Hydrogen-bonded polymeric materials with high mechanical properties and high self-healing capacity

Abstract

Microcracks appear in polymer materials during long-term service, which can further propagate into large cracks and lead to failure of materials. In addition, the management of polymer waste pollution is also a major problem in the current society. Fortunately, polymer materials with self-healing ability can be prepared by mimicking the self-repair mechanism of living organisms, thus effectively prolonging the service life. The introduction of reversible interactions not only endows materials with self-healing ability but also facilitates material recycling. This review primarily discusses the strategies and methods for synergistically improving the mechanical performance and self-healing ability of polymer materials based on hydrogen bonds, including introducing multiple hydrogen bonds, increasing hydrogen bond density, controlling the phase separation degree, enhancing molecular chain mobility, achieving the synergistic effects of hydrogen bonds with other reversible bonds, and synthesizing polymer chains with special topological structures. In addition, we also discuss the self-healing mechanisms based on both experimental and simulation results.

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Jianglong Li

Jianglong Li was born in 1999 in Gansu Province, China. He obtained a bachelor's degree in polymer materials from Jiangsu University in 2022 and is currently a master's student in Materials Science and Engineering at Jiangsu University, under the guidance of associate professor Yijing Nie. His research interests are experiments and simulations related to polymer self-healing mechanisms.

Xiaoyu Du

Xiaoyu Du was born in Dali, Yunnan Province, she received her bachelor's degree in materials from Jiangsu University in 2021 and is currently a master's candidate in Jiangsu University under the supervision of associate professor Yijing Nie. Her research interests are research and development of self-healing polymers with dynamic bonds.

Aofei Zhang

Aofei Zhang was born in 1999 in Henan Province, China. He obtained a bachelor's degree in polymer materials from Jiangsu University in 2022 and is currently a master's student in Materials Science and Engineering from Jiangsu University, under the guidance of associate professor Yijing Nie. His research interest is the development of self-healing elastomers containing reversible non-covalent interactions.

Jianlong Wen

Jianlong Wen was born in 1997 in Shanxi Province, China. He obtained his bachelor's degree in polymer materials from Jiangsu University in 2020 and is currently a graduate student in Jiangsu University under the guidance of associate professor Yijing Nie. His research interest is molecular simulations of effective regulation of multiple network structures in polymer crystallization and elastomers.

Lang Shuai

Lang Shuai was born in 1999 in Guizhou Province, China. He obtained a bachelor's degree in polymer materials from Jiangsu University in 2021 and is currently a master's student in materials science and engineering from Jiangsu University, under the guidance of associate professor Yijing Nie. His research interest is the simulation of functional polymers containing dynamic covalent bonds.

Sumin Li

Sumin Li received her PhD degree from Jiangsu University (China) in 2014. Currently, she is working at school of materials science & engineering, Jiangsu University, as a professor. The research of her group covers a wide range, including novel porous materials for energy and environment related applications.

Maiyong Zhu

Maiyong Zhu received his PhD degree from Yangzhou University (China) in 2011. In 2012, he started independent research work at school of materials science & engineering, Jiangsu University (China), as an assistant professor. In 2015, he was promoted to an associate professor. In 2020, he worked as a visiting professor at Kyoto University (Japan) under the support of the China Scholarship Council. Currently, the research of his group covers a wide range, including green strategies for synthesizing advanced functional materials for energy/environment related applications.

Yijing Nie

Yijing Nie received his BS degree in polymer materials in 2007 and MS degree in polymer materials in 2010 from Sichuan University under the supervision of Professor Guangsu Huang. Then, he received his PhD degree in polymer chemistry and physics in 2013 from Nanjing University under the supervision of Professor Wenbing Hu. Since 2013, he joined the School of Materials Science and Engineering in Jiangsu University, as a lecturer. In 2016, he became an associate professor in Jiangsu University. During 2019–2020, he served as a visiting scholar conducting research work at the University of Göttingen (Germany) under the supervision of Professor Marcus Müller. He combines experiments and molecular simulations to investigate self-healing in polymers, polymer crystallization, and structures and properties of rubber nanocomposites. Now, he is the director of department of polymer materials and engineering and the deputy director of institute of polymer materials, and is elected as the Excellent Young Researcher for the Qing Lan Project of Jiangsu Province of China.

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1. Introduction

The use of natural polymers by humans can be traced back to a long time ago. However, it was not until the middle of the nineteenth century that polymer materials began to be studied and rapidly developed.¹ Among them, functional and intelligent polymer materials are the emerging fields that have gradually developed since the 1960s,² and self-healing polymer materials belong to the most concerned category of intelligent polymer materials. When polymer materials are in use, microcracks may appear inside the materials, and even fracture may occur. This can lead to a decrease in material performance or the occurrence of material failure, resulting in economic losses and even safety accidents. In the case of living organisms, when a break or fracture occurs, it can be repaired spontaneously. Therefore, based on the concept of biological self-repair, self-healing design has been applied to the preparation of polymer materials.^3–7 This allows the materials to spontaneously recover from microcracks and other damage that occur during service, not only extending the service life and achieving greater cost-effectiveness for polymer materials, but also preventing sudden failures during application. Especially in special fields such as aerospace where it is difficult to replace or repair damaged materials,^8–10 replacing or repairing damaged materials requires a significant amount of cost. Thus, the preparation of self-healing polymer materials with excellent performance is extremely important. In addition, the waste of traditional polymer materials can cause serious pollution to the natural ecosystem and environment.^11–13 Moreover, in the current context of dwindling resources, recycling and reusing waste materials is the optimal solution to address the waste issues.^14–18 Interestingly, intrinsic self-healing can achieve these two goals, which not only contributes to the extension of the material lifespan but also utilizes the reversible bonding characteristics for recycling and reusing.

In a general sense, self-healing polymers can be divided into extrinsic and intrinsic self-healing polymers.^19–21 The so-called extrinsic self-healing polymers refer to polymer materials that achieve self-healing behaviors through the addition of external repair agents, such as microencapsulated self-healing polymer systems,^22,23 hollow fiber self-healing polymer systems,^24–26 and polymer systems with microvascular networks.^27–29 Intrinsic self-healing polymers refer to polymer materials with reversible interactions between polymer chain segments, including reversible covalent bonding,^30–32 hydrogen bonding,^14,33–35 π–π stacking,^36,37 metal coordination,^38–41 ionic interactions^42–45 and so on, to achieve self-healing behaviors. Comparatively, extrinsic self-healing polymers have the advantages of simple design and better repair effectiveness. The healed polymer materials also exhibit high mechanical strength. However, there is a major disadvantage that when the repair agents are consumed, they can no longer be healed again, and the number of self-healings is limited. In contrast, intrinsic self-healing polymers do not have these limitations. Due to the presence of reversible interactions, polymers can undergo infinite self-healing according to theoretical investigations.^14,27,46

It should be noted that there is a contradiction between self-healing ability and mechanical performance.^47,48 In general, improving self-healing capacity comes at the expense of mechanical strength. On the one hand, self-healing requires sufficient mobility of chain segments (good flexibility), which may impose negative impacts on mechanical strength. On the other hand, the strength of reversible interactions is relatively low, thus leading to a decrease in the mechanical properties of polymer materials. In contrast, the introduction of strong intermolecular interactions will effectively enhance the mechanical strength of polymers, but may cause a decrease in the self-healing properties.^38,49–51 This review discusses the progress of research work on intrinsically self-healing polymeric materials based on hydrogen bonding interactions. Hydrogen bonding is one of the most common interactions in intrinsic self-healing polymers.^52,53 Introducing hydrogen bonding interactions in polymer materials can impart self-healing ability at room temperature.^14,54 However, the bonding energy of hydrogen bonds is smaller than that of covalent bonds,⁵⁵ and thus introducing hydrogen bonds in polymer materials can reduce mechanical properties. Therefore, research on synergistic enhancement of the healing ability and the mechanical performance has received widespread attention.^56–63 This review focuses on exploring the ideas and methods to simultaneously improve the mechanical performance and self-healing ability of polymers (Fig. 1). The self-healing ability is mainly quantitatively reflected by the healing efficiency, which is the ratio of specific mechanical properties (e.g., tensile strength, elongation at break, or toughness) of materials measured before and after healing, expressed as a percentage.^54,64

Fig. 1 Concepts for improving the self-healing ability and the mechanical properties of hydrogen-bonded self-healing polymer systems.

2. Synergistic improvement of mechanical performance and healing capacity

2.1 Increasing the strength of the hydrogen bonding network

In fact, the increase in the mechanical properties of polymers and the enhancement of the self-healing capability are a set of contradictory relationships. To improve the mechanical properties of a certain polymer, such as its mechanical strength, it is inevitably necessary to restrict the mobility of molecular chains. However, this precisely hinders the improvement of self-healing capability, as self-healing requires sufficient mobility of polymer chains to allow them to interpenetrate and form reversible bonds to reconstruct the cross-linked network system. We know that enhancing mechanical properties involves introducing stronger interactions and restricting the movement of polymer chain segments, while self-healing relies on chain segment diffusion and bond reformation. Then, what factors affect both the self-healing ability and mechanical properties of polymers? In other words, which factors can be regulated to achieve the synergistic improvement of the self-healing efficiency and mechanical strength? Based on the strategies proposed in some literature, this paper explores the factors influencing the self-healing ability and mechanical performance of polymers, as well as the methods to synergistically improve the mechanical performance and self-healing ability.

