catalysts. The first systems developed by Leibler et al. consisted in a commercial epoxy resin (DGEBA) crosslinked with a mixture of di- and tri-functional fatty acids, with Zn+2 to catalyze the transesterification [21, 93]. They showed that a thermal treatment at moderate temperature (100–150 °C) produces a very good welding between two pieces of the vitrimer. Other catalysts tested by the same authors include triphenylphosphine (PPh3) and triazobicyclodecene (TBD) [94]. TBD showed an excellent catalytic activity, similar to Zn+2. TBD and Zn+2 are the most frequent catalysts used for epoxy-carboxylic acid vitrimers [21, 81, 93, 95–103]. These vitrimers were used to study other relevant aspects of DCBs. For instance, Legrand and Soulié-Ziakovic studied how the dynamic bonds can be harnessed to obtain a better adhesion between the matrix and glass fibers [99]. We addressed the generation of a crosslinked polymer from difunctional precursors, at the expense of leaving a fraction of soluble smaller chains [104], which several authors have done [81, 95–97]. Other catalysts that proved to work for accelerating the transesterification reactions in vitrimers include tertiary amines [105], imidazoles [106] and Sn+2 [107]. Also several uncatalyzed systems demonstrated self-healing ability, though higher temperatures or longer times are needed [108–110]. Figure 1.11 shows some examples of self-healing processes of epoxy-carboxylic acid vitrimers. Epoxy-anhydride networks with Zn+2 as transesterification catalyst have also been studied as vitrimer systems though to a lesser extent [21, 93]. Epoxy-anhydride networks have higher glass transition temperatures, and higher modulus (at room temperature) when compared to epoxy-carboxylic acid ones. However, the – OH concentration, which was proved to be a critical parameter for the transesterification reactions rate [21], is much lower in the former (the epoxy-anhydride reaction yields ester groups, hence another source of hydroxyls is needed).
Figure 1.10 (I) Mechanopatterning of two different samples of thiol-ene based elastomers. (a) Using a mask with concentric rings. (b) Using a mask with horizontal lines in a two step process, rotating the mask 30° between the irradiation steps. Reprinted with permission from Ref. [84]. Copyright (2011) John Wiley & Sons, Inc. (II). (a)–(c) As-prepared, cut and self-healed sample of a TDS crosslinked polymer. (d) Stress–strain curves of samples with different healing times. (e) Dependence of the elongation at break with the healing time. Reprinted with permission from Ref. [92]. Copyright (2012) John Wiley & Sons, Inc.
A wide variety of functional groups such as disulfide [111–114], hydroxyurethanes [78], vinylogous urethanes [115–117], anhydrides [118], dioxaborolanes [119], triazolium salts [120], Schiff bases [121, 122], acylhydrazones [123], among others [124] were successfully used as dynamic crosslinks following the groundbreaking work of Leibler and colleagues, with varying degrees of healing efficiencies. Figure 1.12 shows some examples of vitrimers based on different chemistries.
An important aspect to be considered is the temperature needed to activate the exchange reactions that allows the network to flow. A characteristic temperature Tv can be defined, below which the exchange reactions are frozen, or take place only at rates low enough to be dismissed. Above Tv, the exchange reactions proceed at an appreciable rate, and the material flows, and therefore its usage should be always limited to temperatures below Tv. If Tv is low enough to enable self-healing at room temperature, then it would also be expected that the polymeric network can be easily deformed permanently at the same temperature [125]. A balance between good mechanical properties and dimensional stability at the service temperature on one side, and quick self-healing at reasonable temperatures on the other is crucial for a coating whose purpose is to protect the substrate. This compromise can be well illustrated by some disulfide based vitrimers with Tv values below room temperature [111] that also show acceptable mechanical properties. The authors claimed that the formation of quadruple H bonding between urea groups is the key to achieve such behavior, providing an additional healing mechanism, as well as crosslinks that increase the elastic modulus. Most vitrimers, however, show much higher Tv values, up to above 100 °C [18]. The activation energy of the exchange reactions (Ea) is another important parameter, which offers a measure of the sensitivity of exchange reactions rate to temperature. Typical values range from a few tens of kJ/mol to more than 120 kJ/mol. Both Tv and Ea values strongly depend on the type and amount of catalyst used in the material [94, 117].
Figure 1.11 (I) Transesterification reaction between two β-hydroxyester groups. (II) Zn + 2 catalyzed DGEBA-Pripol1040 vitrimers: (a) lap-shear tests fixture; (b) force displacement curves for vitrimers with different catalyst loads. Reprinted with permission from Ref. [21]. Copyright (2012) American Chemical Society. (III) ESO-CA vitrimers without external catalyst: (a) modified lap-shear test stress–strain; (b) optical microscopy images of the fracture surfaces. Reprinted from Ref. [108] with permission from The Royal Society of Chemistry. (IV) Stress–strain curves for virgin and healed DGEBAcarboxylic acid vitrimer with tertiary amines as transesterification catalyst. Adapted from Ref. [105]; Copyright (2019) with permission from Elsevier.
Figure 1.12 (I) Vitrimer with disulphide exchangeable bonds; cut and healing sequence. Reprinted from ref. [111] with permission from The Royal Society of Chemistry. (II) Vitrimer based on vinylogous urethane dynamic crosslinks; recycling process and mechanical tests. Reprinted with permission from Ref. [115]. Copyright (2015) John Wiley & Sons, Inc. (III) Vitrimer with Schiff base dynamic bonds; optical microscopy images of a cut and healed sample and stress–strain curves for samples of the material after multiple recycling processes. Reprinted from Ref. [122]; Copyright (2016) with permission from Elsevier.
1.4 Remote Activation of Self-Healing
Remote activation of the healing process in polymeric coatings can be performed by adding proper nanostructures to the matrix [126, 127]. Metallic nanostructures such as nanoparticles, nanorods, and nanowires [128, 129], carbon nanotubes (CNTs) [130], graphene [131] and some organic and inorganic compounds [132, 133] are known to absorb energy from electromagnetic radiation of different wavelengths and efficiently transform it into heat. This property makes them excellent candidates to produce the temperature increase needed to trigger the self-healing remotely. The mechanisms underlying the absorption of electromagnetic radiation to generate heat are out of the scope of this chapter, and will not be described here, but there are numerous articles and reviews that cover this issue, including those cited above.
Carbon nanostructures were the first to
