High-Performance Reversibly Cross-Linked Polymers With Healability And Recyclability

As a kind of fundamental materials,polymer materials play an indispensable role in industry and daily life,and their usage has surpassed metallic and inorganic materials.With the extensive use of polymer materials,the problems of resource waste and environmental pollution are becoming increasingly serious,which has become a global problem hindering the development of human society.The main reason for this predicament lies in the fact that polymer products,such as vulcanized rubber,epoxy resins,usually have a stable covalent bond cross-linked network structure to meet service requirements.These materials cannot be repaired once they are damaged during use,ending up as waste.It is challenging to recycle those waste polymers and most of them are then discarded or incinerated.Therefore,the development of sustainable polymeric materials that possess attributes,such as longterm service stability,ability to repair mechanical damage,and recyclability is particularly crucial and urgent for the development of sustainable society.In recent years,the construction of reversible cross-linked polymers by supramolecular forces and dynamic covalent bonds has gained considerable attention.These polymers possess sustainability owing to the dynamic nature of their network,which provides them with self-healing properties,recyclable properties,and closed-loop recyclable properties.However,those reversible cross-linked polymers usually exhibit unsatisfactory mechanical properties and improving their mechanical properties remains a key challenge in the field of polymer science.Additionally,imparting functionality to reversible cross-linked polymer materials is crucial for their practical application.This paper addresses the above issues by preparing reversible cross-linked polymer with high mechanical strength through the regulation of reversible interaction force,rigidity of polymer chain segments as well as the aggregation structure of polymer chain.This approach successfully endows the material with sustainable abilities such as self-healing,recycling,and closed-loop recovery abilities,followed by the introduction of functions namely conductivity and flame-retardancy.The specific research contents are as follows:1.Mechanically robust skin-like elastomers with integrated damage resistance,damage-tolerance and self-repairing have been prepared through biomimetic skin design.The elastomer was prepared by using poly-ε-caprolactone(PCL)as the main chain and cross-linked with hydrogen bond arrays.Hydrogen bond arrays exhibit a high binding energy,and can disintegrate to efficiently dissipate energy under external forces,endowing the elastomer with excellent toughness and tear resistance,i.e.,damage resistance.Stretch-induced crystallization of PCL segments occurs once PU-ASC suffers from large deformation,imparting high mechanical strength to resist damage,i.e.,damage resistance.The values for fracture strength,toughness,and fracture energy of PU-ASC are as high as ~73MPa,~375 MJ m-3,and ~161 k J m-2,respectively.The high-performance conductive elastomer(PU-ASC-IL)with integrated damage resistance,damage resistance,and healability can be obtained by loading ionic liquid with a loading ratio of 50 wt%.The ionic conductivities,fracture strength,toughness,and fracture energy of the conductive elastomer are ~1.9 × 10-4S cm-1,~23 MPa,~164MJ m-3,and ~74 k J m-2,respectively.Based on the dynamic nature of hydrogen bonds,both PU-ASC and PU-ASC-IL can completely repair mechanical damage at high temperatures.2.Elastomer with enhanced mechanical properties was fabricated using a dually cross-linked polyurea(PUA).PUA is a multi-block polyurea,comprising polyamide acid(PAA)and poly-ε-caprolactone(PCL)as the backbone,which are interlinked by imine bonds.The PAA chain segments are aggregated into phaseseparated hard phases,which act as nanofillers together with the hydrogen bond arrays formed by ASC to crosslink the amorphous PCL segments.Both the cross-links can deform or even dissociate under external forces to efficiently dissipate energy,thereby enhancing the overall mechanical properties and damage tolerance of PUA compared to PU-ASC.The values of fracture strength,toughness,and fracture energy of PUA are ~72 MPa,~434 MJ m-3,and ~307 k J m-2,respectively.Based on the dynamic nature of hydrogen bonds,damaged PUA can spontaneously heal mechanical damages at high temperature.PUA can also be used to prepare a high-performance self-healing conductive elastomer by loading the ionic liquids.3.High-performance flame-retardant plastics with closed-loop recovery capability were prepared using rigid chain segment polyimide(PI)as the building block.The reversible cross-linked plastic(PI-CP)was prepared by Schiff-base reaction between amino-terminated polyimide(PI-NH2)and aldehyde-terminated cyclophosphazene(CP-CHO).CP-CHO,as a six-arm cross-linking agent,gave PI-CP a high degree of cross-linking,and in combination with the rigid PI chain segments,provided the material with excellent mechanical strength.The fracture strength and Young’s modulus of PI-CP were ~115.6 MPa and ~2.5 GPa,respectively.Based on the dynamic nature of the imine bond between cyclophosphazene and PI under acidic conditions,the discarded PI-CP could be depolymerized into monomers in acidic tetrahydrofuran solution,which could then be repolymerized into new plastics with the same mechanical strength as the original material.The combination of rigid PI and flame-retardant cyclophosphonitrile gives PI-CP excellent flame retardancy,exhibiting a limiting oxygen index(LOI)of 49% and achieving the V0 level in the UL94 fire test.