Self-Healing Smart Materials. Группа авторов
materials can be defined as a smart material that responds to stimulus to extend its service life by restoring mechanical integrity after damage. The capacity for self-healing is usually observed in animals and plants. Any industrial component with this capacity would be beneficiated in the presence of defects in service, such as micro cracks [2, 3]. The goal of developing self-healable materials is to extend the long-term durability by effectively removing any local damage that occurs during its service life [4–8]. Another advantage of self-healable rubber is that can prevent catastrophic failure [9].
Self-healable materials can be classified as autonomous (do not need a stimulus to conduct the self-healing process), or non-autonomous, in which a stimulus is required to trigger the self-healing mechanism. Some of the most common stimuli are light, mechanical, chemical or heat (Figure 3.1).
Other possible classifications of self-healing materials are as extrinsic and intrinsic types, according to the healing mechanism [10, 11]. The first one is characterized by the storage of the healing agents in nanotubes, microcapsules, or other micro-containers, which can be released after the matrix suffers damage [12, 13]. Intrinsic type refers to those polymers containing covalent or non-covalent either reversible bonds, which activate by external stimulation after damage. Dynamic covalent bonds can be disulfide bonding [14] and reversible Diels–Alder reaction (DA) [15], whereas dynamic non-covalent bonds can be metal–ligand coordination [16, 17], hydrogen bonds [18, 19], hydrophobic and π–π interactions [20–22]. When the self-healing mechanism is due to dynamic bonds, the mechanical strengths are weaker than those systems with permanent covalent bonds. To overcome this drawback, some authors proposed the incorporation of crystalline domains within the polymer matrix to increase the toughness of the system [23–26].
Figure 3.1 Classification scheme of self-healing systems.
Covalent bonds are also capable to self-cure partially when the compound is formulated in a way to obtain high content of di- and polysulfides bridges and a low crosslinks density that permits the rearrangement of the bonds that had been broken at the healed interface. Sometimes, this kind of healing mechanism is preferred over hydrogen bonds, because the last one breaks easily when a mechanical effort is applied [2].
When thermal activation for self-healing is used, systems based on DA reaction are the most common. This reaction consists in a [4 + 2] cycloaddition between an electron poor dienophile and an electron rich diene, producing a cyclohexene adduct which could be reverted via retro-DA reaction (rDA) using high temperature conditions (see Figure 3.2). This reaction presents fast kinetics and soft reaction conditions [27].
Amide is another important bond type for self-healing applications, but it requires extreme conditions to be reversed. A reversion method includes the use of bulky substituents that promotes the breakage of the amine bond.
Wool et al. [28] analyzed the mechanism of intrinsic self-healing polymers and summarized that the process takes five phases: (i) surface rearrangement, (ii) surface approach, (iii) wetting, (iv) diffusion and (v) randomization during which the longitudinal chain diffusion is responsible for crack healing.
3.2 Self-Healing in Elastomers
Figure 3.2 Schematic representation of thermally reversible Diels–Alder (DA) reaction of a diene with a dienophile.
3.2.1 Self-Healing Mechanism
There are four mechanisms to trigger the self-healable process in materials: heat induced, light induced, mechanochemical and encapsulation (Figure 3.3). Mechanochemical self-healing and encapsulation do not need a stimulus and the sample can be healed in a variety of conditions. The non-autonomous heat and light triggered self-healing systems have been more developed [2]. Each mechanism is described briefly.
3.2.1.1 Heat Stimulated Self-Healing
Heat induced self-healing can be performed in reversible or dynamic chemical bonds, such as DA reaction, radical based systems or supramolecular interactions: ionic interactions, halogen or hydrogen bonds, metal–ligand interactions, π–π interactions or host-guest interactions [29].
The DA reaction is the most commonly used mechanism when it comes to thermo-reversible self-healing based on covalent bonds [30, 31]. Authors describe the healing process in four steps: a) the material is heated up to break the DA bonds, b) part of thermal energy becomes in kinetic energy that increases the mobility of chains, promoting the filling of the voids; c) cool the sample, allowing the formation of DA bonds; and d) the mechanical properties of the material are recovered.
3.2.1.2 Light Stimulated Self-Healing
There are different methods to promote the self-healing process induced by light. The first one consists of using a photo crosslinking reaction between molecular parts that are photo reactive. Another method uses rebuild that links up polymer chains on both side of the cracks. Covalent bonds like disulfides, allyl sulfides or trithiocarbonates are susceptible to this stimulus. The mechanism consists of breaking these covalent bonds and make reactive radicals which are combined to form new bonds.
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