Polymeric materials are three-dimensional networks formed by the cross-linking of linear polymer chains. During stretching, the crimped linear polymer segments between the crosslinks can be stretched like springs. After the release of tension, the molecular chain can be restored to its original state of distortion. Therefore, to make the polymer material elastic, the cross-linking interactions between the polymer chains must be sufficiently strong. Otherwise, the cross-linking sites are easily damaged when they are stretched, resulting in the material being broken. For self-healing materials, these cross-linking effects cannot be too strong, otherwise they cannot be repeatedly broken and formed under mild conditions, and thus have no self-repairing properties. Therefore, the two aspects of the conflict between the elasticity and self-repairing properties of the material (Fig. 1).
Figure 1. The the incompatibility between mechanical robustness and dynamic healing
We have effectively solved this problem by using the unique features of coordination bonds. We integrate strong and weak coordination bonds in the same polymer. The strong coordination bond is adjacent to the weak coordination bond. When it is stretched, the weak coordination bond breaks to dissipate the energy, and the strong coordination bond still maintained so that the material does not fracture, thus leading high stretchability. On the other hand, due to the combination of strong and weak coordination bonds, the coordination structure is highly dynamic and can be rapidly formed after being destroyed, so that the material can be healed spontaneously after being damaged (Fig. 2). At the same time, this material has a good application in artificial muscles. The research results was published in Nature Chemistry 2016, 6(8), 619-625. In addition, we have used moderately coordinated bonds or intermolecular interactions to cross-link macromolecules, and have also obtained a series of highly elastic self-repairing materials (Macromol. Rapid Commun., 2016, 37, 952-956；2016, 37, 1667-1675).
Figure 2. Highly elastic self-healing polymer.
When the number of crosslinks sites between the linear polymer segments is large and the cross-linking interactions are strong, the movement of the polymer segments is hindered and the material is rigid, that is, it is not easily deformed. However, if the cross-linking action is too much and too strong, the inter-diffusion between the damaged interfaces is not effective, so the material can not be self-healed. Therefore, the rigidity and self-healing properties of the material are also two aspects that are incompatible with each other.
We first use a "fewer but better" strategy to design rigid self-healing materials. The boron-oxygen bond (B-O) is one of the strongest covalent chemical bonds, and its bond energy is as high as 515 kJ/mol. Therefore, the introduction of boron-oxygen bond can help to increase the strength of the material. At the same time, the boron-oxygen bond is hydrolyzed and broken in the presence of water, and can be reformed once the water is removed. Therefore, it can be used to produce materials that are repaired by water. Based on this, we cross-linked polymethylsiloxane chains with Boroxine units to obtain a high-strength material with a small number of boron-oxygen bonds. The Young’s module of this polymer is over 200 MPa, and can withstand 450 times its own weight without breaking (Figure 3a). At the same time, the material can be quickly repaired with the aid of water. The research results were published in Advanced Materials 2016, 28, 8277–8282.
In addition, we can also use “weak but abundant” bonds to obtain rigid self-healing materials. As we all know, hydrogen bonds are weak interactions, but a large number of hydrogen bonds can produce very strong materials. For example, hard biomass such as shells, shrimp claws, and bark is obtained by cross-linking protein chains through hydrogen bonds. At the same time, water molecules can form strong ice through hydrogen bonds at low temperatures. Based on these phenomenon, we designed a short-chain polymethylsiloxane monomer with a large number of carboxyl groups in the side chain and used it to coordinate with metal zinc. Since the carboxyl group is a hard base and the metal zinc is a border acid, the coordination bond formed between the metal zinc and the carboxylic acid is relatively weak according to the principle of soft-hard acid-base theory in coordination chemistry. The use of a large number of such weak coordinating bonds to crosslink the polymethylsiloxane monomer gives a very high mechanical strength material. Its Young's modulus is as high as 500 MPa, and it can withstand 500 times its own weight without being bent (Figure 3b). Research papers were recently published in Nature Communications, 2018, 9:2755.
Figure 3. Stiff and healable polymer based on boroxine bonds.
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