The Effects Of Pore Structure On The Oxidative Stabilization Of Polyacrylonitrile Fiber

Polyacrylonitrile (PAN) fiber is one of the most suitable precursors for carbon fibers. Among the manufacture of carbon fibers, the oxidative stabilization of a precursor fiber is an essential step in making the precursor infusible at high-temperature carbonization. The process is also considered the most important because it largely governs the final structure and mechanical properties of the carbon fiber.This work based on synchrotron radiation small-angle X-ray scattering techniques to achieve the pore structure (average pore size, porosity) and quantitative characterization of PAN fibers. Compared with the other testing methods, this method was of high precision, the negative effects were small, and the production process was simple. It was important to acquire the accurate characterization of the PAN fibers' pore structure to syudy the oxygen diffusion process during the pre-oxidation and the mechanical properties of carbon fiber.In addition, the TG, DMA results showed that, the thermal properties changed greatly between the different atmosphere. After heat treatment in the nitrogen, there were only intramolecular cyclization occuring, intermolecular cross-linking did not occur. But in the air, these two reactions simultaneous occured. This work introduced the porosity parameters based on the previous model, aimed to establish the quantitative relationship between the pore structure of PAN precursor and oxygen diffusion coefficient and made clear the effect of the kinetic of oxidation reaction. On this basis, the samples were tested under different atmosphere by DSC to understand the oxidation heat volume. The relationship between the oxygen diffusion coefficient and the pore structure including the porosity and average pore size was built up. The final oxygen content coincided with pore structure of the raw fibers.There were chemical and physical changes occurring significantly during the thermal reaction of the PAN precursors. The original linear molecular structure transformed to a ladder-shaped, heat-stable structure. In order to suppress the contraction and improve the orientation degree and the final mechanical properties, it was nessaccery to exert a certain drawing. This work found that, in the pre-oxidation process, as the temperature increased, the pore structure were constant. When imposing a certain draeing, the fiber orientation degree and crystal size were increased, while the porosity and average pore diameter declined, thus affecting the diffusion of the oxygen. Due to inadequate pre-oxidation, the fiber structure severely damaged in the carbonization process, leading to the ultimate mechanical properties of fibers decreased.So as to the diffusion of oxygen, When the polyacrylonitrile fiber porosity≥5% and average pore size≥1.25nm, the fiber could be oxidized smoothly.