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

Abstract

Despite extensive studies on structural changes during the carbonization process of pitch-derived fibers, an ac-curate 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 ? is divided into three sequential regimes: Regime I -disruption of stacked polyaromatic mesogens with fluctuations in elasticity from 300 to 600 ?; Regime II -full-scale transformation with enhancement in orientation from 600 to 800 ?; and Regime III -development of semi-crystalline carbon structures with elongation of microvoids from 800 to 1200 ?. By examining the viscoelastic properties of pitch-derived fibers during heat treatment below 600 ? (Regime I), we found that the maximum softness of the pitch-derived fibers is achieved at 500 ?. This is due to the decrease in crosslink density between stacking structures, indicating that the crosslink density below 600 & DEG;C is a significant contributor to the formation of carbon crystallites.