Gas Permeability and Characterization of Thermally Rearranged Polymers Based On 3,3'-Diamino-4,4'-Dihydroxy Biphenyl (AHB) and 2,2'-Bis-(3,4-decarboxyphenyl) Hexafluoropropane Dianhydride (6FDA)

Thermally rearranged (TR) polymers, which are typically polybenzoxales 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.  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.  Upon 79% TR conversion, AHB-6FDA bearing acetate ortho-functional groups showed high CO₂ permeability (369 Barrer) and a pure gas CO₂/CH₄ selectivity of 25, which surpassed the upper bound.  Free volume also increased with extent of TR conversion. 

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. 

Finally, 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.