Multilevel Enhancement In Interfacial Properties Of High-modulus Carbon Fiber Reinforced Resin Matrix Composites

The interface science of carbon fiber reinforced resin matrix composites has always been a frontier research topic.As a link between fiber and matrix,the interface of composites affects the process of stress transfer,crack propagation and failure mode in the composites,so interfacial properties directly affect and even control the macroscopic properties of composites.However,three obstacles to obtain excellent interfacial properties of high-modulus carbon fiber(HMCF)composites were as follows:the inert surface of HMCF,low modulus of matching resin matrix and weak bonding of interphase.In this paper,the method of nanometer infiltration behavior on HMCF surface after treatments was established.The interphase formation mechanism and interface enhancement mechanism were proposed based on the design and preparation of "secondary stiffening" for high modulus epoxy matrix.A novel interphase of composite was designed via molecular assembly of naphthenylimide(NDI)on HMCF surface.The multilevel enhancement in interfacial properties of HMCF composites was realized through increasing infiltration of HMCF surface,stiffening resin matrix and optimal design of interphase in composites.1.Base on the surface characteristics of pristine HMCF(p-HMCF),anodized HMCF(a-HMCF)and sized HMCF(s-HMCF),a method of nanometer infiltration combining molecular simulation and experimental verification was established,the effects of the surface physical and chemical properties of HMCF on the infiltration behavior of resin,interface structure and interfacial properties of composite were systematically investigated.Compared with p-HMCF,a-HMCF and s-HMCF had higher surface chemical activity and surface energy,lower ratio of depth to width of surface groove,and improved infiltration of the resin performance.Continuous interphase and better interfacial bonding were achieved for a-HMCF and s-HMCF composites,but voids with size of 80 nm were found at the interface of p-HMCF composite,so its interfacial shear strength(IFSS),transverse fiber bundle tensile(TFBT)strength and interlaminar shear strength(ILSS)were lower than those of the first two composites.Molecular dynamic simulation of nanometer infiltration behavior on HMCF surface showed that increasing the degree of surface functionalization and reducing the ratio of groove depth to width could eliminate interfacial bubbles,optimize interfacial structure and further improve interfacial bonding between fiber and resin.2.The tailored-modulus epoxy matrix(TD,TCD and MTCD)was prepared by the molecular design of "secondary stiffening".The effects of matrix modulus on interfacial,mechanical and space performance of carbon fiber composites were studied.The crosslinking density and rigidity of crosslinking network were effectively improved through in-situ molecular stiffening of amide acid(AA),and the mechanism model of resin stiffening was established.The thermal endurance and mechanical properties of high-modulus matrix(MTCD)were higher than those of TD and TCD,and the tensile modulus of MTCD reached 4,375 MPa.High-modulus matrix improved the interfacial bonding and failure mode of the composite interface,which made TFBT strength and ILSS of the composites increased by 92%and 40%,respectively.AS a result,the tensile,flexible and compression properties of the composites were improved and the space performance met ASTM standard.3.As for HMCF/TD,HMCF/TCD and HMCF/MTCD composites,interfacial elements,modulus distribution and chemical structure in composites were studied using in-situ analysis methods.The influencing factors of interfacial diffusion degree of the resin system were analyzed,and the mechanism of interphase formation and interface enhancement of the composites was discussed.Compared with HMCF/TD,the thicknesses of interphase of HMCF/TCD and HMCF/MTCD composites were extended successively,modulus transition effect was more obvious,and the diffusion degree of interfacial chemical structure was improved.Compared with TD,TCD and MTCD had low viscosity,good infiltration and slow curing speed,resulting in high degree of diffusion with the sizing layer and interphase forming mechanism of "swelling and diffusing of sizing layer".HMCF/TCD and HMCF/MTCD with wider interphase and higher modulus matrix produced"modulus transition layer",making the formed interface with strong ability of stress transfer and high interfacial performance.4.A novel interphase was constructed by molecular assembly of naphthenylimide(NDI)on the graphite-like structure of HMCF surface.Mechanism of molecular assembly on HMCF surface was discussed.Surface properties and interfacial properties of pristine,commercial and NDI assembled HMCF composites were evaluated.Active assemblied nano-sheet on HMCF surface via ?-? stacking of NDI increased surface roughness and content of active functional groups,and the surface energy of NDI assembled HMCF was increased by 59%and 23%than that of pristine and commercial HMCF,respectively.TFBT strength and IFSS of NDI assembled HMCF composite were increased by 64%and 31%than those of commercial HMCF composite,respectively.Besides covalent bonds,?-? stacking was introduced to increase interfacial bonding,and NDI nano-sheet played a role in rivet and deflection of crack at interface,realizing the interface enhancement of HMCF composites.