2.1.1 Introducing multiple hydrogen bonds. Introducing multiple hydrogen bonds to enhance the strength of hydrogen bonds is one of the effective methods to improve the mechanical properties and healing ability of materials.^65,66 Compared to a single hydrogen bond, multiple hydrogen bonds have higher bond energy, which can provide polymers with better mechanical properties. However, the introduction of multiple hydrogen bonds not only results in the appearance of stronger physical crosslinking, but also leads to the ordered arrangement of polymer chains, which may in turn restrict the mobility of polymer chain segments, and weaken the self-healing ability of self-healing polymers.⁶⁷

Therefore, this creates a pair of contradictory relationships. On the one hand, due to the higher bond energy of multiple hydrogen bonds, broken hydrogen bonds may more easily pair up with each other, potentially improving the mechanical performance and self-healing capability. On the other hand, the multiple hydrogen bonds inherent in the original polymer reduce the mobility of chain segments, which will limit the random association and reorganization of hydrogen bonds, and have a negative impact on the self-healing ability. Therefore, for the method of introducing multiple hydrogen bonds, the self-healing ability of polymers should be determined by the combined effects of the two factors (the improved binding ability of free hydrogen bonds, and the reduced segment mobility). Furthermore, there are some possible methods to increase hydrogen bond strength without causing a decrease in segment mobility. (I) Introducing structural units with high steric hindrance effects in the hydrogen bond sequence segments of polymer chains to prevent the excessive density of the hard domains formed by multiple hydrogen bonds, and then improve the mobility of chain segments; (II) introducing multiple hydrogen bonds at the end of polymer chains or in the polymer side chains to reduce the limitation of multiple hydrogen bonds on the mobility of polymer main chains.^67–69

For example, Yuan et al. utilized functional glycylglycine methyl ester as a chain terminator to prepare a waterborne polyurethane (PU) elastomer in which triple hydrogen bonds can be formed between polymer chains (Fig. 2(a)).⁷⁰ This material has good self-healing ability (Fig. 2(b)), and after 8 h of healing at room temperature, the tensile strength and strain were recovered to 1.18 MPa and 1240%, respectively, with a healing efficiency of up to 91.5%.

Fig. 2 (a) The triple hydrogen bonds formed between the molecular chains in PU elastomers; (b) the healing process of polymers.⁷⁰ Reproduced with permission from ref. 70, Copyright 2022, Elsevier.

Fu et al. designed a dynamic hard domain structure by introducing a chain extender with a highly asymmetric cycloaliphatic structure.⁷¹ This approach is used to modulate the packing density of multiple hydrogen bonds, thereby achieving a synergistic improvement in both mechanical performance and self-healing capability. Furthermore, to validate the significant effect of this high steric hindrance chain extender, comparisons were made with linear chain extenders lacking steric hindrance and those containing benzene rings. As shown in Fig. 3, a polyurethane–urea supramolecular elastomer (PPGTD–IDA; poly(propylene glycol) with tolylene 2,4-diisocyanate terminated (PPGTD) and isophorone diamine (IDA)) with steric hindrance was able to fully recover its mechanical performance after 48 h of healing, whereas the healing efficiencies of PPGTD–HAD (without steric hindrance; 1,6-hexanediamine (HAD)) and PPGTD–PDA (containing benzene rings; p-phenylenediamine (PDA)) were only 11.63% and 0.1%, respectively. Moreover, extending the healing time had no positive effect on the latter two.

Fig. 3 (a) Molecular structures with steric hindrance effect (green), no steric hindrance (red), and containing a benzene ring (blue). (b) Optical microscopy images of the wound healing process during the self-healing process.⁷¹ Reproduced with permission from ref. 71 Copyright 2020, John Wiley & Sons.

Zeng et al. introduced 2-ureido-4[1H]-pyrimidinone (UPy) groups with quadruple hydrogen bonding⁷² into side chains of poly(butyl acrylate) (PBA) copolymers (Fig. 4).⁷³ The fractured PBA–UPy films can fully recover their self-adhesion strength to 40%, 81%, and 100% in 10 s, 3 h, and 50 h, respectively, under almost zero external load. Zhang et al. used biologically derived carboxyl cellulose nanocrystals (C-CNCs) with chitosan (CT)-modified epoxy nature rubber latex to construct multiple hydrogen bonds and nanostructured conductive networks to design sensors with high sensitivity and self-healing capability, which exhibited a high healing efficiency of 93% after the third healing process.⁷⁴

Fig. 4 Schematic of (a) the chemical structure of PBA–UPy, (b) hydrogen bonds between two UPy groups, and (c) polymer chains functionalized with UPy groups.⁷³ Reproduced with permission from ref. 73, Copyright 2014, John Wiley & Sons.

The introduction of multiple hydrogen bonds into polymers plays a role in synergistically improving the mechanical performance and self-healing ability of polymers. The interaction strength of multiple hydrogen bonds is greater than that of a single hydrogen bond,⁷⁵ which contributes to the improvement of the mechanical properties and healing efficiency.

Li et al. investigated the influence of hierarchical hydrogen bonding composed of PU, urea, and UPy moieties (single, double, and quadruple hydrogen bonding) on the self-healing ability and mechanical properties of polymers.⁷⁶ Poly(tetramethylene ether)glycol (PTMEG) was used as a soft segment, and dicyclohexylmethylmethane-4,4′-diisocyanate (HMDI) with a symmetric alicyclic structure as a hard segment. Then, polymer elastomers with both excellent self-healing capability and mechanical performance can be prepared by adjusting the ratio of soft and hard segments (Fig. 5). After fracture and healing for 24 h at 100 °C, the tensile strength and toughness of the healed polymer reached 44 MPa and 345 kJ m⁻³, respectively, and the self-healing efficiency reached 90%, demonstrating the presence of excellent self-healing ability and mechanical properties.

Fig. 5 Illustration of the self-healing and self-recovery process of an elastomer by the dynamic reformation of its modular structure consisting of hierarchical hydrogen-bonding interactions.⁷⁶ Reproduced with permission from ref. 76, Copyright 2018, John Wiley & Sons.

The use of quadruple hydrogen-bonded supramolecular polymers (SMPs) as substrates for thin film electrodes has been reported by Bao and co-workers.⁷⁷ SMPs are composed of soft chain segments (poly(tetramethylene)glycol and tetraethylene glycol) and strong reversible quadruple hydrogen bonding cross-linkers (Fig. 6(b)). The former contributes to the formation of soft phases of SMPs, while the latter provides ideal mechanical performance for SPMs. By varying the amounts of cross-linkers, the properties of polymers can be controlled, and then soft, stretchable, and tough polymer elastomers can be obtained (Fig. 6(a)). It was found that the SMP containing 20 mol% UPy units exhibits excellent self-healing ability and mechanical properties. It has high toughness, a fracture energy of 30 000 J m⁻², and a healing efficiency of 88% after 48 h of self-healing (Fig. 6(c)).

Fig. 6 (a) The chemical structure of supramolecular polymers (SPMs-0-3, where n represents the modulation of mechanical properties by varying the number of UPy cross-links). (b) Cartoon representation of the proposed mechanism for highly stretchable SPMs. (c) Stress–strain curves of both pristine and self-healing samples at room temperature after three different healing times of 6, 24, and 48 h.⁷⁷ Reproduced with permission from ref. 77, Copyright 2018, American Chemical Society.

PU elastomers introduced with catechol functionalized units and molecular weight adjustable soft segments were reported by Zhou et al.⁷⁸ This design leads to the formation of double hydrogen bond and single hydrogen bond motifs between polymer chains, enabling the coordination of polymer mechanical properties and self-healing capabilities. For instance, the fracture strength increased from 1.3 MPa to 5.7 MPa, and the healing efficiency improved from 14.9% within 2 h to 96.7%. Li et al. used highly asymmetric aliphatic cyclic diamine and isophorone diisocyanate (IPDI) as hard segment components to obtain room temperature self-healing PU elastomers.⁷⁹ The hydrogen bonds with different strengths formed between urethane bonds and urea bonds provide the driving force for the self-repair of PU at room temperature. The highly asymmetric aliphatic cyclic diamine plays an important role. On the one hand, when the molecular chain network is subjected to external strain, the aliphatic cyclic diamine can quickly transfer external forces due to the rigidity of the rings, allowing the entire molecular chains to bear the strain together, and thereby improving the modulus of the polymer. On the other hand, the steric hindrance effect can regulate the packing density of hard segments, enabling chain segments to have good mobility and thus improving the healing efficiency. The material exhibits excellent mechanical properties and self-healing performance, with a maximum tensile strength of 3.85 MPa and an elongation at break of almost 3000%. After 12 h of healing at room temperature, the mechanical properties of the material can be fully restored. Yang and coworkers coordinated the self-healing ability and mechanical properties of polymeric materials (P-TDI-IP; poly(oxy-1,4-butanediyl) abbreviated as P, 2,4′-tolylene diisocyanate abbreviated as TDI, and isophorone diisocyanate abbreviated as IP) by introducing stronger dual hydrogen bonds.⁸⁰ As shown in Fig. 7, TDI can form dual hydrogen bonding motifs between molecules, enhancing the mechanical strength of the polymer and giving it excellent elasticity. However, IPDI only forms single hydrogen bonding motifs due to steric hindrance. The weak single hydrogen bonds can be easily broken and reformed, giving the polymer rapid self-healing ability and high elongation. The material obtained using this method has a mechanical strength of 1.3 MPa, a tensile strain of 2100%, and a healing efficiency of 97% after 6 h at room temperature.

