Effects of ortho Functional Groups on the Thermo-Oxidative Cross-Linking Process and Gas Separation Properties of Phenolphthalein-Based Polyimides

Polyimides containing lactone rings can be thermo-oxidatively cross-linked to enhance gas separation properties and antiplasticization performance. Degradation of the lactone ring at high temperature is crucial to generate free radicals for cross-linking. Determining the thermal stability of the lactone ring is required to set an appropriate heating protocol for thermal cross-linking. To elucidate the relation between polymer structure and the thermal stability of the lactone ring, we prepared two phenolphthalein-based diamines, AMPB and AHPB. These diamines were reacted with 6FDA and DAM to form three copolyimides using azeotropic distillation and chemical imidization. This was used to design copolyimides containing lactone rings and different ortho functional groups including −OH, −OCH3, and −OOCCH3. By characterizing the physiochemical properties of these copolyimides and performing molecular simulations, we discovered that the stronger the electron-donating ability of the ortho groups, the higher the temperature at which the lactone ring began to degrade. Copolyimides with a relatively weak electron-donating groups, −OH and −OOCCH3, could be oxidatively cross-linked at 275 °C, while the copolyimide with a −OCH3 group could be only cross-linked at a temperature above 325 °C. Moreover, the gas separation property, mechanical properties, and plasticization resistance were improved after cross-linking.