Gas permeation and physical aging properties of iptycene diamine-based microporous polyimides

• Intrinsically microporous polyimides from triptycene and extended iptycene analog.
• Additional ring causes increase in permeability with minor changes in selectivity.
• Both polyimides demonstrate slight ~15% losses in permeability after 150 days.
• Selectivites were stable after 150 days physical aging.
• Plasticized at ~10 bar CO2 partial pressure under 50:50 CO2/CH4 feed.

The synthesis and gas permeation properties of two 6FDA-dianhydride-based polyimides prepared from 2,6-diaminotriptycene (6FDA–DAT1) and its extended iptycene analog (6FDA–DAT2) are reported. The additional benzene ring on the extended triptycene moiety in 6FDA–DAT2 increases the free volume over 6FDA–DAT1 and reduces the chain packing efficiency. The BET surface area based on nitrogen adsorption in 6FDA–DAT2 (450 m2 g−1) is ~40% greater than that of 6FDA–DAT1 (320 m2 g−1). 6FDA–DAT1 shows a CO2 permeability of 120 Barrer and CO2/CH4 selectivity of 38, whereas 6FDA–DAT2 exhibits a 75% increase in CO2 permeability to 210 Barrer coupled with a moderate decrease in selectivity (CO2/CH4=30). Interestingly, minimal physical aging was observed over 150 days for both polymers and attributed to the high internal free volume of the shape-persistent iptycene geometries. The aged polyimides maintained CO2/CH4 selectivities of 25–35 along with high CO2 permeabilities of 90–120 Barrer up to CO2 partial pressures of 10 bar in an aggressive 50:50 CO2:CH4 mixed-gas feed, suggesting potential application in membranes for natural gas sweetening.

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