1 Introduction
Silicone materials mainly refer to polysiloxanes with Si — O — Si bonds as the main chain and organic groups connected to Si atoms, which are the fastest growing elemental organic polymer materials. Because of the hybrid structure of "inorganic structure" and "organic group", and because the bond length of Si — O — Si bond is longer than that of common C — C bond and the energy of Si — O — Si bond is higher, polysiloxane has incomparable comprehensive properties compared with most organic polymer materials, such as electrical insulation, high and low temperature resistance, aging resistance, radiation resistance, weather resistance, low surface energy, hydrophobicity, gas permeability, physiological inertness and so on
[1⇓⇓~4]. Silicone materials have been widely used in transportation, health care, aerospace, electronics and electrical, light industry, textile, machinery manufacturing, construction, daily necessities and other fields, and have become a unique and important system in the new material industry
[5⇓⇓⇓⇓~10]. However, due to the low mechanical strength of silicone materials, they are vulnerable to external stress in the process of processing, transportation and use, resulting in the loss of material function and the shortening of life. In addition, traditional polysiloxane is crosslinked into silicone resin and silicone rubber by silanol polycondensation, hydrosilylation and other ways. Because of the irreversibility of these crosslinked networks, it is difficult to recycle once it is used and discarded, resulting in serious waste of resources and environmental pollution. Therefore, the design and development of environmentally friendly, high-performance and multifunctional silicone materials will effectively expand their wide applications in the fields of smart coatings and electronic devices
[11].
Inspired by the DNA repair of living organisms at the molecular level and the healing process of skin and bone at the macroscopic level in nature, scientists have designed and developed biomimetic self-healing or self-repairing materials. The material can refill and bond cracks through a certain action mechanism, thereby realizing self-repair of damage, effectively prolonging the service life of the material, improving the safety of the material and reducing waste, and has extremely important scientific significance and application value
[12⇓⇓~15]. After more than 20 years of development, dozens of self-repairing materials and repair systems have been reported, but according to the different repair mechanisms, self-repairing materials are mainly divided into two categories: foreign aid type and intrinsic type
[16⇓~18]. The external-aid self-repairing material is a material in which microcapsules, hollow microspheres, and microtubule networks loaded with repairing agents are embedded in a substrate in advance. When a certain area of the material is damaged, the microcapsules or hollow microtubules near the area will release the repairing agents due to structural damage, and the repairing agents will solidify to repair the damaged area (Figure 1A)
[19]. This repair mechanism has a wide range of applications and excellent repair effect, but it has some shortcomings, such as the performance of the matrix material will be affected by the buried plant, and the area where the repair agent is exhausted no longer has the ability of self-repair. On the other hand, the intrinsic self-healing material is a dynamic crosslinking network formed by crosslinking the molecular chains of the matrix material through dynamic covalent bonds or supramolecular interactions, and under the action of appropriate external stimuli,The dynamic cross-linked network can reversibly exchange or dissociate and recombine to achieve self-repair of the material (Figure 1b), which can be repeated indefinitely in theory
[20⇓~22]. In addition, due to the reversible "dissociation" and "recombination" characteristics of the dynamic crosslinking network, the intrinsic self-healing materials are also endowed with efficient solid phase regeneration ability, which solves the problems of difficult recycling of traditional thermosetting polymer materials and heavy pollution of recycling process. Through the rational design of dynamic crosslinking network, materials can also be endowed with new other functions to meet the needs of specific fields
[23⇓~25].
Self-repairing silicone materials have become the research focus of self-repairing materials because they have both the self-repairing function of self-repairing damage and the excellent comprehensive properties of silicone materials. However, whether it is external self-repair or intrinsic self-repair, it often needs to be stimulated by external stimuli such as ultraviolet light, temperature, solvent, etc. External stimuli are largely the driving force for materials to achieve damage repair, affecting the effectiveness of material damage repair. In addition, different stimulus factors have their own advantages and disadvantages, and the application fields are different. For example, the temperature of the system is controlled by heating, which is easy to operate and implement, but the controllability is poor and the energy consumption is high. Ultraviolet irradiation has the advantages of low energy consumption and good space-time controllability, but it usually only acts on the surface of the coating or sample. Therefore, based on the different external stimuli in the repair process of self-repairing silicone materials, this paper will start from the two aspects of external and intrinsic self-repairing silicone materials, but focus on the intrinsic self-healing silicone materials, and introduce its research progress, especially the frontier research results in the past five years. Finally, the future research direction of self-repairing silicone materials was simply analyzed, in order to lay a theoretical foundation for the further research and wide application of self-repairing silicone materials in the future.
2 External-aid self-repairing silicone material
The external-aid self-repairing silicone material is mainly prepared by embedding microcapsules, microspheres, and the like filled with a repairing agent into a polysiloxane substrate in advance, and when the substrate material is damaged and cracks are generated, the repairing agent embedded in the substrate resin in advance is released to fill and bond the damage through a chemical reaction
[26]. According to the different chemical reaction mechanism between the repairing agent and the matrix material, the external self-repairing silicone materials can be divided into hydrosilylation, hydrolytic condensation curing and photoinitiated crosslinking
[11].
2.1 Hydrolysis-condensation type self-repairing organosilicon material
Chen et al. Used polystyrene microcapsules to encapsulate fluoroalkylsilane FAS13, and used FAS13 to modify nano SiO
2 to obtain fluorinated nano SiO
2(FMS), and then mixed photocatalyst nano TiO
2(P25), microcapsules and FMS with polysiloxane emulsion to prepare microcapsule-type self-repairing waterborne hydrophobic coating
[26]. When the surface of the coating is damaged, FAS13 is released by the degradation of polystyrene microcapsules catalyzed by P25 under ultraviolet irradiation, and FAS13 spontaneously migrates to the damaged surface to repair the damaged area through the hydrolytic condensation reaction of siloxane groups. The results showed that after 40 H of accelerated wear test, the damaged surface of the coating could be covered by FAS13 again, and its superhydrophobicity was restored. After six wear accelerated weathering cycles, the prepared coating still has self-repairing superhydrophobicity. On this basis, Anahita et al. Developed a double-microcapsule self-repairing silicone material, using silicone hydroxyl-terminated polydimethylsiloxane (PDMS-OH) as the repairing agent and dibutyltin dilaurate (DBTL) as the catalyst, and encapsulating the repairing agent and the catalyst in melamine-urea-formaldehyde (MUF) microcapsules respectively by in-situ emulsion polymerization technology (Figure 2a) to obtain PDMS-MUF and DBTL-MUF microcapsules
[27]. When the particle size of PDMS-MUF and DBTL-MUF microcapsules was 56 and 42 μm, respectively, and the shell thickness was 100 ~ 200 nm, the composite structure was uniform and compact, and the microcapsules had good processability. When microcracks are formed, the microcapsules are easily broken to release the repair agent, and the repair efficiency is 55% ~ 67% after 24 H of repair at room temperature (Figure 2b). The repair agent with PDMS as the silicone substrate can ensure the uniformity of the structure and maintain its surface characteristics.
2.2 Silicon-hydrogen addition self-repairing organosilicon material
Hydrosilylation reaction has the characteristics of high efficiency, easy control and mild reaction. Sottos et al. Reported a double-microencapsulated self-healing silicone elastomer based on hydrosilylation reaction
[28]. Two kinds of urea-formaldehyde resin microcapsules were embedded in PDMS matrix in advance, one was coated with platinum catalyst and vinyl-terminated polydimethylsiloxane, and the other was coated with hydrogen-containing silicone oil. When the elastomer was damaged and the microcapsules were broken, the repair agent and catalyst released from the two microcapsules were mixed and hydrosilylation occurred under the siphon effect, and the repair efficiency was higher than 75% after 48 H at room temperature. The repaired PDMS elastomer can not only recover 70% ~ 100% of the original tear strength, but also improve the tear resistance of the material with the addition of microcapsules.
