Recent developments in material design have seen an exponential increase of polymers and polymer composites that can repair themselves in response to damage. In this review, a distinction is made between extrinsic materials, where the self-healing property is obtained by adding healing agents to the material to be repaired, and intrinsic materials, where self-healing is achieved by the material itself through its chemical nature. An overview of the crosslinking chemistries used in self-healing materials will be given, discussing the advantages and drawbacks of each system. The review is not only aiming to enable researchers to compare their ongoing research with the state-of-the-art but also to serve as a guide for the newcomers, which allows for a selection of the most promising self-healing chemistries.
Furan-based thermoset polyurethanes have been prepared in a one-pot fashion with the ability to selfmend under mild temperature conditions, by making use of a Diels−Alder shape-memory assisted self-healing (DASMASH) approach. For this, thermoreversible covalent bonds, obtained by Diels−Alder chemistry, are introduced as cross-linkers into a polycaprolactone (PCL) containing polyurethane material. It is demonstrated that, after introduction of a crack into the PUthermoset, Diels−Alder bonds preferentially break, regenerating free furan/maleimide functional groups, while the shape memory effect favors the crack closure at temperatures above the melting point of PCL, simultaneously resulting in a reformation of the reversible cross-links. The reversibility and shape memory ability of the materials were optimized and studied by FTIR, 1 H NMR and tensile measurements. Different compositions were used to properly understand the role and influence of each component. The polyurethane materials healed at 50°C after mechanical damage induced by either the application of a large tensile deformation or by performing controlled macro/micro scratches with a depth sensing indenter. Online FT-IR monitoring provided a kinetic description of the system reversibility for numerous cycles. Furthermore, mechanical recovery with complete disappearance of the microscratches was accomplished after multiple cycles of large tensile deformation. The results were not only confirmed by an optical inspection and scanning electron microscopy, but also with confocal microscopic mapping, by comparison of the cross-section profiles of the microscratches before and after healing.
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