Oxygen Diffusion And Structural Evolution During Stabilization Of Polyacrylonitrile Fibers

During thermal stabilization of polyacrylonitrile fibers,complex reactions that affect the quality of carbon fibers occur in the fibers.In order to understand the reaction mechanism,influence factors and the relationship between structure and performance during the reactions,several techniques such as differential scanning calorimetry(DSC),thermal gravimetry(TG),fourier transform infrared spectroscopy(FTIR),elemental analyzer,X-ray diffraction(XRD),electron probe X-ray microanalyzer(EPMA)were used to systemically study the effect of chemical modification on the structure and performance of polyacrylonitrile fibers,the change of oxygen content and the influence factors,the evolution of "skin-core" structure during thermal stabilization of PAN fibers,and the development of chemical structure and crystal structure.The complex of Mn⁷⁺and the carbon in C≡N form in PAN fibers modified by potassium permanganate.The permanganate ions act as a catalyst to accelerate the formation of a ladder structure and to reduce the temperature of cyclization by about 10℃,and to reduce the time required for thermal stabilization.After modification, new peak in the IR spectrum was found at 2340cm^-1,which can be ascribed to C=C conjugation structure.The activation energy E of modified fibers reduces from 314 to 277KJ/mol.Modification of PAN fibers with KMnO₄ also results in the change of crystal structure.Chemical reactions take place in the original PAN fibers during modification,resulting in the increase of Bragg spacing,and the decrease of crystal size and crystallinity.O,H,C and N content changes distinctly during the stabilization of PAN fibers. At the initial stage,the increasing rate of oxygen content is very slow and the variation of C,N,H content can hardly be detected.With the rise of temperature,the O content increases rapidly and the C,H,N contents decrease gradually. Investigation on the atomic ratio of carbon and other elements indicates that H is released in the form of H₂O and H₂ at the earlier stage,and in the form of CH₄,NH₃ at later stage,and N is released as HCN.Itaconic acid can initiate the cyclization reaction at low temperature and slower the exothermal reactions in the fibers,while AIA initiate the cyclization reaction at lower temperature.The more the monomer content in PAN fibers,the lower the initiation temperature.Oxygen plays an important role in the reaction of the thermal stabilization.The DSC curves obtained at air and argon indicate that the distinct difference of the reactions during thermal stabilization.Double peaks were observed under oxygen atmosphere,and single peak appeared under argon atmosphere.The oxygen content depends on temperature at earlier stage of thermal stabilization,and on time at later stage.EPMA study on cross section of fibers obtained at different stabilization stages indicates that the formation of "skin-core" structure is related closely to the stabilization technology.At lower temperature,the reaction of stabilization is slow, and the skin structure of the fibers is loose.Oxygen can easily diffuse into the core, and a symmetrical structure forms along radial direction.With the increase of the stabilization temperature,the hetty compact structure generates,resulting in the unsymmetrical "skin-core" structure.The structure becomes severe with the rise of the stabilization temperature and the prolongation of the stabilization time.So an appropriate technology should be employed to relieve the skin-core structure during thermal stabilization.The structure evolution of the fibers during thermal stabilization was also studied.The infrared spectrum indicates that the absorptions at 2922cm^-1(due to CH₂ vibration)and 2245 cm^-1(due to C-N vibration)weaken with the rise of temperature,and the absorption resulted from C=N enhances.The absorption peak due to C-O vibration begin to enhance at 250℃.At lower temperature,reactions occur mostly in amorphous zone at slow rate.A long time is required to extend into the order zone,so the crystal structure changes very slowly.When the temperature rises to 245℃,the diffraction peaks at 2θ=17°and 2θ=29°in the XRD pattern begin to weaken markedly,and a new broad peak appears at 2θ=25.5°,whose intensity enhances with the rise of temperature and the prolongation of time.The crystallinity and the crystal size decrease obviously,and the Bragg spacing widens slowly.At final stage,the peaks at 2θ=17°and 2θ=29°almost disappear thoroughly, while the peak at 2θ=25.5°becomes stronger.The netty structure becomes compact, and the aromatization index reaches to a high level.