Structural characteristics of polyacrylonitrile (PAN) fibers during oxidative stabilization

Abstract The structure change of PAN fibers during oxidation in the range from 20 to 300°C under relaxed or tension conditions for various durations has been investigated by using small-angle x-ray scattering (SAXS), wide-angle x-ray scattering (WAXS) in situ, thermal Fourier transformation infra-red spectroscopy (TFTIR), differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA). The results indicate that below 180°C the structural change is rather small. From 180 to 240°C the ordered phase content decreases gradually as a result of thermal degradation and oxidation but the original backbone structure remains. Also, the means of WAXS in sit, we obtain the linear thermal expansion coefficient of the (100) interplanar distance, which is 8·3 × 10 −5 deg −1 , respectively for temperature below or above 95°C. After heating above 240°C, the cyclization occurs violently and a new planar trapezoid structure is formed. From the rate of decrease of the ordered phase content at different temperatures, the value of the cyclization activation energy is 29·4 kcal mol−1. Based on these results, a modified model of two-phase structure is proposed and elucidated in detail for the PAN fiber.