Thermal and structural stability of Poly(p-phenylene terephthalamide) fibers

Poly(p-phenylene terephthalamide) (PPTA) fibers possess excellent thermal resistance, but their thermal transitions and structural evolution under repeated ultrafast cycling remain unclear. Here, as-received and dried DuPont K29 fibers were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), fast scanning chip calorimetry (FSC), wide-angle X-ray diffraction (WAXD), and confocal Raman spectroscopy. Both samples showed a similar thermal decomposition temperature of approximately 580 °C, while the as-received fibers exhibited an additional mass loss of 2.2% due to absorbed moisture. Conventional DSC results indicated that the high-temperature endothermic response was strongly coupled with thermal decomposition, whereas FSC technique at a scanning rate of 20,000 K/s suppressed decomposition sufficiently to resolve apparent melting and subsequent crystallization. The endothermic peak temperatures of the as-received and dried fibers were 550 and 580 °C, respectively, and the corresponding crystallization temperatures were 516 and 518 °C. During the early stages of thermal cycling, the dried fibers retained apparent melting heat more effectively. After 25 thermal cycles, their apparent melting heat decreased by 25.5% relative to the second heating scan, while the as-received sample exhibited a loss of approximately 52.4% of the apparent melting heat. Moreover, the crystallizable ordered structures were lost more than 95% for both samples after 60 cycles, demonstrating thermal cycling-induced cumulative damage. Following the first FSC cycle, the crystallinity determined by WAXD and Raman spectroscopy increased from 47.2% to 55.4% and from 46.2% to 58.3%, respectively, yet the axial orientation level decreased evidently. Thus, a single ultrafast heating-cooling cycle promoted recrystallization into a more ordered but less oriented structure, whereas repeated exposure to 800 °C progressively destroyed meltable and crystallizable ordered domains.