Fig. 7 (a) Molecular structure of P-TDI-IP synthesized with TDI, IP and poly(oxy-1,4-butanediyl). (b) Supramolecular network of P-TDI-IP. (c) Cartoon depiction of P-TDI-IP networks with hydrogen bonding of different strengths.⁸⁰ Reproduced with permission from ref. 80 Copyright 2020, Elsevier.

Guo et al. introduced T-type chain extenders with multiple hydrogen bonds into PU chains to regulate the degree of phase separation and chain mobility, and prepared self-healing elastomeric materials with good mechanical properties (Fig. 8(a)).⁸¹ They compared the self-healing efficiencies of polymeric materials without side chains (Fig. 8(c)) and with side chains (Fig. 8(b)), and found that the self-healing efficiency of polymeric materials without side chains was lower. This suggests that the flexibility of T-type side-chain hydrogen bonding can regulate the stiffness of the polymer material, which is more favorable for self-healing. The presence of multiple hydrogen bonds improves the mechanical properties of the material so that the maximum stress of the prepared samples is 3.14 ± 0.05 MPa and the elongation at break is 3200% ± 160%. In addition, the self-healing efficiency of the material is 96.7% after 8 h of repair at room temperature.

Fig. 8 (a) Schematic illustration of hydrogen bonding in polyurethane networks, (b) healing efficiencies of T-type side chain-containing polyurethanes at different times, and (c) healing efficiencies of polyurethanes without side chains at different times.⁸¹ Reproduced with permission from ref. 81 Copyright 2021, John Wiley & Sons.

Our group recently reported the preparation of a PU elastomer by introducing UPy units at chain ends (PU-UPy).⁸² In comparison with the PU elastomer, which does not contain UPy units (PU-600), PU-UPy exhibits higher mechanical strength. PU-600 only contains single hydrogen bonds between carbonyl groups and urethane NH groups, whereas PU-UPy contains stronger quadruple hydrogen bonds, resulting in stronger interactions between polymer chains and higher mechanical strength. When healing temperatures are low, the healing ability of PU-UPy is weaker than that of PU-600 due to the greater intermolecular interactions in PU-UPy, which limits chain diffusions during the healing process. In contrast, when healing temperatures are increased and polymer chains have sufficient mobility, PU-UPy will exhibit higher healing efficiency, indicating that the introduction of quadruple hydrogen bonds can improve the healing efficiency at appropriate healing temperatures. In addition, the introduction of multiple hydrogen bonds can also result in the enhancement in polymer mechanical strength.

The above description indicates that the introduction of multiple hydrogen bonds can enhance the interactions between polymer chains, thereby improving the mechanical strength and toughness of polymer materials. Furthermore, in the healing process, multiple hydrogen bonds exhibit a contradictory yet unified dialectical relationship with healing efficiency. On the one hand, strong intermolecular interactions can limit the mobility of polymer chain segments and somewhat restrict the healing ability. On the other hand, strong intermolecular interactions facilitate the reformation of broken bonds, which can better restore the mechanical properties of polymer materials. Under some appropriate conditions (such as relatively high temperatures), the introduction of multiple hydrogen bonds is beneficial to the simultaneous improvement of the healing efficiency and mechanical properties of polymer materials, achieving stability in the use of polymeric materials. Table 1 summarizes the methods to balance the mechanical performance and self-healing ability in networks containing multiple hydrogen bonds.

Table 1 The research studies on the self-healing ability and mechanical performance in networks containing multiple hydrogen bonds

Methods balancing mechanical properties and self-healing ability	Mechanical properties before healing			Healing conditions	Healing efficiency	Ref.
	Tensile strength	Elongation at break	Toughness
H-Bonded thiourea arrays are geometrically nonlinear (less ordered), and no crystallization occurs.	45 ± 8 MPa	393 ± 5%	—	Room temperature compression for 6 h	100%	83
Glycine methyl ester capped to improve chain flexibility and enable triple hydrogen bond formation.	1.29 MPa	—	—	Room temperature for 8 h	91.50%	70
Introduction of a highly asymmetric alicyclic structure within the chains restricts crystallization and results in a relatively loose internal structure of the chains.	4.83 MPa	2010%	65.49 MJ m^−3	Room temperature for 48 h	100%	71
Quadruple hydrogen bonds are in the side chains, and structural units with spatial site resistance in the main chains can reduce the order of the chains.	48.5 ± 6.3 MPa	—	386.5 ± 19.6 MJ m^−3	100 °C for 2 h	90%	84
The main chain contains quadruple hydrogen bonds to increase mechanical properties. Combination of flexible soft segment polytetramethylene glycol and IPDI with spatial site resistance for increased chain motility.	0.91 MPa	17 000%	—	Room temperature for 48 h	88%	77
Introduction of UPy units containing quadruple hydrogen bonds in side chains reduces the restriction on the mobility of the main chains, providing a strengthening and toughening effect.	15.34 MPa	762.3%	69.1 MJ m^−3	Room temperature for 2 h	83.9%	85
Introducing an appropriate amount of UPy units with end-capping can prevent cross-linking in the middle of chain structure, while the high steric hindrance effect of IPDI can hinder crystallization and improve the mobility of the chains.	3.02 ± 0.10 MPa	1508 ± 85%	—	Room temperature for 1440 min.	97%	86
The main chain uses asymmetric cycloaliphatic units and 2′-deoxythymidine side chains to increase the steric hindrance effect, while quadruple hydrogen bonds and sextuple hydrogen bonds contribute to the improvement of mechanical strength.	15.3 MPa	—	100.75 MJ m^−3	40 °C in ethanol, for 6 h	89.50%	87
The high steric hindrance effects of IPDI and isophorone diamine (IPDA), combined with the biphenyl structure of 4,4-methylenedianiline (MDA), reduce the packing density of hard domains, which enhances the mobility of the chains. The interaction of single and double hydrogen bonds imparts mechanical properties and self-healing capabilities.	17.93 MPa	109.88%	58.3 MJ m^−3	Room temperature for 2 h	95.93%	88
1,8-Methanediamine (MD) with a highly asymmetric cycloaliphatic structure in the main chains results in a loose packing density of hard domains, which enhances the mobility of the chains. Additionally, multiple hydrogen bonds provide excellent mechanical properties and self-healing capabilities.	30.3 MPa	1114.60%	126.4 MJ m^−3	Room temperature for 12 h	90.80%	89
Using highly asymmetric structural units with steric hindrance effects in hard domains results in a relatively loose packing density of multiple hydrogen bonds, achieving a balance between mechanical properties and self-healing capabilities.	3.64 MPa	2803%	—	Room temperature for 12 h	95.20%	90

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2.1.2 Increasing the density of hydrogen bonds. The self-healing ability of hydrogen-bonded self-healing polymers stems from the reversible interactions of hydrogen bonds, and meanwhile hydrogen bonds can also serve as physical cross-linking points in polymer networks, thus playing a crucial role in the mechanical performance of polymer materials. Therefore, increasing the density of hydrogen bonds in polymer networks is also a strategy to synergistically enhance the self-healing ability and mechanical properties of polymers. For linear hydrogen-bonded PU elastomers, the density of hydrogen bonds can be controlled by manipulating the main chain length of linear diols. For example, when the content of the chain extender is constant, a larger value of n in the diol HO–X_n–OH leads to a lower density of hydrogen bonds in polymer networks. For side-chain hydrogen-bonded PU elastomers, the density of hydrogen bonds can be controlled by adjusting the content of side-chain hydrogen bonds.

Zhang et al. introduced dopamine into aminoethyl methacrylate, where the phenolic groups on dopamine further increase the density of hydrogen bonds, thereby enhancing the mechanical properties and self-healing capabilities of the polymer. The tensile strength is 1.9 MPa, the fracture elongation is 5000%, and the efficiency reaches 86% after 8 h of healing at 25 °C.⁹¹ The impact of hydrogen bond density in polymer networks on the mechanical properties and self-healing ability of elastomeric biomaterials has been reported by Feng's group.⁹² Biodegradable polyurethane ureas (PUUs) were synthesized using poly(ε-caprolactone) diol (PCL) and L-lysine ethyl ester diisocyanate (LDI), with L-lysine ethyl ester dihydrochloride as a chain extender (LEED). By adjusting the ratios of the chain extender LEED to PCL, polymer samples with different hydrogen bond densities were prepared. Tests on mechanical strength and elongation at break revealed that samples with lower hydrogen bond densities exhibited almost no self-healing ability at 37 °C, making it difficult to obtain accurate healing efficiency values. In contrast, samples with higher hydrogen bond densities maintained a tensile strength of 4.23 MPa and an elongation at break of 627% after 30 minutes of healing. This indicates a close relationship between the self-healing capability of the polymer and hydrogen bond density, confirming that an increase in hydrogen bond density indeed enhances the material's self-healing ability.

