino-4,4’-dihydroxy biphenyl and 2,2’-bis-(3,4-decarboxyphenyl) hexafluoropropane dianhydride (AHB-6FDA).  Thermally rearranged (TR) polybenzoxazoles (PBO) were obtained by thermally treating the polyimides between 350 and 450^oC.  The the gas transport properties of AHB-6FDA are compared with those of its isomer based on 3,3-dihydroxy-4,4’diamino-biphenyl and 2,2’-bis-(3,4-decarboxyphenyl) hexafluoropropane dianhydride (HAB-6FDA).  Both polyimides and the corresponding TR polymers showed similar gas transport properties. 

The effects of polyimide precursor ortho-positioned functional groups on gas transport properties were also investigated.  Acetate-containing AHB-6FDA displayed much higher gas permeability and comparable selectivity relative to hydroxyl-containing AHB-6FDA for both the polyimide and its corresponding TR polymers.  Thus adding bulkier groups at the ortho-position improves gas transport performance in these polymers.