Design Of Rigid-flexible Interphase Based On Polyimide Copolymers And Study On The Interfacial Properties Of M40X Composites

The interphase design of M40X composites is the research hotspot of interphase engineering of carbon fiber reinforced composites（CFRP）due to the integration of high strength and modulus of M40X carbon fiber.The optimum interphase of M40X composites should buffer the modulus difference between CF and resin matrix as well as alleviate the interfacial residual stress from the discrepancy in thermal expansion coefficient,which synchronously improves stress transfer efficiency and energy absorption capacity of the interphase.To mitigate the problem,the construction of rigid-flexible interphase with block polyimide copolymers were proposed in the paper.The effect of the rigid/flexible proportion on the physicochemical properties of M40X surface and interfacial performance of composites was studied.The microstructure of the rigid-flexible interphase and the interfacial reinforcing mechanism were explored and the effective regulation of the rigid-flexible interphase was achieved.（1）Three kinds of polyimide copolymers(PAE₂₀,PAE₁₀ and PAE₅)with different molar ratios of rigid/flexible segments were synthesized and the molecular flexibility was verified via molecular dynamics simulation（MD）.PAE₂₀,PAE₁₀ and PAE₅ were applied to modify p-CF via surface sizing for preparation of PAE₂₀-CF,PAE₁₀-CF and PAE₅-CF.Compared with p-CF,the chemical activity and wettability of PAE₂₀-CF,PAE₁₀-CF and PAE₅-CF were improved.MD results indicated that the increasing flexible proportion contributed to improving the distribution uniformity of PAE copolymers on the CF surface.（2）The interfacial properties and fracture micromorphology of p-CF/EP,PAE₂₀-CF/EP,PAE₁₀-CF/EP and PAE₅-CF/EP monofilament,bundle,unidirectional composites were characterized,which was compared with the interfacial shear stress of the MD interphase models of the composites.Compared with p-CF/EP,PAE₂₀-CF/EP and PAE₅-CF/EP composites,the IFSS,TFBT strength,ILSS and interfacial toughness of PAE₁₀-CF/EP composites showed the maximum value,which was attributed to the crack passivation and deflection as well as the energy absorption from the optimal rigid-flexible proportion,inducing the strongest cohesive failure.Correspondingly,the MD results showed that the highest ISS was also achieved by PAE₁₀-CF/EP composites,which was ascribed to the optimal ratio of flexible segments that ensured the stability of crosslinked network structure between rigid segments and epoxy matrix through the vicarious displacement.Based on the study of the interfacial structures of transit-modulus layer and the microscopic distribution of copolymer chains in the interphase,the reinforcement mechanism of the rigid-flexible interphase was proposed:the scarcity of flexible segments led to the constrained mobility of copolymer chains,which limited the interfacial energy absorption efficiency;the excessive flexible segments induced the random diffusion copolymers,which weakened the stability of interfacial structures,thus resulting in the interphase slippage;the optimal rigid-flexible copolymers effectively shielded and buffered the crack propagation via the stable‘rigid-crosslinking’and‘flexible-entanglement’double networks,which achieved the balance between the interfacial stiffness and toughness of M40X composites.