Fu et al. reported the synthesis of a glassy PU (GPU) with high hydrogen bond density by self-assembly using IPDI and pentaethylene glycol as the raw materials.⁹³ Due to the presence of a large number of loosely stacked weak hydrogen bonds, the glass transition temperature (T_g) of GPU is higher than room temperature. However, GPU exhibits a rapid self-healing behavior at room temperature. After only 10 min of healing under external force, it can recover a tensile strength of 7.74 ± 0.76 MPa, and after 60 min, it can basically restore its original mechanical strength. Similarly, Wu's group also prepared glassy polymers with a hyperbranched structure that can be self-healed at room temperature.⁹⁴ The new hyperbranched polymers can exhibit astonishing instant self-healing, and the tensile strength can recover to 5.5 MPa within 1 min. Both the work of Fu's group and Wu's group demonstrated that there are still many free N–H motifs/branch units/terminals that have strong mobility and can move locally under the glassy state, which is beneficial for the self-healing behaviors.

However, Konkolewicz et al. raised an interesting question: are self-healing polymers and elastomers always tougher with more hydrogen bonds?⁹⁵ In other words, polymer materials containing too many hydrogen bonds may exhibit weak self-healing ability. The increase of hydrogen bond density can obviously increase the mechanical strength or modulus of polymer materials, but can also lead to the decrease of polymer chain mobility, which will further cause the reduction of polymer self-healing efficiency. Previously, our group performed Monte Carlo simulations to investigate the effect of reversible interaction site density on the self-healing process of polymer systems, as shown in Fig. 9(a).⁸² It was found that the dependences of the contents of polymer beads, chain centroids and reversible interaction sites in the crack region on interaction site density are different. Three regions exist in Fig. 9(b). In the first region, the increase of interaction site density causes the increase of the contents of beads and chain centroids, and the number of interaction sites. This means that the improvement in interaction site density can promote the different microstructure reconstructions in the crack region, which can result in the increase of self-healing ability. In the second region, the improvement in interaction site density causes the increase in the contents of beads and the number of interaction sites, but the decrease of the contents of chain centroids. Namely, chain diffusion would be hindered due to the further increase of interaction site density. Under this condition, the change of the self-healing ability depends on the competitive outcome of the two phenomena mentioned above (the decrease in the chain centroid content, and the increase in the bead content and the number of interaction sites). In the third region, the improvement in interaction site density induces the decrease of the contents of beads and chain centroids. That is, if interaction site density is very high in polymer systems, the diffusion of both segments and chains will be restricted, which may lead to the decrease in self-healing ability. These simulation findings are in agreement with the experimental results of Zhang et al.⁹⁶ They found that the tensile strength of polymer materials increases with the increase of the hard segment content from 20% to 50%, but the self-healing efficiency decreases. The decrease in the self-healing ability is attributed to the fact that the number of hydrogen bonds increases and the degree of cross-linking becomes larger with the increase of the hard segment content, thus blocking the movement of chain segments, and resulting in the decrease of the self-healing ability. In addition, our group further combined molecular simulations and machine learning to study the contributions of different structural reconstructions to polymer self-healing ability.⁹⁷ It was found that chain interpenetrations in the crack region make a great contribution to the self-healing of polymers.

Fig. 9 (a) Morphology changes of system 2 during healing at T* = 0.5. (b) Dependences of final normalized contents of chain centroids and beads, and the final number of interaction sites on interaction site density.⁸² Reproduced with permission from ref. 82, Copyright 2023, Royal Society of Chemistry.

Tu et al. designed elastomeric materials with low and high-density hydrogen bond functional regions in polymer networks to explore the contradictory relationship between self-healing efficiency and mechanical strength.⁹⁸ Long-chain polyether amine (red) and short-chain polyether amine (green) copolymerize with IPDI (Fig. 10(a)). The hydrogen bond density formed by the long-chain polyether amine portion is small, providing the polymer with self-healing capability as a soft phase, while the hydrogen bond density of the short-chain polyether amine portion is higher, imparting certain mechanical properties to the polymer as a hard phase (Fig. 10(b)). Then, in this way, by simultaneously constructing regions with different hydrogen bond densities in PU systems, the synergistic improvement of material mechanical strength and self-healing ability can be achieved simultaneously.

Fig. 10 (a) Synthesis process of self-repairing PU. Long chain polyether amine (red) and short chain polyether amine (green). (b) High hydrogen bond density region composed of long chain polyether amine (red) and low hydrogen bond density region composed of short chain polyether amine (green).⁹⁸ Reproduced with permission from ref. 98 Copyright 2023, Elsevier.

Overall, there is a dialectical relationship between the increase in hydrogen bond density and the self-healing ability of polymers. On the one hand, the increase in hydrogen bond density strengthens the exchange effect between hydrogen bond complementary motifs, increases the number of free hydrogen bonds, and provides a stronger driving force for the reassociation of broken hydrogen bonds, thereby facilitating the self-healing. On the other hand, the increase in density of hydrogen bonds will limit the mobility of polymer segments, which is detrimental to the self-healing ability. However, increasing hydrogen bond density usually improves the mechanical properties of polymers by limiting the mobility of polymer chains or absorbing more energy during fracture. Therefore, introducing an appropriate proportion of hydrogen bonds is beneficial for simultaneously improving self-healing ability and mechanical properties, otherwise it will be unbalanced.

2.2 Controlling the degree of phase separation

Wool and O'Connor explored the mechanisms of polymer healing and proposed that the healing process involves five stages: surface rearrangement, surface approach, wetting, diffusion, and randomization.⁹⁹ For intrinsic self-healing materials, reversible interactions between functional groups are crucial as they provide the driving force for healing. Additionally, diffusion is the most fundamental condition for healing to occur, and healing can only occur when molecular chains come into contact with each other and diffuse into each other. It is generally believed that the flexibility or mobility of molecular chains directly determines the self-healing rate of polymer materials.³ Therefore, the mobility of polymer segments or molecular chains plays a significant role in the self-healing process. After polymer fracture, the reformation of reversible bonds is required for material healing. When the mobility of molecular chains or segments at the fracture surface increases, chains can diffuse more deeply into each other, leading to faster bond reformation, and ultimately improving the healing speed, efficiency, and mechanical properties of healed polymers. Some research work has mentioned that the introduction of multiple hydrogen bonds and the regulation of hydrogen bond density can help balance the self-healing ability and mechanical performance of self-healing materials.^100–102 However, when the hydrogen bond density increases or stronger hydrogen bonds are introduced, the polymer mechanical performance can be greatly improved, but this will undoubtedly reduce the mobility of polymer chains and limit the healing efficiency. Therefore, this section specifically discusses methods for adjusting the packing density of hard domains and strategies for enhancing the mobility of polymer chains.

The mechanical properties and self-healing ability of polymer materials based on hydrogen bonds are closely related to the structure of hard domains. Different chain extenders and isocyanates affect the aggregation structure of polymers and thus the self-healing ability and mechanical properties of polymers.^103,104 This is because the structure of hard domains affects the mobility of polymer chains and the strength of intermolecular interactions. Therefore, it would be a good strategy to use isocyanate functional groups and chain extenders with a certain steric hindrance effect, forming a suitable hard domain packing density that will help achieve self-healing polymer materials with high mechanical performance and strong chain segment mobility. Hwang and co-workers reported thermoplastic polyurethanes (TPUs) with different hard domain stacking densities.¹⁰⁵ Hard segments of the materials consist of four different diisocyanates: IPDI (IP; asymmetric alicyclic structure), 4,4′-methylenebis(cyclohexyl isocyanate) (HM; symmetric alicyclic structure), 4,4′-methylenebis(phenyl structure) (M; aromatic structure), and hexamethylene diisocyanate (H; liner aliphatic structure). Ethylene glycol (EG) and bis(4-hydroxyphenyl)disulfide (SS) were used as the chain extender. The TPUs are designated as X–Y, where X and Y denote the abbreviation of the diisocyanate monomer and chain extender, respectively (Fig. 11). Among them, IP-SS exhibits the best self-healing ability, followed by IP-EG and HM-SS, while H-SS and M-SS show the weakest self-healing ability, with no healing capability even at 80 °C. This is attributed to the diisocyanate used in H-SS and M-SS, which leads to the formation of a tightly packed hard domain structure, restricting the mobility of polymer chains and consequently limiting the self-healing ability of the polymer. In contrast, the non-aromatic rings in HM-SS, the asymmetric lipid rings in IP-SS and IP-EG provide better mobility to the polymer chains, ensuring the self-healing ability of the polymer.

Fig. 11 Synthetic routes to TPUs with four different diisocyanates (i.e., IP, HM, M, and H) and two chain extenders (i.e., SS and EG).¹⁰⁵ Reproduced with permission from ref. 105, Copyright 2018, John Wiley & Sons.

Xu et al. utilized the unique asymmetric aliphatic cyclic structure of IPDI and IPDA to construct a loosely packed hard domain structure.¹⁰⁶ Using IPDI and IPDA as the hard segments of the molecular chains, microphase separation is induced through hydrogen bonding between the molecular chains, imparting mechanical strength and toughness to the material. The large spatial hindrance of cycle structures prevents the hard domains from having a high density, facilitating chain segment migration. Moreover, the degree of microphase separation can be controlled by adjusting hard segment contents, thereby achieving the coordination of the mechanical properties and self-healing ability. The materials prepared using this method exhibited a maximum tensile strength greater than 45 MPa, a maximum elongation at break of 850%, and a self-healing efficiency of 95%.

