Preparation,Characterization And Properties Of Polyacrylonitrile/Carbon Nanotubes Composite Fibers Based On DMSO/C₂H₅OH Coagulation System

Due to the simple processing,the low manufacturing cost and the excellent mechanical properties,polyacrylonirtile?PAN?fiber is the most important precursor for carbon fiber.The high quality of PAN fiber is regarded as the foundation of carbon fiber with high performance.Therefore,it is of great importance to study the relationship between the process conditions and microstructures and properties of PAN precursor.During the spinning process,due to the dual diffusions of the solvent and non-solvent driven by the concentration difference in the coagulation bath,the PAN spinning dope is gradually solidified via phase separation induced by a composition change.Thus,the coagulation is the key process to control the microstructure and mechanical properties of the resultant fibers.Due to the coexistence of kinetically-controlled dual diffusion and thermodynamically-controlled phase separation,the coagulation mechanism of spinning dope is very complicated.The construction of a ternary phase diagram will be helpful to the understanding of the thermodynamic phase behavior of the spinning dope during the coagulation process,and thus the clarification of the coagulation mechanism.The microstructure and properties of the fiber are closely related to the phase separation mechanism of polymer/solvent/non-solvent ternary system.To control the microstructure of PAN fiber,it is necessary to study the polymer/solvent/non-solvent phase separation behavior.Temperature and concentration of the spinning dope,temperature and concentration of the coagulation,as well as the coagulation composition are the main factors affecting the fiber's microstructure and properties.Most research works were also focused on these fields.The high PAN concentration of the spinning dope,the high solvent?DMSO?concentration and low temperature of the coagulation medium help us to prepare high-quality PAN precursor for carbon fiber with circular cross-section and few defects.The effects of the non-solvent species on the phase behavior of PAN spinning dope in the coagulation medium and also the microstructure of PAN nascent fibers based on the theoretical phase diagrams are not yet discussed in literature.The researches on the thermal-induced sol-gel transition of PAN/DMSO solution can provide theoretical support for the realization of PAN gel spinning.The thermal-induced sol-gel transition process of PAN/DMSO solution is a single phase transition,which is mainly affected by the heat transfer and can be simply regulated by the temperature.Therefore,high-quality fibers with uniform microstructure can be obtained by realization of the thermal-induced sol-gel transition in the spinning process?this is gel spinning?.The ultra-high molecular weight?UHMW?polymer was used in gel spinning technology in the past,and the concentration of spinning solution is very low,so.the gel spinning of UHMWPAN is not suitable for industrial production.The method of the gel spinning of PAN with conventional molecular weight is an ideal method due to simply dissolving and high PAN solid content.To achieve this,the study focus on the thermal-induced sol-gel transition process of PAN/DMSO solution is needed,which can help us to choose suitable spinning process and manufacture high performance PAN precursor.Carbon nanotubes?CNTs?have excellent mechanical,optical,and electrical performance.Meanwhile,it is similar to polymer chain in the size:diameter in nano-scale and length in micron-scale,thus,it is an ideal filling material for polymer.Therefore,the composite materials prepared by CNTs and polymer raise more and more researchers' attention,and become the research focus of the current material science.CNTs have been used widely in many polymer composites,such as PMMA,PS,PEO,PVA,and so on.The addition of CNTs in PAN fiber does not introduce non-carbon impurities,so the research works of PAN fiber with CNTs enhanced attracted the interests of a large number of researchers home and abroad.Although the CNTs with exciting potential applications in the preparation composite materials with high-strength and light-weight,but there still exist some problems,especially,the high cost of CNTs is the most prominent issue.The price of multi-walled carbon nanotubes?MWCNTs?is a few thousand RMB per kilogram,and the price of single-walled carbon nanotubes?SWCNTs?is as high as tens of thousands of RMB per kilogram.In the practical research and development,improving the enhance efficiency of the carbon nanotubes,thus reducing the amount of carbon nanotubes,is an urgent problem.The main content of this paper is the preparation of PAN carbon fiber precursor enhanced with MWCNTs using new DMSO/ethanol coagulation system.At first,the effects of the non-solvent species?water and ethanol?on the microstructure and properties of PAN fibers as well as the phase separation mechanism during solidification process of the PAN fiber forming were studied.Then,the thermal-induced sol-gel transition process of PAN/DMSO system was studied,which will provide theoretical support for the realization of gel spinning.Hereafter,in order to explore the PAN fiber spinning conditions and manufacture PAN fiber smoothly,the rheological properties of PAN/DMSO spinning solution and PAN/DMSO/MWCNTs