Effect of high-temperature thermo-oxidation on the mechanical and electrical properties of phenylethynyl-terminated polyimide/carbon fiber composite

The thermal degradation behaviors of phenylethynyl-terminated polyimide and their carbon fiber reinforced composites (CFRC) are significant to be evaluated comprehensively as their high-temperature applications are gradually expanded. It is found that ether-containing thermoset polyimide (PI) film shows unique degradation behaviors near glass transition temperature (Tg). Though the triethylamine (Et3N) chemical imidization method is applied, the PI backbone shows thermal stability at 371 degrees C thermo-oxidation condition and the main degradation parts site at the curing structure. More aliphatic (C-C) linkages will be formed in the thermo-oxidation process and they are ascribed to the further crosslinking behaviors of polyene structure. However, the PI film shows stable mechanical properties after thermo-oxidation. Meanwhile, the formation of an amorphous carbon structure is found after thermo-oxidation treatment, which endows the composite with enhanced electrical properties. Though the micro-cracking phenomenon happens at the matrix/CF interface of composite surface and thus degrades its mechanical properties, the in-plane longitudinal and transverse conductivity of the composite perform an increase, factors of 1.13 to 2.56x104 S/m and 2.28 to 454.8 S/m, respectively. Mechanism study indicates that the formation of amorphous carbon components is caused by the structural rearrangement of the degrading polyene curing structure and becomes more in the thermo-oxidation process, not affecting the backbone integrity and mechanical stability of the PI matrix. As a result, thermo-oxidation treatment degrades the mechanical properties of the composite, whereas the formed amorphous carbon component acts as a self-growing carbon-based nano-filling structure at the PI/CF interface and enhances the electrical contact of the carbon fibers.