Preparation And Structure Study Of Polyacrylonitrile/Mesophase Pitch Coaxial Composite Fiber

Polyacrylonitrile(PAN)carbon fibers and mesophase pitch(MP)carbon fibers are two important types of carbon fibers.The former consists of a low graphitization degree of disordered graphite structure,with high tensile strength and relatively low tensile modulus.The latter has a regular graphite-like structure,which has higher tensile modulus,electrical and thermal conductivity compared to PAN carbon fibers,but relatively low tensile strength.To prepare carbon fibers with both high modulus and high strength properties,and minimize structural heterogeneity,which PAN carbon fibers form high degree of graphitization of the cortex and low degree of graphitization of the core layer during graphitization heat treatment,a skin-core composite structure with PAN as the cortex and PAN/MP mixture as the core layer was designed in this paper.PAN/MP coaxial composite fibers were prepared by coaxial wet spinning,and the PAN mass fraction and PAN/MP composition in the spinning solution of the cortex and core layer were optimized,which improved the blending and bonding effect of the skin and core solution,and a gradual transition region was formed at the junction of the skin and core layer.The structure and properties of PAN/MP mixed solutions and coaxial composite fibers were characterized by means of Fourier transform infrared spectroscopy(FTIR),rotational rheometer,optical microscopy(OM),wide-angle X-ray diffraction(XRD),scanning electron microscopy(SEM),and energy dispersive spectroscopy(EDS).The main research results are as follows:(1)There are characteristic peaks of PAN and MP in the PAN/MP mixed solution,and there is an interaction between the two,presenting a good mixed state.The mixed solution of PAN/MP exhibits a phenomenon of shear thinning,and the incorporation of PAN improves the viscofluidity and spinnability of MP.The mixing state of PAN/MP solution is observed by polarizing microscope and optical microscope,and it is found that MP particles are randomly and uniformly dispersed in PAN.The mixture of the two is uniform and stable,providing the possibility for coaxial wet spinning.(2)PAN fibers and PAN/MP composite fibers were spun using different concentrations of dimethylformamide(DMF)aqueous solution as a coagulation bath.As the concentration of DMF increases,the solidification of the solution trickle becomes gentle,the diffusion effect is sufficient,and the fibers fully contract,resulting in a gradual decrease in the diameter of PAN fibers and PAN/MP composite fibers,a decrease in defects such as pores and voids in the fibers,and an increase in the degree of densification.While the concentration is greater than 60%,excessive coagulation bath concentration leads to excessive relaxation of fiber coagulation,resulting in significant deformation and shrinkage of PAN/MP composite fibers,which is not conducive to stable formation.Also different mass fractions of PAN solution in 60% coagulation bath can all obtain PAN nascent fibers with high roundness.(3)The effects of PAN mass fraction on the structure and properties of PAN/MP coaxial composite fibers are studied by using a PAN/MP mixture solution with a PAN mass fraction of 6.0% and MP mass fraction of 12.0% as core layer spinning solution,and PAN solutions with different mass fractions as cortical spinning solutions.It is found that as the mass fraction of PAN in the cortical solution increases from 10% to15%,the crystallinity increases from 24.3% to 31.4%,and the crystal plane spacing of the coaxial composite fiber decreases from 0.529 to0.523 nm,the tensile strength of the coaxial composite fiber increases by122%.This is because of the increase in the mass fraction of PAN,which increases the density of polymer network,and the increase in the crystallinity of the coaxial composite fiber leads to an improvement in the mechanical properties.SEM observations show that with the increase of the PAN mass fraction,the cortex of the coaxial composite fiber transforms from a porous and loose structure to a less porous and dense structure.Based on the difference in the transmissivity of PAN and MP to the same light intensity,gray values of coaxial composite fibers are measured,indicating that a transition region is formed at the interface between the skin and core layers,which is generated by the diffusion of the PAN/MP mixed solution.As PAN mass fraction increases,the viscosity of PAN solutions increases,and the diffusion which from the core mixed solution to the cortical layer is limited,resulting in an increase of 75.2% in the core layer area.When the mass fraction of PAN solution in the cortex is 12%,it is beneficial to form coaxial composite fibers with small diameter and circular shape,which have higher crystallinity and dense structure,and form a continuous transition region at the interface between the skin and core.(4)The effects of PAN/MP composition on the structure and properties of PAN/MP coaxial composite fibers were studied by using12.0% PAN solution as the cortical spinning solution and different composition PAN/MP mixed solution as the core spinning solution.The X-ray diffraction curve of coaxial PAN/MP composite fibers only shows the characteristics of crystalline and amorphous diffraction peaks of PAN.As the mass fraction of MP in the mixed solution increases,the crystallinity of coaxial composite fibers gradually decreases from 27.9%to 26.2%,owing to the intermixing of MP between PAN macromolecules,that weakens the interaction between PAN molecular chains;In the core layer of coaxial composite fibers,spherical MP is dispersed in the network structure of PAN.When the proportion of MP in the PAN/MP mixed solution is high,the core layer is mainly composed of coral structure,and the network structure alternates therein.When the proportion of PAN in the PAN/MP mixed solution is high,the core layer forms a network structure dominated by burrs,and MP small particles are embedded therein;The gray value measurement study finds that the coaxial composite fiber is composed of a lower gray level platform core layer,a gradually increasing gray level transition layer,and a cortex;From the core layer to the cortex,the content of C element gradually decreased and the content of N element increased,which coincided with the gradual decrease of MP content and the gradual increase of PAN content;SEM found that the morphology gradually changed from coral like structure to burr network structure from the core layer to the cortex.Gray value analysis,microregion energy spectrum,and SEM studies confirm that a transition layer structure is formed at the junction of the cortex and core layer,and the size and amplitude of the transition region are related to the viscosity of the PAN/MP mixed solution.(5)The obtained PAN/MP coaxial composite fibers are preoxidized and carbonized to investigate the radial carbon structure of PAN/MP coaxial composite carbon fibers.It is found that the graphitization degree of the core layer of coaxial composite carbon fibers is higher than that of the skin layer in PAN/MP carbon fibers,which proves the scientific design of coaxial composite fiber with MP as core layer.