Formation And Evolution Mechanisms Of Microfibrils Of Polyacrylonitrile Fiber During Dry-jet Wet Spinning Process

The high-quality polyacrylonitrile(PAN)precursor fibers are indispensable for the manufacture of high-performance carbon fibers.The PAN fibers were prepared by dry-jet wet spinning technology and the samples were treated by the ultrathin section and solution etching methods.Their microstructure morphologies were examined by the scanning electron microscope(SEM),atomic force microscope(AFM)and high-resolution transmission electron microscope(HRTEM).The microfibrils were the fundamental structure elements of PAN fibers.In this work,it was used to reveal the existing microstructure morphological types in PAN fibers and elucidate the underlying formation and evolution mechanisms of the microfibrils through analyzing the formation and evolution process of the microfibrils of PAN fibers during spinning process.The coagulation process is one of the most important stages of the microstructure formation and evolution in PAN fibers during spinning process.The microfibrillar network and lamellae were fundamental structures of PAN nascent fibers.During the coagulation process,phase separation,gelation,crystallization,and concentration fluctuations occurred.The densified and elongated domains of the polymer-rich phase were solidified into the microfibrils and they were cross-linked to form the microfibrillar network.The transverse domains that induced by the concentration fluctuation,acted as the precursors of the lamellae,and subsequently the lamellae were stacked closely and regularly.Meanwhile,the crystallization occurred along the whole process,and the molecular chains were folded to form the crystal layers,which were aggregated within the microfibrilsThe PAN nascent fibers and precursor fibers with different coagulation bath drawing ratio were prepared by the one-step and two-step dry-jet wet spinning technology.For two different spinning technology,during the coagulation process,the molecular chains in spinning solution evolved into an interconnected microfibril network.At low drawing ratio,the random interconnected network existed in nascent fibers.Increasing drawing ratio,the network was transformed into the transverse lamellae.Furthermore,the lamellae thickness also decreased.After solution etching of the ultrathin section,the microfibrillar network was random for one-step spinning process,but it was oriented along the fiber axis in nascent fiber with high drawing ratio for two-step spinning process.Additionally,the order and homogeneity of microfibrils in nascent fibers and precursor fibers were improved by increasing drawing ratio,which were benefit to increase fiber tensile strength and modulusThe effect of spinning speed on the microstructures and mechanical properties of PAN fibers and carbon fibers was investigated.When winding speed increased from 63m/min to 95m/min,the fibers still exhibited smooth surfaces,and the diameter and aspect ratio of fibrils increased.Before densifying process,the lamellae were arranged orderly and PAN fibers had larger crystallinity at low spinning speed.After densifying process,the crystal layers were stacked regularly and their crystallinity was larger at high spinning speed,and the resulted precursor fibers had big thermal stability and large dimethylsulfoxide(DMSO)residue value.Meanwhile,the tendency of mechanical strengths of PAN fibers was in accord with their crystallinity changes.Increasing spinning speed brought the negative influence on microstructures and mechanical properties of the resulted carbon fibersFor the purpose of regulating microfibril structures and improving fiber quality,the microfibril orientation of PAN fibers induced by different stretching fields during dry-jet wet spinning process,was investigated.The interconnected microfibrillar network forming in coagulation bath was plastically elongated and gradually developed into the oriented microfibrils,while the transverse lamellae formed.Then the reorganization of the molecular chains brought the remarkable enhancement of fiber crystallinity during the densifying process.Finally,the well-oriented and regular microfibrils appeared due to the deep plastic deformation.The microfibrils in PAN fibers were stacked by crystal layers in an order and tightly manner.Served as excellent reinforcing elements,oriented microfibrils could efficiently improve the fiber mechanical properties.The microfibril morphological changes during spinning process in the longitudinal and transverse direction were characterized,respectively.In the fiber longitudinal direction,the microfibrils were oriented along the fiber axis to form the fiber backbone in PAN fibers.Furthermore,the microfibrils with small size were incorporated into the large fibrils.There were some transverse microfibrils acted as inter-fibril transverse joint bridges to reinforce the connection of the fibril elements.In the fiber transverse direction,the microfibrillar nodes could reflect the distribution of the fibrils in the cross-sections.And the pores existed between the microfibrils.Although the microfibrillar network was elongated and the microfibrils oriented along the fiber longitudinal direction during the spinning process,there were still some transverse microfibril structures.The interconnected microfibrillar network existed in the fiber transverse section.Furthermore,the transverse connection between microfibrils was reinforced.The morphological changes of the microfibril structures in PAN fibers during solution etching process and spinning process were investigated through characterizing the microstructures of PAN fibers.Consequently,the model of PAN fiber structures,the model of the structural changes of PAN fibers during solution etching,and the structural evolution model of the fibrils and crystal layers of PAN fiber during spinning process were established.In PAN nascent fiber,it showed porous network structures and the microfibrils were interconnected to form the network.In PAN precursor fiber,the microfibrils oriented along the fiber axis and they were superimposed by the folded-chain crystal layers.Furthermore,the microfibrils tended to assemble and merge into the fibrils and fibril bundles.During the dry-jet wet spinning process,microfibrillar network was gradually aggregated and merged to form the dense,oriented fibril structures through the plastic stretching and orientation.Meanwhile,the number of the crystal layers in microfibrils increased and the crystal layers arranged more orderly and regularly.