Structure Formation And Evolution Process Of Polyacrylonitrile Fiber By Dry-jet Wet Spinning

The quality of polyacrylonitrile(PAN)precursor is key for the preparation of high performance carbon fiber.The systematic work on PAN fiber evolution of structure and performance during dry-jet wet spinning process was researched to deep understanding of PAN nascent fiber coagulation process and structure evolution in the spinning process.That has the guide to improve the quality of PAN precursor and increase the performance of PAN based carbon fiber.In the paper,X ray diffraction(XRD),scanning electron microscopy(SEM),high resolution transmission electron microscopy(HRTEM),differential scanning calorimetry(DSC),thermogravimetric analysis(TG)are employed.The effect of different spinning dope parameters and coagulation process parameters of coagulation bath on nascent fiber structure and properties,and structural evolution of the PAN fiber during various spinning stage are researched.The coagulation bath of dry-jet wet spinning is the primary and most important stage of precursor structure formation,which has an important influence on the quality of PAN precursor.The effect of molecular weight,solid content,coagulation bath temperature and draft on the structures and properties of nascent fiber is discussed.Coagulation bath temperature has influence on double diffusion process,which has important effect on cross section of nascent fiber.With increase of coagulation bath draft,the crystallinity,crystallite size and rate of boiling water shrinkage of the nascent fiber are improved,molecular chains arrange more tightly,the size and number of the micropore decrease and the nascent fiber prepared has smaller fineness,larger strength,higher rupture elongation and smaller coefficient of variation.With increase of PAN molecular weight and solid content,the density of PAN molecular entanglement in spinning dope increases,which leads viscosity raise.And nascent fiber has smaller titre and higher tensile strength.So spinning dope with high molecular weight and large solid content are benefit to prepare excellent nascent fiber by dry-jet wet spinning process.The microstructures of PAN fibers in various stages during dry-jet wet spinning process and the effect of crystallization on the mechanical and thermal properties of PAN fiber are studied.PAN fibers have smooth surface,less impuritie,fewer surface defects.The crystal size and crystallinity of PAN fibers increase with spinning processing while the crystal structure is more perfect.Stretching can improve the crystallinity,densification and heat setting promote crystal growth.PAN fiber has smaller fineness,larger strength,lower rupture elongation with spinning processing.The crystal structure of precursor has significance influence on mechanical and thermal properties.The perfect crystal structure is benefit to mechanical properties of precursor and restricts cyclization reaction that initial reaction temperature tends to high temperature and helps to prevent the thermal decomposition of PAN molecular chain.Microstructure of PAN fiber was observed by HRTEM.PAN fibers in spinning stages were embedded by epoxy resin and preparation method of ultrathin section was used.The transverse sections vertical to fiber axial and longitudinal sections parallel to fiber axial are obtained.The thickness of slice is 50-70nm,which is conducive to electron beam penetration samples.Among them,the epoxy resin is only used to fix fiber,but it has no enhancement effect to the image contrast.The nascent fiber internal organization is lamellar structure that arranges perpendicular to the fiber axial and there is many pores between lamellae.In the coagulation bath pre-draft stage,the microfibril structure appear which arranges parallel to fiber axial.After multi-stage drawing and densiflcation,microfibrils arrange more closely,which staggers or winds each others,not parallel with diameter of about 30-50nm.The micropore with narrow section shape exists between fibrils,and orients along the fiber axis under stretching.During spinning process,the number of pore decreases,but the pore can not be completely eliminated.