Effect of oxygen content on the preoxidation structure of polyacrylonitrile fibers

Abstract During the preoxidation process of polyacrylonitrile (PAN) fibers, the diffusion behavior of oxygen from skin to core can lead to the formation of radially uneven structure of fibers, which affects the mechanical properties of subsequent PAN carbon fibers. Here we examine the effect of oxygen content on the subsequent structure evolution of fibers with different preoxidation structure, by using 13 C Solid‐state NMR analysis ( 13 C‐NMR), optical density characterization, differential scanning calorimetry analysis, and thermalgravimetric analysis. The results show that increasing the oxygen content during the subsequent heat treatment can effectively reduce the radial structure difference of fiber, its efficiency is related to the preoxidation degree of fibers. For fibers with lower preoxidation degree, namely less heat treatment work ( W ), the radial structure difference disappears when the oxygen content reaches 29%. Further study shows that the improvement of radial structure of preoxidized fibers improved fiber thermal stability, its efficiency is also related to the W of fibers; when the W of fibers is 5, the carbonization yield at 800°C increased from 56.79% to 62.61%, which is 10.25% higher; when the W of fibers is 20, under the same oxygen content range, the carbonization yield increased from 68.97% to 69.60%, which is only 0.913% higher.