Interior morphological feature of PAN nascent fibers and precursor fibers revealed by ultrathin section and solution etching

Polyacrylonitrile (PAN) fiber is a vital precursor for the high-performance carbon fibers. The interior morphologies of PAN nascent fibers and precursor fibers were exposed by the ultrathin section and solution etching, and elucidated by scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results indicated the unequivocal presence of fibril structure features in the PAN fibers. Electron microscope observation revealed different morphological types: microfibrillar network, microfibrils, fibrils, fibril bundles, ribbon fibrils, crystal layers, pore/voids between fibrils, and different transverse connections between microfibrils/fibrils. The microfibrillar network formed in nascent fibers, and the aligned fibrils arranged in precursor fibers. The microfibrils tended to aggregate and amalgamate into the fibrils/fibril bundles, which were interlinked by the transverse connections. The investigation of the detailed interior morphologies and the modeling of the compressive structural types contribute to optimization of the spinning process, thereby regulating the fibril morphology and improving the fiber mechanical properties.