Thermal Behavior And Structural Evolution During Oxidative Stabilization Of Polyacrylonitrile Fibers

Oxidative stabilization of polyacrylonitrile (PAN) precursor fibers is an essential process that affects the quality, carbon yield and production efficiency of carbon fibers. In order to obtain high quality carbon fibers, it is important to carry out deeply study on the thermal behavior and reaction mechanism of PAN fibers, and on the microstructural evolution during the oxidative stabilization process. Several technologies, such as differential scanning calorimetry (DSC), thermal gravimetry (TG), Fourier transform infrared spectroscopy (FTIR), elemental analyzer, wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to systemically investigate from the following aspects: the mechanism of chemical reactions during stabilization; the influence of processing conditions on the changes of composition, structure and the properties of fibers during stepwise rising temperature stabilization; thermal shrinkage behavior, the changes of tension in stabilized fibers and the mechanism of stretching; the dynamics of oxidation reaction and its influence on the "skin-core" structure of stabilized fibers; the evolution of surface morphology, fracture texture and microstructure of PAN precursor fibers during oxidative stabilization process.The analysis of thermal behavior and chemical structure show that the three main reactions do not initiate simultaneously. Dehydrogenization and cyclization reactions begin at the initial stage of stabilization, while oxidation reaction takes place on the precondition of cyclization reaction. Cyclization reaction has induction period that is influenced by temperature and heating rate. The lower is the stabilization temperature, or the faster is the heating rate, the longer is the induction period. In oxidative atmosphere, oxygen hampers cyclization reaction, helpful to take the whole process under control. Itaconic acid (IA) can initiate cyclization reaction at low temperature, alleviate the concentrated exothermal reactions, and avoid chain scission or thermal degradation. The study of dynamics indicates that the reactions in stabilization process are complicated multi-order reactions. The activation energy of poly(AN/1%IA) is 320.7 kJ/mol in air and 174.3 kJ/mol in Argon. The reaction rate constant of overall reactions is much less than that of cyclization reaction.Temperature, time and stretching ratio are the main parameters of stabilization. Studies were carried out on the variations of the composition, structure and properties of fibers during the stepwise rising temperature stabilization. The changes of O, C, H, N content and bulk density of stabilized fibers are strongly depended on temperature and time, but are hardly affected by stretching. At the initial stage of stepwise rising temperature stabilization, the increasing rate of oxygen content and bulk density are very slow, and there is little variation in C, H, N content. With the rising of temperature, the 0 content and bulk density increase rapidly and C, H, N content decrease gradually. At the later stage of stabilization, O content increases with the prolonging of time, but the bulk density declines. Throughout the whole process, the elongation at break of fibers first decreases, then increases and at last decreases again, but the tensile strength of fibers decreases continuously. The results show that elongation at break is related to the flexibility of molecular chains, while tensile strength is depended on the cohesive energy. From WAXD analysis, conclusions can be drawn that stabilization reactions take place in amorphous regions when temperature is below 245°C, and spread to ordered regions with the rising of temperature. As a result, the original crystal structure of PAN transits to amorphous, and a new structure forms at the later stage of stabilization. A new index named stabilization index (SI) is proposed in this paper to characterize the degree of stabilization. Compared with aromatization index (AI), SI is more effective to reflect the structural changes during stabilization. Though stretching has little effect on either cyclization or oxidation reaction, it is conducive to maintain the chain orientation in fibers, and is helpful to suppress the growing of crystallites. The tension in fibers is influenced greatly by stretching ratios, and it is also related to temperature, time and the properties of precursor fibers. The experimental results indicate that under the same stabilization conditions, higher thermal stress generated in such precursor fibers that has low porosity and high crystallinity.Skin-core structure of stabilized fibers is a kind of structural defects that can deteriorate the performance of resultant carbon fibers. The cause of its formation was analyzed from the aspect of oxidation dynamics. The effects on skin-core structure by several factors, such as precursor fibers composition, titre, shape of cross section and stabilization methods, were discussed. The results show that the inhomogeneous distribution of oxygen at the radial direction of fiber is the fundamental reason of skin-core structure, which is caused by lower rate of oxygen diffusion and higher rate of cyclization reaction. It is more liable to obtaining homogeneous stabilized fibers in stepwise rising temperature stabilization than in isothermal stabilization. The shape of cross section has no effect on oxygen diffusion. But the composition and titre of precursor fibers successively influence oxidation reaction with the increasing of temperature. The more content of comonomer, the higher is the oxidation reaction rate. The lower titre of precursor fibers, the more sufficient is the diffusion of oxygen. For the AN/IA copolymer precursor fibers, it is proposed that if the diameter of filament is no more than 8μm, stabilized fibers with no obvious skin-core structure will be obtained by stepwise rising temperature stabilization.SEM study on surface morphology and fracture texture of various stabilized fibers indicates that the fibril structure of precursor fibers is passed down throughout the whole stabilization process. With the development of stabilization, the toughness and diameter of fibrils gradually decrease, along with the closer bonding between fibrils. The skin region of stabilized fibers has high brittleness and dense microstructure, while the core region has high toughness and loose microstructure, which suggests that stabilization reactions develop from the outer to the interior part of fibers. Deeply investigation was carried out on the structure of skin and core regions by TEM. Nanometer crystallites that distribute with no preferable orientation on amorphous matrix are observed in the skin region. While there are lamellar-like structures orientating along the fiber axis in the core region. With the increase of temperature, the crystallite size in the skin regions decreases, the orientation of lamellar-like structures and the combination between lamellas in the core regions decline.The microstructural evolution of PAN precursor fibers during oxidative stabilization process was studied by HRTEM. Lattice fringes images and two kinds of amorphous structures, i.e. maze-like clusters and onion-like spheres, are found coexisted in PAN precursor fibers, which gives experimental evidence for the two-phase structure theory. During oxidative stabilization, great changes take place in crystal regions earlier at 230℃and later in amorphous regions at 255℃. In fibers stabilized at 275℃, new structure that has similar interplanar spacing with d(002) of graphite is observed. In the transverse sections of stabilized fibers, the outer layers of the onion-like spheres interlace with each other and rearrange to form a structure without apparent interfaces. The interplanar spacing in crystal regions increases and the crystallite size decreases as a result of an order-disorder transition that either develops from the outer side of the ordered region to the central part, or impenetrates through the crystal region. In the longitudinal sections of stabilized fibers, the order-disorder transition of ribbon-like structure begins from the end of ribbons and spreads along the interface between ribbons. Though order-disorder transition also takes places in ribbon-like structures, high orientation is maintained, which guarantees high tensile strength and modulus for carbon fibers. The experimental results also show that stabilization reactions progress easier in small crystallites. Therefore, it is important to obtain PAN precursor fibers with smaller crystallites and uniform microstructure.