Repeatable Crack Self-Healing by Photochemical [2 + 2] Cycloaddition of TCE-co-DCE Monomers Enclosed in Homopolymer Microcapsules

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Abstract
Self-healing, an autonomous repairing process stimulated by damage, has recently attracted a great deal of attention in the field of medical and mechanical engineering as well as from scientists, due to its valuable potential applications. However, as the self-healing process is mediated by specific functional materials, practical applications have been limited. Here, we introduce a healable homopolymer microcapsule that can self-heal a crack or cleaved part through a photochemical [2 + 2] cycloaddition process. Microcapsules were prepared through photopolymerization and suspension polymerization, each containing 1,1,1-tris (cinnamoyloxymethyl) ethane (TCE) and 1,1-di (cinnamoyloxymethyl) ethane (DCE) monomers, which act as healing materials. TCE and DCE monomers were polymerized into poly (TCE-co-DCE) without a photoinitiator under illumination. The epoxy specimen embedded with microcapsules showed obvious healing performance during illumination after cracking. From the FT-IR spectra for each step of the healing process, the specimen could be repeatedly self-healed through the reversible process of cyclobutane cross-links to the original cinnamate and vice versa. This work shows an alternative approach using homopolymer microcapsules to accomplish the repeatable self-healing of a crack without interface discontinuity, which could be adopted as a healing substance in various paints.

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