Application And Research Of Schiff Base Vitrimers In Carbon Fiber Recycling And Solid State Electrolytes

Thermosetting resin is a three-dimensional mesh structure.Normal thermoset resin prepregs are difficult to shape after curing,resulting in low reproducibility,waste of resources and environmental pollution.Therefore,the study of reprocessability,degradability and self-healing of thermoset resins has become an important topic in the field of material applications.Vitrimers materials are new polymer materials consisting of dynamic covalent networks with reversible cross-linked structures that can change their topology under external stimuli,thus making it reprocessable and recycle to thermoplastic thermoset resins and their composites.Among them,carbon fiber recycling can effectively solve the cost and environmental problems in carbon fiber applications and improve the sustainability and application value of the material.At present,researchers have successfully achieved the preparation of repeated thermal processing and degradable cross-linked polymers by various reaction methods.Including imine exchange reaction,ester exchange reaction,enamine ester exchange reaction,triazole salt transalkylation reaction,silanol exchange reaction,olefin complex decomposition reaction and disulfide bond exchange reaction.Imine bond has a low topological freezing transition temperature.In this study,Schiff base Vitrimers were used to prepare Schiff base resin/2-layers CF composites with dynamic networks,which successfully achieved the recovery of carbon fibers and improved the interfacial properties of resin/CF.In addition,based on the characteristics of Vitrimers,flexible solid polymer electrolytes（SPEs）with room temperature self-healing properties were prepared in this paper,and the mechanical and electrochemical properties of their solid electrolytes were investigated.The results show that the dynamic topological behavior of Vitrimers materials makes them promising to play an important role in the field of compound recycling and batteries.The main studies are as follows:（1）Polyimine Vitrimers/CF composites were prepared.The glass transition temperature of the polyimine Vitrimers cured with 4’4-diaminodiphenylmethane was 102.2℃,and its tensile strength at break and Young’s modulus reached 80.3 MPa and 2.89 GPa,respectively.the mechanical properties of the polyimide Vitrimers/2-layer CF composites cured with isophorone diamine were slightly lower than those of bisphenol A,and its tensile strength at break and Young’s modulus reached 504 MPa and 5.52 GPa.The polyimine Vitrimers cleaved to white particles in acidic solution（pH=1）at 60℃ for 24 h.The surface morphology and chemical structure were unchanged after carbon fiber recovery as seen from Raman and SEM.（2）Schiff base vanillin epoxy Vitrimers/CF composites were prepared.The glass transition temperature of VE-IPDA/Vitrimers reached 132℃,and the tensile strength at break and Young’s modulus of the 2-layer CF reinforced VE-IPDA Vitrimers composites were higher than those of biphenyl A epoxy resin（E51）/CF and other epoxy Vitrimers/CF composites,reaching 571 MPa and 11.7 GPa,respectively.All VE-IPDA Vitrimers except the4,4-diaminodiphenylmethane cured Vitrimers were clear and transparent in acidic solution（pH=1 HCl:DMF=1:1 v/v）at room temperature.Full recovery of resin and carbon fiber was achieved by adding equivalent amount of IPDA using the principle of imine exchange reaction.The surface morphology and chemical structure of the washed CF after degradation and recovery were unchanged as seen from SEM and Raman maps.（3）Flexible solid state polymer electrolytes（SPEs）with room temperature self-healing properties based on imine dynamic networks were prepared.The effects of crosslinking degree,plasticizer（SN）content and lithium salt（LTFSI）on glass transition temperature（T_g）the lithium ion conductivity（δ）were carefully investigated.With the addition of lithium salt,the T_gvalue of the polymer film was significantly enhanced,and the T_gvalue of the Vitrimers electrolyte with lower crosslinking degree was lower.With the decrease of crosslinking degree（0.66）and the increase of SN content,δshowed a clear increasing trend.Importantly,the original tensile strength of the vitrimers electrolyte cut into two halves with a knife was restored to 74%of the original after self-healing at room temperature.Moreover,the addition of SN resulted in an expanded electrochemical window of up to 5.1 V vs.Li/Li+.The prepared Li|PSFSPE@0.5SN|LiFePO₄cells were able to reach a discharge specific capacity of100 mAh/g after 50 cycles at 0.05 C.