Wang et al. investigated the degree of phase separation and self-healing performance of elastomers using four aliphatic chain extenders.¹⁰⁷ As shown in Fig. 12, OPU represents the use of hexanediol (HDO) as chain extender for the synthesis of PU, while APU, CPU, and SPU represent the use of hexane diamine (HDA), cysteamine (CY), and cystine dimethyl ester (CDE) as chain extenders for the synthesis of poly(urethane-urea), respectively. Among them, OPU is a micro-interphase compatible polymer and the other three are polymers with phase-separated structures. The degree of phase separation and mechanical strength increases in the order of OPU, SPU, CPU and APU. The relaxation time of SPU is smaller than those of APU and CPU, suggesting that the spatial barrier effect of methyl ester groups weakens the degree of microphase separation and enhances the mobility of chains. The relaxation time of CPU is smaller than that of APU because the exchange of disulfide bonds promotes chain migration and weakens the degree of phase separation. However, the change in the self-healing ability is opposite to the change in the mechanical strength for these four samples. That is, the increase of the degree of phase separation will cause the appearance of more hard domain parts with high packing density, and hinder the mobility of chain segments, leading to the improvement of the mechanical strength at the expense of the self-healing ability. Therefore, introducing appropriate contents of hard domains, packing density and bond exchange ability into polymer systems will help improve the chain mobility of polymers, thereby synergistically improving mechanical properties and self-healing ability.

Fig. 12 Synthesis routes for polyurethane (OPU) and polyurethane–urea (APU, CPU and SPU) using four different chain extenders.¹⁰⁷ Reproduced with permission from ref. 107, Copyright 2021, John Wiley & Sons.

Introducing a certain number of flexible blocks into polymer chains will be beneficial for the improvement of polymer chain mobility. A self-healing PU elastomer with high self-healing ability was prepared by our group.¹⁰⁸ Firstly, the PU precursors were first synthesized from polycarbonate diol (PCDL), IPDI and 1,4-butanediol (BPO), and then soft polypropylene glycol (PPG) blocks were introduced at the ends of the polymer chains of the precursors. Compared to PCDL, PPG has a lower T_g,^109,110 and the flexibility of the PPG molecular chains is better, thus improving the mobility of the PU chains. It was found that the best balance of the self-healing ability and mechanical properties was achieved by using PPG-600 (the molecular weight of PPG is 600 g mol⁻¹), as the insertion of PPG with longer lengths would reduce the hydrogen bond density in the PU chains. The PU elastomer obtained by this method shows excellent mechanical strength and self-healing ability, with a tensile strength of 20.1 MPa and a healing efficiency of 79.85% at a healing temperature of 45 °C.

2.3 Multiblock copolymers

Multiblock copolymers include linear and nonlinear block copolymers. Linear block copolymers can be classified based on the number of blocks into diblock, triblock, and multiblock structures, while nonlinear block copolymers include star-shaped, grafted, and branched structures.^111–113 The advantage of multiblock copolymers lies in the use of different blocks with varying T_g, which helps impart both rigidity and flexibility to the polymer. This approach maintains the good mobility of flexible blocks while also providing excellent mechanical properties due to the rigid blocks. Additionally, by adjusting the content of the rigid blocks, one can control the mechanical properties of the block copolymer, ensuring that the copolymer retains good mechanical performance, while the soft segments maintain good mobility. This creates a balance between mechanical properties and self-healing capability, allowing for rapid repair of damage while preserving excellent mechanical performance. Therefore, self-healing polymeric materials with high mechanical properties based on hydrogen bonds can be obtained by designing the structure of multiblock copolymers to contain flexible and rigid blocks in polymer chains and introducing hydrogen bond motifs into different blocks. Guan et al. reported a hydrogen-bonding brush polymer (HBP) using polystyrene backbone chains as hard phases and polyacrylamide (PA-amide) brushes as soft phases.¹¹⁴ The multi-phase design of the soft and hard structure gives the polymer good mechanical properties and self-healing ability. The HBP had good mechanical properties and self-healing ability, with a Young's modulus of 17.3 + 0.3 MPa, a strain at break of 780 + 15%, and a self-healing efficiency of 92% based on the recovery of elongation. Later, they prepared a triblock copolymerized self-healing elastomer using 2-(2-bromoisobutoxy)ethyl methacrylate (BIEM), PMMA and 5-acetamidoamyl acrylate (AAPA), which exhibited higher tensile strength and Young's modulus in terms of mechanical properties, while maintaining good self-healing ability, with a strain recovery of 80% after 24 h of self-healing.¹¹⁵ Similarly, Sun's group used rigid polyimide (PI) as the hard segment and hydrogen-bonded (PUU) as the soft segment in block copolymers to construct a polymer elastomer with a phase-separated structure (PI-PUU) (Fig. 13(a)).¹¹⁶ In this design, the PI segments self-assemble into a rigid nanostructure serving as nanofillers to provide excellent mechanical strength to the elastomer, while the PUU segments containing multiple hydrogen bonds can dissociate under external forces, effectively dissipating energy to enhance the strength and toughness of elastomers. The elastomer developed with this soft-hard structure exhibits outstanding mechanical properties, with a tensile strength of 142 MPa (Fig. 13(b)). Furthermore, this elastomer demonstrates remarkable self-healing capability, with almost complete restoration of its mechanical properties after healing at 50 °C for 24 h. In addition, to verify the excellent recyclability of the elastomer, the elastomer recovered through thermal pressing and solvent-assisted recovery exhibits mechanical properties identical to the original polymer.

Fig. 13 (a) Synthesis route of PI-PUU. (b) The stress–strain curve of PI-PUU shows excellent mechanical strength.¹¹⁶ Reproduced with permission from ref. 116, Copyright 2023, John Wiley & Sons.

Furthermore, Watanabe et al. ingeniously designed multiblock self-healing ion gels that exhibited rapid self-healing at room temperature.¹¹⁷ The block composition of the polymer includes polystyrene (PS) blocks, hydrogen-bonded blocks composed of N,N-dimethylacrylamide (DMAAm), and acrylic acid (AAc) copolymers (PS-b-P(DMAAm-r-AAc)). In the hydrogen bonds of the polymer crown chains, PDAAM acts as the hydrogen bond acceptor and the AAc acts as the hydrogen bond donor (Fig. 14). The pristine ion gel samples exhibited the fracture stress and strain of 0.32 MPa and 400%, respectively, due to the interactions of the coronary chains through multiple hydrogen bonds. The stress–strain curves after 3 h of self-healing were found to be comparable to those of the pristine samples, and the cyclic voltammograms (CVs) of the pristine and healed ion gel sheets obtained at a scan rate of 1000 mV s⁻¹ were almost identical. This indicates that the electrochemical and mechanical properties of the materials can be well recovered after self-healing.

Fig. 14 Schematic illustration and chemical structures of the self-healing micellar ion gel.¹¹⁷ Reproduced with permission from ref. 117 Copyright 2018, John Wiley & Sons.

Xia and colleagues used crystalline dodecanedioic acid and high steric hindrance IPDA as hard segments, with PTMEG as a soft segment.¹¹⁸ The soft and hard segments have different values of T_g, and increasing the hard segment content enhances microphase separation. This allows for the adjustment of the hard segment content to balance mechanical properties and self-healing capability. The highly asymmetric cycloaliphatic structure plays a crucial role, as this non-uniform structure restricts the regular arrangement of polyamide segments, reducing the impact of crystallization on the mobility of the polymer chains. This allows for hydrogen bond network rearrangement and polymer chain movement at lower temperatures. By appropriately adjusting the length of the polyamide segments, the material achieved a tensile strength of 32.0 MPa, a fracture elongation of 1881%, and a toughness of 328.9 MJ m⁻³. After healing at 110 °C for 12 h, the recovery rates for tensile strength and fracture elongation were 67% and 78%, respectively.

A novel multiphase supramolecular liquid crystal elastomer based on liquid crystal block copolymers (LCBCPs) has been reported by Chen et al.¹¹⁹ It is a diblock copolymer composed of side-chain liquid crystal polymers (SCLCPs) and hydrogen-bonded flexible polymers (BCPs), which will form a microphase-separated structure with SCLCP chains embedded in the soft chain matrix. In this polymer structure, the microphase-separated SCLCP provides good mechanical strength and toughness, while the flexible segment with hydrogen bonds plays a role in regulating microphase separation and providing self-healing properties. After 30 min of sample healing at room temperature, the fractured samples can be joined together.

