Microstructure Evolution of Polyacrylonitrile-Based Fibers during Thermal Pre-Oxidation

Abstract

In this work, pre-oxidized polyacrylonitrile fibers are treated with ultrasonic etching and solution etching to produce ultra-thin sections. The evolution of the fibers’ microstructure in the pre-oxidation process is observed, and the transformation model of the microstructure of the pre-oxidized fibers is proposed. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the microstructure changes of the fibers. Fourier transform infrared spectroscopy and X-ray diffraction were used to observe the chemical structure transformation and crystallization degree of the fibers in the pre-oxidation process. The results revealed that pre-oxidized fibers exhibited a smooth surface, while their interior consisted of fibrils. The longitudinal microfibrils were connected by the transverse microfibrils and amorphous regions. The fracture morphology of the fibers shifted from ductile to brittle, and the cross-section gradually became smoother. The linear molecular chain of PAN transformed into a ring structure as pre-oxidation progressed, subsequently leading to the cross-linking of this ring structure into an orderly trapezoidal configuration. The connection between the fibrils was enhanced, and the fiber structure became more compact and stable.