High Strength Self-healing Materials:Design,Synthesis And Applications

One of the important reasons why organisms can continue to multiply in nature is that organisms have their own self-repairing mechanisms when they are injured,such as small to microscopic DNA and protein repairs,and macroscopic skin and bone healing.Inspired by the nature,in the past nearly two decades,researchers have devoted themselves to developing methods and strategies to achieve self-healing materials,so that the materials have the ability to withstand internal or external injuries,and selfhealing materials have emerged.Compared with classic materials,the pristine properties and functions of self-healing materials can be spontaneously recovered when they are damaged.Such materials can effectively extend the service life,reduce the time and cost of maintenance,and improve the safety.The development of self-healing materials has a huge boost to future technologies.This paper aims to advance the development of self-healing materials,focusing on the design,synthesis and application of self-healing materials.By introducing a new type of reversible covalent bonds and dynamic coordination bonds in the polymer system,self-healing materials with excellent mechanical properties were prepared.The mechanical properties and selfhealing properties of the materials were systematically studied,and the application of self-healing materials in conductors,adhesives,3D printing materials,conductive adhesives,and energy absorbing materials was explored.1.Stiff and Healable Polymer Based on Dynamic-Covalent Boroxine Bonds.Stiff and healable synthetic polymers are highly desirable.However,combining stiffness and self-healing property in a synthetic polymer remains challenging as the reduced mobility of the molecular segments in stiff polymers are unfavourable for selfhealing.Herein we report a poly（dimethylsiloxane）（PDMS）polymer exhibiting simultaneously strong（with tensile Young’s module up to 182±15.8 MPa and compressive Young’s module up to 142±9.8 MPa）,stiff（with elongation and compressibility less than 10%at 10 MPa stress）and water assisted self-healing properties.Specifically,we use dynamic-covalent boroxine bond to crosslink the PDMS chain into three dimensional networks.The as-prepared sample is very strong and stiff,and can bear a load of more than 450 times of its weight.As the boroxine/boronic acid equilibrium can be shifted by the removal or addition of water,the polymer can be completely healed upon heating after wetting.Moreover,we demonstrate that our polymer can be used both as self-healing adhesives and in fabricating self-healing semi-transparent conductors.2.Rigid and Healable Polymer Cross-linked by Abundant Zn（Ⅱ）-Carboxylate Interactions.Combination of solid-like properties with fast self-healing is highly challenging due to the slow diffusion dynamics.In this work,we described a new design concept that utilizes weak but abundant coordination bonds to achieve this objective.The new PDMS polymer,crosslinked by abundant Zn（II）-carboxylate interactions,is very strong and rigid at room temperature.As the coordination equilibrium is sensitive to temperature,the mechanical strength of this polymer exhibits rapid and reversible change upon heating or cooling.The soft-rigid switching ability σ,defined as G’max/G’min,can reach 8000 with ΔT=100℃.Based on these features,this polymer not only exhibits fast thermal-healing properties,but is also proved to be advantageous for various applications such as orthopedic immobilization,conductive composites and adhesives,and 3D printing.3.Distinct Mechanical and Self-Healing Properties in Two Polydimethylsiloxane Coordination Polymers with Fine-Tuned Bond StrengthCoordination bonds are effective for constructing high efficient self-healing materials as their strength is highly tunable.In order to design self-healing polymers with better performance,it is important to get a profound understanding of the structureproperty relationships.However,this is challenging for self-healing polymers based on coordination bonds,since many parameters,such as bond energy,bond dynamics,and coordination number will have an essential effect on the mechanical and self-healing properties of the polymer.In this work,we synthesized two poly（dimethylsiloxane）（PDMS）polymers crosslinked by different Zn（II）-diiminopyridine coordination complexes（denoted as PDMS-NNN-Zn,PDMS-MeNNN-Zn respectively）.The two crosslinking Zn（II）-diiminopyridine complexes are similar in coordination numbers and geometries,but defer in coordination bond energy and bond dynamics.As manifested by ITC,rheology,and tensile experiments,we confirm that the coordination bond in PDMS-MeNNN-Zn polymer films is weaker but more dynamic.Consequently,the PDMS-MeNNN-Zn polymer has poorer mechanical strength but higher stretchability and better self-healing properties.The inflicted cracks on PDMS-MeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%.Such fast self-healing properties have never been achieved in self-healing polymers based on coordination bonds.Our results also deliver a clear indication of the important impact of the thermodynamic stability and kinetic lability of coordination complexes on the mechanical and self-healing properties of polymers.Such a comprehensive understanding is helpful for further design of novel synthetic polymers which can achieve an optimal balance between the mechanical strength and self-healing performance.4.Thermodynamically Stable Whilst Kinetically Labile Coordination Bonds Lead to Mechanically Robust Self-healing Polymers.There is often a trade-off between mechanical properties and dynamic healing:strong interactions result in tough but less dynamic systems,precluding autonomous healing,while weak interactions afford self-healing,but yield soft and/or fragile materials.In this work,we have design and synthesized a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum.The Zn-bimcp coordination bond we used in this work has a relative large stability constant(2.2 × 10¹¹)but also exhibits fast and reversible intra-/inter-molecular ligand exchange process.The as prepared Zn-bimcp-PDMS polymer is highly stretchable（can be stretched to 2400%strain）,tough(with toughness of 2.933 ×10⁷ J m^-3),and can be autonomous self-healed at room temperature.Control experiments by varying the metal ions and metal-to-ligand molar ratios showed that the bond strength and bond dynamics play the key role for the material’s mechanical robustness and selfhealing property.The design concepts presented here may represent a general approach to the preparation of self-healing polymers with excellent mechanical properties,while the new polymer reported in this work can find promising applications as anti-impact materials for car crash protection,sportswear,shockproof pad,armored clothing,etc.