Self-healing polymers can be defined as synthetically or artificially created substances that have an internal or self-capacity to repair damages in them without necessarily using human or any external measure for diagnosis. The cost may be brought by fatigue or injury during their course of work, or also due to various environmental conditions. In most cases, the loss so caused negatively impacts the performance of a polymer. The use of self-healing polymers has become widely accepted due to their self-healing capacities, unlike in other polymers or substances that do not carry this feature and therefore require time to time checkups to detect cracks or other damages to maintain an optimal level of functionality in such chemicals.
Due to the self-healing nature of polymers, they have become an integral substance in products such as fibers, films, paints, and rubber. Over time, it has continuously been improved to boost its reliability and period.
Based on the polymer healing mechanism, self-healing polymers are divided into two main groups; the extrinsic and intrinsic polymers and further classified in three broad ways into an either vascular, capsule-based, or intrinsic polymer. In most polymers, however, the self-healing mechanism is a three-step process that some sciences equate to the natural or biological response. Immediately after damage, the first action will be actuation or triggering, which is then followed by an immediate transfer of materials to the damaged part and then finally a chemical repair process to heal the polymer. However, most polymers have this three-step process as their healing mechanism; there are three healing mechanisms applicable to specific polymers. These are; entanglement, polymerization, and reversible cross-linking.
Self-healing polymers yield stress through cleavage bonds. From the molecular study of these polymers, most of the traditional polymers, unlike the relatively new polymers, generate mechanical stress through heterolytic or homolytic bond cleavage. Despite this fact, numerous factors influence how any polymer will yield weight, and these include temperatures, type of pressure, type and level of solvation, and chemical properties specific to a particular polymer. Further molecular studies reveal that stress-induced damages lead to more loss on a polymer and may damage the surrounding polymers, thereby weakening the rubber or fiber as a whole. The resultant large scale injury as a result of the street is called microcrack.
Polymers that yield mechanical stress through the homolytic bond cleavage mainly use two types of’ radical reporters, the PMNB (pentamethylnitrosobenzene) and DPPH (2, 2 - diphenyl-1-picrylhydrazyl). In these bond cleavage, two radical species are born and recombine to heal the stress or damage. However, other homolytic divisions could be started, thereby causing more damage to the polymer.
Through numerous isotope labeling experiments, scientists have observed that some polymers cleavage heterolytically. In this bond, two forms of species, anionic and cationic, are formed, and it is these species that combine to healing stress or damage.
Some polymers such as the Diels-Alder based polymer do not undergo either homolytic or heterolytic bond cleavage. They instead undergo a reversible bond cleavage where mechanical damage cleaves two stigma bonds. By so doing, the stress leads to some more pi-bonded electrons instead of the normal charged or radical moieties.
Unlike other molecular substances, supramolecular polymers are made up of monomers that are interacting non-covalently. Their interactions are through the van der Waals forces, metal coordination, and hydrogen bond. When subjected to mechanical stress, the supramolecular polymers forces of interaction, such as the van der Waals forces, will be disrupted, which in turn leads to polymer breakdown or monomer separation.
Unlike the case with natural polymers, extrinsic polymers are healed through vascular or microcapsule network. In these networks, polymer healing in case of molecular stress or damage occurs due to the release of vascular or microcapsule content into the damaged point, causing a reaction that repairs the polymer and restores its standard functionality.
Though extrinsic polymers are healed through vascular and microcapsule networks, each network functions differently. In a macrocapsule network, the healing agent is sequestered in small capsules that will release a healing agent only when ruptured. On the other hand, the vascular network, the healing agent, is sequestered in hollow channels, which are interconnected in one, two, or three dimensions.