| Carbon fiber reinforced polymer matrix composite material has been widely used in aerospace,military,automotive and other fields because of light weight,high specific strength,high specific stiffness,corrosion resistance,high temperature resistance and other excelent properties.With the rapid development of science and technology,people put forward higher requirements for the properties of composite materials.In the research of improving the properties of composite materials,multiscale reinforcement has become a hotspot in the field of carbon fiber reinforced composites in recent years.However,how to control the multi-scale reinforced structure of carbon fiber surface to improve the interface properties of composite materials has not been studied systematically.In this article,based on the multi-scale idea,designing and preparing polyethyleneimine?PEI?/carbon fiber reinforcement,carbon nanotubes?CNT?/PEI/carbon fiber multi-scale reinforcement and graphene oxide?GO?/PEI/carbon fiber multi-scale reinforcement.And for the first time,by changing the molecular weight of PEI,the mass ratio of CF-PEI to CNT-COOH and the mass ratio of CF-PEI to GO,researching the regulation rules of polymer molecular weight,the grafting density of CNTs and the grafting morphology of GO on the multiscale structure of carbon fiber surface and the interface properties of carbon fiber/epoxy resin composite,and the interface strengthening mechanisms of three kinds of reinforcements have been further analyzed and expounded.The main research contents of this paper are as follows:?1?Preparation of CF-PEI reinforcement by chemical grafting.Meanwhile,to investigate the effect of molecular weight of polymer on multiscale structure and interface properties of composites,PEI macromolecules with molecular weight of 600,1800 and 10000 were grafted onto the carbon fiber surface respectively.The results show that grafting PEI can significantly improve the wettability of carbon fiber in epoxy resin,realize the chemical bond connection between carbon fiber and epoxy resin,enhance the chemical bonding and mechanical meshing between the two phases,and finally improve the interface properties and mechanical properties of the composite.In addition,the molecular weight of grafting polymer can affect the surface chemical structure and micromorphology of carbon fiber,and thus regulate the interface properties of the composite.With the increasing of PEI molecular weight,the molecular chain grows,the steric hindrance effect intensifies,the grafting molecules number on the carbon fiber surface reduces,the polar groups content,the roughness and the surface energy on the carbon fiber surface reduce significantly,leading to the surface wettability of the carbon fiber,and the chemical bonding and the mechanical meshing effect weaken,the interface performance of composite material degrades.When the molecular weight of grafting PEI is 600,the interfacial strength and bending strength of CF-PEI reinforced composites are increased by 38.9%and 36.7%,respectively.Therefore,the interfacial properties of the composites can be regulated by adjusting the molecular weight of the grafting polymer.?2?Using PEI-600 with the best modification effect as the medium,grafting CNTs with excelent performance on the surface of CF-PEI by chemical grafting method to prepare CF-PEI-CNT multi-scale reinforcement.And for the first time by changing the mass ratio of CF-PEI to CNT-COOH?1:1,2:1,4:1,8:1?,to adjust the grafting density of CNTs on the carbon fiber surface.The results showed that the oxygen-containing groups on the CNTs surface increased the surface polarity of carbon fiber,increased the wettability of carbon fiber in the resin matrix,and the carboxyl group?-COOH?and hydroxyl group?-OH?in the carbon nanotube structure reacted with the amino group?-NH2?in the PEI structure to form chemical bonds.CNTs with the special nano-structure can be"embedded"into the matrix resin like a"rivet"in the composite process,forming a strong riveting cooperation and restricting the free movement of epoxy resin molecules near the interface.The grafting CNTs form a transition layer of medium modulus in two phases,which is conducive to the propagation of applied loads and the absorption of failure energy.The CNTs grafting density on the carbon fiber surface can control the interface of the composite performance.When the mass ratio of CF-PEI to CNTs-COOH is 2:1,the high-density CNTs grafting on the carbon fiber surface with different angles maximize the chemical bonding and mechanical riveting between carbon fiber and matrix resin,and improve the interface property and bending property of composites.The IFSS,ILSS and bending strength of composites reinforced by CF-PEI-CNT2 increased by 74.1%,80.1%and 55.2%than unmodified carbon fiber,and increased by 27.7%,30.2%and 18.8%than CF-PEI,respectively.?3?Using PEI-600 as the coupling agent,grafting GO with special structure and properties on the carbon fiber surface by chemical grafting method to prepare CF-PEI-GO multi-scale reinforcement.And for the first time by changing the mass ratio of CF-PEI to GO?10:1,20:1,40:1,60:1?to adjust the GO grafting morphology on the carbon fiber surface.The effects of GO graft morphology on multiscale structure of carbon fiber surface and interfacial properties of composites were studied.The results showed that a large number of polar groups?-COOH and-OH?on the GO surface could significantly enhance the wettability of epoxy resin on the carbon fiber surface.When the GO with lamellar structure is"upright"on the carbon fiber surface,it can"anchor"the matrix like"anchor",which can greatly enhance the mechanical"riveting"effect of carbon fiber and resin matrix.GO with lamellar structure forms an interface layer with medium modulus at the two-phase interface,which can transfer and disperse the applied load evenly and effectively,and consume more energy when the interface is broken.The graft morphology of GO on the surface of carbon fiber can adjust the multi-scale structure on the carbon fiber surface and the interface performance of its composites.When the mass ratio between CF-PEI to GO was 40:1,a large number of GO with moderate size"stand"on the carbon fiber surface,which greatly increases the surface roughness of carbon fiber and enhances its“riveting”effect with the matrix resin.At the same time,this grafting way can increase the polarity area of carbon fiber in the matrix resin,so that more polar groups on the GO surface can participate in chemical reactions and form stronger chemical bonding between the two phases.The interface properties and bending properties of the composites were improved to the greatest extent.The IFSS,ILSS and bending strength of CF-PEI-GO3 reinforced composites were improved by 64.3%,76.4%and 50.9%,respectively,compared with the unmodified carbon fibers,and 19.2%,27.4%and 15.8%respectively,compared with CF-PEI. |
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