composite solution have been investigated.Finally,the PAN/MWCNTs fibers were prepared using new process and their structure and properties were characterized.Carbon fibers with excellent mechanical properties were obtained by pre-oxidation and carbonization treatment of the PAN/MWCNTs fibers.The main conclusions are as follows:1.The ternary phase diagrams of PAN/DMSO/H₂O and PAN/DMSO/C₂H₅OH systems were constructed by turbidity titration and theoretical calculation based on extended Flory-Huggins theory.Homogeneous zone and metastable phase region of PAN/DMSO/C₂H₅OH system were larger than the corresponding regions in PAN/DMSO/H₂O system was founded in ternary phase diagram.The phase separation trajectory of PAN/DMSO/H₂O system and PAN/DMSO/C₂H₅OH system were experimentally determined with dynamic component analysis method and the phase separation mechanism in the spinning process of PAN fiber was demonstrated.For PAN/DMSO/H₂O system,the ternary composition first passes through the metastable zone quickly,and then stayes in the unstable zone for a long time.While the PAN fiber solidification process for PAN/DMSO/C₂H₅OH was in metastable zone completely.PAN fibers prepared use DMSO/H₂O as the coagulant have elliptical cross section and obvious skin-core structure:dense skin,porous core.The fibers solidified in DMSO/C₂H₅OH bath have round cross section,compact structure and no obvious skin-core structure.2.The critical gelation temperature and time of the thermal-induced sol-gel transition of PAN/DMSO solution were determined by dynamic rheology tests,the results showed that increasing the PAN concentration,increasing non-solvent water content,reducing ageing temperature,can all increase the gelation temperature and reduce the gelation time.The gelation temperature of the thermal-induced sol-gel transition of PAN/DMSO solution obtained using falling ball viscosity method was very close to that of dynamic rheological method.There is no change in the refractive index during the thermal-induved sol-gel transition process of PAN/DMSO solution.Two weak endothermic peaks appeared at 40?50? and 60?70? in differential scanning calorimetry curves of PAN gels prepared with different cooling programs,obtained via raising temperature mode,indicateing that the crosslink points of PAN gels are composed of two molecular aggregates with different degrees of order.3.The carboxyl group and hydroxyl group were imported by concentrated sulfuric and concentrated nitric mixed acid treatment of the multi-walled nanotubes?MWCNTs?surface.Thus,modified MWCNTs can be dispersed into the PAN/DMSO solution better.PAN/DMSO/MWCNTs composite spinning solutions were prepared using solution blend method conveniently.The rheological properties of PAN/DMSO solutions with different PAN concentration under various temperatures as well as those of PAN/DMSO/MWCNTs solutions have been studied.The addition of MWCNTs to the PAN/DMSO spinning solution has an impact on the rheological behavior.Specifically,the apparent viscosity,complex viscosity and complex modulus were all increased.Giesekus model was used to fit the apparent viscosity of PAN/DMSO/MWCNTs solutions,and the results showed that the characteristic relaxation times of PAN molecular chain increased after introducing MWCNTs into PAN/DMSO solutions,and the increase was even more obvious at low temperatures.In addition,the angular frequency dependence of the storage modulus of PAN/DMSO/MWCNTs solutions decreased in dynamic rheology tests,suggesting MWCNTs may play as a "gelation accelerator".4.PAN/MWCNTs composite fibers with superior mechanical properties were prepared through the dry-jet wet spinning process using DMSO/C₂H₅OH as the coagulant,supplemented by new technology of stretch in air bath.The surface morphology,crystallization,orientation index and the mechanical properties of drawn fiber were analyzed.The results show that the addition of small amount of MWCNTs will significantly promote the mechanical properties of drawn PAN fiber.MWCNTs may be bonded to PAN chains somewhat due to the hydrogen bonds between MWCNTs and PAN chains.The orientation index of PAN chains increased with the help of MWCNTs in the drawing process,which makes PAN chains tend to pack compactly and form a more perfect crystal structure.The orientation of MWCNTs in PAN/WMCNTs composite fibers can be determined by laser Raman spectroscopy with the appurtenance of polarized film.The total draw ratio was increased significantly using the new technology of stretch in air bath,which enhances the orientation of MWCNTs in composite fibers and thus the efficiency of MWCNTs.5.The PAN/MWCNTs composite fibers during the pre-oxidation process were characterized by FTIR,XRD,EA and other equipments.The results suggested that the relative cyclization index and aromatization index were increased,and there was little change in the oxygen contents between PAN/MWCNTs and PAN fibers.PAN/MWCNTs-based carbon fibers were prepared by successive low temperature carbonization and high temperature carbonization treatment of the obtained pre-oxidized fibers.The morphology,mechanical properties and crystallization of the carbon fibers were also characterized.The results suggested that the introduction of MWCNTs to the PAN precursors is helpful in forming graphite structure and enhancing the mechanical properties of the obtained carbon fibers.