Polyacrylonitrile (PAN) fibers were subjected to continuous thermal-oxidative stabilization and carbonization processes.During the thermal-oxidative stabilization stage at 268℃,oxygen-rich and oxygen-deficient environments were established by controlling the oxygen volume fraction (φO2),and then the oxidized PAN fibers (OF) and carbon fibers (CF) were obtained.Fourier Transform infrared spectroscopy (FT-IR) and wide-angle X-ray diffraction (WAXD) were used to investigate the action mechanisms of oxygen-rich and oxygen-deficient environments,as well as how the chemical reaction degree in PAN affected the crystalline structure and tensile properties of CF.The results proved that the effects of oxygen-rich and oxygen-deficient environments could be divided into two stages:(Ⅰ)as φO2 rose,the intermolecular cyclization and oxidation predominate in the reactions of PAN dominated the reaction,leading to the crystallite size reduction and tensile property enhancement of CF;(Ⅱ)when φO2 further rose,the dehydrogenation became the major reaction in PAN,which caused the crystallite size increase and tensile property deterioration of CF.When φO2 was 23.4% and 22.1%,the CF prepared from PAN copolymer and terpolymer fibers could achieve the optimization of tensile properties,in which the tensile strengths were 3.7GPa and 3.9GPa,and the Young's moduli were 277.1GPa and 254.8GPa,respectively.
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