Design and synthesis of highly stretchable self-healing materials

Abstract

Most natural biomaterials have an intrinsic ability to self-heal upon encountering damages. Therefore, living beings can recover when wounded. To mimic the self-healing properties of natural biomaterials and prolong the lifetime of the material in various applications, many synthetic self-healing polymers which can repair the internal or external damages have been developed. So far, a lot of synthetic polymers have been designed to self-heal by encapsulating healing agents (in microcapsules or microvascular networks, classified as extrinsic self-healing) or incorporating dynamic chemical bonds (including reversible covalent bonds such as alkoxyamine, disulfide, boronic ester and boroxine bonds or bonds formed by Diels-Alder reaction, or non-covalent interactions such as hydrogen bonds, π-π stacking interactions, host-guest interactions, ionic interactions and metal-ligand interactions, classified as intrinsic self-healing) into the polymer matrix. Due to the consumption of the encapsulated agents, the repair in extrinsic self-healing system is generally not repeatable. Therefore, intrinsic self-healing systems based on reversible covalent bonds or noncovalent interactions are preferred. Highly stretchable self-healing polymers are desirable due to their widespread application in flexible and stretchable electronic devices, including transistors, sensors, energy-storage devices, and light-emitting diodes (LEDs). However, the design of such materials is a nontrivial task. Elastomers are typically made up of cross-linked networks of long chains of polymers. The elasticity arises from the ability of the chains to reconfigure under an applied tension. The crosslinking sites between the long polymer chains ensure that the network returns to its original configuration when the tension is removed. Therefore, the cross-linking bonds should be as strong as possible, otherwise they will be disassociated upon tension and a permanent deformation will be resulted. However, for an autonomous self-healing material, weak dynamic bonds should present as crosslinking sites so that they will break first upon damaging and reform to heal. Polymers crosslinked by weak dynamic bonds tend to be soft and viscoelastic. Therefore, it is highly challenging to design materials that simultaneously exhibit good elasticity and autonomous self-healing properties. In this review, we highlighted the recent advances in design and synthesis of highly stretchable self-healing materials. The main strategy for designing self-healing materials, including (1) double network; (2) incorporating nano-fillers; (3) inducing multi-phase separation; (4) introducing sacrificial bonds; (5) increasing the energy of dynamic bonds, have been described. From this review, one could witness that the combination of high stretchability and self-healing can be realized by various strategies. However, it should be noted that most of the reported highly stretchable self-healing polymers exhibit slow elastic recovery behavior, i.e., they can only recover to their original length after relaxation for a long time after stretching. This is reasonable, because the weak and dynamic dynamic bonds between polymer chains break more readily upon stretching but it takes some time for them to reform, which invariably leads to poor recovery behavior. Therefore, it is still challenging to create materials that have both good elastic performance and autonomous healing capability. If self-healing polymer with fast elastic recovery upon stretching can be developed through continuing efforts of scientists, more widespread applications of self-healing materials can be expected.