Microstructural Evolution during Dry-Jet Wet Spinning Postprocessing from Coagulation Bath Fiber to High-Performance Polyacrylonitrile Precursor Fiber

High-quality polyacrylonitrile (PAN) precursor fibers positively affect the achievement of high-performance carbon fibers. To regulate the fiber structure and improve fiber quality, X-ray diffraction, scanning electron microscopy, small-angle X-ray scattering, high-resolution transmission electron microscopy, and atomic force microscopy, among other methods, were performed to investigate the internal microstructural evolution of PAN fibers during the postprocessing of dry-jet wet spinning and its correlation to high-performance acquisition. As the spinning process progressed, the lamellar structure perpendicular to the fiber axis in the as-spun coagulating bath fibers experienced disruption and recombination. At the same time, there was also the occurrence of the orientation arrangement of the PAN molecular chains and the directional extension and merging of microfibers. Eventually, the oriented microfibrils were tightly stacked to form crystalline layers, leading to continuous improvement in the fiber's crystalline structure and an increase in crystallinity and compactness. Moreover, the axial deviation of the internal pores within the fiber decreased continuously, accompanied by the axial elongation and fracture merging of the pores, which would enhance the regularity of axial alignment of microfibers and promote improvement in the fiber's tensile properties. Microfibril may further enhance the mechanical properties of the fiber by impeding the propagation of microcracks.