Self-healing metallopolymers

Detailed investigation of the self-healing properties by scratch testing

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In the last decade several intrinsic self-healing materials have been developed in which the healing mechanism was mainly based on the reversibility of a certain structural element. For this purpose, reversible covalent bonds, e.g., based on the Diels-Alder reaction or weaker non-covalent interactions, e.g., hydrogen bonding, ionic interactions or ?-? interactions, have been successfully utilized. In contrast, only few examples describe the self-healing of polymeric materials based on reversible metal-ligand-interactions, although this concept can be observed in nature (e.g., in mussel byssus threads). The directed interaction between a metal ion and corresponding ligands offers the possibility to integrate a reversible unit into a polymer matrix. If metal complexes are used as structural units to promote selfhealing, a compromise between the mechanical properties and the self-healing behavior must be found. Typically, the strength of the metal-ligand bond is the key factor for both. However, weaker metal complexes would promote the healing while, at the same time, the mechanical stability is reduced. The analysis of the self-healing capacity and the mechanical properties are central in order to design new functional materials. The terpyridine ligand was chosen as a model system to study the parameters for the resulting polymer properties. For this purpose, terpyridine containing polymers were synthesized and subsequently crosslinked with different metal salts, which offers the possibility to understand the influence of the crosslinking unit, the cation and the counterion. The resulting polymeric materials were investigated in detail (e.g., by scratch testing) to obtain a better insight into the self-healing behavior of the material.