Effect of the draw ratio in dry jet-wet spinning on aromatic copolysulfonamide fibers

Aromatic copolysulfonamide (co-PSA) fiber produced via wet spinning exhibits poor mechanical properties. Hence, dry jet-wet spinning was introduced to prepare theco-PSA fiber. Dry jet-wet spinning exhibits the advantage of positive stretching to the extruded fiber before coagulation. The fibers were spun at different jet draw ratios and second draw ratios and were examined for scanning electron microscopy, mechanical properties, sound velocity, small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS). The results revealed that all the fibers exhibited a uniform and dense structure and nearly round cross section at different drawing conditions. The strength and orientation of the fiber increased slightly as the jet draw ratio was increased to the second draw ratio of 2.0. Furthermore, the fibers formed a periodic ordered structure along the fiber axis. However, the strength of the fibers decreased significantly when the jet draw ratio continued to increase and the second draw ratio decreased year-on-year to maintain the total draw ratio at 11.0. Additionally, the fibers exhibited lower orientation because the molecular chain segments could not continue to extend and orient even at larger jet draw ratios, and thus they could not be further well-stretched at subsequent lower stretching ratios. Meanwhile, the periodic ordered structure disappeared and more content of entanglement structure existed in the fibers. During the heat drawing process, the results of SAXS and WAXS indicated that the better ordered molecular chain segments within the fiber were easier to arrange and reconstruct to significantly improve the content of crystalline and mesophase structures in the fibers. Hence, after the heat drawing process, the strengths ofco-PSA-H2 andco-PSA-H3 fibers increased to more than 620 MPa, which is significantly higher than those of commercialized fibers and fibers reported in extant studies.