The Disorientation Behavior Of Polyacrylonitrile Based Carbon Fiber Precursor

Polyacrylonitrile (PAN) based carbon fiber exhibits a lot of mechanical properties due to its highly orientated arrangement of graphite crystallite structure along fiber axis. During the preparation process of carbon fiber, disorientation emerged in heat setting and peroxidation procedure that was not advantage for formation of highly orientated structure. Most of the previous literatures reported characterization and model of PAN precursor orientation structure, and the effects of technological conditions on the orientation parameters, but there was little information about the disorientation behavior of PAN fiber. Therefore, in order to effectively maintain orientation structure of fiber in oxidative stabilization process, as well as to providing a scientific basis for the regulation of precursor orientation structure, it is important to research on the mechanism of disorientation. Thermodynamic and viscoelasticity were used to explore the nature and behavior of disorientation in present study.In this paper, the deduction of the equation for describing the disorientation of PAN precursor was performed based on the establishment of viscoelasticity model. The function of strain and stress related to temperature and time was clarified. On the basis of above, the disorientation behavior of different mobilitive units in PAN fiber chains and the effects of temperature and time on disorientation were obtained by TMA, p-FTIR and WAXD experiments. The disorientation of crystalline and amorphous states were also discussed.The results indicated that:1、Thermodynamic equation of disorientation: dG=du-TdS=σdγ-TdSThe inner-stress and the change of entropy is the source of disorientation.2、The fiber disorientation includes elastic part disorientation and viscous part disorientation. The elastic part disorientation was completed in the process of sample collection, while the disorientation of viscous part will need a longer time to complete.3、Viscoelastic equation of disorientation: It means the stress and strain are the function of temperature、time and the characters of PAN fiber.4、Strain was increasing in thermal environment. Significant decline began at about80℃, the degree of disorientation in crystalline regions and chains increase with higher temperature, the rate of descent was maximum at about150℃.5、With different viscosity of chains, plastic drawing mainly led to molecular chain orientation, and the segmental orientation was obtained by exceeding drawing. With different drawing ratio, exceeding stretching would obtain the fibers with different degrees of disorientation.6、With the increasing of drawing ratio, the storage modulus and loss modulus were higher due to the increasing of orientation. Meanwhile, the inner stress was higher with the higher stretching, which could lead to more quickly decreasing of storage modulus and loss modulus.7、With temperature increased, the βc (110℃) and a (150℃) relaxations occurred, which was ascribed to the molecular motions in crystalline and amorphous phases respectively. The stretching has effect on a relaxation obviously, while make almost no difference to βc relaxation.