Effect of structural evolution in high‐temperature thermal oxidative stabilized polyacrylonitrile fibers on carbonized fibers

Abstract The effect of structure and properties of polyacrylonitrile (PAN) fibers during thermal oxidative stabilization (TOS) stage (especially high‐temperature TOS stage at 270–300°C) on the skin‐core structure of carbonized fibers was studied. The work clarified the reasons for selecting the highest TOS temperature, and also determined the optimal TOS temperature range of obtaining structurally stable carbonized fibers. The changes of chemical groups were quantitatively calculated by a combination of peak‐fitting and second‐order derivative methods. The results showed that the cyclization was dominant in the early TOS stage, oxidation reaction mainly occurred in the post‐TOS stage, and crosslinking structure significantly increased after 250°C. In the high‐temperature TOS stage, cyclization reaction was basically completed, the growth rate of oxygen content slowed down, and dehydrogenation index continued to increase. The microstructure observation found that the grooves on the fiber surface were gradually smooth after low‐temperature and high‐temperature carbonization. The radial structure of A285‐1350 and A300‐1350 fibers have obvious heterogeneity, and diameter dropped below 5 μm. Therefore, carbonized fibers have obvious structural defects when the TOS temperature is above 285°C. The TOS temperature between 200 and 285°C is more conducive to obtain structurally stable carbonized fibers.