Unveiling the transformation of liquid crystalline domains into carbon crystallites during carbonization of mesophase pitch-derived fibers

Despite extensive studies on structural changes during the carbonization process of pitch-derived fibers, an accurate description of the transformation from liquid crystalline domains into carbon crystallites is still limited to a few depictions based on common analytical tools for carbon fibers. We employed small-angle X-ray scattering (SAXS) with model fits for the unification of such disparate measures. The carbonization process below 1200 °C is divided into three sequential regimes: Regime I - disruption of stacked polyaromatic mesogens with fluctuations in elasticity from 300 to 600 °C; Regime II - full-scale transformation with enhancement in orientation from 600 to 800 °C; and Regime III - development of semi-crystalline carbon structures with elongation of microvoids from 800 to 1200 °C. By examining the viscoelastic properties of pitch-derived fibers during heat treatment below 600 °C (Regime I), we found that the maximum softness of the pitch-derived fibers is achieved at 500 °C. This is due to the decrease in crosslink density between stacking structures, indicating that the crosslink density below 600 °C is a significant contributor to the formation of carbon crystallites.