Effects of sub-Tg cross-linking of triptycene-based polyimides on gas permeation, plasticization resistance and physical aging properties

Two triptycene-based diamines, 2,6-diaminotriptycene and 2,6-diaminotriptycene-14-carboxylic acid, were synthesized and reacted with 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to form three polyimides, denoted as 6FDA-DAT, 6FDA-DAT/DATCA (9: 1) and 6FDA-DAT/DATCA (8: 2). The last two carboxylic acid containing polyimides were further heated at a temperature range of 200-450 C to induce decarboxylation crosslinking. Properties including solubility, thermal stabilities, glass transition temperature (T-g), d-spacing, physical aging and gas permeation were measured. Since T-g of 6FDA-DAT/DATCA (9: 1) and 6FDADAT/DATCA (8: 2) were not observable at temperature up to 450 degrees C, all decarboxylation crosslinking reactions were carried out below their T(g)s. 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 6FDADAT/ 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. Therefore, the sub-T-g decarboxylation crosslinked polyimides had great potential for natural gas separation.