The key role of microtexture in the graphitisation of PBO fibre chars as seen by X-ray scattering and transmission electron microscopy

Poly(p-phenylene benzobisoxazole) (PBO) fibres, heat-treated between 900 and 2700 °C, were studied by both small- and wide-angle X-ray scattering, and by HRTEM. As directly imaged by HRTEM, the material treated at 900 °C consists of nanometre-sized graphitic domains with a microtexture that resembles non-graphitisable carbon. However, above 2000 °C, these elongated structures coalesce, yielding highly graphitised lamellar carbon. This graphitisation mode is very different from the conventional progressive graphitisation of lamellar carbon. It is similar to that of polyimide films, and is characterized by a preferential planar orientation of the polyaromatic structural units inherited from the pristine fibre microtexture.