The Forming Process And Removal Method Of Skin-core Structure Of PAN-based Carbon Fibers

Polyacrylonitrile(PAN)-based carbon fibers have not only the inherent characteristics of carbon materials,but also the softness and spinnability of textile fibers.It is a new type of reinforcement with high performance for composites.PAN-based carbon fiber is produced by spinning,heat stabilization and carbonization,however,the skin-core structure is very easy to form and pass on to the carbon fiber in the preparation process,thus affecting the final performance of carbon fiber.Therefore,in order to produce high-performance PAN-based carbon fiber,it is very meaningful to understand the formation process of skin-core structure and master the method of eliminating skin-core structure.In view of the above problems,this paper mainly studies the following contents:(1)Microstructure changes of PAN-based fibers during wet spinningPAN-based copolymer was made into precursor by wet spinning.The structural changes of PAN-based fibers at the key stages of wet spinning are systemically studied by X-ray diffraction(WAXD),scanning electron microscopy(SEM),X-ray scattering(SAXS),atomic force microscope(AFM)and quantitative nanomechanical mapping(QNM).The forming reasons of physical skin-core structure and its genetic characteristics were discussed in detail.According to the WAXD and SEM analysis,the as-spun fibers show a homogeneous three-dimensional fibrillar network structure with random crystallite orientation,low crystallinity.The difference of solidification speed between skin and core results in the formation of physical skin-core structure in the early stage of wet spinning.As the draw ratio increases,the crystallite orientation and crystallinity increase firstly in the skin and then in the core.The uneven force distribution in fiber is responsible for the growth of physical skin-core structure.According to the SAXS and SEM analysis,different sizes of circular holes in the as-spun fibers are randomly distributed in the fibrillar network,and the shape and volume fraction of pores are strongly dependent on the compactness degree of fibrillar network.As the fibrillar network inhomogeneously shrinks,the total pore volume decreases,the pore shape becomes slender,and the density of skin region is higher than that of core region.According to the AFM and QNM analysis,the steam drawing process can improve the fiber network structure,which is the key step to eliminate the physical skin-core structure and prepare homogeneous PAN-based precursor fibers.(2)Microstructural changes of PAN-based fibers during thermal stabilizationPAN-based precursor was made into stabilization fibers by gradient thermal stabilization process.The structural changes of PAN-based fibers at the key stages of thermal stabilization are systemically studied using attenuated total reflection infrared spectroscopy(ATR-IR),AFM,nanoscale infrared spectroscopy(nano-IR),X-ray,SEM,X-ray energy dispersive spectroscopy(EDS),optical microscopy(OM)and nanoscale Lorentz contact resonance(nano-LCR).The forming reasons of chemical skin-core structure and its genetic characteristics were discussed in detail.ATR-IR,AFM and nano-IR reveal the radial distribution of specific functional groups and find that the gradient distribution of oxygen concentration in the fiber results in the chemical skin-core structure.Under oxygen-sufficient conditions,many ladder aromatic rings are formed in the skin.Intermolecular crosslinking presumably occurs through oxygen-induced dehydrogenation reactions.Under oxygen-deprived conditions,large numbers of isolated aromatic rings are generated in the core.Intermolecular crosslinking is dependent on comonomer concentration and Diels-Alder reaction of conjugated olefines.In addition,different fractured and polished morphologies of skin and core may be identified using SEM and OM,providing evidence for radial transformations of crystal structure.Moreover,a more accurate modulus distribution in the fiber was obtained by nano-LCR for the first time,further supporting the proposed intermolecular crosslinking and crystal rearrangement processes and formation process of chemical skin-core structure.(3)Effect of stabilization efficiency on skin-core structureThe fiber with different diameter and stabilization efficiency can be obtained by adjusting the feed pressure,stabilization temperature and time.The influence of fiber diameter and stabilization efficiency on the skin-core structure and properties of PAN-based fibers were discussed in detail using different analytical methods such as DSC,thermogravimetric analysis(TGA),OM,SEM,density gradient column(DGA),Raman,WAXD and tensile test machine.The results of OM and SEM show that the skin-core structure of stabilization fiber will be inherited to the carbon fiber in the subsequent carbonization process,and then affect the final performance of carbon fibers.The fine denier precursor fibers is more suitable for high-efficiency heat stabilization process.The results of SEM and Raman show that increasing the stabilization efficiency can reduce the oxygen diffusion rate into the fiber.When the oxygen diffusion rate is slow,it is easier to form a continuous conjugate layer in the in-plane direction of the fiber,which is beneficial to the improvement of the tensile strength of PAN-based carbon fiber.When the stabilization efficiency increases to a certain extent,the chemical skin-core structure is easy to appear,which will reduce the tensile strength of PAN-based carbon fibers.The results of WAXD and tensile test machine show that not only the tensile strength of carbon fiber is improved,but also the problem of high modulus of high strength carbon fiber is effectively solved by adjusting the stabilization efficiency.