Decarboxylation cross‐linking of fluorescein‐based copolyimides for gas separations

Abstract A fluorescein‐based diamine, 2‐(3,6‐bis(4‐amino‐3‐hydroxyphenoxy)‐9H‐xanthen‐9‐yl) benzoic acid (CADA), was synthesized and copolymerized with Durene and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to form a 6FDA‐Durene/CADA (3:2) copolyimide. The copolyimide was thermally cross‐linked at 300–425°C to form a series of decarboxylated cross‐linked polyimides. Compared with the 6FDA‐CADA polyimide, the incorporation of Durene diamine increased gas permeability of the pristine and the thermally cross‐linked polyimides greatly. The copolyimide cross‐linked at 375°C showed separation performance to the O 2 /N 2 gas pair beyond the 1991 Robeson upper limit, while the CO 2 /CH 4 separation property of the copolyimide cross‐linked at 425°C (PI‐425) was close to the 2008 Robeson upper limit. Moreover, the anti‐CO 2 ‐induced plasticization property of the 6FDA‐Durene/CADA(3:2) copolyimides was improved by cross‐linking. The copolyimides cross‐linked at 350°C and above were not plasticized at a CO 2 pressure up to 30 atm. This study showed that gas transport property of the decarboxylated cross‐linked polyimides could be improved significantly by the incorporation of bulky diamino moiety. The fluorescein‐based copolyimide had great potentials for gas separation applications.