Self-Healing Smart Materials. Группа авторов
tests fixture; (b) force displacement curves for vitrimers with different catalyst loads. (III) ESO-CA vitrimers without external catalyst: (a) modified lap-shear test stress–strain; (b) optical microscopy images of the fracture surfaces. (IV) Stress–strain curves for virgin and healed DGEBAcarboxylic acid vitrimer with tertiary amines as transesterification catalyst."/>
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 be proposed as nanoheaters. Huang et al. used few-layers graphene (FG) to initiate the self-healing process in thermoplastic polyurethane (TPU) [134]. The FG allowed the self-healing to be triggered by three possible methods: IR light irradiation (through the photothermal effect), an electrical current circulating through the material (resistive heating), and the application of an electromagnetic wave, in the range of the microwaves (in this case the FG act as dipoles that absorb the electromagnetic wave and generate heat through dipole distortion). The authors reported that up to 20 successive healing cycles can be obtained by IR light irradiation, with efficiencies over 99%. Recyclable composites with FG and a TPU matrix were also prepared by Fang et al., who also determined that healed and recycled samples have the same conductivity as the virgin undamaged ones [135]. Graphene or graphene oxide were also incorporated to other self-healing systems such as supramolecular elastomers [136], epoxy vitrimers [137], and polyurethanes with DA reversible crosslinks [138, 139]. This permitted the usage of IR light to heat the materials, achieving not only a rapid spatially controlled self-healing, but also a spatial modulation of mechanical properties [136] and an improved mechanical performance [137]. The healing efficiencies measured for these systems were very high, typically reaching values above 90% (Figure 1.13).
CNTs were also applied to achieve indirect heating of self-healing polymeric networks. Yang and coworkers used 0.1 to 0.3 wt.% of multi-walled CNTs (MWCNTs) to trigger the self-healing process in an epoxy matrix (DGEBA with adipic acid and TBD as transesterification catalyst) with infrared light [96]. The polymeric matrix could be efficiently welded in times as short as 30 s for the highest MWCNTs content and 3 min for the lower one, with an irradiation power of 3.8 W/cm2. MWCNTs solar light absorption was also harnessed to develop coatings with self-healing superhydrophobicity, that can be useful to generate steam and produce fresh water [140]. The coatings consist in a mixture of beewax, MWCNTs and polydimethylsiloxane (PDMS), and the superhydrophobicity can be restored thanks to the migration of beewax upon heating. The authors showed that after 20 healing cycles, the contact angle suffered only a minor decrease, from 159.3° to 155.5°. The superhydrophobicity self-healing can be achieved by direct or indirect heating—either through photothermal effect or Joule heating.
Figure 1.13 (I) Healing efficiency of a TPU with different weight fractions of FG triggered by IR light (a), an electric current (b) and an electromagnetic wave (c). Reprinted with permission from ref. [134]. Copyright (2013) John Wiley & Sons, Inc. (II) Stress-strain curves and optical photographs of tensile tests on healed supramolecular elastomers based on polyglycidols with thermally reduced graphene oxide. Reprinted with permission from ref. [136]. Copyright (2017) John Wiley & Sons, Inc. (III) Healing efficiency by direct heating and NIR irradiation on PU-graphene nanocomposites with reversible DA crosslinks. Reprinted from ref. [138]. Copyright (2018) with permission from Elsevier. (IV) Healing efficiency of a PU coating with DA reversible crosslinks with functionalized graphene nanosheets after 1 min of IR light exposure. Reprinted with permission from ref. [139]. Copyright (2019) American Chemical Society.
Metallic nanostructures also absorb energy from electromagnetic waves at specific wavelengths that depend on a number of variables (namely shape and size of the nanostructure, its concentration, the material of the nanostructure and that of the surroundings among others). An interesting advantage over carbon nanostructures such as CNTs and graphene is that very low concentrations of metallic nanostructures are needed. Thus transparent—most times colored—nanocomposites can be obtained, which is a very interesting feature when these materials are considered to be used as coatings. We used different amounts of gold nanoparticles (NPs) embedded in a self-healable matrix to be able to trigger the healing remotely by using a green laser (λ = 532 nm) [141]. The matrix was synthesized from epoxidized soybean oil (ESO) crosslinked with citric acid (CA), and used the β-hydroxyesters generated in the curing reaction as exchangeable bonds [108]. The Au NPs with diameters ranging from 9 to 22 nm and coated with polyvinylpyrrolidone (PVP) were added to the reacting mixture during the synthesis, and the nanocomposites showed absorption peaks centered at around 540 nm with varying intensities. Complete healing was attained when the damaged sample was irradiated with the green laser with a power density of around 1,750 mW/cm2 for 2 h (Figure 1.14-I). An important advantage of the indirect heating through laser irradiation is that when the material is only partially fractured (i.e. there is a ligament binding both sides of the crack) the confined thermal expansion brings the crack surfaces together and contributes to an efficient healing [141, 142]. Zhang et al. used Au NPs embedded in a self-healing matrix of crosslinked poly ethylene oxide (PEO) [142] and in crystalline thermoplastic PEO [143] to trigger the self-healing and the shape