Self-Healing Polymers

Abstract

Self-healing is a very common event for living things. Taking this cue from nature, various self-healing strategies in polymers have been developed. This chapter deals with the concept, strategies, and applications of self-healing polymers. In the thermoplastic section, the lowering of the glass transition temperature (Tg), cross-linking via cycloaddition, and dynamic cross-linking are explored to perform self-healing. Disulfide bonds and the Diels-Alder reaction, studied for self-healing, are discussed. Dynamic covalent bond-based self-healing by using sulfide bonds, CO2, etc. are also included. In the thermoset section, epoxy resin is introduced for its self-healing property using the Diels-Alder reaction, the retro thiol-Michael reaction, and disulfide linkages. Discussion is also extended to thiol-maleimide-based cross-linking for self-healing with dual-sensitive characteristics. The cross-linked acrylate with a thiol-based strategy has applicability in coatings, sealants, and high-performance elastomers. Polydimethyl siloxane-based microcapsules, with excellent applicability in self-healing, are also discussed. A unique load transfer mechanism, very different from conventional composites, can be designed with a liquid-filled solid shell in which the viscosity of the liquid is manipulated. This system comes under self-healing as the stressed solid shell comes back to normal after the load is withdrawn.. Self-healing elastomers with hydrogen bond-mediated healing are discussed in detail. Ionomers have a natural tendency to self-heal after the stress is withdrawn. The chapter includes ion-mediated self-healing of siloxanes, bromobutyl rubber, and metal methacrylate containing elastomers. Schiff base-based hydrogels, enzyme-induced dual network hydrogels add to the variety of topics in this chapter. The magnetic behavior of the hydrogels and injectable hydrogels have promise for practical application. Dual network-based hydrogels, protein hydrogels, cell-loaded hydrogels, etc. with unique self-healing ability are also presented. Advances in hydrogels include supramolecular hydrogels and 3D printing of hydrogels. The discussion of applications in various areas, such as self-healing coatings based on microcapsules, includes microvascular networks containing various liquids, such as PU-based paint, silyl esters, solvents, etc. Layer-by-layer coating with self-healing ability is also a topic here. Other topics include self-healing textiles, self-healing composites, intrinsic self-healing polymers, self-healing nanocomposites, self-healing electrochemical systems, and miscellaneous self-healing systems.