A Short Review on Self‐Healing Thermoplastic Polyurethanes

Abstract

of self-healing will happen only by satisfying four factors: location, temporality, mobility, [17] and intermolecular interaction. The surface of the damaged part needs to contact with each other properly and the dynamic interaction requires time to reform. Self-healing TPUs can be used as coatings, [18] adhesives, [19] biomedical materials, [20] wearable electronics, [21] and this self-healing property can extend the service life and reduce waste. Early selfhealing materials were design to employ encapsulated external healing agents. [22] With conceptualization and development, the classification of healing agents comprises three types: microcapsules, [23,24] hollow fibers, [25,26] and microvascular networks. [27] Limited by the supply of healing agents, multiple cycle repairs cannot be achieved; in addition, the microcontainers may be harmful to the regular service of the matrix, which change the inherent properties of the matrix, fail to respond quickly and cause the healing agent to flow out when the matrix is damaged. To overcome these restrictions, research on self-healing polymers has focused on the incorporation of self-healing mechanism directly in the material by introducing dynamic bonds or interactions (covalent or non-covalent) that can break and reform in a perpetual manner upon appropriate stimuli. The dynamic non-covalent interactions like hydrogen bond, [28,29] shape-memory assisted self-healing, [30,31] host-guest interaction, [32,33] and metal-ligand coordination [34,35] primarily influence the self-healing efficiency. The dynamic covalent bonds such as Diels-Alder bonds, [36,37] disulfide bonds, [38,39] diselenide bonds, [40,41] and ditelluride bonds [42] exert a major effect on the mechanical property of self-healing polyurethanes. However, there is a contradiction between self-healing efficiency and mechanical property. It is challenging to obtain a material with both remarkable mechanical strengths and a high self-healing efficiency. Interest in homogeneous self-healing materials has been growing because they do not need to be combined with therapeutic agents, which avoids many thorny issues including complicated pre-embedding processes, compatibility, and controlled and sustained release. For healing agents-assisted self-healing materials, they are usually thermosetting, and an obvious limitation is that they only support one self-healing cycle. [43] Self-healing elastomers can achieve repair through dynamic molecular interactions between the damaged surface. In this article, we only focus on self-healing TPU elastomer mechanism based on dynamic covalent or non-covalent bonds. Self-healing polyurethanes based on external healing agents are outside the scope of this article.Thermoplastic polyurethane (TPU) is a melt-processing polymer with a high elasticity, high tensile strength, low-temperature resistance, wear-resistance, and corrosion resistance ability. TPU can be used in the automotive industry, electronics, medical supplies, coatings, and sports equipment. Introduction of self-healin...

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“…3). 73 It is worth mentioning that this is the typical morphology of TPEs, and thus not exclusively for polyurethane-based elastomers.…”

Self-Healing Materials for Electronics Applications

“…One of the great benefits of a chain exchange reaction is that it entails a lower temperature for activation of the SH mechanism than the Diels–Alder reaction [ 135 ]. Disulfide bonds tend to undergo exchange reactions in which two adjacent S–S bonds are broken and restructured via ionic intermediates or free radicals, where fracture recovery can be initiated at low temperatures via oxidation, heating, or photolysis [ 95 , 136 , 137 , 138 , 139 , 140 ].…”

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