The evolution of radial structure of polyacrylonitrile monofilament under force field.

The generated sp2 hybrid conjugate structure of a C atom, which resulted from the chemical reaction affected by temperature and time, is an effective six-membered ring planar structure of the final carbon fiber. This kind of hybrid conjugate structure determined the formation of the final structure and mechanical properties of carbon fiber. In this paper, the formation and evolution of sp2 hybrid conjugated structures of PAN precursor during thermal stabilization were investigated by Raman, UV-vis and 13C-NMR methods. The results indicated that with the increase of stabilization temperature, the degree of the sp2 hybrid conjugated structure of stabilized PAN fiber increases “linearly”, while the content of the sp2 hybrid carbon atoms increases with “S-type”. The final sp2 hybrid conjugated ring structure is mainly composed of single-ring, double-ring, triple-ring, and double-bond structures. Compared with the time factor, the temperature effect plays a decisive role in the formation of the sp2 hybrid conjugate structure.

Because of the strong chemical inertness of high‐modulus carbon fiber (HMCF), the surface treatment of HMCF becomes less effective. In this work, multistage anodization method was applied to modify HMCF. NH4HCO3 and (NH4)2SO4 were used as electrolytes to study the influence of electrolyte type on the oxidation effect in the multistage oxidation process. The HMCF was treated by multistage anodization in the following three ways: NH4HCO3 → NH4HCO3, (NH4)2SO4 → (NH4)2SO4, and NH4HCO3 → (NH4)2SO4. In order to compare with the multistage anodization methods, HMCF was also anodized by NH4HCO3 and (NH4)2SO4 using single anodization method, respectively. The treated HMCFs were characterized in detail using X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), and dynamic contact angle analysis, respectively. The results of XPS exhibited that HMCF modified by multistage anodization showed more functional groups on its surface. More sp2 hybridized bonds were converted into sp3 hybridized bonds after modification by multistage anodization, especially when (NH4)2SO4 was used as electrolyte. The results of SEM/AFM indicated that HMCF modified by multistage showed a greater surface roughness. Interlaminar shear strength (ILSS) was used to observe the interfacial properties of HMCF between epoxy resin. The results showed that ILSS of untreated carbon fiber composite was only 27.3 MPa; it was increased to 71.8 and 78.6 MPa after treated with single anodization by NH4HCO3 and (NH4)2SO4, respectively. After modification by multistage anodization, the ILSS increased in different degrees compared with single anodization method. Especially, the ILSS reached 93.1 MPa when the method of using NH4HCO3 followed by (NH4)2SO4 was used to treat HMCF.