He et al. reported that polymer elastomers with self-healing, transparent and conductive properties can be prepared by the copolymerization of acrylic amide/choline chloride (AAm/ChCl) and maleic acid/choline chloride (MA/ChCl),¹²⁰ as shown in Fig. 15. The presence of a poly(AAm/ChCl) hard block can cause the increase of mechanical strength and T_g, while the presence of a poly(MA/ChCl) soft block is beneficial for the improvement in the flexibility of polymer networks. The multiple hydrogen bond interactions between hydroxyl, carboxyl and amino groups give the elastomer good self-healing ability. The mechanical properties of the elastomer can be controlled by adjusting the molar ratio of AAm to MA. When the molar ratio of AAm : MA is 1 : 1, Young's modulus of the poly(AAm/ChCl-co-MA/ChCl) film is 0.1 MPa, which is equivalent to that of traditional soft rubber. It is worth noting that the material has a wide range of healing temperature and good healing ability. After repairing for 72 h at −23 °C, the repair efficiency was 73 ± 2% (Fig. 15(c)). As the temperature increases, the interaction between molecular chains increases. At 20 °C, the healing efficiency of the polymer film could reach 91 ± 2%. It reaches 94 ± 2% at 60 °C. Its wide self-healing temperature range gives the material the potential to still exert its self-healing effect in harsh environments. Thus, taking advantage of the superiority of multiblock copolymerization methods, the molecular structure can be flexibly designed to synergistically improve the self-healing ability and mechanical properties of polymeric materials.

Fig. 15 (a) Demonstration of the healing process and (b) chemical structure of acrylic amide (AAm)/choline chloride (ChCl) and maleic acid (MA)/ChCl type polymerizable deep eutectic solvent (PDES) commoners and their copolymerized poly(AAm/ChCl-co-MA/ChCl) elastomers. (c) The healing efficiency at different temperatures was 73 ± 2% even at −23 °C. Reproduced with permission from ref. 120 Copyright 2020, American Chemical Society.

2.4 Multiple reversible networks

Hydrogen bonds have lower bond energy compared to covalent bonds.⁵⁵ Therefore, introducing hydrogen bonds into polymers to achieve self-healing will inevitably sacrifice the mechanical properties of polymers to some extent. In recent years, self-healing materials based on non-hydrogen bonding interactions have been widely reported.^{3,21,54,121–128} Therefore, it may be considered to explore the resolution of the conflict between the mechanical properties and self-healing ability in hydrogen bond-based self-healing polymers by combining hydrogen bond networks with other reversible networks. Self-healing polymers with only strong network cross-linking lack energy dissipation, while those with only weak network cross-linking are prone to creep, have low stability, and exhibit low elasticity.¹²⁹ In a dual-network structure composed of a hydrogen bond network and a dynamic bond network (reversible dynamic covalent bonds, metal coordination bonds or ionic interactions), the hydrogen bond network, being weaker, serves as sacrificial bonds to dissipate energy and enhance the material's toughness. The other stronger network improves the material's mechanical strength and stability.¹³⁰ Additionally, since both the networks are dynamic, their reorganization under appropriate stimuli enhances the material's self-healing capability. Table 2 shows the data of the mechanical performance and self-healing capability of the systems based on the combination of the hydrogen bond network and other dynamic networks.

Table 2 Polymers based on the combination of the hydrogen bond network and other dynamic networks

Healing mechanism	Mechanical properties before healing			Healing conditions	Healing efficiency	Ref.
	Tensile strength	Elongation at break	Toughness
Hydrogen bonds and dynamic covalent bonds	46.4 MPa	655%	109.1 MJ m^−3	60 °C for 2 h	90.30%	131
	47 MPa	720%	—	70 °C for 1 h	100%	132
	6.04 MPa	527%	6.91 MJ m^−3	60 °C for 2 h	95%	133
	6.3 MPa	1957%	—	130 °C for 5 min	93%	134
	7.9 MPa	585.90%	—	120 °C for 10 s, and then room temperature for 120 min	91.8%	135
	6.3 MPa	—	—	60 °C for 6 h	93.80%	136
	100 MPa	521 ± 5%	—	120 °C with solvent-assisted healing for 1 h	94.40%	137
	16.31 MPa	885%	69.8 MJ m^−3	Room temperature for 2 h	80.50%	138
	15.34 MPa	762.30%	69.1 MJ m^−3	Irradiated for 10 min in a photochemical reactor	95.00%	85
Hydrogen bonds and coordination bonds	9.1 MPa	989%	62.1 MJ m^−3	60 °C for 24 h	78%	139
	86.2 MPa	777.10%	—	Solvent-assisted repair at room temperature for 36 h	71.00%	140
	14.15 MPa	477 ± 6%	47.57 MJ m^−3	Room temperature for 12 h	95%	141
	76.37 MPa	839.10%	308.63 MJ m^−3	40 °C for 6 h	91.72%	142
	11.3 MPa	1336%	82.6 MJ m^−3	Room temperature for 24 h	91%	143
Hydrogen bonds and ionic bonds	5.15 MPa	—	—	Room temperature for 24 h	82%	144
	5.6 ± 0.7 MPa	1260 ± 20%	—	70 °C for 1 h, and then room temperature for 24 h	77 ± 7%	145

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2.4.1 Hydrogen bonds and reversible dynamic covalent bonds. Reversible covalent bonding includes the Diels–Alder reaction,^146–149 disulfide bonding,^150,151 urea bonds,^121,152,153etc. Combining reversible covalent bonding with the hydrogen bonding network to synergistically improve the mechanical properties and self-healing ability of materials has been studied by many scholars recently.^{135,154–160}

Disulfide bonds can undergo bond exchange at moderate temperatures,¹⁶¹ and thus synergistic enhancement of the healing efficiency, toughness, and tensile strength of polymeric materials through the introduction of disulfide and hydrogen bonding has been extensively reported.^{158,162–167} For example, Liu et al. used reversible disulfide bonding in combination with hydrogen bonding to prepare polymeric materials with excellent mechanical properties and high self-healing efficiency.¹⁶⁸ The large number of hydrogen bonding interactions in polymer networks give the material high stiffness, and the dynamic exchange of disulfide bonds may increase the mobility of chain segments, thus affecting the diffusion rate. This coexistence of hydrogen bonding and disulfide bonding in the structure may accelerate the initial or interfacial adhesion,¹⁶² and the disulfide bond exchange reaction promotes chain interdiffusion, thus changing the polymer crosslinked network.¹⁶⁹ Due to the existence of two reversible bonds, the polymer has good self-healing ability at moderate temperature, and the self-healing efficiency can reach 94%. At the same time, it has good tensile properties, with Young's modulus up to 112 MPa and toughness up to 81 MJ m⁻³. Wu et al. designed a polymer material by introducing asymmetric lip-ring structures adjacent to aromatic disulfides into the main chains of PU to regulate molecular mobility and phase morphology.¹³¹ This elastomer, referred to as PU-HPS, was prepared using PTMEG, IPDI, and hydroxyphenyl disulfide (HPS). It was found that the storage modulus (E′) decreases with increasing HPS content over the entire test temperature range. Additionally, T_g also decreases with increasing HPS content. This indicates that the introduction of exchangeable disulfide bonds enhances molecular mobility and reduces the stiffness of PU-HPS, which is beneficial for self-healing performance. Since the content of disulfide bonds can control molecular mobility, the migration rate of the PU-HPS chains can be adjusted by varying the HPS content. This material exhibits excellent tensile strength (46.4 MPa) and a high toughness (109.1 MJ m⁻³). After 24 h of healing at 60 °C, the healing efficiency of the polymer can reach 90.3%. Therefore, the introduction of dynamic disulfide bonds facilitates the increase in molecular chain mobility at moderate temperatures. The combination of hydrogen bond networks and disulfide bond networks synergistically enhances the mechanical properties and self-healing ability of the polymer.

The utilization of hydrogen bonding networks in synergy with Diels–Alder bonds to improve the mechanical properties and self-healing ability has been also reported by several scholars.^{135,170–173} For example, Picchioni et al. reported the introduction of Diels–Alder bonds and hydrogen bonds in polymers, which allows the material to withstand multiple healing cycles without losing any mechanical properties, even for the healed samples.¹⁷⁴ They suggested that this may be attributed to the apparent contribution of hydrogen bond interactions to the faster recovery of the modulus in these systems. Zhang et al. designed polymer elastomers with an IPN structure.¹⁷⁵ One network contains hydrogen bonds of UPy dimer interactions, which produce noncovalent interactions, and the other network contains dynamic covalent bonds of Diels–Alder (Fig. 16). The elastomers exhibit both good mechanical properties (the tensile strength is around 6 MPa) and self-healing ability (the self-healing efficiency can reach 100% at 90 °C for 10 min).

Fig. 16 Design of interpenetrating polymer networks (IPNs), where one network contains dynamic covalent bonds (D–A bonds) and the other network contains dynamic non-covalent bonds formed by UPy groups (hydrogen bonds).¹⁷⁵ Reproduced with permission from ref. 175 Copyright 2019, Royal Society of Chemistry.

In addition, there are several reports on the synergistic effect of hydrogen bonds with dynamic imines.^176–178 For example, Chen and colleagues introduced dynamic imines and hydrogen bonds into polymer networks to explore the contradictory relationship between mechanical performance and self-healing ability.¹³⁶ They compared a polymer without dynamic imine (PHT) with a polymer containing dynamic imine (PHTI) and found that the self-healing efficiency of PHTI significantly increased while maintaining good mechanical performance. The mechanical strength of PHTI containing dynamic imine can reach 14 MPa at room temperature. Further analysis of the impact of dynamic imine bonds on self-healing revealed that after 6 h of healing at 40 °C, the healing efficiency of PHTI can reach 93.8%, while PHT does not show a significant improvement in self-repair ability due to heating. This suggests that the combination of hydrogen and imine bonds optimizes the self-healing process, attributed to the active dynamic reaction process of imines at room temperature.^179,180 Therefore, two synergistic effects of hydrogen bonds and dynamic imine bonds exist: (1) the increase in the mobility of polymer chain segments, and (2) the reattachment of the hydrogen bonds and the recombination of the dynamic imines. Thus, the introduction of dynamic imines can realize the improvement of self-healing properties without sacrificing mechanical strength. Song et al. also reported the use of hydrogen bonds combined with dynamic imine bonds to improve material properties.¹⁸¹ The self-healing efficiency of the prepared samples after 12 h of healing at different temperatures is 24.2% (40 °C), 31.3% (50 °C), 87.2% (70 °C), and 93.2% (100 °C), respectively. It is evident that as the temperature increases, the dissociation ability of hydrogen bonds increases, promoting segment movements, activating the exchange and reversible decomposition of dynamic imine bonds,^182,183 and thereby improving the self-healing efficiency.

