Study on the relationship between chemical structure transformation and morphological change of polyacrylonitrile based preoxidized fibers

In this work, a new research perspective on the relationship between chemical structure transformation and morphological change of polyacrylonitrile (PAN) precursors during the preoxidized process was provided. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were used to analyze the morphologies of preoxidized fibers. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermal gravimetry (TG) were used to characterize the chemical structure, crystal structure and thermal properties, respectively. The chemical structure and morphology of the same region of the fiber were obtained together by nano infrared spectroscopy atomic force microscope (NanoIR2-FS), which was convenient to study the relationship between them. The results showed that the difference of chemical structure between the skin and core region increased in the early stages of preoxidation and decreased in the late stages of preoxidation. The change of morphology was consistent with the chemical structure. Microfibrils along the fiber axis and lamella structures perpendicular to the fiber axis were observed, and the morphology became denser due to the transformation of chemical structure. In addition, the cross-sectional fracture morphologies of the preoxidized fibers changed and the crystallization degree of the preoxidized fibers decreased.