Microstructure Analysis Of Polyacrylonitrile Fiber By Dry-jet Wet Spinning

High quality polyacrylonitrile(PAN)precursor is the precondition for preparing high performance carbon fiber,while the density of nascent fiber is the key to the quality of precursors.The systematic work on the structure of PAN nascent fiber and precursors was researched,which has an important theoretical guiding significance for improving the density of PAN nascent fiber and the quality of precursors.In this paper,PAN nascent fibers and precursor prepared by dry-jet wet spinning process were studied.High-resolution transmission electron microscopy(HRTEM)and scanning electron microscopy(SEM)were used to observe and analyze the morphology of fiber.X-ray diffractometry(XRD),fourier transform infrared spectroscopy(FTIR),the specific surface area and mesoporous/microporous analyzer and other test analysis techniques were employed to characterize the fiber structures.The effects of different drying conditions and air-gap distance and coagulation bath drafting on the structure of PAN nascent fiber were systematically studied.The microstructure of fiber was studied by using ultra-thin sectioning technique.The PAN precursors were separated by the methods of skin-core separation,ultrasonic etching and solvent etching to study the structure of the fibrils,and the fiber model was established.PAN nascent fiber is a coagulated filament collected from the first coagulation bath,which contained a certain amount of solvent,while the test instruments used to characterize the fiber structure required the sample to be dried.Therefore,it is necessary to dry the fiber.In this work,acetone,ethanol and water were respectively used to soak,clean and dry the PAN nascent fibers,and the freeze drying method was used to prepare dry samples.The effect of different dry conditions on the structure and properties of PAN nascent fiber were researched.The skin of PAN nascent fiber treated with different solvents and freeze-drying was smooth,and the fiber cross-section morphology treated with acetone,ethanol and freeze-drying was loose.There is a pore structure inside PAN nascent fiber,and the size of pore belongs to micropore and mesopore.The crystalline and functional group structure of the fiber didn't change after treated by different solvents.The fracture of PAN nascent fiber belongs to ductile fracture,At the initial stage of stretching,the stress-strain curve of fiber conforms to Hooke's law.With the extension of time,the fiber undergoes a large yield with less tension.The effects of different air-gap distances and coagulation bath drafting on the PAN nascent fiber structure were investigated.The skin of PAN nascent fiber prepared by dry-jet wet spinning was smooth.Both the cross-section of the PAN nascent fiber with air-gap distances of 3 mm and 5 mm had an obvious three-dimensional network structure,which was formed by the interwinding of the PAN molecular chains.The fiber cross-section with air-gap distances of 8 mm and 10 mm presented a spot-like structure with uniform distribution.With the increase of the coagulation bath draft ratio,the diameter,denier and swelling capacity of the nascent fiber decreased.The PAN nascent fiber with different draft ratios were embedded with epoxy resin and the embedded blocks were obtained.The ultra-thin longitudinal sections of PAN nascent fibers were prepared by ultra-thin sectioning technique.The slice thickness was about 70 nm.The DMSO solution was used as the etching solvent to dissolve and etch the fiber sections.The microstructures of PAN nascent fiber sections before and after being etched were characterized by HRTEM.A three-dimensional network structure with no orientation distribution appeared in the nascent fiber sections with draw ratios of 1.8 and 2.5.Lamellar structures closely arranged perpendicular to the fiber axis appeared in the PAN nascent fiber sections with draw ratios of 3.2,3.8 and 4.5,the thickness of the lamellar structure decreased with the increase of the draw ratio.After etching in a DMSO solution,it was found that microfibrils appeared inside the fiber sections under the drafting of 1.8 and 2.5,and a microfibril network structure was observed inside the fiber sections with draw ratios of 3.2,3.8 and 4.5.PAN precursor prepared by dry-jet wet spinning was taken as research object.The PAN precursor were separated by the methods of skin-core separation,ultrasonic etching and solvent etching to realize the fibrillation of the precursor.Fibril was successfully dissected from fiber in the process of colloidal stripping.The fibril diameters were between 200 and 400 nm,and the fibrils were connected by an amorphous structure.After etching,fibrils appeared in precursor ultrathin section with diameters of 80-200 nm.There were microfibrils in the precursors,and the diameters of microfibrils ranged from 20 to 40 nm.Fibrils were composed of microfibrils.