Reactive Molecular Dynamics Simulations Of The Thermal Stabilization Of Carbon Fiber Precursors

Due to its excellent properties,carbon fiber has been widely concerned and applied,and its demand has been increasing.The production line of carbon fiber is long,and the heat stabilization process of its precursor is the dominant factor that limits the yield.However,the thermal stabilization process of precursor is very complex,including various physical and chemical changes.Most of the existing studies focus on the macro level,and the micro level is seriously inadequate.Therefore,in this paper,molecular dynamics simulations are used to explore the microscopic process of thermal stabilization of the precursor at the atomic scale and analyze the reaction mechanism.The main research contents and results are as follows:(1)The interaction and reaction process of amino radicals,hydroxyl radicals and oxygen radicals in plasma with polyacrylonitrile(PAN)chains under the conditions of thermal stabilization were studied by molecular dynamics simulations.The results show that amino radicals have little effect on the thermal stability of PAN molecular chain;hydroxyl radicals can react with PAN molecules,which can not only introduce oxygen or hydroxyl into the backbone,but also open the triple bonds in the nitriles into double bonds to activate the nitriles.This is helpful for the formation of heat-resistant trapezoid structure in the heating process of the precursor.The activity of oxygen radicals is too high,it can transform the cyano C-N triple bond into C-N double bond rapidly,but it can also oxidize the main chain quickly,which leads to the breaking of the chain into short segments,and is not conducive to the formation of a continuous trapezoid structure.(2)In order to explore the effect of side groups on the cyclization of carbon and nitrogen,the thermal stabilization process of PAN chain with hydroxyl and hydrogenated nitrile group was simulated,and the effect of dehydration on the cyclization of carbon and nitrogen was analyzed.Three kinds of C-N cyclization modes are observed in the simulation:one is to remove a water molecule between the hydroxyl and the hydroxyl on the adjacent nitrile group to form a C-N six-member ring;the other is to directly remove a water molecule between the hydroxyl and hydrogen on the adjacent nitrile groups to form a C-N seven-member ring;the third is to remove a water molecule between the hydroxyl on the nitrile group and the hydrogen on the backbone carbon to form a C-N six-member ring.These results show that the reasonable introduction of hydroxyl groups on the cyano group can increase the length of the cyano group,increase the collision probability between the cyano groups,promote the cyclization of carbon and nitrogen,and accelerate the thermal stabilization process.(3)In order to understand the microscopic mechanism of drafting in the process of heat stabilization of the PAN precursor,the molecular dynamics process of PAN chain was simulated,and the influence of drafting on the conformation of PAN precursor in the process of heat stabilization was analyzed.The results show that the cyano group of PAN chain is spirally oriented without drawing,and there is a large steric resistance between adjacent cyano groups.The application of drawing can not only prevent the reorientation and contraction of the PAN chain in the precursor,but also shorten the distance between adjacent cyano groups on the backbone to a certain extent,which may increase the reaction rate of carbon nitrogen cyclization.However,it is not that the greater the stretching force,the smaller the distance between adjacent cyanogens,but there is an optimal stretching force.When the stretching force exceeds this value,the distance between adjacent cyanogens will increase due to over stretching.(4)The concept of using polyvinyl chloride(PVC)as a new raw material to prepare carbon fiber was explored.The results show that PVC precursor will decompose under the action of heat,and it is difficult to form cross-linking or trapezoid structure which suggests that it is difficult to be thermal stabilized,so it is not suitable to be used as carbon fiber raw material.