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 memory function with a green laser (λ = 532 nm; up to 7,500 mW/cm2). The addition of gold microparticles (~1,300 nm) was demonstrated as another efficient method to use visible light (λ = 808 nm) to produce the indirect heating and activate the self-healing process in a DGEBA-sebacic acid-TBD crosslinked matrix [144]. The authors observed that photothermal heating is far more efficient than direct heating to achieve a good healing (Figure 1.14-II). Similar findings were made in other self-healable polymers, including those with CNTs [96], showing one of the advantages of the indirect heating through photothermal effect.
Figure 1.14 (I). (a)–(b) Optical microscopy images showing a healed sample, and stress–strain curves for the virgin and healed samples. (II) (a)-(b) Optical microscopy images of a crack healed through direct heating and by light irradiation, and the corresponding stress–strain curves for a DGEBA–sebacic acid–TBD vitrimer with gold microparticles. Reproduced with permission from Ref. [144].
Anisotropic 2- and 3-dimensional nanostructures have also been found useful to induce the needed indirect heating. These nanostructures usually display more than one surface plasmon resonance frequency, thus being capable of act as heaters when illuminated with light sources of several wavelengths [128]. Silver nanowires (NWs) were introduced within polymeric films based on polycaprolactone (PCL) and poly(vinyl alcohol) (PVA) [145]. Illumination with infrared light produced the indirect heating needed to heal the damaged film, and led to a complete recovery of its conductivity, which is also enabled by the presence of the Ag NWs. Figure 1.15 shows the experiment demonstrating the self-healing. Chen et al. used a combination of GO, Au NPs and Ag NWs to induce the self-healing process in a PCL based conductive membrane through irradiation with a green laser (λ = 808 nm) [146]. In this case, the healing was achieved thanks to the interdifussion of the electrospun PCL fibers, allowed by the heating produced by the photothermal effect, and reaching high efficiency levels (91.3% of its conductivity and 90.7% of its tensile strength were recovered after one cut-healing cycle).
Figure