Guo et al. Prepared microcapsules with two different core materials by in-situ polymerization using polyurea formaldehyde (PUF) as the shell material. Microcapsule I was loaded with Sylgard 184 Part A components, namely vinyl-terminated polydimethylsiloxane and platinum (Pt) -based catalyst, and microcapsule II was loaded with hydrogen-containing silicone oil as the crosslinking agent (Figure 3A)
[29]. The two are blended with the PDMS substrate in an appropriate proportion to construct a self-repairing silicone rubber. When cracks appear in the material, the microcapsules break and release the repairing agent, and Sylgard 184 Part A reacts with the hydrogen-containing silicone oil to form a cured product to fill and repair the cracks (Fig. 3B). When the weight ratio of microcapsule II to microcapsule I was 0. 6 and the total content was 10 wt%, the maximum self-healing efficiency of PDMS composite at room temperature could reach 70%, and the repair effect was also significantly improved by prolonging the repair time.
2.3 Photocrosslinking self-repairing organosilicon material
Photocurable self-repair mainly refers to the introduction of photosensitive groups with photoreactivity into the silicone substrate in advance. When the material is damaged, the photocurable groups can be reacted by ultraviolet irradiation to achieve repair. Compared with hydrolytic polycondensation reaction and hydrosilylation reaction, the photocrosslinking reaction has the advantages of high speed and mild conditions. Chung et al. First synthesized urea-formaldehyde resin microcapsules encapsulated with methacryloxy-terminated polydimethylsiloxane (MAT-PDMS) and photoinitiator benzoin isobutyl ether (BIE), and then mixed the microcapsules into poly (dimethylsiloxane-co-methylphenylsiloxane) substrate to prepare a photocurable self-healing silicone coating
[30]. When the coating is damaged and the microcapsule is broken, MAT-PDMS and BIE will be released. Under the irradiation of ultraviolet light or sunlight, the free radicals produced by BIE can initiate the photocrosslinking of MAT-PDMS to repair the damaged area. The results of water permeability test and chloride ion penetration test show that when the coating is used as mortar protective layer, the repaired area can still effectively protect the mortar from water, chloride ion and other deterioration factors after the microcracks of the coating are repaired by sunlight for 4 H.
In order to realize the deep curing reaction of photocrosslinked coatings, Zheng et al. Designed a UV/hydrolytic polycondensation double crosslinking self-repair system
[31]. A polysiloxane containing both acryloyloxy (CH
2=C(CH
3)COO—) and silanomethoxy (—SiOCH
3) was first synthesized as a repair agent, and then encapsulated in microcapsules with chitosan reduced graphene oxide (CS-RGO) as the shell, and finally the microcapsules were embedded in epoxy-modified silicone resin to form a double-crosslinked self-repair system. When the repair agent flows out, the acryloyloxy group is rapidly cross-linked and cured under ultraviolet irradiation, while the silicon methoxy group can undergo hydrolysis-condensation reaction in air to form Si-O-Si bonds to achieve deep cross-linking and curing of the damage (Fig. 4). In addition, the silicon methoxy group can also have a condensation reaction with the hydroxyl group on the surface of the resin matrix, so that the polysiloxane and the matrix have a chemical bonding effect, thereby increasing the bonding force with the matrix. Under the synergistic effect of ultraviolet light and moisture (ultraviolet light irradiation for 3 min, placed at room temperature for 24 H), the efficient self-healing of cracks was achieved, and the repair efficiency of the sample was higher than 92%. The self-repairing coating also has excellent mechanical properties and anti-corrosion properties, and has great application prospects in the protection of marine facilities such as ships, bridges, pipelines and the like.
In addition, Wang et al. Used sodium alginate-calcium chloride as the wall material and tung oil as the core material (TO @ CA), which was dispersed in the silicone rubber coating TO prolong the service life and related properties of the silicone rubber anticorrosive coating
[32]. At the same time, the TO @ CA/SR composite coating was reinforced and modified by filling SiO
2 nanoparticles TO achieve self-healing of microcracks without affecting the mechanical properties. The results showed that the self-repair efficiency could reach 87. 43% when the average particle size of the microcapsules was 64 μm, the dosage was 10 wt%, and the repair time was 12 H at room temperature.However, the reaction rate was slow because the active oxygen in the air was used to promote the double bond crosslinking in tung oil. This study is of great significance for slowing down the corrosion rate of marine equipment, prolonging its service life and improving its safety.
In recent years, the hot direction of foreign aid self-healing silicone materials is mainly the design and development of new microencapsulation technology. The design of a suitable microencapsulated self-repairing system should not only consider the ease of encapsulation and the environmental stability of the material, but also ensure the reactivity of the repairing agent after damage. Once triggered, a good self-repairing system should be able to respond quickly. In addition, the microcapsules must be uniformly dispersed in the polymer matrix to release the core material into the cracks in time in case of cracking, so as to eliminate the adverse effects on the properties of the matrix material as far as possible. However, due to the limited amount of microcapsules implanted in the external self-repairing silicone materials, the repair efficiency of the materials still needs to be improved, and these materials can only achieve a limited number of repairs at the same damage site. Therefore, researchers began to focus on intrinsic self-healing silicone materials.
3 Intrinsic self-repairing silicone material
Intrinsic self-repairing silicone materials mainly use the dynamic reversible chemical structure of polysiloxane crosslinking network to achieve multiple repair processes.The dynamic reversible structure (such as dynamic covalent bond and dynamic non-covalent bond) is introduced into the molecular chain or the end of the molecular chain of the linear polysiloxane to endow the crosslinked network with the dynamic reversible characteristic; At the same time, combined with the mobility of polymer chain segments, the material can achieve structural damage and performance repair spontaneously or under external stimulation
[33,34]. Different reversible structural units have different stimulus response conditions (Fig. 5), and the research progress of intrinsic self-healing silicone materials will be introduced according to different stimulus response conditions.
3.1 Thermally responsive self-healing silicone material
3.1.1 Thermally responsive self-healing organosilicon materials based on hydrogen bonding
Hydrogen bond is one of the most widely studied supramolecular chemical interactions, which has attracted much attention because of its reversibility, directionality, high speed and sensitivity. Reversible hydrogen bond interactions have been widely used in self-healing materials due to the reversible break-recombination reaction of hydrogen bonds under thermal excitation. Usually, a single hydrogen bond is weak, but when multiple hydrogen bonds are crosslinked at the same time, it can produce greater binding energy and stability, such as ureidopyrimidinone (Upy), carbamate, ureido and so on, which are representative motifs of multiple hydrogen bonds
[35⇓⇓~38]. When the material is damaged, the weak hydrogen bond dissociation can effectively dissipate energy, and when the damaged surfaces are combined, the hydrogen bonds on the surface can be reformed through the diffusion movement of the polymer chain, so as to realize the self-repair of the damaged area
[39].
In DNA molecular structure, two deoxynucleotide chains form the famous double helix structure through the complementary pairing of bases, and the main force comes from the hydrogen bonding between them. This biological hydrogen bonding interaction is widely used in self-repairing DNA hydrogels, but the preparation process is complicated and the mechanical strength is poor
[40,41]. Jiang et al. Reported a cross-linked polydimethylsiloxane elastomer based on guanine paired with cytosine
[42]. Guanine and cytosine were photosensitized by the reaction with isocyanoethyl methacrylate (ICEMA), respectively, and then mixed with the copolymer of IPDI and bisaminopropyl-terminated PDMS and photoinitiator 1173, and cured by ultraviolet irradiation to obtain self-healing PDMS elastomer GC-PDMS. Due to the introduction of complementary bases, GC-PDMS has the ability of thermal reversible self-repair, and the repair efficiency can reach 75% after multiple repairs. This work provides a reference for the formation of silicone elastomers by incorporating bases rather than DNA fragments into PDMS, and the preparation process is simple, effective and solvent-free, which is expected to be applied in a variety of fields.
Zhang et al. Developed a self-healing coating composed of polydimethylsiloxane-based polyurea (PDMS-PUa) and a small amount of organic antifouling agent (DCOIT) (Figure 6a)
[43]. The coating has good adhesion to the substrate, which is attributed to the breaking and reorganization of the hydrogen bonds in the urea group. The coating shows good self-healing performance in both air and artificial seawater at room temperature (Fig. 6B). When the content of organic antifouling agent reaches 10.0 wt%, the self-healing efficiency can reach 100%. Increasing the temperature can also accelerate the healing process. In addition, the coating also realizes the controlled release of the organic antifouling agent. Marine field tests showed that the coating had excellent antifouling/decontamination properties even under static conditions for more than six months (Fig. 6 C).
