Gas permeation properties of thermally rearranged (TR) isomers and their aromatic polyimide precursors

• Characterizes gas permeability and free volume as a function of TR conversion.
• Examines the influence of ortho-position functional group on gas transport.
• Explores the effects of meta/para configuration on gas transport.

This study explores the influence of meta/para and ortho-position functional group structures on gas transport properties of two isomeric polyimides and their thermally rearranged (TR) polymers. The diamine isomers, 3,3′-dihydroxy-4,4′-diamino-biphenyl (p-HAB) and 4,4′-dihydroxybiphenyl-3,3′-diamino-biphenyl (m-HAB), were polymerized with 2,2′-bis-(3,4-dicarboxy-phenyl) hexafluoropropane dianhydride (6FDA) to form two aromatic polyimide isomers. These polyimides were also prepared with either hydroxyl or acetate groups at the ortho-positions to the imide ring, and the polyimides were partially converted to the corresponding polybenzoxazoles by thermal treatment at various temperatures and times. Single gas permeabilities of CH4, H2, O2, N2 and CO2 were measured at 35 °C and upstream pressures ranging from 3 atm to 17 atm. Polyacetylimides and their corresponding TR polymers showed higher gas permeability and lower selectivity than polyhydroxyimides and their TR analogs, respectively. The effect of isomerism on gas transport properties was also investigated. Para-connected polyhydroxyimide had higher fractional free volume, and thus, gas permeability coefficients than meta-connected polyhydroxyimide. Interestingly, in contrast to the meta/para effects previously reported in the literature for linear aromatic polymers, the meta-linked polyacetylimide had higher permeabilities and lower selectivities than its para-linked analog, presumably due to steric hindrance from the acetate groups that may inhibit the rotational mobility of the phenylene units in the para-linked isomer. For TR polymers derived from either polyhydroxyimides or polyacetylimides, para-configured polymers had higher gas permeabilities than meta-configured analogs. Finally, polyimide precursors for TR polymers can be sensitive to synthesis and casting conditions, and this study demonstrates how small variations in synthesis and casting procedure can have a significant effect on physical and gas permeation properties.

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