3.2.1.3 Mechanochemical Self-Healing
This mechanism is particularly interesting in poly-urea and polyurethane urea based systems, in which the amine bonds is present [32]. The reversion of the amine bond can form ketene, which is highly reactive for the reversible interaction. Therefore, the isocyanate can be used as a reactant with an amine group to form a urea bond. This type of bonds is stronger compared to hydrogen bonding.
Rekondo et al. [33] developed a self-healing poly (urea urethane) that contains both amino and quadruple hydrogen bonds. A sample with those characteristics can be completely separated and bonded by physical contact, exhibiting a rapid self-healing ability at room temperature due to the combination of both bonds type.
3.2.1.4 Encapsulation
There are different strategies to contain the healing agents inside the material such as hollow fibers or a microvascular network filled with a crosslinking agent, microspheres or microcapsules of crosslinking agent and superparamagnetic nanoparticles, as is shown in Figure 3.3 [34]. The mechanism to heal the sample is similar in cases of hollow fiber, microspheres or microcapsules and microvascular network: once the crack propagates, the container is broken and releases the crosslinking agents; they fill the crack and then repair the structure. In the case of super paramagnetic nanoparticles dispersed in the matrix, the self-healing mechanism is activated through the application of an external magnetic field that induces a vibration in the particles. Therefore, the temperature increases and facilitates the self-healing.
Figure 3.3 Self-healing illustration of common encapsulation methods, (a) hollow fibers inside a polymer, (b) Microspheres/microcapsules, (c) microvascular network, and (d) superparamagnetic nanoparticles (Reprinted from Yang et al. [34], open access).
3.2.2 Characterization of Healing Process
The healing efficiency can be calculated as the ratio between the tensile strength results for the healed and original sample according to Equation (3.1) [35–38]:
This relation also works for others mechanical properties such as failure stress, failure strain, energy at break or J-integral at crack initiation [36].
Raman Spectroscopy can be used following the evolution of the three main spectral bands: and C–C stretching (υ C–C = 1,590 cm−1), C−S stretching (υ C–S = 650 cm−1), S−S stretching (υ S−S = 500 cm−1) [35], which are related to the chemical structure of the crosslinked material.
In addition, there are optical methods to characterize the healed sample. Optical microscope and digital cameras are used to analyze the healing process [39] and also scanning electron microscopy (SEM) to study the healing of the scratches [40] as can be seen in Figure 3.4. The figure shows two samples of a compound, which were cut and submitted at different conditions to recover the mechanical properties: room temperature (that take 17 h) and 60 °C (that take 7 h).
Figure 3.4 SEM images to trace the healing process of the scratch. (Reused with permission and modified after Peng et al. [40]).
3.3 Particular Cases in Different Elastomers
3.3.1 Natural Rubber (NR)
Natural Rubber (NR) is obtained from rubber plants through coagulation process being the most used type the Hevea brasiliensis. NR is chemically composed by cis-1,4-polyisoprene which presents exceptional properties like superior tear resistance, high resilience and fatigue resistance.
Some authors have studied the reprocessability of epoxidized natural rubber (ENR), adding different fillers. Cao et al. reported the preparation and characterization of ENR modified by a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized cellulose nanocrystals (TOCNs), generating free carboxyl groups, which serve as reinforcing fillers and also as cross-linking agents via epoxy-acid reaction [41]. The linkages between the rubber chains and the reinforcement particles consisted in β-hydroxyl ester, as can be seen in Figure 3.5. The interfacial ester bonds were formed between epoxy groups and carboxyl groups and that was probed by FTIR results.
Figure 3.5 Illustration of the TOCNs cross-linked ENR network based on epoxy-acid reaction, using zinc acetate (Zn(OAc)2) and 1,2-dimethylimidazole (DMI) as additives (Reprinted with permission from Cao et al. [41]).
The covalent rubber network presented high fracture strain (>600%) and tensile stress (>5 MPa). Furthermore, the network with exchangeable β-hydroxyl ester bonds at rubber–TOCNs interface can change the network topology via trans-esterification reactions. The samples achieved up to 80% self-healing efficiency.
Xu et al. developed recyclable and self-healable ENR/citric acid-modified bentonite (CABt) composites (Figure 3.6) [42]. CABt presents numerous carboxyl groups on surface, which react with ENR through exchangeable β-hydroxyl ester linkages, as can be seen in Figure 3.7. Meanwhile, the inherent stickiness of ENR matrix and the low crosslinked network facilitate transesterification reactions of β-hydroxyl ester linkages and chains diffusion, which make ENR/CABt composites recyclable and healable.
Some studies have focused on the DA reactions because of its reversible crosslinking property. Trovatti et al. [43] report the furan-modified NR reversible crosslinking. A comparison between the furan-modified NR 1H-NMR spectra and its retro-DA de-crosslinked rubber gives evidence of the reversibility of the reaction. Tanasi et al. [27] studied the use of a DA reaction to obtain a NR reversibly crosslinked and with self-repair capacity. NR was crosslinked via DA reactions following the procedure shown in Figure 3.8: i) NR reacted with maleic anhydride to produce NR-g-MA; ii) furan moieties were grafted to the rubber generating NR-g-furan; iii) pending furans were crosslinked with a bismaleimide, obtaining thermoreversible bridges which crosslinked NR matrix (NR-DA). Authors explained through phenomenological model that the reversibility of the crosslinks could be achieved by dynamic mechanical analysis complemented with chemical analysis. The results show that the rubber compound present mechanical properties and a crosslinks density comparable whit those of a vulcanized NR with low sulfur content, that represent a healing efficiency greater than 80% at low strain.
Figure 3.6 Schematic diagram of chemical reaction between ENR and CABt (Adapted with permission from Xu et al. [42]).