Lee et al. Designed a polysiloxane-based polyurethane (PDPU) porous elastomer based on oxime-urethane-urea block by using triboelectric effect (Fig. 7A)
[44]. The PDPU elastomer is composed of urea-oxime-carbamate units, so that the molecular chain has better fluidity to enhance the re-entanglement after dissociation, the dynamic exchange of oxime-carbamate bonds on the crack surface,Reversible dissociation and association occur, while the low glass transition temperature of the polymer imparts high mobility to the molecular chains, resulting in excellent mechanical properties and healability (fig. 7B). The completely cut PDPU can heal well after heating at 100 ℃ for 4 H, showing an output voltage comparable to that before damage. Moreover, this porous PDPU generates electrical energy under periodic compression through the interaction between the entrapped air and the elastomer matrix (Figure 7 C). Triboelectrification of positively charged entrapped air (or other gases) with PDPU, inducing electron transfer from the gas to the solid polymer for power generation, is promising for built-in wearable sensors for self-powered intelligent soft robots and healthcare monitoring (Figure 7 d).
Skov et al. Introduced UPy into polydimethylsiloxane system and prepared a hydrogen bond-based thermoresponsive self-healing elastomer P (PDMSMA-co-UPyMA)
[45]. The elastomer can completely repair the damage when heated at 70 ℃ for 1 H. In addition, P (PDMSMA-co-UPyMA) was prepared by adding 20 wt%Fe
3O
4 magnetic particles into P (PDMSMA-co-UPyMA) to make a remotely controlled repairable magnetic composite, and the magnetic composite cut off and then reconnected together was placed in an alternating magnetic field to heat up the material for repair, and a good repair effect was obtained after 2 H of repair. It is generally believed that hydrogen bonding interactions fail in water or moisture environments, and Yao et al. Reported a self-assembled PDMS elastomer with water-enhanced repair properties
[46]. (UP) 3T oligomer was prepared by copolymerization of ureidopyrimidinone with isocyanate group (UPy-NCO), bisaminopropyl terminated PDMS oligomer and hexamethylene diisocyanate trimer. Through the quadruple hydrogen bonds between the UPy of the (UP) 3T end group, a high-density three-dimensional cross-linked network can be assembled (fig. 8). In the process of assembly, UPy dimers will further assemble into hard laminates with high crystallinity, which will be separated from the soft hydrophobic PDMS chains to form a semi-crystalline polymer network with microphase separation. The high crosslinking density and semi-crystallinity not only endow (UP) 3T with excellent mechanical strength, but also (UP) 3T has water-enhanced repair characteristics, and the mechanical strength can be completely restored after repair in hot water at 70 ℃ for 5 min. This is due to the low water permeability of PDMS phase, which allows water molecules to exchange with UPy units through the polymer network, but limits the water retention capacity of the polymer network. The hydrophobic PDMS phase is not affected by water and also positions the UPy microphase to ensure the reversible dissociation and reorganization of hydrogen bonds between UPy units. The prepared water regulating and multifunctional repairable materials have potential applications in the fields of actuators, smart coatings and biomedicine.
3.1.2 Thermally responsive self-healing silicone materials based on DA reaction
The Diels-Alder (DA) reaction is a [4 + 2] cycloaddition reaction between a diene and a dienophile, a typical example of which is the reversible reaction between a furan derivative and a maleimide derivative
[47]. Xia et al. Reported a biocompatible self-healing PDMS elastomer based on DA reaction
[48]. Polydimethylsiloxane (PM) with maleimide side chains was directly crosslinked with furan-terminated functional siloxane (FS) to prepare heat-repairable silicone elastomers (fig. 9). The mechanical properties of PDMS elastomer can be adjusted by changing the molecular weight and maleimide content of PM. The DA reaction and reverse reaction at different temperatures endow the PDMS elastomer with good self-healing and solid phase regeneration capabilities, and the healing efficiency of the elastomer can reach 95% after heat treatment at 80 ℃ for 24 H. In addition, cytotoxicity evaluation and animal subcutaneous embedding experiments show that the elastomer has good biocompatibility and has potential application value in biomaterials such as artificial skin, artificial prosthesis and tissue engineering scaffolds.
Zheng et al. Reported a more convenient method to prepare self-healing PDMS based on DA reaction
[49]. Firstly, a diepoxide with two DA bonds was synthesized as a crosslinking agent, and then it was crosslinked with aminopropyl-terminated PDMS to form self-healing PDMS. The tensile strength of the prepared elastomer can reach 0.80 MPa, and the elongation is more than 400%. The introduction of DA bonds into PDMS not only improves its mechanical properties, but also endows the elastomer with excellent thermal response self-healing and solid phase regeneration properties, and its self-healing and remolding efficiencies are both above 90%. On this basis, Jiang et al. Introduced graphene into the cross-linking system of PDMS modified by grafting of maleimide-terminated PDMS with furan. The introduction of graphene greatly improved the strength of the material, and the cross-linking network composed of DA bonds also endowed the material with self-healing properties
[50]. In order to improve the agglomeration of nanofillers in composites, it is often necessary to modify the surface of nanofillers to enhance the interaction between nanofillers and polymer molecular chains, but this strong interaction will restrict the movement of molecular chains, thus affecting the repair performance of materials. In order to solve this problem, they grafted furan groups on the surface of graphene, which can react with the polymer matrix to increase the interaction between graphene and the matrix, but do not affect the thermal response self-repair behavior of the composite (Fig. 10), because this strong interaction can be damaged together when the material is heated for repair
[51]. The repair efficiency of the nanocomposite containing 35 wt% graphene was maintained at about 92% after three consecutive self-repair experiments, and the nanocomposite also showed excellent recovery and elasticity under cyclic pressure, which can be used to develop wearable pressure sensors for use in complex situations.
3.1.3 Thermally responsive self-healing organosilicon materials based on metal-ligand interaction
The coordination bond is composed of the non-covalent interaction between the metal ion and its surrounding organic ligand, and the strength of this interaction can be adjusted by the combination of different metal ions and ligands. The reversible dissociation and association processes of coordination bonds endow polymers with highly dynamic properties, which have been applied to self-healing materials
[52⇓~54]. You et al. Incorporated dynamic Fe
3+- triazole coordination bonds into the backbone of polydimethylsiloxane (PDMS) to obtain a highly elastic polymer that can be thermally repaired at mild temperatures
[55]. The coordination strength between Fe
3+ and 4H-1,2,4-triazole group is low, which can be dissociated and reconstructed quickly, endowing the material with high stretchability and excellent repair performance, and the repair efficiency is 94. 3% after heating at 60 ℃ for 20 H.
Rao et al. Reported a self-repairing dielectric PDMS elastomer based on metal-ligand interaction, which provides an idea for the design of stretchable dielectric materials based on metal-ligand interaction in the future
[56]. The main chain of the elastomer has a 2,2 '-bipyridine-5,5' -dicarboxamide structure, and the metal-ligand interaction formed by adding Zn
2+ has kinetic instability, which endows the PDMS elastomer with the ability of rapid self-repair at room temperature. A repair efficiency of more than 70% was obtained for the damaged sample at room temperature for two days or at 90 ° C for several hours, which is attributed to the higher degree of mobility of the polymer chain and the highly dynamic nature of the metal ion-bipyridine coordination bond at high temperatures. When PDMS elastomer containing FeCl
2 or ZnCl
2 as crosslinker is integrated into organic field effect transistor as gate dielectric, it has higher dielectric constant compared with the transistor with common PDMS as gate dielectric, and has no hysteresis transmission characteristics. The low ionic conductivity in PDMS and the strong Coulomb interaction between metal cations and Cl
- anions prevent anions from migrating under gate bias. The transistor with PDMS elastomer as the dielectric with FeCl
2 as the crosslinker is fully stretchable and exhibits ideal transport characteristics, and the gate leakage current is not high even after 1000 cycles of stretching at 100% strain.
