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

Abstract

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 (Tg), d-spacing, physical aging and gas permeation were measured. Since Tg of 6FDA-DAT/DATCA (9:1) and 6FDA-DAT/DATCA (8:2) were not observable at temperature up to 450 °C, 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. Therefore, the sub-Tg decarboxylation crosslinked polyimides had great potential for natural gas separation.