Study On The Structural Evolution And Properties Of Polyacrylonitrile Fibers During Stabilization

During oxidative stabilization, the chemical structure, crystal structure, morphological structure and properties of polyacrylonitrile (PAN) fibers change violently, and the structure defects emerge easily. Therefore the studies on the structure evolution and the relevance between structures and properties are quite important for optimizing the production techniques and improving the mechanical properties of carbon fibers. Optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectrum were used to systemically investigate the evolution of chemical structure, crystal structure, fibril structure and skin-core structure of wet-spinning and dry-jet wet-spinning PAN fibers during stabilization, meanwhile, the changes of properties were studied as well. The regularity of structure evolution and property change of PAN fibers during stabilization and the relevance between structure and properties were discussed.The research suggests that the chemical structure of PAN fibers changes drastically. The cyclization reactions of C=N experience a stage of slow reaction and violent reaction afterwards, and C=N transforms into C=N and C=C conjugate structures. When stabilization temperature exceeds245℃, pyrolysis of C=N occurs instead of cyclization reaction due to high temperature. Dehydrogenation lasts throughout the stabilization process. C-O group is formed with oxygen and carbon from cyclization structure when stabilization temperature reaches225℃, and oxygenolysis of part of C-O groups occurs when temperature is higher than255℃. The spinning method and temperature gradient distribution have no influence on the variation of chemical structure during stabilization, but exert an influence on the contents of functional groups.The crystal structure of PAN fibers and stabilized fibers was studied by XRD, and the results indicated that the crystal structure changed obviously as the stabilization progressed. During early stage of stabilization, part of molecular chains in amorphous region reorganize and transform to ordered structure, leading to gradual increase in crystallite size as the temperature rises. But when temperature is higher than225℃, the quasicrystal structure begins to be deformed because of being heated. Hence the crystallite size and crystallinity decrease gradually. It can be concluded that the stabilization reactions firstly take place in amorphous region and then extend to crystal region. In the later stage of stabilization, a new diffuse diffraction peak at25.5°emerges, which indicates the forming of a new structure. The variation of crystal structure of dry-jet wet-spinning PAN fibers is similar to wet-spinning PAN fibers during stabilization. The stabilization temperature is an influential factor to the crystal structure, and prolonging stabilization time can produce an obvious effect on the crystal structure when the temperature is high enough for the start of the cyclization reaction.The PAN fibers and stabilized fibers were etched ultrasonically, and the changes of chemical structure and crystal structure were studied before and after ultrasonic etching. The results show that the ultrasonic etching has no effect on the transformation of chemical structure during stabilization process, but has some influence on the crystal structure. After ultrasonic etching, the crystallite size and crystallinity of PAN fibers and stabilized fibers decrease in different degree. The crystallite size and crystallinity of stabilized fibers heated at lower temperature reduce slightly, while the intensity of peaks at17°nd29°, crystallite size and crystallinity of stabilized fibers heated at around240℃all drop sharply. It is considered that the enhanced non-crystalline structure at240℃is damaged severely under ultrasonic etching.The fibrils in PAN fibers and stabilized fibers were separated by the method of ultrasonic etching. The results suggest that using90wt%DMSO as solvent is favorable to fibril separation. Separated fibrils appear in PAN fibers and stabilized fibers of early stabilization stage. When temperature reaches245℃or even higher, fibrils compactly combine with each other, and no separated fibrils appears, which suggests cyclization and crosslinking of molecular chains. The diameter of single fibril decreases gradually as stabilization progresses, which is related to the drawing and tensile force caused by heat shrinkage during stabilization process. Fibrils join with each other by amorphous structure, and split with each other as amorphous structure among fibrils is dissolved by ultrasonic etching. Under ultrasonic etching, the voids among fibrils are enlarged. Subsequently, the solvent enters and spreads among the fibrils, which leads to the separation of fibrils.It can be clearly seen that grooves are present on the surface of wet-spinning PAN fibers and the stabilized fibers, and the grooves tend to be narrower and shallower as stabilization progresses. These grooves are the macro behavior of fibrils arranging on the fiber surface. The fibril dimension of dry-jet wet-spinning PAN fibers and the stabilized fibers is smaller compared with wet-spinning PAN fibers and the stabilized fibers, and the groove depth and width of dry-jet wet-spinning PAN fibers and the stabilized fibers are also obviously smaller.100wt%DMSO is an effective solvent for PAN. The stabilized fibers heated at different temperatures were etched in100wt%DMSO, and the etched morphology of stabilized fibers was observed. The wet-spinning PAN fibers stabilized at195℃and225℃are etched severely. The fibers stabilized at225～245℃shows low corrosion resistance, and the core is dissolved by ultrasonic etching due to lower stability compared with the skin, which leads to significant morphology difference between the skin and the core. Mieropores are formed in the core of the fibers stabilized at255℃and265℃generating a loose structure. The skin region is more compact and stable, and the fiber shows insolubility and infusibility. Some micropores and voids appear in the core region of dry-jet wet-spinning PAN fibers stabilized at220～230℃, while the skin region is more compact. The stabilized fibers heated at higher temperatures have no morphology difference between the skin and core region.The phase structure of skin and core of stabilized fibers were studied comparatively by Raman spectrum. It shows that the characteristic peaks of graphite-like structure appear in stabilized fibers heated higher than240℃The difference of Raman spectrum and parameters between the skin and the core of stabilized fibers suggests the difference of chemical structure. Along with the occurring of cyclization reaction, the aromatic heterocycle structure of the skin and the core is slightly different. It is confirmed that the organic functional groups and non-carbon atoms of aromatic heterocycle structure in the core are less than that in the skin. After oxidative reaction begins, oxygen diffusion from skin into core is hindered by the aromatic heterocycle structure. So the skin is stabilized fully, while the core is stabilized insufficiently, which leads to the aggravation of skin-core structure. The reaction process of oxidation after cyclization is the main causation of skin-core structure in stabilized fibers. Oxygen can diffuse sufficiently in fine fibers under the condition of slow or small gradient elevation of temperature, and the cyclization and oxidation reactions can overlap, so the skin-core structure will be alleviated.Drawing during stabilization causes the decrease of linear density. But the bulk density increases as the stabilization temperature rises, which leads to the increase of linear density. The linear densities of stabilized fibers are influenced by the two factors above. Throughout the whole stabilization process, the elongation at break of fibers first reduces, then increases before it decreases again, which is related to the ordering transformation of molecular chains in amorphous region in the early stabilization, the looseness of molecular chains by non-crystallization in the middle stage of stabilization, and the formation of compact ladder structure in the later stage of stabilization. The tensile strength of fibers decreases continuously as stabilization progresses, which is related to the decline of cohesive energy caused by the transformation from the strong polarity C=N to the weaker polarity C=N.The dry-jet wet-spinning PAN fibers have the similar variation law of mechanical properties to the wet-spinning PAN fibers during stabilization process. But the linear densities of dry-jet wet-spinning PAN fibers and the stabilized fibers are lower than that of wet-spinning PAN fibers and the stabilized respectively, while the elongation at break and tensile strength of dry-jet wet-spinning PAN fibers and the stabilized fibers are higher than that of wet-spinning PAN fibers and the stabilized.