The distribution of oxygen element in the cross section of polyacrylonitrile (PAN) precursor fibers and oxidized fibers has been investigated by using a scanning transmission X-ray microscope (STXM) with a spatial resolution of 30 nm

combined with dual-energy contrast imaging method.By using the optical density ratio of the corresponding pixels in the two STXM images

the STXM experimental results indicated that the oxygen is homogeneously distributed in the pristine PAN fibers.During stabilization process

two important reactions occur and change the chemistry of PAN structure.In the skin section of PAN fiber

oxidation converts it into a conjugated and hetero-aromatic structure in the presence of oxygen.While in the core section of PAN fiber

cyclization turns linear PAN into a six-member ring structure due to the absence of oxygen.After treatment

an increase in the oxygen concentration in the skin of the PAN fibers is clearly observed while it remains constant in the core of PAN fibers

clearly demonstrating the diffusion process in the treatment.The propagating velocity of oxygen along the radical direction is about 0.033m/min.This research shows that the STXM is a powerful technique to characterize the internal microstructure of the PAN pre-oxidized fibers

and therefore will help to rationally understand the evaluation mechanism of the skin-core structure.