Zuo et al. Designed a rigid and repairable organosilicon material based on abundant weak metal-ligand interactions
[57]. Carboxyl modified polydimethylsiloxane (PDMS-COOH) was synthesized by hydrosilylation and hydrolysis, and the organosilicon material PDMS-COO-Zn was prepared by the formation of Zn(Ⅱ)-COO
- coordination interaction. The material can achieve rapid and reversible changes in mechanical strength by heating or cooling (Figure 11A), is a rigid polymer at room temperature, and becomes a viscous liquid when heated to 120 ℃.Because of this thermally responsive modulus change, the material can undergo rapid repair and remodeling, and the samples that are reconnected together after being cut off almost completely recover the initial morphology after being heated at 80 ° C for 4 H (Fig. 11 B). This rapid softening and hardening makes it promising to replace the traditional plaster casting in orthopedic fixation surgery, while combining the advantages of light weight, easy disassembly and recyclability. In terms of 3D printing, the thermal response repair characteristics of the material enable it to print different modules for thermal response splicing, breaking through the limitation that the size of 3D printing products is limited by the size of the printer (Figure 11c).
The mechanical properties and dynamic self-repairing ability of self-repairing materials are usually contradictory. In order to obtain excellent self-repairing ability, it is necessary to construct a weak polymer network, which will lead to the decline of mechanical properties of materials. In order to solve this problem, Bao et al. Designed a thermodynamically stable but kinetically unstable coordination crosslinking self-repairing elastomer, which was prepared by coordination crosslinking of polydimethylsiloxane containing ligand (2,6-bis (imino) methyl-4-chlorophenol) with Zn (II) (Fig. 12)
[58]. The ligand provides two equivalent coordination sites, which cannot coordinate with one metal ion at the same time due to steric hindrance, and the exchange of coordination bonds can occur intramolecularly or intermolecularly. The coordination structure is highly dynamic at room temperature and has a large association constant. The elastomer has excellent mechanical properties and room temperature repair properties, and can be almost completely repaired at room temperature for 24 H. This study provides a new design concept for the preparation of materials with both excellent mechanical properties and self-healing properties, which is expected to be applied in energy-absorbing materials.
Bai et al. Prepared a colorless, transparent and repairable PDMS elastomer by introducing Zn(Ⅱ)-COO
- coordination into PDMS
[59]. The elastomer shows excellent optical transparency and good mechanical strength, the maximum tensile strength can reach 3 MPa, and the elongation at break can reach 1000%.This is due to the fact that ionic aggregates have different sizes and stiffnesses. Small and weak ionic aggregates can preferentially sacrifice energy dissipation, while large and hard ionic aggregates endow elastomers with high strength and elasticity. Based on the dynamic characteristics that the Zn(Ⅱ)-COO
- coordination interaction dissociates upon heating and recombines after cooling, the mechanical properties of the sample are almost completely restored after 1 H of repair at 100 ℃, showing great potential as a reusable adhesive.
Photonic elastomers with structural color and self-healing ability are ideal materials for visual force or strain sensors, but their preparation is still challenging. Zhu et al. Introduced isotropically arranged monodisperse nanoscale SiO
2 into a supramolecular PDMS elastomer based on the coordination interaction between amino-terminated PDMS and Ce (Ⅲ), so that the elastomer could change its color like a chameleon when stretched by external force, and the color was independent of the angle
[60]. At the same time, the dynamic Ce(Ⅲ)-NH
2 coordination also endows the elastomer with self-healing ability, which can ensure that the elastomer can play a long-term stable role in practical applications.
3.1.4 Thermally responsive self-healing silicone materials based on imine bond
Yu et al. Prepared a transparent, highly stretchable self-repairing PDMS elastomer by using aminopropyl-terminated PDMS as the substrate and 1,3,5-triformylbenzene as the crosslinking agent to form imine bond crosslinking through amine-aldehyde condensation
[61]. The preparation method is simple, and the elastomer exhibits good visible light transmittance (80%), high stretchability (≈ 700%), and self-repair ability at room temperature. Liu et al. Also used aminopropyl-modified PDMS to react with terephthalaldehyde to form imine crosslinks to prepare highly transparent self-healing PDMS
[62]. The elastomer has a strong self-healing ability, and 95% of the original properties can be recovered after 24 H at room temperature. It also shows a high transmittance of 91% and selective fluorescence quenching of Fe
3+. Tian et al. Formed a glass-like PDMS with an imine crosslinked network via an amine aldehyde condensation reaction between terephthalaldehyde, Tris (2-aminoethyl) amine, and aminopropyl terminated PDMS (fig. 13A)
[63]. After the film with cracks was treated at 100 ℃ for 30 min, the cracks gradually disappeared, which was attributed to the dynamic imine bond on the surface of the cracks. With the movement of the molecular chain at high temperature, the broken imine bond could be reformed, and finally the cracks were healed. Due to the multiple stimulus response characteristics of the imine bond, this glass-like PDMS can be reshaped after soaking in water at room temperature (Figure 13 B), in addition to its excellent thermal response damage self-repair and solid phase regeneration capabilities, which provides the possibility of green reprocessing of PDMS.
Feng et al. First synthesized two aldehyde modified tetraphenylethylene derivatives with different fluorescence properties, and then used them as crosslinking agents to react with aminopropyl-terminated PDMS(NH
2-PDMS-NH
2) to prepare two PDMS elastomers with excellent fluorescence properties, mechanical properties, thermal stability, and self-healing and recycling properties
[64]. The results showed that the mechanical properties of the elastomer were affected by the functionality of the aldehyde modified tetraphenylethylene derivative and the molecular weight of the aminopropyl-terminated PDMS. At the same molar ratio of crosslinking agent to NH
2-PDMS-NH
2, with the increase of the molecular weight of the soft segment of PDMS, the number of crosslinking points decreased, and the crosslinking density decreased, which led to the decrease of the tensile strength of the elastomer. At the same molecular weight and aldehyde to NH
2-PDMS-NH
2 molar ratio, the elastomer prepared by the crosslinking agent with lower functionality exhibited superior mechanical properties due to the more rigid tetraphenylethylene skeleton structure in the crosslinked network. Due to the dynamic nature of the imine bond, the crosslinked network can be quickly rearranged under different stimulus conditions, so that the elastomer can not only quickly repair damage at room temperature, but also be recycled through solvent processing and heating molding, and the fluorescence characteristics of the elastomer will not change significantly.
3.1.5 Thermally responsive self-healing silicone materials based on ionic bonding
An ionic bond is one that results from the mutual coulombic attraction between oppositely charged ions and can often provide very strong bond strength, excellent elasticity, and self-healing due to the clustering of a large number of active bonds. Li et al. Reported a self-healing silicone elastomer based on ionic bonds, and synthesized a polysiloxane precursor (PDMS-g-COOH) containing carboxyl groups by thiol-ene reaction, and then formed dynamic ionic bonds by the reaction of carboxyl groups with Zn
2+ to prepare a thermoplastic silicone elastomer (Fig. 14)
[65]. Based on COO
-/Zn
2+ ionic crosslinking, the PDMS-g-COOH/ZnO elastomer can be recycled, and its mechanical properties remain basically unchanged after multiple recycling. In addition, the cut elastomer can recover to 83.5% of the original strength after being kept at 80 ℃ for 4 H. This study provides a simple and feasible method for the preparation of high-performance functional silicone rubber.
At present, the self-repairing silicone elastomer is difficult to achieve both light curing and self-repairing properties, and the self-repairing elastomer prepared by thermal curing is generally opaque and has poor compatibility. Our research group prepared a silicone transparent elastomer with fast UV curing and excellent self-repair property through the reversible/irreversible hybrid double network constructed by the photoinduced click reaction of PDMS-SH and vinyl-terminated silicone oil and the thermal reversible dynamic ionic crosslinking of carboxyl silicone oil and amino silicone oil (Fig. 15A)
[66]. The repair of silicone elastomer can be achieved by the dynamic dissociation and recombination of ionic bonds in the elastomer, and the repair efficiency is 97%. In addition, the silicone elastomer has excellent recycling performance (Figure 15B), which can recover 90% of the strength, and the recycled elastomer still has excellent self-repair performance, and the repair efficiency can reach 90% after multiple repairs.
