The Effect Of The Orientation Structure Of Polyacrylonitrile Precursor On The Thermal Stabilization Process

The axial mechanical properties of polyacrylonitrile (PAN) basedcarbon fiber was originated from its orientation structure. During thestabilization process, which was one of the key process for the manufactureof high performance carbon fiber from the PAN precursor, complicatedphysical and chemical transition of the oriented PAN molecular chains wereoccurred and it was very important for the formation of pseudo-graphitecrystalline structure and properties of the resulted carbon fiber. In presentstudy,the effect of orientation structure of PANprecursor on the stabilizationprocess, including the structure transformation and the extent of cyclizationwas investigated. The characteristic correlation between the orientationstructure and the mechanism of stabilization process was revealed.Stretching was performed during the stabilization process and the effect ofwhich on the structure as well as properties of final carbon fiber was alsostudied. The author believes this study can contribute to the development of the research field of high performance carbon fiber.During the thermal stabilization process, from the viewpoint ofmicroscopic, the evolution of the orientation structure was related to thecyclization of the nitrile side group. The orientation degree of crystalline partwas increased because of the rearrangement of molecular chains under thestress, which was caused by the cyclization started from the amorphous part.At the same time, the molecular chains in the pseudo-crystalline region couldbe regularly arranged and lead to the increase of crystallinity and crystallitesize. As for the environment factors, the evolution of aggregation structurewas influenced by the time and temperature as well since the cyclizationprocess was different.When the heat treatment temperature was180oC, thecyclization degree was low so that the induced crystallization and orientationof crystalline part was not obviously.After heated at200oC for30minutes,with the increasing of the cyclization degree, the above phenomenon couldbe observed gradually.In the case of being heated at220oC,at the beginning,the crystallinity, crystallite size and orientation degree of crystalline partwere increased rapidly because of the induced crystallization andrearrangement of molecular chains. However, with the extension of heatingtime, the crystalline molecular chains started to participate the cyclizationand lead to the destruction of the whole crystalline structure, the crystallinity,crystallite size and orientation degree were decreased. When the heatingtemperature reached to240oC, the cyclization occurred in both amorphous and crystalline region rapidly, the crystalline structure factors abovedecreased obviously.When the stabilization process was performed in air, since thecyclization could be initialized by oxygen, the induction period of thecyclization was reduced so that the transition of aggregation structure wouldoccur at relative lower temperature.The effect of orientation structure of PAN precursor on the exothermalof stabilization reaction was investigated on the basis of differential scancalorimeter measurements. Initially, the cyclization was constrained and theexothermic heat flow was relatively lower if the orientation of PANfiber ishigher. With the increasing of temperature, the confinement of theorientation structure was decreased since the oriented chain participate thecyclization gradually. In the middle and last stage of stabilization, theendothermic peak was sharper for the PAN fiber with higher orientationdegree.The formation of cyclizated structure was also related to the orientationstructure. The measurements of13C-NMR indicate that relatively more andlarger aromatic layer structure, which was benefit to the mechanicalproperties of carbon fiber such as elongation, tensile strength, tensilemodules, was easy to obtain from the PAN precursor with higher orientation.As mentioned above, the cylization reaction was related to theorientation structure of PANfiber, and it was also affected by the stretching during stabilization. Comparing to the stretching during precursor processingstage, the stretching during stabilization below200oC is more useful toincrease the orientation degree of crystalline part. With the increase ofheating temperature, the orientation degree of crystalline part was increasedwith stretching ratio, while the tendency was slow down. The cyclizationdegree and aromatic layer structure was relatively larger for the PANfiberwith higher orientation degree. On the other hand, stretching after heattreatment at200oC for8min has almost no effect on the orientation structureof crystalline part.The pseudo-graphite structure and mechanical properties of so obtainedcarbon fiber were measured. It is easy to obtain larger and more regulararomatic layer structure by performing higher stretching during thermalstabilization while the pseudo-graphite structure is more homogeneous andgraphite degree is higher, which is very useful to improve the mechanicalproperties of final carbon fiber.