Polymers Of Intrinsic Microporosity Polyimides Based On Triptycene: Synthesis And Gas Separation Performance

Polyimide is one of the most commonly used membrane material because of its good thermal stability,chemical stability,mechanical performance and gas selectivity.But most polyimides are insoluble and infusible.Moreover traditional polyimides have many deficiencies,such as lower gas permeability and poor anti-CO₂ induced plasticization.To resolve these issues,we introduced triptycene building block into polyimides' backbones to improve both the solubility and gas permeability.Cross-linking was also adopted to increase the anti-CO₂ induced plasticization.Here,triptycene-based diamine,2,6-diaminotriptycene?DAT?,was synthesized and reacted with 4,4'-?hexafluoroisopropylidene?diphthalic anhydride?6FDA?and pyromellitic dianhydride?PMDA?to form two polyimides,denoted as 6FDA-DAT and PMDA-DAT.Properties including molecular weight,solubility,thermal stabilities,free volume fraction?FFV?glass transition temperature?Tg?,d-spacing,gas permeation,anti-CO₂ induced plasticization and physical aging were measured.The results showed that incorporate triptycene in the polyimides chain could hinder the chain packing efficiency,thereby increasing the FFV,d-spacing,thermal stabilities and solubility.These two polyimides had good gas separation performance and high Tg?>450??.Meanwhile,bulky CF3 group of 6FDA also had a similar effect as triptycene did,i.e.6FDA-DAT has higher gas permeability than PMDA-DAT.However,6FDA-DAT showed a poor anti-CO₂ induced plasticization property.So triptycene-based diamines with carboxylic acid,2,6-diaminotriptycene-14-carboxylic acid?DATCA?,was synthesized and reacted with DAT and 6FDA to form two copolyimides,denoted as 6FDA-DAT/DATCA?9:1?and 6FDA-DAT/DATCA?8:2?.These two copoloyimides were further heated at a temperature range of 200 to 450 ? to induce decarboxylation crosslinking,and a series of properties were measured.The results showed that 6FDA-DAT/DATCA?9:1?and 6FDA-DAT/DATCA?8:2?had a lower FFV and d-spacing attributed to hydrogen bonding induced by carboxylic acid groups,and decreased the gas permeability of copolyimides.All decarboxylation crosslinking reactions were carried out below their Tgs,gas permeabilities of 6FDA-DAT/DATCA?9:1?and 6FDA-DAT/DATCA?8:2?increased with decreased selectivities after crosslinking.No plasticization were observed for crosslinked 6FDA-DAT/DATCA?9:1?and 6FDA-DAT/DATCA?8:2?at a CO₂ pressure up to 30 atm for pure gas and a partial CO₂ pressure up to 20 atm for?CO₂:CH4 1:1?mixed gases.In addition,crosslinked polymers exhibited significantly reduced physical aging rate.