The modification before the stabilization process could tune the exothermic behavior and the structural evolution of PAN fibers during stabilization. In this study, we demonstrate that a rapid thermal pretreatment in nitrogen can effectively mitigate the exothermic behavior of PAN fibers, such as decreasing the initial temperature, broadening the exothermal peak, and decreasing the nominal heat release during heating the fibers in air. The color of fibers has shown gradual changes from white to light yellow, yellow and brown during thermal pretreatment in nitrogen with the increase of pretreating temperature and time. The differential scanning calorimetry (DSC), Fourier Transform Infrared Spectrometer (FTIR), X-ray diffraction (XRD), and Thermogravimetric Analysis (TG) characterization have been applied to analyze the thermal properties, chemical and physical structural difference between PAN, and thermally pretreated PAN fibers. The thermal pretreatment of PAN fibers in nitrogen could induce cyclization, dehydrogenation, and cross-linking reactions, in which the cyclization play an important role on improving the cyclization index of stabilized PAN fibers. Meanwhile, the pretreatment can result in noticeable changes of the aggregation structure of PAN fibers, as indicated by the increase of crystallinity and crystalline size. These structural modifications can benefit the main cyclization reaction during stabilization and enhance the carbon yield in resultant carbon fibers. The rapid thermal pretreatment in nitrogen could increase efficiency of modification on PAN fibers, and that could save much time and energy. It is beneficial to manufacture low-cost carbon fibers and to spread the applications of carbon fibers.

In this work, a continuous and rapid atmospheric plasma setup was developed for rapidly modifying the surface of PAN-based carbon fibers (CFs). The interlaminar shear strength (ILSS) of CFs increased from 64.9 to 80.0 MPa with 60 s plasma treatment. Further mechanical and surface structural characterizations revealed that the effect of plasma was different, depending on the treatment time. When the treatment time was lower than 15 s, the effect of plasma was mainly on physically etching the surface of CFs, and the ILSS of CFs increased rapidly. Further extending the plasma treatment time did not increase surface roughness but promoted the addition of oxygen-containing functional groups on the surface of CFs, corresponding to a slower growth rate of ILSS. The atmospheric plasma was generated via a dielectric barrier discharge (DBD) method, and its energy intensity was significantly lower than that of plasma generated under low pressure. Accordingly, a mechanism was proposed for the plasma treatment of CFs: atmospheric plasma was not strong enough to simultaneously etch all the carbon atoms on the surface of CFs; therefore, carbon atoms on the graphitic plane were selectively etched, followed by the attaching of oxygen-containing functional groups on the exposed carbon sites caused by etching.

Oxygen has significant influence on both the chemical and physical structural evolution of PAN fibers during rapid stabilization.