Formation Mechanism Of The Defects In Polyacrylonitrile Precursor During Preoxidation By Small Angle X-ray Scattering

The mechanical properties of Polyacrylonitrile(PAN)based carbon fibers which belongs to brittle materials are determined by the defect structure which mainly comes from PAN precursor and the pre-oxidation process.Therefore,study on the formation and evolution of defect structures in the pre-oxidation process is one of the main technical tasks to improve the mechanical properties of final carbon fibers.Small angle X-ray scattering(SAXS)technique was already applied to characterize the defect structure of PAN fibers because of its simple sample preparation method and low requirements for sample morphology.However,previous studies have some disadvantages such as diversification of defect models and complex data processing methods.The defect structure lacks a unified and systematic research system of SAXS.In this thesis,a spindle-shaped rotating body model of the defect structure of PAN fibers was established.The characterization method and data analysis system of SAXS technology were further developed.The porosity ?v,gyration radius Rg,average correlalation cutting length Iv,volume of voids V,specific surface area S/V and fractal dimension scale D of the defect structure within PAN fiber under different temperature and time conditions were obtained.The results indicated that:1.The defect structure of PAN fiber was formed and developed with the increase of temperature.At the initial stage of pre-oxidation,the porosity of the fiber would decrease,since the crystallinity was increased by the enhacement of PAN molecular motion.Small defects were merged into big defects and the total volume of defects remained basically unchanged,and the shape of the defect became more regular.After that,the amorphous region began to be cyclized.The generation of new defects reduced the average size of defects and increased the porosity of fibers.At the same time,the axial chemical shrinkage of the fiber reduced the volume of the defect and the shape of the defect became more complicated.Finally,the crystalline region began to be oxidized and cyclized.The porosity of PAN fiber increased?The radial size and volume of the defect became larger.The chemical shrinkage kept the average diameter of the defect constant and the regularity of the defect shape became higher.2.The defect structure of PAN fiber was also affected by reaction time.The fiber structure bacame unstable because of the secondary crystallization,cyclization and axial shrinkage of the fiber in the early amorphous region.In the radial direction,the defect size increased slightly and the volume of the defect decreased slightly.The overall size and porosity fluctuated within a small range.The surface structure of the defect became more complicate.In the later stage,the crystalline region began to be oxidized and cyclized.The degree of chemical shrinkage became larger.The porosity of the fiber,the radial size and volume of the defect increased,and the average size became smaller.The surface structure of the defect became more regular.3.On the basis of the mechanism of the formation and evolution of the defects of PAN precursor in the pre-oxidation process,a model of the mechanism of defect formation was established.