Study On Electrochemical Reduction Grafting Modified Carbon Fibers And Their Interfacial Properties With PEEK Resin

In recent years,carbon fiber reinforced thermoplastic polymer composites have made significant progress.However,the interface interaction between polyetheretherketone（PEEK）and carbon fiber（CF）is weak in practical applications,which makes it difficult to enhance the interfacial strength of the composite material by regulating the processing and forming process.In order to enhance the interface interaction between PEEK matrix and carbon fiber,researchers have conducted a lot of work from five angles of wetting,mechanical interlocking,interface crystallization,secondary/chemical bond binding,and diffusion.Among the five interactions,chemical bond interaction theoretically can provide the best modification effect.However,the establishment of chemical bonds requires complex chemical treatment of carbon fiber,harsh reaction conditions,and the chemical bonds（ester,amide,etc.）generated are unstable at high temperatures.Therefore,this paper proposes a covalent bond interface modification strategy that does not require complicated carbon fiber modification,has a fast and mild reaction,and is bond-stable.It uses diazonium salts as reactive groups and grafts PEEK onto the surface of carbon fibers through electrochemical reduction.The feasibility of grafting PEEK to modify carbon fiber by electrochemical grafting was explored,and the grafting behavior and rules of polymers with different grafting positions on the surface of carbon fiber were systematically studied,as well as the effect of the molecular weight and concentration of grafted polymers on the interface strength of CF/PEEK composite materials.Details and results are described below.（1）PEEK with amino side groups was synthesized via nitration and amination reactions.PEEK diazonium salts were obtained by reacting with tetrafluoroborate nitroso salts（NOBF₄）.For the first time,polymer graft modification of carbon fiber was achieved using PEEK diazonium salts.The grafting behavior of PEEK side chain diazonium salts with different melt indexes and concentrations was studied,and the optimal conditions were optimized.The results showed that low molecular weight PEEK diazonium salts had a higher ion conductivity and effective grafting capacity.The interfacial shear strength（IFSS）between carbon fibers and the polyetheretherketone matrix reached 75.1 MPa after grafting,an improvement of 78.9%over unmodified carbon fibers.This preliminary study demonstrates the feasibility of electrochemically grafting carbon fibers using PEEK diazonium salts and its effect in improving the interfacial strength between CF/PEEK.（2）Due to the steric hindrance effect of the diazonium salt side chain in PEEK,the interference of multiple grafting sites,and the large molecular weight,the enhancement of interfacial strength between carbon fiber and PEEK resin is limited.To address these issues,a low molecular weight PEEK end-group diazonium salt that is soluble in conventional organic solvents was designed to optimize the grafting effect and obtain a stronger interface.The solubility structure of this end-group diazonium salt can be recovered by hydrolysis to restore crystallization ability.The study on grafting behavior found that under the same conditions,the effective grafting amount of the end-group diazonium salt was increased by 70.1%compared to the side-chain diazonium salt.However,the precipitation of polymer diazonium salts can wrap around carbon fibers and inhibit subsequent electrochemical grafting at high concentrations.The force-displacement curve of the polymer chain pulled out of the carbon fiber surface by a probe was recorded by single-molecule force spectroscopy technology.The breaking force of the molecular chain（1890 p N）demonstrated the formation of a strong covalent bond between the polymer and the carbon fiber.After PEEK end-group diazonium salt grafting treatment,the interfacial shear strength（IFSS）between carbon fiber and PEEK matrix was further increased to 97.3 MPa,which was 130.3%higher than that of untreated CF/PEEK composite materials.This indicates that the changes in the grafting sites and molecular weight significantly improve the interfacial interaction between the PEEK matrix and the carbon fiber.（3）To further enhance the grafting efficiency and avoid the precipitation of polymer diazonium salt hindering subsequent electrochemical grafting,a research plan was designed to graft phenylethynyl groups onto the surface of carbon fibers and achieve in-situ crosslinking.Using small molecule phenylethynyl diazonium salt with high grafting efficiency to modify carbon fibers,phenylethynyl end-capped soluble polyaryletherketone（PEEK）was designed and synthesized for coating and modifying carbon fibers.After acidification and 410℃heat treatment,in-situ crosslinking reaction occurred between the phenylethynyls in the coating agent and the grafted carbon fibers,constructing a covalent bond-strengthened interfacial phase.After heat treatment,the crystalline structure in the interfacial phase was preserved to some extent,enhancing the interfacial effect.The interfacial shear strength（IFSS）between the modified carbon fibers and the PEEK resin matrix increased from 43.4 MPa in the untreated composite to 103.1 MPa in the modified composite,while the interlaminar shear strength（ILSS）increased from 58.4 MPa to 97.4 MPa.These results indicate that the in-situ crosslinking reaction between the phenylethynyl-modified carbon fibers and the crosslinking groups in the coating agent effectively enhances the interfacial strength between the carbon fibers and the PEEK matrix,and the combined effect of crystallization and crosslinking in the interfacial phase can further strengthen the interfacial effect.The electroreduction process,coating process,and heat treatment process all can be carried out under existing production conditions of carbon fibers and composites,and the processing cycle is short,which provides the possibility of continuous production of this composite material.