New aspects in the oxidative stabilization of PAN-based carbon fibers

Oxidative stabilization studies on PAN-based copolymer fibers indicate the presence of at least two distinct reactions occurring at temperatures below 380 °C. Techniques such as thermal stress, percentage shrinkage, differential scanning calorimetry, wide-angle X-ray diffraction and small-angle X-ray scattering have been used to elucidate the mechanisms involved. Reactions initiate in the amorphous part of the copolymer at temperatures below 200 °C and contribute to the major portion of the macroscopic shrinkage. This corresponds to the first “amorphous” peak observed in DSC. Crystalline morphology is largely maintained during this stage, although considerable randomization of crystal lamellae take place. Reactions propagate to crystalline components at higher temperatures and proceed to completion at ~380 °C, giving rise to the second “crystalline” DSC peak. Additionally, a broad shoulder on the second peak is observed. Although its origin is not clear, it is speculated that it corresponds to certain intermolecular cross-linking reactions. The importance of using copolymer PAN as a carbon fiber precursor is outlined. Additionally, new models for morphological development during stabilization are proposed. Macroscopic shrinkage along the fiber axis is seen to be primarily an entropy-driven process, with “chemical” effects serving to only modify the entropie response. Intramolecular cyclization reactions serve as the basis for initial stabilization, with intermolecular cross-linking occurring in the presence of oxygen above ~300 °C for oxidatively stabilized fibers.