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The changes in performance during thermal-oxidative aging process of the aromatic co-polysulfonamide (co-PSA) fibers over a broad temperature range from 250 °C to 320 °C have been investigated. In addition, the mechanism of thermal-oxidative aging process has been studied by using structural information obtained from the fibers at varying length scales. The results showed that a significant reduction in tensile strength was observed compared with that of initial modulus during aging process. Macroscopically, thermal-oxidative aging mainly causes color changes of fibers and thermally induced macro defects begin to appear only at 320 °C for 100 h. On a micro level, the crystal structure of fibers remained stable and did not show significant changes expect that aging at 320 °C. In addition, thermo-degradation as well as crosslinking has been observed primarily in amorphous region. With the increase of temperature and time duration, the crosslinking became more dominant and crosslinking density increases. Correspondingly, the fibril length decreases due to degradation and then increases due to the formation of crosslinked structures within the fibers. The results suggest that molecular degradation is the main cause of strength loss and the formation of crosslinking structure within the fibers contributes to the retention of modulus and improvement of creep resistance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44078.

The structure and performance changes of aromatic copolysulfonamide (co-PSA) fibers that occurred during wet spinning process have been studied. While using different length scale characterization, including scan electron microscopy (SEM), wide-angle X-ray scattering (WAXS), and small-angle X-ray scattering (SAXS), it was found that the molecular chains of co-PSA formed an isotropic network during coagulation which further lead to extension and orientation of these chains during the subsequent stretching. As a result, only after heat stretching and heat setting the molecular chains tended to pack into crystal lattice in the fibrils. This gave rise to a much denser structure along the spinning line and the glass transition temperature of co-PSA fibers increased a little after heat setting. Before heat stretching, the co-PSA fibers were in amorphous state, and only the amorphous orientation was observed within the fibers. After heat stretching at the temperature higher than T-g, the fraction of amorphous region decreased, and the crystal structure formed in the fibers, which became more perfect during heat setting. The structure development during spinning process contributed toward the improvement of thermo-mechanical stability, tenacity and modulus of the co-PSA fibers. (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42343.