Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

HAB-6FDA polyimide was synthesized from 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) and 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) by a two-step polycondensation method with chemical imidization. This polyimide was used as the precursor to prepare thermally rearranged (TR) polymers. The rearrangement reaction was performed at temperatures from 350 to 450°C. Based on mass loss during the TR process, TR conversion levels as high as 76% were observed. CO ₂ permeability increased from 12 Barrer in the precursor polyimide (HAB-6FDA) to 410 Barrer in the TR polymer prepared at 450°C, while pure gas CO ₂/CH ₄ selectivity decreased from 42 to 24. Both diffusivity and solubility increased following the rearrangement process, but the change in gas diffusivity was the largest contribution to the increase in permeability. From the polyimide to the sample heated at 450 for 30min, CO ₂ diffusivity increased by approximately a factor of 20 while solubility increased by a factor of 1.7. Similar changes were observed for other gases. Fractional free volume increased from 15% in the polyimide precursor to 20% in the TR polymer rearranged at 450°C. The HAB-6FDA TR polymers exhibited higher CO ₂ permeability than other classes of polymers with similar free volume, suggesting a free volume distribution in these TR polymers that favors high permeability.