HRTEM microstructures of PAN precursor fibers

Ultrathin sections prepared from both transverse and longitudinal PAN precursor fibers were examined by high-resolution transmission electron microscopy. Ordered and amorphous phases coexist in these sections, providing verification for the two-phase structure of PAN fibers, however, onion-like spheres and crystallites predominate in the transverse sections, and highly oriented structures are dominant in the longitudinal sections. The formation of the onion-like spheres can be ascribed to intramolecular and intermolecular dipolar nitrile group interactions. The larger area of oriented structures in the longitudinal sections is due to the coalescence of several ribbons with the approximative orientations. Some defects, such as pores and voids, have also been observed. These examinations provide not only some direct structural evidence but also some information for optimizing spinning technology and for deliberately controlling the microstructures to obtain PAN precursor fibers applicable to fabricating high strength carbon fibers.