Exploring The Structure Of PAN Nascent Fiber And Fibril Structure Of Precursor

The wet spinning and dry jet-wet spinning is an important process of the preparation of polyacrylonitrile based carbon fiber, the solidification molding process of polyacrylonitrile spinning dope and the structure of the nascent fiber has the important influence onthe structure of precursor, preoxidation fiber and the final performance ofcarbon fiber. The formation mechanism of the nascent’s structure in the first coagulation is of great importance.First, to understand what kind of treatment can effectively keep the internal real structure of nascent fiber, we explore the impact of ductile fracture and brittle fracture at low temperature on the cross-sectional morphology of primary fiber, the results shows that the brittle fracture at low temperature can preserve the internal morphology of nascent fiber.The nascent fiber were washed with water, ethanol, acetone then natural drying, respectively, and freeze-drying machine to dry directly.By observing surface and cross-sectional morphology of the primary fiber, the results shows that these processing methods can effectively keep the morphology of the fiber. IR and TG/DSC shows that the structure and thermodynamic properties have not been affected by some different treatments. The stronger bonding force between solvents and PAN by compared the crystallization performance of nascent fiber under different treatment, the better crystallization properties.Secondly, the nascent fibers with different molecular weight have ultrasonic etching process to explore factors that influencing the internal structure of the dry jet-wet spinning fiber. The concentration of etc hing liquid increase with the increase of molecular weight, the surface structure of the fiber with three-dimensional reticular structure turns into micro fibril structure, the layer structure of nascent fiber become dense. With ultrasonic etching different solid contents of nascent fibers, the concentration of the etching liquid increased with the increase of solid content, changes of cortex similar to the changes rule of molecular weight. Comparing the different draw ratio of nascent fiber, the crystallization properties rise with the increase of draw ratio, the diameters of nascent fiber decrease with the increase of draw ratio by observing images of the cross-sectional structure, many holes exist on the cross-sectional of primary fiber. Along the radial, the structure of nascent fiber is characterized by the structure of three-dimensional network, the structure of nascent fiber has a certain orientation along the axial. By ultrasonic etching the different air layer height of nascent fibers, the cortex structure of fiber becomes more dense with increase of the air layer height. It is difficult to etching fiber’s surface as the decrease of air layer height. The crystallization performance of nascent fiber is better when the air layer height is 5-8 mm. Analysis for thermodynamic performance of the nascent fiber with different air layer height, the thermodynamic temperature of cyclization reaction and oxidation reaction has no obvious changes. To verify the three-dimensional reticular structure of nascent fibers, the primary fiber is handled with the ultra-thin slice process. Micro pore distribution of the cross-sectional and longitudinal section show that the internal structure of nascent fiber is loose, the structure of nascent fiber is characterized by the three-dimensional network structure without orientation along the radial and the existence of the dense cortex. Etching ultra-thin slice’s cross-sectionalof the primary fiber, residue is observed in the three dimensional network structure which plays a role of connecting the dense material. There are many dense ordered layered structure along the axial direction with the obvious orientation. PAN macromolecular chain which connected the layered structure can be seen in the picture after etching treatment.In addition, the unformed fibers of wet spinning and dry jet-wet spinningin the first coagulation bath were studied respectively, surface of the unformed fiber has more groove structure in the first coagulation bath, the cross-sectional shape of wet spinning is like "kidney". Each part of cross-sectional nascent fiber in wet spinning and dry jet-wet spinning is like the three-dimensional network structure. After dry jet-wet spinning, the crystallization properties of unformed fibers changes irregularly. The crystallization performance of unformed fiber in the wet spinning bath decreses with the increasing distance of jet mouth which decided by the movement of double diffusion in the first coagulation bath. The chemical group in each part of the nascent fiber in wet spinning and dry jet-wet spinning has no changes. The average elongation at break of unformed fiber in the wet spinning is higher than the fiber in dry jet-wet spinning. The average fracture strength is lower than the fiber in dry jet-wet spinning.At last, the ultrasonic etching PAN precursor of wet spinning and dry jet-wet spinning respectively, the themorphology of precursor by different treatment concentration of DMSO when etching time 6h has been observed, the best etching concentration for PAN wet spinning precursor is 89% DMSO which clearly shows the crystal structure and the prototype of the three-dimensional network structure after excessive etching. The best etching concentration for PAN precursor by dry jet-wet spinning is 89% DMSO. Density cortex of surface’s structure became ribbon fibrillar by etching process, the ribbon fibrillar became multiple micro fibril by etching, respectively. The lamella structure of the micro fibril can be clearly seen, small amount of micro fibril structure drops on copper mesh. Etching 3h、6h、9h for wet spinning and dry jet-wet processing PAN, respectively, PAN precursor can be fully and effectively etched after 6h, which clearly shows the surface and structure information of the precursor.