Evolution Of Fibril Structure Of Dry-jet Wet Spinning Polyacrylonitrile Pre-oxidized Fiber

Polyacrylonitrile precursor undergo complicated physical and chemical changes during the pre-oxidation process,and their crystal structure,chemical structure,morphological characteristics and mechanical properties have undergone dramatic changes.Fibers are prone to structural defects during the pre-oxidation process.Therefore,the study of the structural evolution of the fiber during the pre-oxidation process and the relationship between structure and performance are beneficial to improve the production process of carbon fiber,thereby improving carbon fiber mechanical properties to produce higher quality carbon fiber.In this paper,gradient pre-oxidation of the polyacrylonitrile precursor produced in the laboratory is performed in six temperature zones,using a variety of analytical testing technologies systematically studied the crystalline structure,chemical structure,fibril structure and mechanical properties of the dry-jet wet spinning filaments during the pre-oxidation process.The correlation of the structure and performance of the pre-oxidized fiber are discussed,and the hypothesis of the fiber structure model is proposed.The crystalline structure of the pre-oxidized fiber has changed significantly.During the entire pre-oxidation process,the pre-oxidation reaction expanded from the amorphous region to the crystalline region,and the diffraction peaks around 20?17°and 29° gradually weakened;at the initial stage of pre-oxidation,the molecular chain of the amorphous region rearranged,thus the quasi-crystal structure is grown,which increases the grain size.When the pre-oxidation temperature reaches 230?,heating changes the quasi-crystal structure to an amorphous structure,which gradually reduces the grain size and crystallinity.When the pre-oxidation process progressed to the later stage,the diffraction peaks around 2???17° and 29° gradually disappeared,and new diffraction peaks appeared at 20?25.5°,indicating that a new order structure was formed.It was found that dehydrogenation,cyclization and oxidation reactions occurred during the pre-oxidation process.The dehydrogenation reaction started from the early stage of pre-oxidation and went through the entire pre-oxidation process;A portion of C?N underwent a cyclization reaction and transformed into a conjugated structure of-C=N-and-C=C-;During the pre-oxidation process,the macromolecular chain changed from a linear structure to a cross-linked ring structure.Raman spectroscopy test showed that the sp3 hybridized carbon atoms in the pre-oxidized fiber were converted to sp2 hybridized,indicating that the crosslinking and dehydration of the PAN molecular chain generated the C=C structure.The R value gradually decreased,indicating that the degree of order gradually increased,and the aromatic trapezoidal structure gradually improved during the pre-oxidation process.The pre-oxidized fibers were etched in different concentrations of dimethyl sulfoxide(DMSO)solution for different time,and it was found that the fibers could not be dissolved,indicating that the dense oxide layer on the surface of the pre-oxidized fibers had strong corrosion resistance.The fiber was embedded with epoxy resin and embrittled with liquid nitrogen,and the structure of the fiber cross section was observed under SEM.The brittle fracture fiber has a granular cross section,indicating that the fibril unit in the fiber structure is destroyed alone,and the tough fracture indicates that there is a strong cohesion between the fibrils,and the molecular chain of the pre-oxidized fiber has changed from linear to trapezoidal.When studying the fibril structure of the fiber,a combination of ultra-thin slice preparation and DMSO solvent etching was used.It was found that when the concentration of dimethyl sulfoxide(DMSO)solution was 90wt%,it was helpful to separate the fibril structure in the ultra-thin slices of pre-oxidized fibers.The fibers in the pre-oxidation stage separated the fibril structures parallel to each other;when the pre-oxidation temperature reaches about 240?,the separated fibril structure no longer appears in the fiber.This is due to the fact that in the late pre-oxidation,the molecular chain of the fiber is cyclized,and the cross-linking into a trapezoidal structure makes the fibril gradually become one.After the DMSO solvent contacts the surface of the fiber,it diffuses into the inside of the fiber through the pores between the fibrils,and the amorphous part between the fibrils is first dissolved in the solvent etching,so that the fibrils are exposed.The amorphous part between the platelets also dissolves,leaving platelets perpendicular to the fiber axis.The fibrils are composed of microfibrils,and the microfibrils are composed of plate crystal structure and amorphous interlayer perpendicular to the fiber axis.During the pre-oxidation process,the elongation at break of the fiber showed a trend of increasing first and then decreasing.In the early stage of pre-oxidation,as the pre-oxidation temperature increased,the pre-oxidation reaction gradually expanded from the amorphous region to the crystalline region.The quasi-crystalline structure of the fiber changed to amorphous due to heat,and the molecular chain structure becomes relaxed,resulting in an increase in elongation at break.In the late stage of pre-oxidation,the molecular chains were fully cyclized and cross-linked to form a dense trapezoidal structure,the rigidity of the fiber was increased and the elongation at break is reduced.As the pre-oxidation progressed,the highly polar C?N group gradually transformed to the C=N group,which make the tensile strength gradually decrease.