Investigation Of The Formation Mechanisms Of Polyacrylonitrile (PAN) Precursors For Carbon Fibers

Carbon fibers not only combine high tensile strength and high tensile modulus with low weigh,but also exhibit exceptional thermophysical properties and excellent damping characteristics.Due to their excellent performances,carbon fibers are widely used as reinforced fibers for various advanced composite materials in all kinds of fields.Polyacrylonitrile（PAN）fibers are the most widely used precursors for producing carbon fibers for a variety of advantages.Carbon fibers made from PAN precursors have several beneficial features,which make them become a mainstream in carbon fiber industries.Particularly widely used as high-performance materials, PAN-based carbon fibers become more attractive for industries,military and production of high quality sporting goods.There are large gaps in the properties of PAN-based carbon fibers between the homemade and foreign products.PAN precursors have been the key factor for producing high performance carbon fiber.There are mainly two kinds of methods used to spinning PAN precursors recently and they are wet spinning and dry-jet wet-spinning using DMSO as solvent.In both spinning processes,the fiber-forming is mostly occurred in the coagulation bath where the phase separation,coupled diffusion and thermal conduction take place,thereby leading to the precipitation of the polymer and the formation of the fiber.There has been a great deal of fundamental work on the formation mechanism of PAN fiber in both spinning processes.But their efforts are primarily focused on coupled diffusion or just discussed the phase behaviors for PAN-DMSO-H₂O systems. However,the formation mechanisms of PAN fiber for these two spinning methods are not clear up to date.In fact,it is likely that incorporation of phase separation phenomena into coupled diffusion would be more predictive of fiber properties and offer more areas of future investigation.This thesis divides into two major parts.In the first part,the phase separation thermodynamic model and diffusion model for PAN-DMSO-H₂O system are described.Based on these models,the diffusion coefficients of solvent and nonsolvent and their ratios are measured,and the ternary phase diagrams are determined respectively by calculation or experiments.In the second part,by discussing the ternary phase diagrams and the diffusion coefficients of solvent and nonsolvent and their ratios in wet spinning and dry-jet wet spinning processes,the formation mechanisms of PAN fiber are described.In chapter 2,based on the Flory-Huggines theory,the thermodynamics theory of bicomponent system was expanded and tricomponent differential equations were set up for PAN-DMSO-H₂O system.In chapter 3,the methods to detennine the interaction parameters of nonsolvent-solvent,nonsolvent-polymer and solvent-polymer were introduced and a ternary phase diagram was calculated.Then In chapter 4,the phase separation behaviors of PAN-DMSO-H₂O systems were investigated with cloud point method and an ideal linearized correlation of cloud point curve was found.According its LCP equation,the components of cloud points with higher polymer concentration were extrapolated to produce a whole experimental binodal curve which was corresponding to the calculate result.The phase separation trajectory was determined by measuring components of PAN as-spun fiber spun by wet spinning or dry-jet wet spinning methods.Therefore,a series of ternary phase diagrams,with experimental binodal curve and phase separation trajectory and calculated spinodal curve,were determined.It is the first time to determine a complete ternary phase diagrams of polymer-solvent-nonsolvent systems for wet spinning and dry-jet wet spinning.In addition,these researchers shed a new light on the experimental method of determining the ternary phase diagram which includes phase separation trajectory and the method can be applied in wet spinning fiber or film of other polymer-solvent-nonsolvent system. In the ternary phase diagram for PAN-DMSO-H₂O,with an increasing in temperature and the molecular weight of PAN,the binodal curve became close to the PAN-H₂O axis.Changes in binodal curve shifted the miscibility gap and therefore maximaed locations.The spinning conditions,like the concentration of coagulation bath,percentage of water in spinning dope,jet stretch and height of gas gap,had no effect on binodal curve,while affect obviously on phase separation trajectory.In chapter 5,based on the diffusion model deriving from Fick’s law,the diffusion coefficients of solvent and nonsolvent and their ratios were determined by dynamic component analysis method.And then the diffusion mechanisms in both spinning processes were studied.An increasing