Superstrong And Ultratough Supramolecular Elastomers Based On Hydrogen-bonded Crosslinking

Supramolecular elastomers are a class of elastomeric materials with rubber properties constructed by cross-linking polymer segments based on supramolecular non-covalent interactions.The dynamic reversibility of non-covalent interactions endows supramolecular elastomers with healable and recyclable properties,which prolongs the service life and cycle of the elastomers,as well reduces the waste of resources and environmental pollution due to the difficulty to recycle and degrade of covalent cross-linking-based elastomers,which is of great significance to the construction of a sustainable society.In recent years,although a larges number of supramolecular elastomers with healable and recyclable properties have been developed,its development prospects still faces the following problems:(1)Supramolecular elastomers constructed based on non-covalent interactions always exhibit limited mechanical strength(<20 MPa)due to the weak non-covalent bonds,which greatly limits their application in practical production and life.(2)Supramolecular elastomers often exhibit poor thermal stability and solvent resistance,resulting from the dynamic reversibility of their internal non-covalent interactions.Meanwhile,a larges number of water-sensitive polar groups in supramolecular elastomers lead to a significant decline in mechanical properties under aqueous or high humidity environments.In this paper,in allusion to the above-mentioned problems in the development of supramolecular elastomers,we took the linear polyurethane based on hydrogen bond cross-links as the research object,and carried out the following work:1.In view of the generally weak mechanical strength of supramolecular elastomer,inspired by the structure of spider silk,we designed and fabricated a healable and recyclable supramolecular elastomer with super strength,ultra high toughness and excellent crack tolerance.By introducing acylsemicarbazide(ASCZ)and urethane(Urethane)groups into the hard segment of poly(urethane-urea)(Supra PU)elastomer,as well adjusting the spacer(alicyclic or aromatic six-membered ring)between the ASCZ and Urethane groups,the hydrogen bond density and the size of the hydrogen bond domain in the elastomer were meticulously engineered,resulting in the preparation of the superstrong,ultratough and healable elastomer with excellent crack tolerance.As a result,the target Supra-PU elastomer had the highest tensile strength?75.6 MPa,ever recorded for polymeric elastomers known,and exhibited a true stress at break as high as?1.21 GPa that was comparable to that of typical spider silk.Meanwhile,the superstrong Supra-PU elastomer still maintained a very high elongation-at-break of?1520%,resulting in an enormous toughness of?390.2 MJ/m~3,which was?2.4 times higher than that of typical spider silk.The superstrong and ultratough Supra-PU elastomer showed superb crack tolerance with an unprecedently high fracture energy of?215.2 k J/m~2,which was even greater than that of metals and alloys.Moreover,the dynamic rupture and reformation of the hydrogen bond crosslinks could be activated by heating,endowing the Supra-PU elastomer with healability and recyclability.In this work,the supramolecular elastomer materials constructed based on non-covalent cross-linking had extremely high mechanical strength,which broke the traditional understanding of the low mechanical strength of supramolecular elastomers,providing a reference for designing healable and recyclable materials with excellent mechanical properties.2.Aiming at the problem of poor hydrothermal stability and solvent resistance of supramolecular elastomer,we prepared high-strength and high-toughness supramolecular elastomers with excellent hydrothermal stability and solvent resistance via designing and regulating the molecular structure of elastomer.We introduced a crystalline polytetrahydrofuran(PTMEG)into the soft segment of the polyurethane elastomer,as well as an ASCZ group into the hard segment,adjusting the spacer(alicyclic or aromatic six-membered ring)between the ASCZ groups,resulting in the dense ordered aggregation of hard segments and highly crystalline soft segments in elastomer,finally prepared the target supramolecular elastomer.While placed in a humidity environment of 93%for 30 days,the elastomer still keept a high tensile strength of 44.0 MPa,exhibiting a high moisture resistance.Meanwhile,the elastomer showed excellent thermal stability which tensile stress as high as 17.4 MPa at 120?.Furthermore,it demonstrated excellent solvent resistance,which maintained the mechanical properties after soaking in various polar and non-polar organic solvents(eg.dimethyl sulfoxide,N-methylpyrrolidone,chloroform)for one day.Interestingly,the elastomer could be dissolved in polar solvents at high temperatures,endowed with remodeling properties,and the mechanical properties remain unchanged after multiple remolding.In this work,a method for designing healable and recyclable supramolecular elastomer with high hydrothermal stability and solvent resistance was proposed,which provides a new idea for solving the problems of poor hydrothermal stability and solvent resistance of supramolecular elastomers.3.Based on the in-depth understanding of supramolecular non-covalent interactions,more and more healable and recyclable supramolecular elastomers have been prepared,however there is still a lack of research on the correlation mechanism between the structure and properties of supramolecular elastomer.In this work,we constructed a polyurethane model system based on hydrogen bond crosslinks,systematically studied the regulation of molecular structure on interchain hydrogen bond interaction and microphase separation structure,as well as established an accurate correlation between the structure and properties of polyurethane elastomers based on hydrogen bond cross-linking.First,we took diisocyanate and chain extender of different structures as raw materials,precise controled the number of hydrogen bond sites in the polyurethane hard segment and the spacer structure between hydrogen bond sites(including:aliphatic chain,fat ring,aromatic ring structure)via control variable method,then a series of polyurethane elastomers based on hydrogen bond cross-linking were successfully prepared.Subsequently,a series of characterization were carried out to demonstrate the hydrogen bonding interactions between the segments of the above-mentioned elastomers and their microphase-separated structures.Their mechanical,thermal,healable and remodeling properties were tested as well.Finally,through systematic comparative analysis,the accurate correlation had been summarized and established,where between the molecular structure,chain structure,microphase structure and other multi-scale structures and properties of polyurethane elastomers based on hydrogen bond cross-linking.The specific rules are as follows:(1)Increasing the number of hydrogen bonding sites in the hard segment could enhance the mechanical strength and toughness of the elastomers.(2)As improving the flexibility of the spacer among the hydrogen bonding sites in the hard segment(flexibility:aliphatic chain>aliphatic ring>aromatic ring),the synthetic elastomers exhibited dramatically increased degree of hydrogen bonding,meanwhile enhanced thermal stability and resilience,while increasing the size of the hydrogen bond domain,the thermal stability of the elastomer is further increased,and the repair time of the elastomer is also increased.(3)Moderate degree of hydrogen bonding of the polyurethane elastomers could improve the tensile strength and tear resistance of elastomers.However,when it became excessive,the elastomer was prone to stress concentration during stretching process,resulting in the decreased mechanical strength and tear resistance.(4)All of the polyurethane elastomers based on hydrogen bond cross-linking exhibited the properties of remolding and recycling.The work in this chapter established an accurate correlation between the structure and properties of polyurethane elastomers based on hydrogen bond cross-linking,which provided guidance for the customized preparation of healable and recyclable supramolecular elastomers with specific mechanical properties.