insulation, good lightweight performance, good flexibility and excellent shock absorbent. Zhao et al. [60] developed silicone foam composite with self-healing properties. The concept consists in a material of acquiring different shapes without any fracture. Authors formed hybrid foam with shear stiffening gel (STG) and Methyl vinyl silicone rubber (MVQ) which maintained its shape. Expandable microspheres were used as blowing agent. These microspheres have two structures: a low-boiling volatile hydrocarbon core wrapped in the thermoplastic shell that when heated permits to the liquid inside the core to evaporate. During this process, the thermoplastic shell swells, expanding almost to hundred times. The authors evaluated the self-healing capacity after performing cuts on the samples and then bonding the surfaces together. STG samples were able to stick together, evidencing a very good viscous state. Through tensile tests, a recovery of only 11.1% was determined after healing. In the case of the elongation at break, the recovery was about 34 %. The mechanical properties of the self-healing samples are shown in Figure 3.23 (samples were named considering different MVQ proportions).
Chen et al. [61] synthetized a poly(dimethylsiloxane) (PDMS) elastomer with sacrificial hydrogen bonds and dynamic imine bonds. The self-healing mechanism was characterized through FT-IR, tensile test and dynamic mechanical analysis. The experimental processes and synthesis of the specimens were made in a two-step approach, where segmented PDMS products (named as PDMS-U) was obtained by reacting aminopropyl terminated polydimethylsiloxane (AP-PDMS) and 4,4’-methylenebis-(cyclohexylisocyanate) (HMDI). Then, isophthalaldehyde (IPAL) was added to form imine groups and obtain the self-healing materials with urea and imine groups (PDMS-UI). The samples varies in the molar ratio of urea to imine groups, and the samples are designated as PDMS-UI-x (x = 1–4), being x proportional to the ratio value.
Figure 3.23 Tensile stress–strain curves of self-healed composite foams with different amounts of shear stiffening gel (Adapted with permission from Zhao et al. [60]).
Comparing the self-healing capacity of the samples, there is a decrease in the property with the increment of urea (Figure 3.24). Authors determined that the self-healing capacity depends on the concentration of imine groups.
Sun et al. [62] worked with dielectric elastomers to simulate artificial muscles. Amino terminated polydimethylsiloxane (PDMS-NH2) and a polymethylvinylsiloxane (PMS-g-COOH) were synthetized, in which hydrogen and ionic bonds would act as physical reversible bonds during the crosslinking process and, then, grant self-healing characteristics. The silicone supramolecular network (SiR-SN) was formulated by incorporating different proportions of PDMS-NH2 and PMS-g-COOH. Self-healing properties were evaluated through tensile test on thin elastomer films. Previous to the test, samples were cut in half and, then, clipped together to reattach the surfaces. Then, the specimens were heated 1, 2 and 5 h at 80 and 100 °C. Figure 3.25 shows the mechanical characterization of the samples. Authors stated that, after thermal treatment, all samples showed the ability of self-healing. They attributed different self-healing mechanisms according to the temperature used: at 80 °C leads to the reformation of hydrogen bonds, while at 100 °C there is a conversion of the hydrogen bonds into ionic bonds. In this case, there is a big difference between the hydrogen bond proportions of the composites. A better tensile strength is observed for composites with more hydrogen bonds, because it allows to more links to convert into ionic ones. Finally, an electric field is applied in the samples in order to simulate the final application conditions. In those samples self-healed at 80 °C a mechanical recovery of 100% is obtained when an electric field is applied. Even so, they explain that the samples treated at 100 °C show a greater hardness but without reaching a 100% recovery.
Figure 3.24 Evaluation of self-healing efficiency of PDMS-UI-x composites for different healing times (x varies between 1 and 4, being a value proportional to the molar ratio of urea to imine groups) (Adapted with permission from Chen et al. [62]).
Figure 3.25 Stress–strain curves for a silicone supramolecular with different PMS-g-COOH to PDMS-NH2 ratio: (a) SiR-SN 0.1/1; (b) SiR-SN 0.2/1; (c) SiR-SN 0.5/1 before and after healing under different conditions (Reprinted with permission from Sun et al. [62]).
3.3.6 Polyurethanes
Thermoplastic polyurethane (TPU) is a block copolymer consisting of alternating sequences of hard and soft segments, characterized by a high resilience and resistance to impacts, abrasion and tear. The TPU formulation includes diisocyanates, polyols, and chain extenders. The type and proportion of each of them have a significant influence on the final properties of the material. This system exhibited self-healing ability if dynamic disulfide bonds [63–65] and hydrogen bonding [66, 67] were introduced in its structure.
Based on these characteristics, several self-healing TPU elastomers have been designed by introducing dynamic covalent bonds and non-covalent interactions into the chains. Mainly, the most used mechanisms for self-healing TPU are based on dynamic disulfide bonds [68–70] and hydrogen bonding [71, 72].
Rekondo et al. [73] stated that the dynamic chemistry of disulfide bonds plays a vital role in self-healing processes. Metathesis of aromatic disulfides at room temperature is possible through the use of a tertiary amine as the catalyst. In this case bis(4-aminophenyl) disulfide was used, which is a diamine molecule characterized by an aromatic disulfide (Figure 3.26) as a dynamic crosslinking agent in poly(urea–urethane) systems. Author analyzed the use of equimolar amounts of (bis(4-aminophenyl) disulfide (1a) and bis(4-methoxyphenyl) disulfide (2) in deuterated DMSO (Figure 3.26). It was tested with 0.1 equivalents of tertiary amine (NEt3), and the equilibrium was achieved in few minutes. If the reaction is tested without NEt3, metathesis initiated in few minutes and the equilibrium is achieved in 22 h (Figure 3.26). With the aim to test that the reaction is catalyzed by primary aromatic amines present in 1a, the tests were carried out taking into account 1b and 2, without NEt3. The system reaches equilibrium in 24 h if the same amount of moles of 1b and 2 are used, confirming that a catalyst is not necessary for the exchange reaction to occur.
Figure 3.26 Reversible metathesis reaction of aromatic disulfides 1a–b and 2 (Adapted with permission from Rekondo et al. [68]).
Nevejans et al. [74] proposed the self-healing concept in coatings based on waterborne poly(urethane-urea) containing aromatic disulfide dynamic bonds. Authors expressed that self-healing coating material should present two opposite properties: i) high strength and ii) molecular mobility. The influence of the flexibility of the two aromatic disulfides and its concentration on self-healing efficiency was explored. The two aromatic disulfides are shown in Figure 3.27 being S2(Ph(CH2)3OH)2 designated as S3 and the more flexible alternative S2(Ph(CH2)6OH)2 is named S6.
The mobility of the different compounds was characterized through the relaxation time (in dynamic experiments) and the self-healing ability through the time needed
