Radial Distribution Of Chemical Structure Of Polyacrylonitrile Monofilaments And Its Evolution During Stabilization Process

The thermal stabilization process of poly-acrylonitrile(PAN)precursor fibers is a very important stage for the manufacturing of high-performance carbon fibers.During this stage,PAN precursor is transformed from organic linear chains to ladder structure,which provides thermal stability for the fiber.The structural basis for the subsequent carbonization process was established.The structural defects formed in the thermal stabilization will be further developed in the subsequent carbonization and graphitization processes.It will ultimately affect the mechanical properties of the final carbon fibers.The formation of structural defects during thermal stabilization is due to the different substance and heat transferring rate along radial direction.As a result,the structural distribution within PAN monofilaments along radial direction could be observed with the progressing of thermal stabilization.Nowadays,the study on the structural distribution along radial direction during the thermal stabilization of PAN precursor mainly focused on the morphology of skin-core structure.Nevertheless,the original reason for the formation of skin-core morphology is the characteristics chemical structural distribution that resulted from the difference of the progressing of reaction along radial direction of PAN precursor monofilaments.However,the analysis method for the chemical structure distribution has not been established until now since the dimension is small.Therefore,it is important to analyze the difference and evolution mechanism about the chemical structure along the radial direction of the fibers during the thermal stabilization process.The purpose of this study is to reveal the mechanism of the chemical structure formation and the influence of external factors on the radial structure of PAN stabilized fiber.It is important to improve the mechanical properties of final carbon fiber.In present thesis,various characterization methods for analysis of the chemical structure along radial direction of PAN precursor monofilaments were established.EPMA was applied to investigate the distribution of oxygen and nitrogen content at the cross section of the PAN monofilaments.The oxidation degree and the distribution of nitrogen-containing structures within the cross section of stabilized monofilaments was studied.Nano-IR is used to study the distribution of functional groups,including C=N,O=N,CH2,CH,etc.,along radial direction on the cross-section.Furthermore,the distribution curves of the elements and function groups along radial direction were obtained by the line scanning of EPMA and Nano-IR experimental results.In this way,quantitatively characterization method for revealing the radial distribution of chemical elements and functional groups was established.The effect of temperature and time on the radial distribution of chemical structure of PAN monofilaments during thermal stabilization process was studied based on the established methods.Besides,Nano-indentation method was applied to study the distribution of modulus within the monofilaments.The correlation between the radial chemical structure distribution and the modulus was obtained.The results indicated that:(1)The difference in the degree of oxidation of PAN monofilaments along radial direction was enlarged as the stabilization temperature increasing.Since the progressing of the oxidation reaction and heat diffusion exists radial distribution,the dehydrogenation and cyclization reactions in the fiber initially take place at the boundary between the skin and core regions followed by occurring focused on the core part due to the reaction heat accumulation.When the temperature increase furthermore,due to the higher concentration of oxygen in the skin,dehydrogenation and cyclization reactions was taken place rapidly in the skin part.As a result,the chemical structure tends to be homogeneous along radial direction.(2)The thermal stabilization time is important to the chemical structure distribution along radial direction of fibers.Because of the difference of cyclization and dehydrogenation reactions progressing along radial direction of fibers with various heat treatment times,it can be found that the cyclization reaction could be occurred at lower temperature if the heating time is longer.With the extension of stabilization time,the difference of chemical structure along radial direction is smaller(3)The correlation between the modulus and chemical structural distribution along the radial direction of the fiber was studied.It can be found that the modulus variety and the cyclization reaction process showed similar tendency but the radial difference of the modulus is slightly smaller than that of cyclization degree.Then the cyclization reaction in the process is the main factor affecting the distribution of modulus.