Study On The Structural Interrelationship Of Polyacrylonitrile Precursor Fibers To Carbon Fibers

In this paper, the structural evolution and interrelationship from PAN nascent fibers to carbon fibers was systematically investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Fourier transform infrared spectroscopy(FTIR), X-ray diffraction(XRD), differential scanning calorimetry (DSC) /were also been used for fibers characterization.In PAN wet-spinning process, the counterdiffusion of the solvent DMSO and coagulation reagent H₂O caused the obvious skin-core structure of PAN nascent fibers, which had compact skin and loose core with pores. It has been found that the temperatures of coagulation baths would greatly influence the cross section and skin-core texture of PAN fiber. With high concentration of coagulation baths, higher temperature increased the rate of counterdiffusion, which led to circular cross section, reduction of defects in the core and higher crystallization. Suitable collapse process and steam drawing process could make the fibers more homogeneous. XRD showed that the crystal structure was initially formed in nascent fibers. The regularity of molecular chains and crystallization were gradually improved by the latter processes. The crystallization, the orientation and the tensile strength of self-made precursor fibers were higher than those of Japanese fibers. However, the properties of the resultant self-made carbon fibers were lower than the latter, which might be accounted for the influence of the non-crystal structure. The images of SEM and TEM indicated that the skin-core structure of PAN precursor fibers was inherited from that of nascent fibers. PAN precursor fibers were composed of four parts. The sheet-like skin, which was compact and homogeneous, had high crystallization and highly oriented structure. The core with low crystallization and some voids was loose, somewhat disorderly and unsystematic. Moreover, the precursor fiber had a pillar-like layered structure along the fiber axis. The average thickness of each layer increased gradually from the skin to the endothecium. Meanwhile, a structural model of PAN precursor fibers has been built.The Japanese precursor fibers had broader exothermic regime and double separated exothermic peaks, which might lead to better cyclization reaction and structure. The result of FTIR spectra of PAN fibers and the stabilized fibers revealed that cyclization reaction took place before dehydrogenation reaction.—C(?)N groups were gradually transformed to cyclization structure. The changes of oxygen element content and density of the fibers in the stabilization process consisted with relative cyclization index (η). When the temperature was lower than 200℃, oxidation reaction and cyclization reaction were not intense. However, the chains of amorphous phase near crystal area became flexible, which started vibrating, rotating and could form new crystallites. Combing FT-IR analysis, cyclization reaction firstly took place in amorphous phase.The images of SEM and TEM revealed that PAN stabilized fibers had the analogical structure as PAN precursor fibers. The cross section was also skin-core morphology, with a concentric circles structure. The white outmost layer presented the thin dense skin, followed by the cortex, the endothecium and the core. The skin consisted of stacked sheets, which were almost perpendicular to the fiber axis. The cortex had the same pillar-like layered structure as that of PAN precursor fibers. In the core, the structure became disorderly with microvoids. This indicated that the compactness and regularity decreased from the skin to the core. Moreover, there was an indistinct interface between the cortex and the endothecium. The structure of the cortex, which was more regular than that of the endothecium, had some oriented crystallites caused by those unstabilized PAN crystallites. The images of HRTEM showed that many global particles existed in the stabilized fibers. The outer of global particles were amorphous phase, which was annular structure. The inner of global particles had some crystallite areas, which was not completely stabilized. Therefore, The size of crystallites in PAN precursor fibers would severely influence the structure of stabilized fibers. Diminishing the size of crystallites in PAN precursor fibers is a key factor for high performance carbon fibers.Study on the pre-carbonized fibers and carbon fibers by FT-IR suggested that remnant—C(?)N groups continued cyclization reaction in pre-carbonization process, where the ratio of dehydrogenation reaction was higher than denitrogenation reaction. The N hexahydric rings gradually dehydrogenized, denitrogenized and formed C hexahydric rings in carbonized process. XRD analysis indicated that the smaller d₀₀₂, thicker Lc and higher crystallization of T-700 carbon fibers led to the higher tensile strength. The images of SEM and TEM showed that the microstnicture of the carbon fiber could be also represented as a microcomposite, which was composed of four parts from the skin to the core. The high density of sheets packing in its outer skin and the loose structure in the core should be noted. The compact skin of carbon fiber consisted of stacked carbon layers, forming small coherent units (nano-crystallites) in the stacking direction. The stacking direction of layers is preferentially perpendicular to the fiber axis. There were also some pillar-like layered structures forming the cortex of carbon fiber. With the complicated chemical reactions in the stabilization and carbonization processes, the space between layers decreased gradually from precursor fibers to carbon fibers.HRTEM analysis suggested that there were strip microstructure and global microstnicture in PAN-based carbon fibers. The strip microstructure of carbon layers resembled that of PAN molecular chains in precursor fibers. The global microstructure of carbon layers encircled the center, which was similar to that of the stabilized fibers. This indicated that the microstructure of PAN molecular chains had significant structural interrelationship.Surface defects of fibers have transmissibility, such as grooves, scratches and holes, which can be greatly reduced through ameliorating manufacture processes, circumstance and equipment precision. Interior defects have close interrelationship, such as skin-core multilayer morphology, loose core and holes, which can be weakened by adjusting coagulation process, the temperature and time of stabilization process.From the study on the structural interrelation and heredity of defects, it can be concluded that alleviating the defects, such as skin-core morphology, loose core and deliberately optimizing molecular chain structures and improving the orientation of stacked carbon layers are essential to obtain high strength carbon fibers.Acrylonitrile-ammonium itaconate copolymer {P[AN-co-(NH₄)₂IA]} was fabricated by free-radical solution copolymenzation of acrylonitrile (AN) and ammonium itaconate [(NH₄)₂IA]. Due to preferable hydrophilicity and facilitating cyclization reaction in stabilization process, P[AN-co-(NH₄)₂IA ] fibers could increase the draw-ratio in the spinning process and in the stabilization process to attenuate some property-limiting facts in precursor fibers and carbon fibers. The optimized final draw-ratio (12 folds) in the spinning process and stretching ratio (10%) in the stabilization process were introduced. Moreover, (NH₄)₂IA could significantly help in reducing the initiation temperature and heat evolved, broadening exothermic peak and restraining the main molecular chains of PAN from rupture during stabilization process. The tensile strength of P[AN-co-(NH₄)₂IA]-based carbon fibers could reach 3810 MPa. The improvement was due to fewer surface defects, better interior morphology, higher degree of orientation and graphitization.