Polyacylsemicarbazide (PASC) polymers based on hydrogen bonding and dynamic covalent bonding were reported by Xia et al.¹³⁷ The polymer is endowed with good mechanical properties and self-healing ability, and the fracture stress can reach 100 MPa and the healing efficiency can reach 94.4%. Moreover, the polymer showed good recycling performance, and the composites prepared by the solvent decomposition method could still maintain good mechanical properties after recycling.

Synergistically enhancing the mechanical properties and self-healing capabilities of polymers based on hydrogen bonding and reversible covalent bonding outside of the above have also been widely reported. For example, an acylhydrazone bond with a hydrogen bond,¹⁸⁴ the combinations of hydrogen bonds with dynamic boronic ester bonds,^185–188 hydrogen bonds with dynamic urea bonds,^189–192 and so on were also used.

2.4.2 Hydrogen bonds and metal-ligand coordination. In recent years, the strategy of using hydrogen bonding networks and metal-ligand bonding networks to synergistically improve the self-healing ability and mechanical properties of self-healing polymers has also been widely investigated,^193–199 providing a good strategy for balancing the relationship between the two properties. Comparatively, coordination bonds are stronger than hydrogen bonds.^8,200 The presence of coordination bonds can effectively enhance the mechanical strength of polymers. Under external forces, the weaker hydrogen bonds can reversibly break to dissipate energy, thereby improving the toughness of the polymer. Additionally, both types of reversible bonds contribute to the self-healing ability of polymers.^196,199 Therefore, the combination of these two reversible bonds provides a promising strategy for achieving self-healing polymers with high mechanical performance and self-healing capability. Zhang et al. combined hydrogen bonding with metal ion coordination networks to obtain a polymer with spherical nanostructures, demonstrating excellent mechanical properties and healing capability.²⁰¹ The mechanical strength reached 30.6 MPa, and even at −23 °C, the healing efficiency could reach 85.7%. This is attributed to the movement of small mobile units such as polymer branch units and end groups, which undergo secondary relaxation, imparting the polymer with excellent low-temperature self-healing ability. Li et al. combined a hydrogen-bonded network with a metal-ligand-bonded network to obtain a dual physical crosslinked network of polysiloxane elastomers with a healing efficiency of up to 98% at room temperature.²⁰² Zhang's group employed a strategy combining hierarchical hydrogen bonding networks (single, double, and quadruple hydrogen bonds) with Zn²⁺ coordination networks to investigate the simultaneous enhancement of mechanical properties and self-healing ability.¹⁴¹ As shown in Fig. 17, the formation of Zn²⁺ coordination can significantly increase physical crosslinking of the polymer material. When large deformation occurs, the dynamic dissociation and recombination of coordination interactions can result in a constant non-covalent crosslinking connection that restricts the sliding of polymer chains, thereby promoting an increase in strength, elongation, and toughness. Hydrogen bonds in the polymer network, as weak dynamic bonds, can quickly reconfigure upon fracture to dissipate strain energy, endowing the elastomer with excellent self-repair and stretchability. Strong quadruple hydrogen bonds with high binding energy, as powerful dynamic bonds, contribute to the construction of a robust molecular network, greatly improving the material's robustness and elasticity. Therefore, by combining hierarchical hydrogen bonds with coordination bonds, a supramolecular elastomer with a tensile strength of 14.15 MPa, a toughness of 47.57 MJ m⁻³, and a Young's modulus of 146.92 MPa was obtained. In addition, their group reported another polymer elastomer based on hydrogen bonding and Zn²⁺ coordination bonding, which showed a stronger mechanical strength (76.37 MPa) and toughness (308.63 MJ m⁻³) and reached a healing efficiency of 91.72% after the self-healing at 40 °C for 6 h.¹⁴² Yang et al. synthesized elastomeric materials containing hydrogen bonds and aluminum coordination bonds.²⁰³ Similarly, weaker hydrogen bonds dissipate strain energy and impart high stretchability and excellent self-healing properties to polymers, and aluminum coordination bonds act as strong dynamic bonds that contribute to the formation of a strong network, thus obtaining a combined strategy of strong dynamic coordination bonds and weak dynamic hydrogen bonds to achieve synergistic enhancement of mechanical properties and self-healing ability. The polymeric material obtained through the dual dynamic bonds possesses a tensile strength of 2.6 MPa, a toughness of 14.7 MJ m⁻³, an elongation at break of 1700%, and a self-healing efficiency of 90%.

Fig. 17 Schematic modeling displaying the evolution of the dynamic dissociation and association of hierarchical H-bonding interactions and metal coordination bonds during stretching.¹⁴¹ Reproduced with permission from ref. 141, Copyright 2023, Elsevier.

Our group has also investigated the combination of a Zn²⁺ coordination network with a hydrogen bonding network to obtain interpenetrating network polymer materials (CPU-m%; m denotes the percentage content of quadruple hydrogen bonds in the interpenetrating network polymer) with excellent mechanical properties and self-healing capabilities.¹²⁵ These interpenetrating networks include multiple hydrogen bonding networks (single and multiple hydrogen bonds), coordination bonding networks, flexible segments, and covalent cross-linking networks (Fig. 18(a)). The multiple hydrogen bonding networks, metal coordination bonding networks, and flexible segments contribute to the self-healing ability of the polymer, while multiple hydrogen bonding networks, metal coordination bonding networks, and covalent cross-linking networks contribute to the enhancement of the mechanical properties. By combining reversible networks with other networks synergistically, polymer materials with outstanding mechanical properties and self-healing capabilities have been obtained, with a tensile strength of 40.52 MPa (Fig. 18(b)), a self-healing efficiency of 86.87% at 45 °C for 2 h, and a self-healing efficiency of 94.84% at 65 °C for 1 h (Fig. 18(c)).

Fig. 18 (a) Detailed schematic diagram of the structure of interpenetrating network systems with coordination interaction. (b) Stress–strain curve of interpenetrating polymer networks with hydrogen bonds and coordination bonds. (c) Stress–strain curves after 1 h of healing at different temperatures versus the original material stress–strain curves.¹²⁵ Reproduced with permission from ref. 125 Copyright 2023, Elsevier.

2.4.3 Hydrogen bonds and ionic interactions. Compared to hydrogen bonds, the interaction strength of ionic bonds is greater,²⁰⁴ and the electrostatic attraction between ions also facilitates self-healing. Therefore, synergistically enhancing mechanical properties and self-healing capability through the interaction of hydrogen bonds and ionic bonds is also a feasible approach. Wang designed an ionic self-healing elastomer (SEI) with synergistic interactions between hydrogen bond networks and ion-coordinated networks using dicarboxylic acid, diethylenetriamine, and zinc ions (Fig. 19).¹⁴⁴ The prepared material exhibits a tensile strength of 5.15 MPa, and the self-healing efficiency calculated based on tensile strength and fracture elongation reaches 82% and 96%, respectively, after 24 h of healing. In the ionic network and hydrogen bond network, rich hydrogen bonds form between the amide oligomers, while ionic bonds aggregate to connect multiple amide oligomer chains, creating ionic physical cross-linking points. The oligomers provide sufficient chain segment mobility to the network, while the strong physical cross-linking points from ionic bonds enhance mechanical properties, thus simultaneously improving both mechanical performance and self-healing capability. Böhme et al. modified brominated poly(isobutylene-isoprene)rubber (BIIR) with compounds containing an imidazole and a uracil portion as modifiers.¹⁴⁵ A flexible linker was employed to connect the imidazole and uracil to enhance the mobility of the molecular chains. The reaction between BIIR and the imidazole moiety forms ion groups associated with ionic clusters, while the uracil groups associate with a bifunctional linker containing two diacetamide pyridine moieties through triple hydrogen bonds, thereby achieving physical cross-linking of the rubber. The synergistic effect of hydrogen bonds and ion bonds can cause the improvement of the mechanical properties. The prepared rubber material exhibits a tensile strength of 5.6 ± 0.7 MPa, a fracture elongation of 1260 ± 20%, and a self-healing efficiency of 77 ± 7%.

Fig. 19 Principles of synthesis of ionic self-healing elastomers.¹⁴⁴ Reproduced with permission from ref. 144, Copyright 2020, Elsevier.

