Effect of nitrogen pretreatment on the skin‐core structure of thermal oxidative stabilization polyacrylonitrile fibers

Abstract

Polyacrylonitrile precursor was pretreated under nitrogen to study the effect of the extent of cyclization of pretreated fibers on the chemical structure, crystal structure and skin‐core structure of thermal oxidative stabilization (TOS) fibers. Based on the FTIR spectra, the cyclization degree of pretreated fibers was quantitatively calculated by a combination of peak‐fitting and second‐derivative operations. Combining the results of wide‐angle X‐ray diffraction, Raman spectroscopy and scanning electron microscope analysis, pretreatment was beneficial to increase the cyclized structure of TOS fibers, but inhibited the oxidation reactions in the TOS process. As the increase of cyclization degree of pretreated fibers, the oxygen content and crystal size of TOS fibers decreased and the crystal interplanar spacing increased; in the skin layer of TOS fibers, the ID/IGratio increased and brittle fracture area decreased. When the cyclization degree of pretreated fibers was in the range of 10%–35%, the skin‐core structure of TOS fibers was weakened, which demonstrated that proper pretreatment before TOS process can improve the skin‐core structure.

Accelerating the stabilization of polyacrylonitrile fibers by nitrogen pretreatment

“…The oxidation of the cyclization structure on the fiber surface reduces the further diffusion of oxygen into the fiber core, which may be the reason for this heterogeneous evolution 14 . At the same time, this oxidation reaction is controlled by diffusion, 31 the residence time of NOPF in the air is shorter than that of OPF, resulting in insufficient oxidative cross‐linking, loose structure and poor mechanical properties of the core 32 . Therefore, the force transmitted by the skin layer and the core is uneven when the fibers is fractured, and the core is easy to break and pull out.…”

“…14 At the same time, this oxidation reaction is controlled by diffusion, 31 the residence time of NOPF in the air is shorter than that of OPF, resulting in insufficient oxidative cross-linking, loose structure and poor mechanical properties of the core. 32 Therefore, the force transmitted by the skin layer and the core is uneven when the fibers is fractured, and the core is easy to break and pull out. In severe cases, the cross-section will be completely destroyed (Figure 4d1).…”

Effects of ambient pressure on structural characteristics of polyacrylonitrile pre‐oxidized fibers

“…In the known technical means, the pre‐oxidized skin‐core structure can be reduced and homogenized by reducing the diameter size of the fiber or adjusting the oxidation temperature/time spectrum. Ge et al 21 reported that after 90 min of treatment at 260°C, skin‐core radial structure of the fibers appeared, whereas after 120 min of treatment at 240°C, the radial distribution became uniform, indicating that lower temperature and longer time can improve the homogeneity of the radial distribution of pre‐oxidized fibers. Ge et al 22 pretreated PAN precursor fibers under nitrogen, which had homogenous radial distribution of pre‐oxidized fibers.…”

“…Ge et al 21 reported that after 90 min of treatment at 260 C, skin-core radial structure of the fibers appeared, whereas after 120 min of treatment at 240 C, the radial distribution became uniform, indicating that lower temperature and longer time can improve the homogeneity of the radial distribution of pre-oxidized fibers. Ge et al 22 pretreated PAN precursor fibers under nitrogen, which had homogenous radial distribution of pre-oxidized fibers. Chen et al 23 reported that with the increase of stretching ratio, the skin area became larger, improving the uniformity of radial structure.…”

Effect of heat treatment on the radial structure of coaxial polyacrylonitrile/mesophase pitch composite fiber

“…16,17 During pre-oxidation of MP fibers, dehydrogenation and oxidative coupling occur; resulting in formation of oxygen-containing functional groups with higher melting points (such as carboxyl groups, hydroxyl groups, and acid anhydrides) and the release of small molecule gases (such as water, carbon dioxide, and carbon monoxide), forming a more stable fused-ring aromatization structure than the original fiber. 18,19 The pre-oxidation of PAN and MP is carried out in air, in which oxygen gradually diffuses from the outside to inside of the fiber, resulting in oxidation. During this process, the degree of pre-oxidation in the radial direction of the fiber might vary.…”