New insights into orientation distribution of high strength polyacrylonitrile-based carbon fibers with skin-core structure

The continuous changes in orientation angle (OA) across longitudinal sections of high strength polyacrylonitrile-based carbon fibers with circular cross sections were investigated by transmission electron microscopy. A contrast ring was observed in some cross-sections, displaying two contrast zones in the longitudinal section. Two types of OA distribution features were found in the absence or presence of contrast zones. Contrast zones were confirmed as the symbol of the skin-core structure and the boundaries between the skin and core. The high preferred orientation of the basal planes was first identified in the contrast zone, which presented an OA inflexion point (OIP) in the OA plot. The OIP was also detected in the contrast zone for the self-prepared carbon fibers. The OIP and contrast ring shifted towards the core as the preoxidation time increased. For a preoxidation time of 8 h, both the OIP and contrast ring disappeared. A mechanism, based on the stress-induced alignment of the basal planes, was proposed to account for the formation of the OIP.