Preparation of High-Temperature Resistant Polyimide Fibers by Introducing the p-Phenylenediamine into Kapton-Type Polyimide

To improve the heat resistance of polyimide (PI) fibers for application in harsh environments and establish a correlation among the chemical structure, fabrication performance, and material properties, a simple and rigid diamine, p-phenylenediamine (p-PDA) was incorporated into the Kapton-type PI synthesized from pyromellitic dianhydride and 4,4-diaminodiphenylmethane (ODA). The comprehensive properties of these co-PI fibers were systematically investigated to assess the impact of p-PDA addition. Two-dimensional wide-angle X-ray diffraction (WAXD) was used to investigate the evolution of the aggregation structure of the co-PI fibers during the processing. The thermogravimetric analyzer (TGA) test shows that the incorporation of p-PDA improves the heat resistance of polyimide fibers, with the 10 wt % weight loss temperature (T10%) ranging from 582 to 605 °C and the maximum decomposition temperature (Tmax) of 611–635 °C for the co-PI fibers with different p-PDA contents. Additionally, the potential degradation mechanism of the PI fibers was examined by utilizing pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and other thermal analyses. By introducing p-PDA, the content of O element (ether bond in ODA) in the system decreases, leading to a reduction in oxygen free radicals from ODA during the decomposition process of polyimides. The decrease in active species can cause a decrease in the decomposition rate and improve the heat resistance of the polyimide fibers. The study of the thermal decomposition mechanism of polyimides provides a valuable foundation for the preparation of high-performance polymer fibers with enhanced thermal resistance and excellent overall performance.