3.1.6 Thermoresponsive self-healing silicone materials based on disulfide bond
Disulfide bond (S — S), as one of the dynamic covalent bonds, is a hot research topic in various fields. In the case of cracking or stress damage, the self-healing behavior based on S — S exchange (fracture and reformation of S — S) requires response to alkaline pH, free radical agents, light, or temperature
[67⇓~69][70⇓~72][73]. For example, in an alkaline environment, the generated thiolate ion will attack the S — S bond, thereby generating a new S — S bond and thiolate anion. In the presence of a free radical agent or light, the S — S bond is broken and exchanged to form a new bond. Zhang et al. Reported a disulfide bond (HEDS) modified PDMS-based polyurethane (PDMS-PU), which has good thermal stability, tensile properties and repair properties (Fig. 16)
[74]. With the increase of PDMS content, the decomposition temperature of PDMS-PU elastomer increased by 34 ℃, and the elongation at break increased gradually, up to 1204%, and the self-healing efficiency of PDMS-PU increased with the increase of disulfide bond content. Because the disulfide metathesis reaction is accelerated with the increase of repair temperature under thermal stimulation, the repair efficiency of PDMS-PU elastomer is more than 80% when heated at 90 ℃ for 12 H or 120 ℃ for 3 H, which can be used as the substrate of flexible self-repair electronic products in the future.
Guo et al. Prepared a room temperature rapid repair silicone elastomer by reacting diepoxy-terminated PDMS with 4,4 '-diaminodiphenyl disulfide, a crosslinking agent containing an aromatic disulfide bond
[75]. Due to the efficient exchange reaction of aromatic disulfide bonds at room temperature and the excellent molecular chain motion ability of polysiloxane, the elastomer has excellent self-repairing ability at room temperature, and the repairing efficiency is more than 95%, even if the surface is aged or the two parts are in overlapping contact, the repairing efficiency at room temperature is only slightly reduced. Due to its good mechanical strength and high stretchability, this elastomer can be used as a matrix material for stretchable and wearable electronic devices.
Zou et al. Reported a transparent self-healing silicone elastomer based on a reversible disulfide bond
[76]. They used bis (3-hydroxypropyl) -terminated polydimethylsiloxane as the soft segment, isophorone diisocyanate (IPDI) as the hard segment, aromatic disulfide (HPDS) with dynamic reversibility at room temperature as the chain extender, and added fumed silica and carbon fiber to construct a room-temperature self-repairing ablation-resistant composite PIH (Fig. 17A).
The disulfide bond at the fracture surface can effectively carry out the exchange reaction, which is an important factor to achieve self-repair. The excellent chain mobility of the soft segment is also conducive to the generation of self-repair and microphase separation structure, thus improving the heat resistance. The hard segment shows moderate rigidity and acts as a physical crosslinking point to provide sufficient mechanical properties. A proper amount of disulfide bond can make the elastomer have the best mechanical strength, fast room temperature self-healing efficiency and excellent thermal stability, but the disulfide bond is a part of the hard segment, and too much hard segment will reduce the fluidity of the molecular chain and reduce the self-healing efficiency. Among them, the self-healing efficiency of the film PIH-12.5 with HPDS content of 12.5 mmol is as high as 92.9% after 3 H of repair at room temperature (Fig. 17b), and the composite prepared with this substrate can withstand the flame erosion at 1300 ℃ (Fig. 17c), which provides a new idea for expanding the application of self-healing materials in the aerospace industry.
3.1.7 Thermal Response Self-healing Silicone Materials Based on Siloxane Balance
Cyclic polysiloxane is subjected to ring-opening polymerization under the action of a catalyst, and the obtained polymer can be degraded and the molecular chain is rearranged to realize dynamic crosslinking, so that the cyclic polysiloxane can be used for preparing a self-repairing silicone elastomer. McCarthy et al. Reported a self-healing silicone material based on siloxane balance
[77]. The material is a crosslinked organosilicon polymer containing a dimethylene crosslinking bond and an active silanolate end group, which is generated by octamethylcyclotetrasiloxane (D
4) and bis (heptamethylcyclotetrasiloxane) ethane (bis-D
4) under the catalysis of ammonium tetramethylsilanolate. The active silanolate end group can catalyze the thermochemical equilibrium between isomers and cyclic oligomers in the material network, and the material can be repaired or reshaped by heating at 90 ℃ for 24 H. They believe that any crosslinked PDMS elastomer can catalyze the breaking and reorganization of the silicon-oxygen bond in the network by adding an alkaline catalyst, resulting in a dynamically crosslinked silicone elastomer with self-healing ability. Recently, Opris et al. Also prepared dielectric elastomers with polar side chains by anionic ring-opening polymerization of D
4 and cyanopropyl-modified D
4 catalyzed by tetra-n-butylphosphine hydroxide
[78]. With the help of the active silanolate at the end of the molecular chain of polysiloxane, the main chain of polysiloxane can be continuously rearranged under heating conditions, thus endowing the elastomer with self-repairing properties and showing application prospects in artificial muscles.
Based on this, Huang et al. prepared an elastomer by mixing aminopropyl-terminated PDMS, trimethylolpropane triacrylate (TMPTA), and tetramethylammonium hydroxide (TMAH), and then crosslinking the mixture by amino-olefin Michael addition reaction via heating (Figure 18 a)
[79]. Due to the addition of the alkaline catalyst TMAH, the siloxane bonds in the elastomer network are activated. Based on this dynamic siloxane balance, the cut elastomer can recover 91% of its original strength by heating, and its original shape can be restored even if the elastomer is cut into pieces (Figure 18 B).
How to prepare silicone materials with both rigidity and repairability is still a challenging problem. To solve this problem, Zhao et al. Reported a hard but reversible Si — O — Si inorganic-organic hybrid network polymer synthesized from aminopropyl-functionalized polysilsesquioxane and triethylamine (TEA), and prepared a coating with excellent transparency, hardness, flexibility, and rapid self-healing properties by filling soft poly (methyl-3,3,5-trifluoropropylsiloxane) micelles
[80]. The highly crosslinked continuous network gives the coating a hardness (H = 0.83 GPa) higher than that of most polymers, while the uniformly dispersed micelles reduce the Young's modulus (E = 5.89 GPa) to a value as low as that of ordinary plastics, thus combining excellent hardness and flexibility. Because TEA can block the hydrogen bonding between aminopropyl and silanol, more aminopropyl groups are free and have stronger mobility. With the help of the self-catalysis of the free aminopropyl group, the breaking and recombination of the Si — O — Si bond can be effectively promoted (Fig. 19 a), so that the material has excellent self-healing performance in the ordinary environment. When the scratch coating is placed above a 45 ℃ water bath, the exposed hydrophilic polysilsesquioxane network absorbs water vapor and rapidly initiates a reversible reaction of Si — O — Si bonds. Due to the cleavage of the Si — O — Si bond, the generated polysilsesquioxane block can flow into the scratch, and the scratch (~ 50 μm) gradually becomes blurred and quickly disappears within 4 min (Fig. 19 B). This nanocomposite strategy shows great advantages in achieving transparency, amphiphobicity and programmable versatility at the same time, and is expected to be applied to various fields.
3.1.8 Thermally responsive self-healing organosilicon materials based on π-π stacking interaction
π-π stacking is a weak intermolecular interaction between two relatively electron-rich and electron-deficient aromatic rings, which has a wide range of applications in supramolecular self-assembly, molecular recognition and crystal engineering. Hayes et al. Studied the supramolecular interactions between pyrenyl derivatives and polymers containing imide groups and found that they adopt folded conformations in order to maximize the number of face-to-face interactions between electronically complementary structures
[81]. Based on this π-π stacking interaction, they fabricated a novel thermoresponsive supramolecular material (Fig. 20).
This supramolecular material is composed of polyimide, which can produce π-electron-deficient "tweezer-type" acceptors through chain folding conformation, and linear PDMS with π-electron-rich pyrene groups, which are cross-linked through π-π stacking interactions. The damaged region of the material can be repaired by breaking the π-π stacking interaction by heating and reforming the π-π stacking interaction after cooling.
Brook et al. Prepared coumarin-grafted PDMS by azide-alkyne cycloaddition reaction. The π-π stacking dimerization of coumarin head to tail can form a physical crosslinking structure. By changing the content of coumarin, the coumarin-grafted PDMS can be transformed from a low-viscosity liquid into a high-viscosity liquid, elastomer or brittle solid
[82]. This π-π stacking interaction can be broken by heating, so this PDMS has the ability to be remodeled by heating.
