Complementary X-ray scattering and high resolution imaging of nanostructure development in thermally treated PBO fibers Original Research Article

Structural changes in poly(p-phenylene benzobisoxazole) (PBO) derived carbon fibres (CFs), treated in the range 900 °C ⩽ T ⩽ 2700 °C, are investigated by small- and wide-angle X-ray scattering (SAXS and WAXS), and by high-resolution transmission electron microscopy (HRTEM). SAXS detects rough interfaces within the fibres, normal to the fibre axis and belonging to a fibril structure. HRTEM and SAXS also reveal nanometre-sized elongated voids spaced semi-regularly along the fibre axis. With increasing temperature, longitudinal packing becomes denser and the broad reflections in the WAXS region sharpen into the reflections characteristic of graphite. Preferential axial orientation of the graphitic layers is due to the internal fibril structure of the fibre, which itself is governed by the draw axis of the parent PBO fibre and improves substantially on going from 2400 °C to 2700 °C. In-plane reflections in the parallel direction are well resolved, demonstrating tri-periodic ordering within the graphitic sheets (“true” graphitization, with hkl reflections in place of the hk bands of bi-periodic turbostratic carbons). This study of PBO-based CFs is the first detailed analysis of the nanostructural changes that occur during the graphitization process of 1D carbons.