The effect of thermally induced chemical transformations on the structure and properties of carbon fiber precursors
The transformation of functional groups and development of radial structural heterogeneity and fibre properties during the thermal stabilization of polyacrylonintrile (PAN) precursor fibers was quantitatively defined for the first time using high resolution spectroscopic imaging techniques. The infrared imaging of isothermally treated fiber cross-sections reveals the radial distribution of specific functional groups (CN, C=N, CH2, CH and C=O) that forms the ladder polymer structure, the most critical stage in the precursor stabilization process. Apparently, it was found that the cyclization reaction of PAN polymer chains occur at a faster rate in the core of the fiber during heating where it further selectively promoted the dehydrogenation reaction. On the other hand, the conversion of sp3 to sp2 hybridized carbon atoms was found to be higher around the skin layer compared to the core of the fibers, thus providing evidence for different cross-linking mechanisms in these regions. The simultaneous occurrence of these reactions due to the excess heat developed in the core followed by chain crosslinking led to the evolution of radial heterogeneity in the fibers. It was also found that the mechanical properties were built upon the structural transformations and the variation in the modulus across the fiber cross section further confirmed the reaction mechanism.