3.1.9 Thermally responsive self-healing silicone materials based on multiple dynamic structures
Traditional self-healing materials can only be activated under a single specific condition. With the deepening of research, smart polymers with a single stimulus response can no longer meet the demand. Different non-covalent interactions and dynamic covalent bonds have different characteristics, for example, disulfide bonds can undergo exchange reactions under specific wavelengths of ultraviolet light, and DA bonds can be dissociated and recombined by thermal stimulation in different temperature ranges. Therefore, by changing the combination of dynamic structures, materials can be endowed with characteristics such as photothermal dual-response self-healing or degradable. In addition, the synergistic response of multiple dynamic structures may improve retrievability and healing potential to meet complementary needs. At present, heat-induced self-repair is the most commonly used pathway because it is easy to operate and can precisely control the repair conditions, but in the case of remote activation, a single thermal stimulus response is not ideal. To overcome these problems, it is necessary to combine different stimuli to meet diverse needs. UV-induced self-healing materials are becoming another popular alternative because of their low cost, environmental friendliness, remote activation, and spatiotemporal controllability. Disulfide compounds produce sulfur radicals under the stimulation of ultraviolet rays, which drive the polymer chain to move through exchange or metathesis reactions to restore the original mechanical strength of the material and achieve self-repair. Therefore, while pursuing self-healing under mild conditions, researchers are also looking for self-healing silicone materials with multiple stimulus responses and excellent comprehensive properties to meet the needs in different fields.
Zheng et al. Reported a PDMS elastomer based on a dual dynamic covalently crosslinked network of aromatic disulfide and imine bonds
[83]. The elastomer was prepared by crosslinking aminopropyl-terminated PDMS with 1,3,5-triformylbenzene and 4,4 '-diaminodiphenyl disulfide. In the PDMS network, the weaker aromatic disulfide bonds act as sacrificial bonds, while the stronger imine bonds act as semi-permanent crosslinking points, and the prepared elastomers exhibit excellent stretchability and self-healing properties at room temperature. More importantly, due to the presence of imine crosslinks, PDMS elastomers can be degraded in the presence of acids, stronger nucleophiles, or competing monofunctional aldehydes, which can be used to develop reusable adhesives. They applied this network of dual dynamic covalent bonds to nanocomposites to prepare degradable, reprocessable, and self-healing PDMS/carbon nanotubes (CNTs) nanocomposite elastomers
[84]. The elongation at break and tensile strength of the composite elastomer can be adjusted by adjusting the content of aromatic disulfide bonds and carbon nanotubes. The composite elastomer has excellent self-repair ability, and the self-repair efficiency reaches 95% after repair at room temperature for 12 H. In addition, the mechanical properties of the composite elastomer are not significantly reduced after multiple recycling and reprocessing. More importantly, the elastomer can be degraded in the presence of trifluoroacetic acid, O-ethylhydroxylamine, or benzaldehyde to achieve complete recovery of carbon nanotubes. The rapid development of flexible transparent electronic devices has put forward higher requirements for substrate materials, not only for high transparency, but also for good self-healing ability, high mechanical strength and toughness.
Gu et al. Developed a foldable and repairable transparent PDMS film by constructing a layered cross-linked network and double dynamic bonds
[85]. They synthesized a novel urea-containing chain-extended PDMS (LPx) from low molecular weight aminopropyl-terminated PDMS. LPx is physically cross-linked through the formation of hydrogen bonds between urea bonds, and LPx presents better rigidity. Subsequently, a hyperbranched polysiloxane (HPSi) with multiple amino groups was linked to LPx through an isocyanate containing an exchangeable aliphatic disulfide bond (SS-NCO) to prepare a transparent self-healing LPx-SS-HP film with a layered crosslinked network and double dynamic bonds. The good stiffness and toughness of LP
2-SS-HP are attributed to hydrogen bonding and covalent cross-linking of HPSi at room temperature, which can "unlock" segmental motion through both disulfide exchange and hydrogen bond dissociation in response to thermal stimulation in the network, and LP
2-SS-HP has excellent self-healing properties. The tensile strength, elongation at break and toughness of the spliced LP
2-SS-HP samples recovered to more than 90% of their original values after heating at 100 ℃ for 30 min. LP
2-SS-HP has outstanding flexibility. Compared with polyimide (PI) and polyethylene terephthalate (PET) of the same size, PI and PET have irreversible creases after two folds, while LP
2-SS-HP has no obvious creases. The LP
2-SS-HP has good transparency, and the transmittance of the film with any thickness is more than 80% in the visible region. This series of films achieve high strength and modulus without affecting their elongation at break, and are expected to be applied to flexible transparent self-repairing electronic products.
Lin et al. Reported a controllable self-healing tough silicone protective material
[86]. They used 4,4 '-dicyclohexylmethane diisocyanate (HMDI) and isophthalaldehyde (IPAL) with aminopropyl-terminated PDMS to prepare silicone elastomers with adjustable self-healing properties and mechanical properties through a stepwise synthesis method. The formed cross-linked hydrogen bonds improve the mechanical strength of the material, while the imine bond with excellent dynamic properties at room temperature endows the material with self-healing ability. It is found that when the ratio of urea bond to imine bond increases, the strength of the material increases, but the self-healing ability decreases, which is attributed to the increase of cross-linked hydrogen bonds, which limits the movement of molecular chains. When the ratio of urea bond to imine bond is equal, the elastomer has the ability of rapid self-repair at room temperature and significant anti-corrosion ability for metals.
Zheng et al. Fabricated a robust, repairable, and reprocessable nanoscale SiO
2/PDMS composite by simultaneously constructing dynamic inter-ethylene urethane cross-links and sacrificial Zn(Ⅱ)-NH
2 coordination interactions in the material matrix
[87]. The surface of SiO
2 modified by amino group will participate in the crosslinking reaction and form dynamic bonds at the interface between the filler and the matrix. The cross-linking bond of m-vinyl urethane makes the material have good high temperature self-healing ability and reprocessing ability, and the coordination interaction formed between the Zn
2+ and the primary or secondary amino group with the sacrificial bond effect significantly improves the mechanical properties of the composite. In addition, Zn
2+ can also act as a Lewis acid catalyst to accelerate the exchange reaction of the m-vinyl urethane crosslinking bond, thereby improving the reprocessability and repairability of the PDMS composite. The composite material also shows good shape memory ability and shows great application potential in the field of smart materials.
Zheng et al. Reported a high mechanical strength, repairable and reprocessable conductive carbon black/silicone rubber nanocomposite based on dynamic imine bond and sacrificial coordination bond
[88]. In this study, aminopropyl-modified PDMS was crosslinked by imine bond and Zn(Ⅱ)-NH
2 coordination interaction, and conductive carbon black was added to prepare conductive carbon black/silicone rubber nanocomposites. It has been shown that following the energy dissipation mechanism of sacrificial bonds, the introduction of coordination interactions significantly improves the strength, stretchability and toughness of the material without affecting the self-healing ability or reprocessing ability. In addition, the addition of carbon black, in addition to improving the electrical conductivity of the nanocomposites, can also promote the cleavage of sacrificial bonds and significantly improve the energy dissipation efficiency.
Huang et al. Reacted 2-formylphenylboronic acid with ZnCl
2 to form a coordination compound CHO-Bz, and aminopropyl-modified PDMS was reacted with CHO-Bz and glutaraldehyde to prepare a silicone elastomer with a crosslinked network combining imine bonds and coordination interactions, which realized the high toughness and rapid self-healing ability of the elastomer at room temperature
[89]. By changing the ratio of imine bonds and coordination interactions in the network, the toughness of the elastomer can reach 0.67 MJ·m
-3, which is higher than that of silicone elastomer with a single imine network. In addition, the elastomer has excellent self-repair ability, and the repair efficiency can reach 94% after 24 H of repair at room temperature. In addition, they also prepared composite elastomers with good superhydrophobicity and self-cleaning properties by mixing nano-SiO
2 fillers into elastomers. After the surface of the material is damaged, the material can be quickly repaired at room temperature to restore the superhydrophobicity.