in the temperature and concentration of coagulation bath increased the diffusion coefficients of solvent and nonsolvent but decreased their ratios.To some extent,the diffusion coefficients could reflect the solidifying rate of PAN fiber as well as their ratios.But it was not possible to predicte exactly the solidifying rate of PAN fiber and fiber’s structure only by the diffusion coefficients or their ratios.In chapter 6 and chapter 7,incorporating of phase separation phenomena into coupled diffusion the formation mechanisms of PAN fiber in wet spinning and dry-jet wet spinning were discussed,respectively.There was an intrinsical difference between these two formation mechanisms.The fiber-forming of the latter occurred in two kinds of medium:air gap and DMSO solution and the fiber’s profile,structure and properties was results of the coupled diffusion of solvent and nonsolvent and the phase separation of the PAN polymer in two kinds of medium.The rate of coupled diffusion and phase separation mechanism in dry-jet wet spinning were different from those in dry-jet wet spinning that take place just in DMSO solution. In wet spinning,the coagulating process of PAN fiber divided approximately into two stages.During the first stage,the filament precipitated rapidly,simultaneously with binodal decomposition,which resulted in compact nodular structure;during the second stage,the coagulating rate decreased obviously and there mainly occurred spinodal decomposition.The structure was a gel network of interconnected polymer fibrils separated by voids.Annual rings of final PAN as-spun fiber spun with wet spinning method appeared like the layer of wood when viewed in cross section by SEM.Generally,duo to the influences of spinning parameters,there were four kinds of fiber layer structure:skin-skin structure,all-skin structure,all-core structure, typical skin-core structure and one layer of skin with several layers of core structure. Similar to the structure in wet spinning process,the radial structure of PAN as-spun fiber spun with dry-jet wet spinning method had layer structure,but it was not very clear.At the same time,we demonstrated the formation mechanisms of fiber cross-section shape both in wet spinning and dry-jet wet spinning.The fiber profile was mainly determined by diffusion rate of solvent and nonsolvent.Thus,the fiber cross-section shape could be predicted by analyzing the ratio of diffusion coefficient of solvent to that of nonsolvent:the more the ratios tended to 1,the more circular the fiber profile was.It was found that an appropriate increasing,in the concentration of coagulation bath,percentages of PAN and water in the spinning dope,jet stretch（approximately 0.9 for wet spinning and 2.0 for dry-jet wet spinning）and height of gas gap （approximately 10 mm）,would lead to a mild precipitation of PAN fiber.The fiber would have compact homogeneous structure of a gel network of interconnected polymer fibrils,thin titre,high crystallinity and orientation and high intensity.There were two opposite effects of temperature on fiber structure:（1）with appropriate percentages of water in the spinning dope,increasing the temperature of coagulation bath caused better structure and properties.（2）Without water in the spinning dope, the structure of PAN fiber would become worse as an increasing in temperature.Given the structure,there are fiber structure implications that dictate the physical properties of PAN fiber and determine fiber property differences brought about by different spinning parameters.The experimental results were corresponding to the predictive properties of PAN fiber.By controlling spinning parameters,such as high concentration and low temperature of coagulation bath and spinneret with small diameter,the PAN fiber with all-skin structure,thin titre and high intensity could be obtained.With appropriate percentages of water in the spinning dope,appropriately high concentration and temperature of coagulation bath,the PAN fiber with all-core structure and circular cross-section shape would be obtained.This type of fiber could be used for ideal PAN precursor.According to these results demonstrated above,in chapter 6 we reported a method which would be more predictive and controlling of fiber to produce the PAN fiber with requisite structure and properties applying in carbon fiber industries.Using this method we successfully obtained homogenous structure and higher properties.These results and methods studying the fiber formation mechanism represented in this thesis can be applied not only to develop further PAN precursor with circular profile and non-circular profile but also to investigate the formation mechanism of other chemistry fiber or film spun with wet spinning or dry-jet wet spinning methods.