Study On Surface Modification Of High-modulus Carbon Fiber Based On Molecular Assembly And Interfacial Properties Of Their Composites

Current dual processing technique of anodic oxidation and sizing waterborne epoxy has not completely solved the surface inertness of high-modulus carbon fiber(HMCF)from its high-degree graphitization,and the interphase of their composites cannot effectively alleviate the modulus saltation between HMCF and epoxy matrix and act as the "hub" of load transfer.Aiming at solving the low load-transfer efficiency and weak failure form of HMCF composites caused by the vulnerable interphase,two-dimensional assembled layer constructed by molecular assembly of naphthalenediimide(NDI)and three-dimensional assembled layer constructed by hybrid molecular assembly of naphthalenediimide/hydroxylation multi-wall carbon nanotubes(NDI/MWNT)on HMCF surface were proposed in this paper.Modulus intermediate layer formed by afore-mentioned multi-dimensional assembled layer with modulus platform was studied,and the design and control of interphase structure and the interface enhancement of composites were achieved.(1)Amino terminated naphthalenediimide(NDI)was prepared by the design of molecular structure and utilized as sizing agent to modify Pristine HMCF for preparing NDI@HMCF.Compared with Pristine HMCF,the surface chemical activity and roughness of NDI@HMCF were improved.HMCF surface was used as substrate to assist nucleation and molecular assembly of NDI,and the two-dimensional assembled layer was constructed by the flat ensiform assemblies on NDI@HMCF surface.The mechanism of molecular assembly was proposed as follows:After nucleation from the ?-? interaction between the naphthalene ring of NDI and the sp2 hybridization domain of HMCF substrate,the H-type ?-? interaction between NDI led molecules to horizontally aggregate and the hydrogen bond between terminated amino groups drove molecules to regularly arrange.(2)MWNT and NDI sizing agents were successively utilized to modified Pristine HMCF for preparing MWNT@HMCF and NDI/MWNT@HMCF.Compared with MWNT@HMCF,the surface chemical activity and roughness of NDI/MWNT@HMCF were improved.The entangled MWNT network was established on MWNT@HMCF surface.HMCF and MWNT were used as joint substrate to assist dual nucleation and hybrid molecular assembly of NDI,and the three-dimensional assembled layer was constructed by the organic and inorganic hybrid structure in which one end of the oblique ensiform assemblies were embedded in the MW/NT network on NDI/MWNT@HMCF surface.The mechanism of hybrid molecular assembly was proposed as follows:After dual nucleation from the ?-? interaction between the naphthalene ring of NDI and the sp2 hybridization domain of MWNT and HMCF joint substrate,the H-type?-? interaction between NDI led molecules to obliquely aggregate under the constraint of MWNT network and the hydrogen bond between terminated amino groups drove molecules to regularly arrange.(3)The interfacial properties and fracture morphology of Pristine HMCF,MWNT@HMCF,NDI@HMCF and NDI/MWNT@HMCF unidirectional,bundle and monofilament composites were compared respectively,and the change of structure,modulus and carbon content of various interphase region were characterized by TEM,f-AFM and EDS,based on which the interface enhancement mechanisms of composites were proposed.Compared with Pristine HMCF and MWNT@HMCF composites,ILSS?TFBT strength and IFSS of NDI@HMCF and NDI/MWNT@HMCF composites were increased.The weakest adhesive failure was occurred in Pristine HMCF and MWNT@HMCF composites,while the stronger bulk failure was occurred in NDI@HMCF composite,and the strongest cohesive failure was occurred in NDI/MWNT@HMCF composite.Modulus intermediate layer with modulus platform was formed by two-dimensional and three-dimensional assembled layer in NDI@HMCF and NDI/MNVNT@HMCF composites.The mechanisms of interface enhancement were proposed as follows:Two-dimensional assembled layer acted as "armor" to prevent cracks propagating onto HMCF surface,and three-dimensional assembled layer simultaneously acted as "armor"and "anchors" to further transfer stress concentration from HMCF surface into interphase.