Design Of Rigid-flexible Interphase Based On Naphthalimide With Different Flexible Chain Lengths And Interfacial Properties Of High-modulus Carbon Fiber Composites

Balancing the strength and toughness of interphase is a hot issue in interphase engineering for carbon fiber composites.The huge discrepancy in the modulus of high-modulus carbon fiber(HMCF)and epoxy matrix,which limited the stress transfer uniformly of HMCF composites.And the residual stress cannot be released during curing and cooling of composites,due to the mismatch of thermal expansion coefficient between HMCF and resin.To solve the problem,the construction of rigid-flexible interphase with different structures by three kinds of rigid-flexible copolymers with different flexible chain lengths were proposed in this paper.The correlation mechanism between flexible chain length and modulus gradient,flexibility and residual stress of interphase was studied.The interfacial mechanical properties and strengthening mechanism of three interphases were compared.The design of interphase were achieved.(1)The rigid-flexible copolymers of difunctional L-NDI,M-NDI and four functional S-NDI with different flexible chain lengths were synthesized via the design of molecular,which were obtained by substituting the imide sites of naphthalimide with polyether amines with polymerization degrees of 33.2,5.6and 5.3,respectively.CF-L-NDI,CF-M-NDI and CF-S-NDI were prepared during sizing with L-NDI,M-NDI and S-NDI to modify CF-desized.In contrast to CF-desized,the roughness,wettability and surface energy of CF-L-NDI,CFM-NDI and CF-S-NDI were increased.Among them,CF-S-NDI was the highest.(2)The Interfacial properties and fracture morphology of CF-desized/EP,CF-(L-NDI)/EP,CF-(M-NDI)/EP and CF-(S-NDI)/EP monofilament,bundle and undirectional composites were compared.The modulus,deformation and thermal residual stress of interphase were analyzed and the strengthening mechanism of interphase was proposed.Compared with CF-desized/EP composite,IFSS、TFBT strength and ILSS of CF-(L-NDI)/EP,CF-(M-NDI)/EP and CF-(S-NDI)/EP composites were increased.The weakest adhesive failure was occurred in CF-desized/EP composite,while the partial adhesive failure was occurred in CF-(L-NDI)/EP composite,and the cohesive failure was achieved in CF-(S-NDI)/EP composite.The enhancement mechanism of interphase was proposed: with the decrease of the flexible chain length and the increase of the functionality,the sizing agent chemically combined with the resin matrix to form interphase dominated by rigid components and supplemented by flexible components.The rigid components allowed the stress to transfer more uniformly,and the plastic deformation of the flexible component released the residual stress.