a self-healing composite based on the dispersion of microcapsules with a healing agent in a polymeric matrix. The adhesion between the capsule and the matrix plays a very important role, since it is directly related with the load transfer to the microcapsule, and to its ability to release the healing agent [22, 49]. Another disadvantage is that once the healing agent has been consumed in one or multiple repairing events, the material loses its self-healing feature. This last disadvantage is one of the most important differences in comparison with intrinsic self-healing systems, as we will show in the next section.
Figure 1.7 SEM images of the scratch of (a) BS-55, (b) CS-55, (c) SH-55, (d) BS-60, (e) CS-60, and (f) SH-60 after 12 h of UV irradiation (wavelength: 310 nm; power: 582 W/m2). Reprinted with permission from Ref. [49]. Copyright (2019) American Chemical Society.
1.3 Intrinsic Self-Healing Polymer Coatings
The expression intrinsic self-healing polymers designates a wide variety of systems with autonomous or induced healing ability. A large portion of them are polymeric networks with either reversible or dynamic bonds, and are nowadays the most relevant kind of intrinsic self-healing thermosetting polymers with potential use as coatings. The use of reversible H-bonds to prepare self-healing supramolecular polymers received a great impulse following the first report of the synthesis and use of 2-ureido-4[1H]-pyrimidinone (UPy) for that end [52, 53]. UPy is a functional group that associates through a quadruple H bonding, displaying an unprecedented high association constant (Figure 1.8-I). It can be introduced in the main chain or in pendant ones of linear or branched polymers with different chemistries, and since its development by Sijbesma et al. [52] it has been used in multiple polymers with different applications, including self-healing coatings [54–56]. This versatility, that allows the introduction of UPy groups in a number of polymeric backbones, is one of its most important advantages. The high association constant provides good mechanical strength at low temperatures, but at high temperatures the constant drops dramatically, allowing the mending of the material, as shown in Figure 1.8-II. Leibler et al. developed self-healing supramolecular rubbers by using different multiple hydrogen bonding groups [57–59]. They used a mixture of fatty diand trifunctional carboxylic acids condensed with diethylene triamine and afterwards reacted with urea to yield oligomers bearing H bonding groups with different functionalities that performed the same function as UPy, achieving self-healing even at room temperature [60].
Some researchers raised some concerns that some supramolecular rubbers may not display an adequate solvent resistance to be used as coatings, and overcame this drawback by mixing supramolecular with covalent bonds [61–65]. While the introduction of covalent bonds makes a more resistant and mechanically sound material, its backside is a decrease of the self-healing capacity, needing more time and/or higher temperatures to repair a damage in the materials.
Figure 1.8 (I) 2-ureido-4[1H]-pyrimidinone (UPy), and a scheme showing its association-dissociation process. (II) Self-healing of a UPy-based coating at 80 °C for 2 min: (a) and (b) optical microscope images of the coating before and after healing, and (c) SEM image of the healed coating. Reproduced with permission from Ref. [55]; Copyright (2018) John Wiley & Sons, Inc.
Networks built entirely from covalent bonds can also display self-healing ability, if some of the covalent bonds are of reversible nature. The use of reversible crosslinks based on the Diels–Alder (DA) chemistry (Figure 1.9-I) dates back to 1979 [66, 67]. Nonetheless, the use of DA crosslinks to produce self-healing crosslinked polymers was not explored until several years later [17, 68]. Chen and coworkers used multifunctional dienes and dienophiles to produce a reversible network that underwent depolymerization when heated to 120–150 °C [69]. Researchers from the same group improved some aspects of their materials by using a different maleimide to avoid the use of solvents and obtain colorless polymers, more suitable to use as coatings [70]. They identified the DA adducts as the responsible for the crack propagation, due to the relatively low amount of energy needed to break them [69, 70]. Therefore, it can be expected a very good degree of healing once the same covalent bonds are restored through a convenient thermal treatment. The temperature requirement to produce the retro-DA can be tuned by selecting the diene–dienophile pair, covering a wide temperature range from room temperature or mild heating [71, 72] to above 250 °C [73, 74]. Figure 1.9-II shows an example of DA based self-healing material. One of the weaknesses of the DA-based intrinsic self-healing thermosetting polymers is that during the healing process the material loses its crosslinked structure. An alternative to overcome this drawback is the use of the dynamic equilibrium inherent to the DA adducts. Using this approach, Zhang and colleagues demonstrated plastic deformation at low temperatures in a DA-based polymer, without complete de-polymerization, but they did not offer self-healing proofs [75].
Figure 1.9 (I) Diels Alder reversible reaction. (II) (a) Synthesis of the tetrafunctional monomer (1); (b) formation of the DA-based network (3); (c)–(e) healing/recycling process. Reprinted with permission from Ref. [72]. Copyright (2014) John Wiley & Sons, Inc.
Dynamic covalent bonds (DCBs), in contrast to reversible bonds, can be used to produce self-healing intrinsic polymers that maintain their cross-linked structure even at high temperatures during the healing process [19, 76, 77]. The exchange reactions between dynamic bonds need to be activated through an external stimulus. The most usual one is heat, but some dynamic bonds respond to light, and in a lesser extent to other stimuli such as mechanical stress [78–80] or solvents [81, 82]. Light responsive DCBs based on the sulfur chemistry were extensively studied by C. Bowman’s group [83–89]. The healing mechanism of these materials relies on bond exchanges through addition-fragmentation chain transfer. These polymers are capable not only of self-healing, but also of undergoing selective plastic deformation through the use of appropriate masks. This can be used to create 2D patterns on the material, as shown in Figure 1.10-I. Matyjaszewski and co-workers also used light-activated reactions to produce self-healing polymers. Their polymeric networks have either trithiocarbonate (TTC) [90, 91] or thiuram disulfide (TDS) [92] moieties acting as the dynamic crosslinks. Mechanical tests performed on the latter showed that around 90% of the mechanical strength could be recovered by visible light irradiation (Figure 11.0-II). Some of the advantages of the use of light to trigger the self-healing are highlighted by the authors: the healing can be performed at room temperature, preserving the substrate (especially useful when it cannot withstand higher temperatures); no solvents are needed; the air atmosphere does not interfere with the healing process nor it produces any degradation by oxidation, since the temperature remains low.
Thermally activated DCBs can also be harnessed to obtain self-healing polymeric networks. Hydroxyesters, obtained from the reaction of an epoxide with a carboxylic acid and capable of undergo transesterification reactions, were first studied by Leibler and colleagues, who coined the term “vitrimers” to name these self-healing networks [21, 93, 94]. Though many different chemical groups have also been proposed as alternatives [18, 19], vitrimers based on the transesterification reaction remain the most widely studied ones. A variety of epoxy and poly-carboxylic acid precursors
