Self-Healing Materials: A Review of Mechanisms and Applications

The deterioration of adhesive performance is often attributed to the formation of internal cracks. Due to the inherent difficulty in timely detection and repair of micro-cracks, the mechanical properties of materials are compromised, leading to a reduction in service life and limitations in their application range. Therefore, the ability to promptly repair cracks, particularly through the realization of self-healing materials, holds significant importance for enhancing the utilization efficiency of materials.

The concept of self-healing originates from the remarkable healing process observed in biological skin damage, which involves the release and subsequent polymerization of repair agents stored within the material or the reformation of reversible interactions within the material itself. Self-healing materials possess the remarkable ability to perceive changes in the external environment and respond appropriately, ultimately restoring their original performance. This unique characteristic positions them as a widely applicable and highly sought-after type of smart material.

Self-healing materials can be broadly categorized into two distinct types: exogenous and intrinsic. Exogenous self-healing involves the strategic incorporation of composite functional substances into the material to facilitate self-healing. Common repair methods employed in this category include the utilization of microcapsules and hollow fibers. The repair mechanism in these methods involves the addition of microcapsules containing monomers to the material. When the material sustains damage, the propagating cracks release the monomers, which then come into contact with the catalyst present in the material matrix. This interaction triggers polymerization, ultimately leading to the repair of the damage. However, this approach has inherent limitations, as the repair agents encapsulated within the microcapsules are finite. Once these agents are depleted, the material loses its self-healing capabilities.

Conversely, intrinsic self-healing involves providing energy to the material to enable covalent or non-covalent interactions within the material itself, facilitating self-healing. In the absence of external repair agents, reversible interactions are introduced into the polymer matrix, bestowing upon it dynamic characteristics and responsiveness to the surrounding environment, thereby achieving self-healing. Compared to exogenous self-healing materials, the most notable advantage of intrinsic self-healing materials lies in their theoretical potential for multiple self-healing events.

Therefore, ESOF-Ix, due to the incorporation of Diels-Alder bonds, is expected to exhibit the characteristics of intrinsic self-healing materials. To validate this hypothesis and assess the self-healing capabilities, ESOF-I1.0 was intentionally subjected to a scratch, creating a surface crack. Subsequently, the material was subjected to heating at 130℃ for varying durations. As depicted in Figure 5, the crack gradually diminished as the heating time increased, demonstrating exceptional self-healing performance. This phenomenon is attributed to the disruption of Diels-Alder bonds at elevated temperatures. This disruption leads to a breakdown of the cross-linked network structure, enhancing the mobility of the chains, allowing them to move between the two interfaces of the scratch. Upon cooling, the Diels-Alder bonds reform, reconstructing the cross-linked network structure and effectively repairing the scratch. However, it is important to note that the relative positions of the chains have changed during this process.