Study On The Formation Of Ring Layer Orientation By Polarized Raman Spectra

PAN-based carbon fiber has become the focused functional and structural material for its excellent mechanical properties. The good mechanical performance of carbon fiber is inseparable with its orientation structure. The investigation on orientation structural evolution during the preparation of PAN-based carbon fiber has been the main research point of scientist and it is the theoretical basis and guidance of technological optimizing during the preparation of carbon fiber process.After pro-oxidation and carbonization process, PAN fiber transformed into carbon fiber, and molecular structure transformed from molecular chain orientation into ring layer orientation. In the process of heat treatment, carbon fiber ring plane started to form. However, X-ray diffraction could not measure the inheritance and evolution of the orientation structure due to the crystalline structure disappeared. And it is limited for the study on the evolution of orientation structure during pro-oxidation process.According to the domestic and foreign researchers, the study on the orientation structure of PAN precursor and carbon fiber were carried out so far. Molecular orientation of PAN precursor can be characterized by dichroic method and acoustic method. Wide angle X-ray diffraction (WAXD) also have an important role to the research on the crystallite orientation of fiber. Whereas, in the later period of stabilization, the crystalline structure of fiber disappeared and the color turned dark. There was some limitation of the orientation characterization during the later period of pro-oxidation. In order to study the evolution of orientation structure from pro-oxidation process to carbonization process and provide scientific basis for the fiber orientation of controlling stabilization period as well, it is necessary to find a feasible measurement method to characterize the orientation of PAN fiber during different period.In this paper, by using polarized Raman method, the effects of temperature and tension on the depolarization degree during pro-oxidation and carbonization process were investigated. Combining with X-ray diffraction, we have investigated the crystallite orientation, the formation of ring layer orientation and the relationship between th e orientation of ring layer and crystallites were also studied. The results showed that:1. During the process of thermal stabilization, the size of the fiber was shrinked due to the disorientation and the cyclic shrinkage. At the final, the orientation of the crystalline region disappeared, and the orientation of the molecular chain remained at about 0.82.2. According to the measurements of deducing different fiber samples polarized Raman and the relationship between depolarization degree and anisotropy factor k2 was derived.Anisotropy factor k2 is defined as the ratio of the anisotropy invariant and isotropic invariants, the establishment of the relationship between depolarization degree and ring layer orientation, so as to establish the polarized Raman characterization method for study on the orientation structure. This method was applied to the study on the evolution of the orientation structure of PAN precursor.3. During stabilization process, the ladder ring plane began to form. Anisotropic factor k2 is in a slow change of the initial stage of pre-oxidation, when the intramolecular reaction plays a dominant role. The ladder ring plane orientation and the original molecular chain orientation changes little. Up to 240? k2 fall rapidly due to the intermolecular reaction and the structural regularity of fiber molecules is gradually destroyed. The linear molecular transformed into disordered and irregular ladder cyclic polymer. The ring plane orientation decreases rapidly and causing fiber size shrinkage.4. In low temperature carbonization process, the ring plane gradually transform to the six membered ring layer. The orientation and crystallite increased with the increasing of temperature and tension. In the initial stage of 1000 temperature carbonization, with the thermal decomposition reaction, the escaping of small molecular gas by-products results in the formation of defects in fiber, the crystallite orientation is slower than the ring basal plane orientation. Starting from 680?, non-carbon elements was removed, the ring plane along the fiber axis was rearranged, the crystallite orientation is greatly increased.5? In the high temperature carbonization stage, tension promoted the rearrangement of the fiber internal ring plane, with the higher tension, the ring plane along the fiber axis rearrangement can more easily, the orientation became better. Condensation reaction make the ring plane gradually change to the six membered ring layer, non-carbon elements was further removed, ring layer along the fiber axis rearrangement and orientated. The orientation of crystallite increased with the increasing temperature and gradually grow and perfect. In the high temperature carbonization stage, the orientation structure in carbon fiber gradually perfected and formed a regular structure. The orientation of ring layer and crystallite increased with the increasing of temperature and showed consistent change trend.In this paper, by using polarized Raman method, the formation and evolution of ring layer orientation during pro-oxidation and carbonization process were investigated. And it has important meaning for the controlling of ring layer orientation during the manufacturing process of optimized carbon fiber.