Feng et al. Prepared high-strength, repairable, and thermally conductive 3D graphene/PDMS composites selectively cross-linked by borate bonds and UPy hydrogen bonds through molecular design
[90]. By optimizing the reversible interaction, the maximum strength of the material is 7. 33 MPa and the self-healing efficiency is 97. 69% ± 0.33% when the molar ratio of boroxine to UPy is 1 ∶ 3. In addition, they also used UPy modified graphene aerogel to prepare a composite with high mechanical strength (39.63 ± 3.73 MPa) and good thermal conductivity (0.826 W·m
-1·K
-1), and the cut composite recovered the mechanical properties (78.83% ± 2.40%) and thermal conductivity (98.27% ± 0.13%) after repairing at 40 ° C for 6 H.
Zhang et al. Introduced aromatic disulfide bonds and strong and weak hydrogen bonds into PDMS at the same time, and designed a silicone elastomer with high stretchability and self-repair in harsh environments
[91]. The strong hydrogen bond endows the material with elasticity, while the weak hydrogen bond can dissipate energy through reversible fracture and recombination when the material is deformed by force. In addition, the aromatic disulfide bond acts as a sacrificial bond, which makes the material have high stretchability, even up to 14000% of the original length. The exchange of aromatic disulfide bonds and the synergistic effect of multi-strength hydrogen bonds endow the elastomer with rapid autonomous self-healing ability in harsh environments, including room temperature, low temperature of − 40 ° C, underwater, supercooled water (30% NaCl solution at − 10 ° C), strong acid (pH = 0) and strong base (pH = 14) (Fig. 21). In general, water can affect the self-healing effect of self-healing materials based on hydrogen bonding, but they found that the elastomer did not significantly absorb water and reduce mechanical properties after immersion in water for 24 H, thanks to the hydrophobicity of PDMS molecular chains.
Achieving good mechanical properties and fast and efficient self-healing properties at room temperature remains a challenge for self-healing silicone elastomers. To solve this problem, Jiang et al. Reported a robust, self-repairable silicone elastomer composed of multiple dynamic bonds
[92]. In this system, the sacrificial aromatic disulfide bonds and weak hydrogen bonds are able to dissipate strain energy and achieve rapid reorganization and rearrangement of the network at room temperature, endowing the elastomer with rapid self-healing and high stretchability. While the coordination of Fe
3+ with catechol can adjust the mechanical strength of the elastomer and ensure the elasticity. Based on the synergistic effect of multiple dynamic bonds, the elastomer has a high elongation at break of 1100%, a tensile strength of 1. 11 MPa, and a rapid self-healing ability at room temperature (3 H, healing efficiency of 96%). The stretchable electrodes and strain sensors developed based on this elastomer (Fig. 22), open the way for potential applications in flexible electronics and Microsystems.
Lin et al. Reported a conductive PDMS/nanosilver composite elastomer suitable for wearable sensors
[93]. The matrix of the composite material is a self-repairing PDMS elastomer with dynamic disulfide bonds, coordination interaction between Zn
2+ and catechol, and weak hydrogen bonds between catechols, which is prepared by reacting aminopropyl-modified PDMS with 3,3 '-dithiodipropionic acid and protocatechuic acid to form primary crosslinking, and adding ZnCl
2 to form coordination interaction for further crosslinking. The PDMS/AgNPs composite elastomer with excellent mechanical properties, conductivity, antibacterial property and self-healing property was prepared by adding nano-silver (AgNPs) conductive filler to further reinforce the PDMS elastomer. The elastomer with 10 wt% AgNPs has excellent tensile strength (2.4 MPa) and high stretchability (strain at break 1762%). Based on the synergistic effect of multiple dynamic bonds in the PDMS network, the composite elastomer has excellent self-healing ability at room temperature, and the repair efficiency reaches 91. 2% after 24 H at room temperature. Moreover, the combination of the bactericidal effect and hydrophobic surface of AgNPs and Zn
2+ endows the conductive elastomer with efficient antibacterial properties.
Lin et al. Reported a self-healing silicone elastomer with both acid and thermal responses
[94]. The elastomer is prepared from aminopropyl-terminated polydimethylsiloxane through a two-step reaction to synthesize tetrahydrazide-terminated PDMS, and then the tetrahydrazide-terminated PDMS reacts with terephthalaldehyde to form hydrazone crosslinking bonds. Because the acylhydrazone bond can be reversibly exchanged under the catalysis of acid, the elastomer can realize the repair of damage through the catalysis of acid even at room temperature. The repair efficiency can reach 85% by coating the surface of the material with acetic acid and leaving it at room temperature for 48 H.
Our research group designed a high-strength recyclable self-healing silicone elastomer based on a dual asymmetric dynamic crosslinking chain structure
[24]. Utilizing diethylenetriamine-propyl modified polydimethylsiloxane (PDETAS) as a matrix and p-carboxybenzaldehyde (FBA) as a crosslinking agent, one of the crosslinking involves a strong covalent imine or aminal bond that maintains structural stability,The other involves weak non-covalent ionic bonds, which endow the material with the ability to redissolve, and the cross-linked chain formed has an asymmetric structure, showing the characteristics of supramolecular materials with stable structure at room temperature and rapid repair at moderate temperature (Fig. 23a). Due to the synergistic contribution of dynamic ionic bond, imine bond and aminal bond, the mechanical properties of the cut elastomer (PDETAS-FBA) can be restored to more than 95% of the original after a certain period of time at 100 ℃. The sheared elastomer can be regenerated in solid phase after molding at 80 ℃ and 10 MPa for 2 H (Fig. 23b), and the regenerated sample can maintain excellent mechanical properties after three cycles of regeneration. This work provides a new idea for the preparation of silicone elastomers with both mechanical properties and self-healing properties.
3.2 Photoresponsive self-repairing organosilicon material
Illumination is an environmentally friendly, immediate response, efficient and low-energy stimulation. With the increasing demand for low energy consumption and environmental protection, traditional thermal responsive self-healing coatings and adhesives are gradually replaced by UV-responsive curing products
[95⇓~97].
Disulfide not only can realize self-repair under temperature stimulation, but also has the characteristics of easy response to ultraviolet radiation stimulation, and has the characteristics of environmental friendliness, simple operation, controllable time and space, and high efficiency and energy saving
[98]. Disulfide compounds produce sulfur radicals under the stimulation of ultraviolet radiation, which drive the polymer chain to respond quickly through exchange or metathesis reactions to achieve self-repair, and have become a hot research topic in recent years. Our group has introduced a simple method to endow commercial silicone elastomers with solar-driven catalyst-free self-healing and solid-phase regeneration properties
[99]. To improve the mechanical strength and dynamics of PDMS elastomers, we synthesized a novel disulfide-containing MTQ resin (SS-MTQ). SS-MTQ was used to co-condense with hydroxy silicone oil (HO-PDMS-OH) and silane coupling agent bis (triethoxysilylpropyl) disulfide (Si75) to produce a silicone elastomer containing disulfide bonds (fig. 24).
It has been found that disulfide bond exchange through a free radical mechanism can be triggered by ultraviolet light in sunlight, and even in non-polar solvents and at light intensities as low as 40 mW·cm-2, sunlight can still effectively promote disulfide bond exchange. This indicates that even in the weak polar polymer of silicone elastomer, the change of sunlight throughout the day can promote the exchange reaction of disulfide bonds in silicone elastomer, which endows silicone elastomer with self-repair and solid-phase regeneration properties under sunlight.
Wang et al. Used a thiol-terminated sulfur-containing heterochain polysiloxane (P-SH) as the base polymer and three thiol-containing compounds or polymers, including pentaerythritol tetrakis (β-mercaptopropionate) (PETMP), octa (3-mercaptopropyl) silsesquioxane (POSS-SH), and poly (mercaptopropyl) methylsiloxane (PMMS) as crosslinking agents,Disulfide-containing self-healing silicone elastomers (SEs) were prepared by oxidative coupling reaction, which directly formed disulfide bonds during the crosslinking process
[100]. The reversible nature of the disulfide bond is easily triggered by various conditions, and the silicone elastomer can self-heal after being cut into pieces and heated at 150 ℃ for 2 H or irradiated by ultraviolet for 30 min, with a self-healing efficiency of more than 70%. In addition, they can be used as an adhesive for glass sheets and can withstand a weight of 200 G (Figure 25), and this adhesion is reversible and can be repeated many times. This simple and effective strategy can be extended to construct other self-healing elastomers based on thiol-containing base polymers and crosslinkers.
