Study On The Structural Evolution Of Polyacrylonitrile(PAN)-Based Precursor Fibers During Heat Treatment Process

The preparation of polyacrylonitrile(PAN)based-carbon fibers mainly including thermal oxygen stabilization and carbonization.A large number of chemical reactions such as cyclization,dehydrogenation and oxidation that occured in the thermal oxygen stabilization process,which leads to the recombination process of the original microcrystalline aggregation structure inside the precursor fibers,at the same time,the chemical reaction result in the linear molecular chain transformed into the ladder like carbon ring structure gradually and increasing the thermal stability of fibers,which could withstand high temperature in the subsequent carbonization process;In the carbonization stage under nitrogen atmosphere,the heat-resistant ladder like plane structure formed in the thermal oxygen stabilization stage further undergoes thermal condensation and crosslinking reactions.A large amount of non-carbon elements were eliminated,and the carbon elements were gathered gradually,two-dimensional turbostratic graphite structure were formed which increased the compactness of fiber.After the precursor fibers undergoes the thermal oxygen stabilization and carbonization stages,the internal macromolecular structure undergoes an essential transformation from organic to inorganic,the thermal oxygen stabilization and carbonization stages were related to each other,and the structure formed in the stabilization stage determines the quality of the final carbon fiber,therefore,to prepare high-performance carbon fibers,it is important to study the evolution of the chemical structure and physical microstructure of PAN during the heat treatment of carbon fibers.Thus,in this paper,the influence of the difference in the microcrystalline structure of the precursor fibers on its thermal behavior during the heat treatment process were studied,and the structure evolution of PAN-based precursor fibers during the gradient stabilization and carbonization stage was discussed in detail.(1)The microcrystalline aggregation structure difference of four PAN precursor fibers were analyzed by wide-angle X-ray diffraction(WAXD),and the influence of the difference of microcrystalline structure on its thermal behavior and thermal stability during the stabilization stage was studied by differential scanning calorimetry(DSC)and thermogravimetry analysis(TGA)techniques and the activation energy of the reaction was studied by combining Kissinger and Ozawa equations,The research results indicated that in a nitrogen environment,both the heat release and rate of release of the precursor fibers was gradually increased with the increase in crystallinity,but the reaction kinetics study showed that the precursor with a small crystallinity has a relatively large dynamic advantages;the exothermic values(?H)in air atmosphere is much greater than that in nitrogen atmosphere,and the tendency of change with crystallinity is opposite to that in nitrogen atmosphere,and the thermal stability gradually decrease as the increase in crystallinity,which indicats that the difference in crystallinity has a greater impact on the thermal stabilization reaction in air atmosphere.(2)The evolution of chemical structure and physical microstructure of PAN precursor during thermal oxygen stabilization and carbonization process was discussed and researched in detail by differential scanning calorimetry(DSC)?fourier transform infrared spectroscopy(FT-IR)?Raman spectroscopy(Raman)and wide-angle X-ray diffraction(WAXD)?X-ray photoelectron spectroscopy(XPS)?scanning electron microscope(SEM).The results indicated that with the progress of thermal stabilization,cyclization,dehydrogenation and oxidation reactions occurs successfully,and the temperature is between 230°C and 250°C,much more active centers was introduced in the macromolecular chain,the cyclization and oxygen uptake reactions are significantly enhanced,and the fiber density increases from1.1852g/cm~3to 1.3844g/cm~3Raman results showed that during the stabilization stage,the ratio values of I_D/I_Gis reduced from 3.39 to 1.99.the chemical reaction leads to a large amount of SP~3hybridized C-C structure converts into SP~2hybridized C=C structure,however,the values of I_D/I_Gshows opposite trends compared with the stabilized stage after low-temperature carbonization,and the ratio values of I_D/I_Gincreased from 2.30 to 2.43,which indicated that a large number of graphite-like structures with defects are formed in the cross-linked network structures after low-temperature carbonization,and the amorphous polyaromatic carbon structures are formed by cross-linking between molecular chains in the low-temperature carbonization stage,but after high-temperature carbonization treatment,the ratio of I_D/I_Gdecreased from 1.92 to 1.59,indicated that high-temperature carbonization makes the graphitic carbon network structure grow gradually,and the degree of graphitization and the order degree of turbostractic layer graphite structure increased.XRD research results showed that as the thermal oxygen stabilization proceed,the microcrystalline structure in the original precursor fibers gradually undergoes a refinement-recombination process,and a large number of chemical reactions make the aggregation structure tend to amorphization.In the carbonization stage,the structure rearranged and formed a new graphite-like amorphous polyaromatic network structure.The SEM research indicated that more obvious grooves and protrusions on the fiber surface along the axial direction are gradually inherited as the heat treatment proceed,but after carbonization,the grooves become shallower,and the protrusions become thinner and uniform.In addition,it can be clearly seen from the SEM results that the fiber diameter becomes smaller gradually as the heat treatment temperature increases.