Development of Phenolphthalein-Based Copolyimides and their Derivative Cross-Linked and Thermally Rearranged Polymers for Gas Separation

Thermally rearranged (TR) polymers have good gas separation property, but the high TR temperature often caused pore collapsing and embrittlement of hollow fiber. We designed two copolyimides by reacting a TR-able 3,3′-diaminophenolphthalein (DAP), and two non-TR able diamines, 2,4,6-trimethyl-m-phenylenediamine (DAM) and diethyl-toluene diamine (DETDA) with hexafluoroisopropylidene diphthalic anhydride (6FDA) using azeotropic distillation. The bulky DAM and DETDA diamines and the hydrogen bonding provided by the OH groups of DAP enabled high glass transition temperatures of Co-PI-DAM and Co-PI-DETDA copolyimides so that the thermal cross-linking and TR reaction realized at sub-Tg. Moreover, compared with the 6FDA–DAP polyimide, copolymerization greatly improved the tensile strength from 35 to 75 MPa (Co-PI-DAM). The cross-linked TR polymers exhibited high separation performance to CO2/CH4 and were not plasticized by CO2 at 30 atm. The two copolyimides show great potential for fabricating TR hollow fiber membranes.