In addition, azo and its derivatives have excellent photoresponse properties, and when exposed to UV stimuli, it can convert light energy into mechanical response through trans-cis isomerization of the azo moiety, thus achieving dynamic photorepair. Li et al. Obtained a self-healing silicone elastomer (Zn(abpy)
2-PDMS) with photoresponse by introducing azopyridine structure into polysiloxane
[101]. Azo and pyridine structures, on the one hand, can coordinate with metal ions, and on the other hand, can undergo cis-trans isomerization under illumination. Under visible light, the azopyridine structure is in a trans conformation and can effectively coordinate with the zinc ion; Under the irradiation of ultraviolet light, its structure will change from trans to cis, which hinders the occurrence of metal coordination. Based on this photoresponse process, when the cut surface of the damaged material is irradiated by 365 and 450 nm light sources, respectively, the cleavage and recoordination of the coordination bonds may occur, effectively promoting the healing of the elastomer (Figure 26). After 3 cutting/repair cycles under mild conditions (40 ° C) and severe conditions (e.g., water and subzero temperature), the resulting photorepair materials still have high ductility (up to 735%), high toughness (16.33 MJ·m
-3), and excellent photorepair efficiency (about 90%). By introducing carbon nanotubes as a conductive layer, the elastomer shows excellent sensing behavior and is expected to be applied in fields such as intelligent robotics and flexible electronics.
3.3 Dielectric-driven self-repairing silicone material
You et al. Prepared a self-repairing elastomer TIA-PDMS based on coordination bond crosslinking by adding CoCl
2 to polydimethylsiloxane containing 4H-1,2,4-triazole groups in the molecular chain
[102]. It was found that the coordination number of the elastomer changed from four to six when immersed in water or methanol, resulting in solvatochromism. With the help of methanol, TIA-PDMS can be repaired at room temperature due to the solvent-induced coordination number change.
Feng et al. First synthesized 4- [ (allyloxy) methyl] -2- (4-vinylphenyl) -1,3,2-dioxaborolane (VPD), and then VPD was mixed with mercaptopropyl-modified PDMS and photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) and subjected to mercapto-ene click reaction under UV light to prepare elastomers based on dynamic borate bonds
[103]. The elastomer has excellent self-healing ability at room temperature. In addition, the healing process can be accelerated by adding a small amount of water dropwise to the damaged area, which benefits from the hydrolysis and dehydration condensation of the borate bond on the damaged surface. Due to the uncontrollable hydrolysis reaction of borate bond, the use of VPD alone as a crosslinking agent is not conducive to improving the properties of materials. In order to solve this problem, a PDMS elastomer with two kinds of crosslinked networks was prepared by adding vinyl-modified polydimethylsiloxane and nano-SiO
2 as reinforcing materials. It is found that the introduction of the second crosslinking network enhances the mechanical properties of the elastomer and also maintains a good self-healing ability.
The movement ability of molecular chain is the key factor to promote the repair. Only when the molecular chain diffuses to the damaged interface of polymer, the repair of polymer matrix can be realized. It is a challenge to realize the self-repair of rigid polymer. In order to solve this problem, Bao et al. Prepared rigid and repairable silicone materials based on dynamic borate bonds
[104]. PDMS was crosslinked by dynamic covalent cyclic boroxine to form a 3D network, which endowed the material with high Young's modulus, tensile strength of 9. 46 MPa and elongation at break of less than 10%. After wetting with water, the boroxine exposed on the surface moves in the direction of dissociation, and the surface of the material becomes viscous and soft, thus showing the ability of water to promote repair. In addition, the heating causes the material to soften, thereby increasing the fluidity of the chain and promoting the self-healing process (Fig. 27), so that the properties of the material can be almost restored to the initial state with the extension of the healing time.
In addition to solvent-driven self-healing, gas is also an excellent driving medium because of its advantages of easy access, rapid transmission and good environmental adaptability. Miwa et al. Proposed a kind of aeroplastic silicone elastomer. First, a modified polydimethylsiloxane with amide groups and carboxyl groups in the molecular chain was synthesized, and then part of the carboxyl groups were neutralized by NaOH, and crosslinked by hydrogen bonds between amide groups and ionic aggregates formed by carboxylic acids and sodium carboxylates
[105]. It is found that the elastomer is aeroplastic, and the storage modulus of the elastomer in CO
2 atmosphere is reduced to less than half of that in air atmosphere, because the CO
2 with higher polarizability penetrates the ionic aggregate, resulting in the plasticization of the ionic aggregate and the increase of the rearrangement rate of the ionic crosslinked network (Fig. 28 a). The rate of ionically crosslinked network rearrangement is increased by a factor of about 30 under CO
2 atmosphere compared to that in air. Under CO
2 atmosphere, the elastomer can be quickly repaired at room temperature (Fig. 28 B), even at low temperature.
4 Conclusion and prospect
After two decades of rapid development, dozens of different types of self-healing materials have been reported, showing broad application prospects in many fields. Future research on self-repairing materials should not only focus on exploring new types and mechanisms, but also on how to realize the real industrial application of self-repairing materials. In order to realize the industrial application of self-repairing materials, there are two key problems that must be solved: one is to ensure that the mechanical properties of self-repairing materials can meet the requirements of practical application. The existing materials with high mechanical strength have low self-healing ability, and the materials with good self-healing ability have poor mechanical properties, so how to design and synthesize materials with both high mechanical strength and excellent self-healing ability is a challenging problem. The second is the contradiction between complex preparation process and large-scale use. Most of the existing self-repairing preparation processes are complex, harsh conditions, and often require a large amount of organic solvents, which makes it difficult to achieve large-scale manufacturing and application. Therefore, how to design a green, low-carbon and simple synthesis process to ensure the large-scale application of self-healing materials is still an urgent problem to be solved.
For self-healing silicone materials, although there are many repair systems based on different dynamic structures, there are still opportunities and challenges. Self-healing silicone materials have the advantages of self-healing of damage that traditional silicone materials can not match, and can be better used in functional coatings, electronic sensors and other sophisticated fields. However, in the external-aid self-repairing silicone material, the microcapsules are difficult to disperse uniformly in the polymer substrate, and the interfacial interaction between the capsules and the silicone substrate will affect the mechanical properties of the substrate itself. The external-aid repairing agent should be easy to encapsulate and have environmental stability, but it can be released in time and react quickly to fill the damage under the action of microcracks. In addition, due to the limited amount of microcapsules implanted in the substrate, the repair efficiency of the material still needs to be improved, and the same damage site can only be repaired once, so how to achieve multiple efficient repair of the same site is still a difficult problem. On the other hand, due to the existence of various dynamic bonds, silicone materials often show structural instability, serious decline in mechanical properties, high temperature resistance and aging resistance. In addition, the self-repair process is largely dependent on external environmental stimuli, which can not achieve active self-repair in a real sense, and due to the fatigue of dynamic bonds under external stimuli, the material properties and repair efficiency are greatly reduced after multiple repairs. Therefore, the future development should focus on the self-repair mechanism, thermodynamics and kinetics of silicone materials, especially the influence of the existence of silicone segments in the crosslinking network on the enthalpy and entropy changes in the repair process, so as to solve the problem from the structure-activity relationship of structure-property. It is hoped that the self-repairing silicone materials with wide applicability and excellent comprehensive properties can be developed through a simple synthesis process, which has important guiding significance for exploring the essence of self-repairing materials and guiding the research and development of new self-repairing silicone materials.
At the same time, the self-repairing function of silicone materials should be effectively coordinated with other functions such as photosensitivity, shape memory, antifouling, anticorrosion and antibacterial, so as to make the self-repairing silicone materials develop in a more intelligent direction.So as to expand the application of self-repairing silicone materials in 3D printing, coatings, special adhesives, medical scaffolds, artificial muscles, electronic skin and flexible devices. In a word, as the existing technical problems and material weaknesses are constantly overcome and solved, it is believed that self-repairing silicone materials will have great potential for development in the future.
National Natural Science Foundation of China(52033011)
National Natural Science Foundation of China(52273104)
Guangdong Basic and Applied Basic Research Foundation(2022A1515011972)
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