Enhancement of polyimide and 3D graphene-polyimide through thermoforming and its effect on mechanical properties and associated creep phenomenon

Polyimides (PIs) are a class of polymers used in diverse applications due to their excellent thermal stability, outstanding chemical resistance, and superb mechanical properties. One of the most widely used PIs is Pyromellitic Dianhydride-Oxydianiline (PMDA-ODA) PI (Kapton^®), which can be purchased as thin films or tubes, thus limiting its use to specific applications. In this work, a novel thermoforming process is proposed which facilitates forming of planar PI and novel three-dimensional graphene PI (3D-C/PI) shapes into 3D contours. Our thermoforming process allows the formation of various shapes, such as helical, angular, or cylindrical profiles, thus expanding PIs' usage to new applications. The effect of the thermoforming process on the PI's chemical composition, mechanical properties, and chemical resistance to oxidizing plasma environment are studied. Also investigated is the creep resistance of the thermoformed PI compared to pristine PI at various temperatures; an analytical creep model has been fitted to the experimental data. Our results show that accelerated creep tests of thermoformed and pristine PI result in the same deformation and that the thermoforming process has no influence on the PI's chemical composition or chemical resistance. We also found that 3D-C/PI presents excellent thermoformability and that thermoformed PI exhibits higher elasticity compared to pristine PI.