Direct measurement of microfibril structures in polyacrylonitrile fibers during carbon fiber manufacturing process

The exceptional tensile strength and modulus of high-performance carbon fibers are determined by the microstructure evolution during the manufacturing process. The comprehension of the internal morphology of polyacrylonitrile (PAN) fibers is crucial for establishing the robust structure–property relationship and achieving superior mechanical properties in the fibers. In this work, a combination method of the ultrathin sectioning and electron microscopy technique was developed and employed for the analysis of internal structure features of the nascent fibers, precursor fibers, pre-oxidized fibers and carbon fibers. The microfibril elements were already formed during the coagulation stage and further developed within the carbon fibers through spinning, thermal stabilization and carbonization processes. Subsequently, the unoriented microfibrillar network underwent a transformation into dense fibrils, and the crystal layers within these microfibrils experienced a conversion into the turbostratic graphite structures. Based on the Nano-IR2-FS results, the morphological changes of the microfibrils were found to be intricately associated with the evolution of chemical structure, implying a strong correction between them. Through analysis of the modulus differences, it became possible to distinguish between the crystalline domains and amorphous regions, facilitating the establishment of a relationship between the mechanical strength and the microfibril structures. This work presented a direct measurement method for unraveling the complex hierarchical structures of polymer fibers, which held great potential for developing high-performance polymer fibers.