Fuente: Journal of applied polymer Lugar: RESEARCH ARTICLE This work investigated the changes in the molecular structure of mesophase pitch during pre-oxidation process, and presented suitable conditions for the formation of stable polycyclic aromatic structures for high yield and high tensile strength mesophase pitch based carbon fibers.

ABSTRACT
Pre-oxidation of mesophase pitch fibers generates a thermally stable structure and enables further carbonization at high temperatures to prepare carbon fibers. Elemental analysis, x-ray photoelectron spectroscopy, Fourier transform infrared, and Raman spectroscopies are employed herein to study the changes in elemental contents and functional groups of fibers pre-oxidized at 180°C–320°C. Mass spectrometry, thermogravimetry, and scanning electron microscopy are performed to examine the changes in the aromatic structure and morphology caused by pre-oxidation and their influence on the resulting thermal stability and carbon fiber's tensile strength. As the pre-oxidation temperature increases, the C and H contents of the fiber decrease, and the O content increases. The methyl, methylene, and aromatic groups form numerous O-containing functional groups, and the polycyclic aromatic molecules become larger with the increasing pre-oxidation temperature. The residual C content of the fiber pre-oxidized below 280°C increases because O is mainly removed by H2O, and the C content decreases above 280°C because O is mainly removed by CO2 during carbonization. The loss of the carbon skeleton in the fibers greatly reduces their mechanical performance. Ultimately, pre-oxidation between 240°C and 280°C provides the highest thermal stability and tensile strength, where the residual C rate is 91.5% and the tensile strength of the carbon fiber reaches 2.22 GPa.