INTRODUCTION Polyacrylonitrile (PAN) has been accepted as the most suitable precursor for producing high performance carbon fibers with excellent properties such as low density, high stiffness and strength and good resistance to chemical and environmental 1-4 . However, the production of polyacrylonitrile-based carbon fiber, including precursor spinning, thermal stabilization and carbonization, is a complicated process. The properties of the reluctant carbon fiber are significantly influenced

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... the parameters of each process, particularly the precursor fibers and thermal stabilization. High performance carbon fibers can only be produced depending on fine precursor fibers with excellent properties and optimized pre-oxidation. To improve the mechanical properties of the precursor fibers and reduce both the induction period and rate of heat release during stabilization have drawn much attention in earlier researches 5-7 . The stereoregularity of polyacrylonitrile, which was beneficial to cyclization during stabilization, was rarely studied 8 . Nowadays, high stereoregularity of polyacrylonitrile could be obtained through acrylonitrile-urea canal polymerization, which was initiated, by γ-ray irradiation, matrix-anionic-polymerization and etc. 9-11 . The iso-polyacrylonitrile precursor with high stereoregularity was an effective way to obtain high performance carbon fiber. Isotactic polyacrylonitrile with triad isotacticity of 0.53 was successfully synthesized by using dialkylmagnesium as initiator. The effects of oxygen concentrating on chemical evolutions and thermal properties of iso-polyacrylonitrile were studied by Fourier transform infrared spectroscopy and thermal gravimetric analysis at different concentration (0, 20, 50 and 80 vol. %). Activation energy of thermal stabilization was obtained under non-isothermal conditions by using iso-conversional methods. The results showed that the amount of un-cyclized nitrile groups increased with oxygen concentration. This indicated that the cyclization was affected by oxygen concentration. Two stages and a large residue mass of iso-polyacrylonitrile pyrolysis during oxidation were detected in the oxygen-containing atmosphere. The activation energies were calculated by Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) iso-conversional method under different oxygen concentration varying from 0 to 80 %.