Study On Microstructure And Chemical Structure Of Dry-jet Wet Spinning Polyacrylonitrile Based Preoxidized Fiber

Polyacrylonitrile(PAN)based carbon fiber has been widely utilized in various fields because of its high strength,low density and corrosion resistance.The production process of carbon fiber mainly included the preparation of PAN precursors,preoxidation and carbonization.Among them,the preoxidized process was a crucial step in the production of carbon fiber.During the preoxidized process,the chemical structure and microstructure of the fiber have changed,and the microstructure would be inherited into the carbon fiber in the subsequent carbonization process.Therefore,the study of the structure and properties of PAN fibers in the preoxidized process was of great significance for improving the quality of carbon fibers.In this thesis,PAN precursor was prepared by laboratory dry-jet wet spinning process,and preoxidized fibers in six temperature zones were prepared by gradient heating preoxidation method.The fiber samples were treated by technique of ultrathin sections and solution etching.Scanning electron microscopy(SEM)and high-resolution transmission electron microscopy(HRTEM)were used to analyze the morphologies of preoxidized fibers.Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD)and thermal gravimetry(TG)were used to characterize the chemical structure,crystal structure and thermal properties,respectively.Through nano infrared spectroscopy atomic force microscope(NanoIR2-FS),the chemical structure and microstructure of the same region of the fiber could be obtained at the same time to characterize the relationship between them.It provided a new research method for studying the relationship between the chemical structure transformation and microstructure change of PAN precursor during the preoxidized process.The most important process parameters in the preoxidized process were temperature and time,which have an important impact on the quality of preoxidized fiber.Two different technologies were used to prepare preoxidized fibers.One was conventional preoxidation(S1),which adopted the traditional heating temperature and time.The preoxidized temperature was 220-255℃.The other was efficient preoxidation(S2),which increased the preoxidized temperature and reduced the preoxidized time.The preoxidized temperature was 250-285℃.In this thesis,the structure and property of preoxidized fibers with two different technologies were studied.The results showed that in the process of preoxidation,the color of preoxidized fiber became darker due to the chromogenic groups generated by chemical reaction.During the preoxidized process,the fibers underwent cyclization,oxidation and dehydrogenation reactions,and the linear macromolecular chain changed into trapezoidal structure.The cyclization degree and cyclization efficiency of fiber of S2 were higher than that of S1,and the cyclization reaction was denser.With the increase of temperature,the crystallinity of preoxidized fiber decreased,and the crystal structure changed obviously.And a new ordered structure was formed.The crystallinity of S2 decreased sharply from third to fourth temperature range,which was related to the formation of a large number of cyclic structures at this stage.The aromatization index(AI)increased gradually during the preoxidized process,and the cyclization degree of the preoxidized fiber increased.After preoxidation,the linear molecular chain of PAN changed into a cross-linked trapezoidal structure,and the thermal stability of the fiber was significantly improved.In addition,the loss weight temperature of S2 was higher than that of S1,indicating that S2 had better thermal stability.During the preoxidized process,the tensile strength of the fiber decreased and the elongation at break increased.From the comparison,it was found that the mechanical properties of S2 were better than that of S1.By characterizing the surface morphology,sectional morphology and internal morphology of preoxidized fiber,the transformation of fiber microstructure in the process of preoxidation was studied.The surface of the preoxidized fiber was smooth and the diameter was about 8μm.The interior presented microfibril structure,which became more dense during the process of preoxidation.After ultrasonic etching with dimethylsulfoxide(DMSO)solution,the preoxidized fiber swelled and wrinkles appeared on the surface.In the late stage of preoxidation,DMSO solvent was difficult to diffuse into the fiber,and the corrosion resistance was significantly improved.The cross-section of the fiber after brittle fracture showed microfibril structure in the fiber.With the increase of temperature,the fracture morphology of the fiber changed from tough fracture to brittle fracture,and the cross-section was gradually flat.There were fibril structure along the fiber axis and lamellar structure perpendicular to the fiber axis in the preoxidized fiber.The difference of morphology between skin and core region gradually increased in the early stage of preoxidation and decreased in the late stage of preoxidation.After etching the ultrathin sections with DMSO solution,the amorphous structure was dissolved and the fibril structure was separated.The longitudinal microfibrils were connected by the transverse microfibrils and amorphous structure,and the interior of the microfibrils was formed by the stacking of lamellae and disordered areas.During the preoxidized process,the linear molecular chain of PAN was transformed into trapezoidal structure,the soluble area of DMSO solution was gradually reduced,and the corrosion resistance of preoxidized fiber was improved.By studying the chemical structure and microstructure of the preoxidized fiber skin and core region,the effect of chemical structure transformation on the microstructure during the preoxidized process was discussed.With the increase of cyclization degree and oxidation degree,the fracture morphology of preoxidized fiber changed from tough fracture to brittle fracture.The difference of morphology between the skin and core region increased in the early stages of preoxidation and decreased in the late stages of preoxidation.The change of chemical structure caused the change of fracture morphology.Microfibrils along the fiber axis and lamellar structure perpendicular to the fiber axis were observed.With the increase of preoxidized temperature,the morphology became more dense.The evolution relationship between chemical structure and morphology of preoxidized fiber skin and core region was studied by NanoIR2-FS and ultra-thin section technology.The higher the degree of cyclization and crosslinking,the more dense the morphology of the fiber.It was found that the change of microstructure was consistent with the rule of chemical structure transformation.The difference of chemical structure and morphology between skin and core region increased in the early stages of preoxidation and gradually decreased in the late stages of preoxidation.Through research and analysis,the morphology transformation model of preoxidized fiber was proposed.The surface of the preoxidized fiber was smooth,and the interior was composed of fibrils.The fibrils were formed by the aggregation of microfibrils of different sizes,including transverse microfibrils and longitudinal microfibrils.The longitudinal microfibrils were connected through transverse microfibrils and amorphous regions.Microfibrils were formed by stacking lamella along the fiber axis,in which ordered crystal layers and disordered regions were arranged alternately.In the early stages of preoxidation,a small amount of PAN linear molecular chains changed to cyclic structure,and cyclic structure began to be formed in the lamella.In the late stages of preoxidation,the cyclic structures were crosslinked into an orderly trapezoidal structure,the connection of microfibrils were strengthened,and the fiber structure became more dense and stable.