Rheology Of Polyacrylonitrile Solution And Relation Between Microstructure And Mechanical Properties Of PAN-based Carbon Fiber

Carbon fibers have excellent mechanical properties and have been widely used as structural or functional material in aerospace,transportation,medical equipment,energy exploration,and entertainment fields.Carbon fibers are mainly obtained through the pyrolysis of organic precursors.According to the types of precursors,there are polyacrylonitrile,pitch,and viscose-based fibers.Among them,polyacrylonitrile（PAN）-based carbon fibers have become the most popular in the carbon fiber industry due to their excellent overall performance.PAN-based carbon fibers are obtained through a series of technological processes such as polymerization of acrylonitrile,spinning,pre-oxidation,and carbonization（graphitization）,and the carbon content is above 90%.The mechanical properties of carbon fibers are directly related to its microstructure.Research and regulation of its microstructure is a hotspot in this field.The aim of this study is to prepare PAN-based carbon fibers with high-performance.Firstly,the processing performance of polyacrylonitrile is studied.By mixing two polyacrylonitriles with different molecular weights,the spinnability of solution was investigated.Second,the microvoids structure of polyacrylonitrile material is explored and evaluated during the molding process.Finally,the relation between the structure and mechanical properties of home-made carbon fibers are analyzed.The main research content is as follows:（1）The study on the rheological behavior of mixing polyacrylonitrile:The rheological behavior of polyacrylonitrile solutions from mixing polyacrylonitriles with the high molecular weight and low molecular weight are studied.The results show that the increase of the amount of polyacrylonitrile with high molecular weight leads to an increase in the density of molecular chain entanglement.Meanwhile,the mixing with a certain proportion of polyacrylonitrile with high molecular weight can increase the inhomogeneous structure in the system.The rheological behavior of polyacrylonitrile with high molecular weight and liquid bridge thinning behavior under different system strains is studied by CaBER.The existance of high molecular weight PAN can extend the life-time of liquid bridge.Under the different system strains,the rheological behavior of polymer solution behaves like Newtonian fluids.（2）Characterization of microvoid by T2 relaxation spectra and discussion:To obtain a compact polyacrylonitrile membrane,the size and pore structure of the polyacrylonitrile material are regulated and controlled by using four different organic alcohols as the coagulation bath.The results show that the resulting membrane structure is affected by the coagulation bath.Due to the different components of the coagulation bath and the spinning dope,a delayed phase separation is caused.And finally,a compact polyacrylonitrile film is obtained.Meanwhile,results manifest that slowing the double diffusion process is beneficial to the formation of the compact structure.Compared to the statistical information of SEM,the results from T2 relaxation spectra are massive,and which are more accurate than that from SEM method.（3）Relation between microstructure and mechanical properties of carbon fiber:The microstructure of carbon fiber with different tensile modulus is studied by the means of Raman spectroscopy,XRD,scanning electron microscopy,small-angle X-ray diffraction,and XPS photoelectron spectroscopy.Comparison and analysis are carried out.The results show that the size of the graphite crystallites increases,the interlayer spacing d_（002）decreases,and the tensile modulus of the carbon fibers increases.As crystallites size inside the fibers increases,the preferred orientation of the graphite crystallites decreases along the fiber axis.Due to the rearrangement and densification of graphitic crystal,the degree of graphitization increases,and the microcrystalline structure in the carbon fibers shifts from a wave-form composed of microcrystals and amorphous to a belt-form with well-defined three-dimensional.The microvoid structure of the fiber also changes as the modulus increases.The presence and growth of microvoids lead to stress concentration around the microvoids,and which is very harmful to the mechanical properties of the carbon fibers.Besides,the graphite crystallites along the fiber axis have a larger size and a high degree of orientation,and which are quite necessary for the improvement of carbon fibers.Thus,it is very important to control the growth of graphite crystallites and create microvoids effectively and reasonably for the manufacture of carbon fibers with high performance.According to the experiments above,only polyacrylonitrile with high molecular weight and less microvoids can fabricate PAN-based fibers with well defined structure.Molecular chains which have highly preferred orientation and regular arrangement are easily form into well-defined graphite crystallite,and it is very useful for the fabrication of polyacrylonitrile-based carbon fibers with high performance.