3. Special chain topology structures

There are polymers with different topologies, such as star polymers, H-shaped polymers, super H-shaped polymers, pom–pom polymers, comb polymers, dendritic polymers, cyclic polymers, and so on.^205–209 Polymers with special topological structures may have unique properties. For example, comb-like or bottle-brush-like topological structures consist of a main chain backbone and numerous side chains. When the side chains are functionalized with units capable of forming hydrogen bonds, the interpenetration between the side chains not only enhances the mechanical properties but also provides the corresponding materials with good self-healing capabilities. Xie et al. prepared hydrogen-bonded bottle-brush polymers by end-capping poly(4-methylcaprolactone) side chains with UPy units.²¹⁰ By adjusting the UPy content, polymers with different moduli can be obtained, and the tensile strength of the sample after self-healing is close to that of the original sample.

Wu's group prepared a novel side-chain brush polymer and enhanced the mechanical properties and self-healing capabilities of the polymer through the mechanical interlocking of the side-chain brushes.²¹¹ Experimental and simulation results indicate that the length and density of the side chains have a significant impact on the mechanical properties and self-healing ability of the polymer. The side chains can form molecular entanglements at appropriate chain lengths, and an optimal side-chain density facilitates mutual penetration of the side chains, which is beneficial for mechanical performance and healing processes. Similarly, based on coarse-grained molecular dynamics (CGMD) simulations, Liu et al. investigated the self-healing mechanism of comb copolymer systems.²¹² The effects of three parameters, namely the spacing between side chains, the average molecular weight of each main chain, and the flexibility of side chains, on the interpenetration of side chains were investigated to understand their influence on the self-healing and mechanical performance. The simulation results revealed that there is a favorable distance between side chains, at which the interpenetration and non-bonded interactions between polymer chains will be facilitated. When favorable spacing between side chains is maintained, it is preferable to choose main chain monomers with smaller molecular weights. Additionally, rigid side chains enhance the stiffness of main chains and promote the overlapping and the formation of entanglements with neighboring chains, thereby facilitating the interpenetration between adjacent side chains, and ensuring good mechanical properties. Due to the interpenetration of side chains and the formation of hydrogen bonds, the corresponding damaged material can be self-healed.

Jiang and co-workers prepared a self-healing elastomer containing gradient-distributed side chains.²¹³ As shown in Fig. 20, the gradient-distributed side chains have different chain lengths, resulting in different spatial hindrances. Therefore, the gradient distribution of the side chains facilitates the interpenetration and self-assembly, increasing chain entanglements and the local ordered structure of PU systems. This leads to the formation of high modulus microdomains and promotes the formation of hydrogen bonds between the side chains and between the side chains and the main chains. As a result, chain segments are more tightly entangled together, forming physical cross-linking points, and thereby enhancing the strength and toughness of the PU elastomer. Additionally, the introduction of the gradient-distributed side chains disrupts the regularity of the PU chains, reducing the aggregation of hard segments and increasing the mobility of segments, which is beneficial for the self-repair of the system. The tensile strength and elongation at break of the PU elastomer are 27.5 MPa and 1147%, respectively. After 48 h of healing at 60 °C, the healing efficiency can reach 94.8%. This demonstrates the superiority of designing polymer materials with special topological structures to achieve a balance between mechanical performance and self-healing ability.

Fig. 20 Topology of the polymer network of polyurethane with gradient distribution of dangling chains.²¹³ Reproduced with permission from ref. 213 Copyright 2023, American Chemical Society.

Hyperbranched polymers have highly branched molecular chains. The outer branch units and end groups of hyperbranched polymers usually have high mobility. The uniqueness of hyperbranched polymers lies in the fact that their properties can be easily tailored by modifying the nature of the end groups.^214–216 Wu et al. reported novel polymers with a hyperbranched structure.⁹⁴ The hydrogen bonding density in the polymers is extremely high, and their T_g is above room temperature. However, although the hyperbranched polymers are in the glassy state at room temperature, they still exhibit excellent instantaneous self-healing ability, with a tensile strength recovery of up to 5.5 MPa within 1 min. This is attributed to the unique structure of the hyperbranched polymers, which not only prevents the ordered stacking of chains, resulting in the coexistence of free and bound hydrogen bonding motifs, but also allows a large number of branching units and end groups to have high mobility at room temperature. This enables the dissociated portions to quickly reform hydrogen bonds on fracture surfaces, giving the hyperbranched polymers the ability of instantaneous self-healing.

Jia and coworkers reported a self-healable, high-toughness polymer elastomer.²¹⁷ The high toughness of the polymer was achieved by constructing quadruple hydrogen-bonded cross-links and polyrotaxanes in the polymer network, in which cyclodextrins (CDs) on the polyrotaxanes could slide along the main chain of the polyethylene glycol (PEG) molecule, as shown in Fig. 21(a). The mechanism of high toughness of the polymer was explained by a combination of experimental methods and molecular dynamics simulations. When the polymer is subjected to external forces, the quadruple hydrogen bonds first cause CDs to slide to the end of the polymer chain (the end groups of the polymer can prevent CD slippage), and then further under the action of external forces, the quadruple hydrogen bonds undergo severe damage, allowing CDs to return to a freely sliding state, thereby increasing the polymer's deformability and toughness. As a result, the polymer exhibits a tensile strain of 2900%, a toughness of 77.3 MJ m⁻³, and a fracture energy of 127.2 kJ m⁻². Additionally, after 24 h of healing at room temperature, the healing efficiency can reach 91% (Fig. 21(b)), attributed to the low T_g, allowing the polymer chains to relax rapidly at room temperature and the rapid exchange of quadruple hydrogen bonds to repair the material.

Fig. 21 (a) Design strategy for high toughness and self-healing elastomers and graphical representation of the structural changes in the elastomeric network during stretching and release. (b) Stress–strain curves of polymer elastomer films healed at room temperature different times.²¹⁷ Reproduced with permission from ref. 217 Copyright 2023, John Wiley & Sons.

4. Micro-mechanisms of self-healing

Researchers are dedicated to studying and resolving the contradiction between improving the self-healing capability and mechanical performance of polymers. The key to solving this issue is to explore the microscopic mechanisms of self-healing, in order to achieve macroscopic control over both mechanical performance and self-healing ability.

Wu et al. cleverly introduced the fluorophore 4-tricyanovinyl-[N-(2-hydroxyethyl)-N-ethyl]aniline (TC1), which exhibits strong aggregation-caused quenching (ACQ) effects, into a self-healing elastomer with a dynamic hydrogen-bonding network. Since TC1 is highly sensitive to the local chain packing environment, it can be used to detect microscopic structural changes during self-healing.¹⁰⁰ When the sample film was cut, the fluorescence intensity of the fractured surface was found to be significantly higher than that of the normal elastomer surface. This indicates that under the external force, the hydrogen bonds and network structure were disrupted, leading to a slight increase in free volume and a subtle decrease in chain packing efficiency. As a result, the intermolecular distance between the polymer chains increased, causing the fluorescence intensity to increase. The researchers then monitored the fluorescence intensity at the same location on the fractured surface over time. They observed a two-stage decrease in the fluorescence intensity. In the first stage (0–4 h), the fluorescence intensity rapidly decreased, suggesting that the network rearrangement during this stage was very fast, and the reduced network structure and decreased chain packing efficiency significantly enhanced the mobility of the polymer chains. In the second stage, the decrease in the fluorescence intensity was much slower, indicating that the reconstruction of the fractured network restricted the mobility of the polymer chains, slowing down the network rearrangement. Furthermore, in the scratch test, the fluorescence intensity around the scratch was significantly higher than that in other areas, suggesting that the network in the scratch region was disrupted, and the migration rate of the chain segments increased. During the recovery process, the fluorescence intensity in the scratch area decreased to the point where it was indistinguishable from other regions, and the scratch was almost completely healed.

Previously, our group used molecular dynamics (MD) simulations to study the changes in the microscopic structures during polymer self-healing, and further explored the quantitative relationship between the microscopic structural changes and the self-healing efficiency using machine learning methods.⁹⁷ A crack model was constructed, as shown in Fig. 22(a). The dynamic diffusion behaviors of polymer segments and entire chains during the healing process were quantified by calculating the normalized contents of beads and centers of mass of chains. The reorganization of dynamic bonds during the self-healing process was quantified by calculating the normalized content of reversible interaction pairs. First, the self-healing behavior of the polymer at a temperature of T* = 1.6 (T_g = 0.65) was studied. Fig. 22(d) shows the morphological changes at different times during the self-healing process. As the healing time increased, the normalized contents of beads, centers of mass of chains, and reversible interaction pairs increased over time, and the reversible interaction pairs increased rapidly during the cooling stage. This indicates that extending the healing time above T_g is beneficial for the diffusion of polymer segments, entire chains, and the reconstruction of reversible interaction pairs, thereby improving the self-healing efficiency. Then, the influence of chain diffusion on self-healing was further explored by healing for the same time at different temperatures. It was found that as the temperature increased, the normalized contents of beads and centers of mass of chains increased, indicating that the migration rate of chain segments and the diffusion ability of entire chains became stronger, and the polymer chains were more easily able to diffuse into the crack region to achieve reconstruction. Fig. 22(b) shows the self-healing efficiency at different temperatures, which can be seen to first increase and then gradually stabilize as the temperature increases, i.e., the self-healing efficiency reaches saturation at high temperatures. However, Fig. 22(c) shows that the normalized contents of beads, centers of mass of chains, and reversible interaction pairs do not exhibit a saturated state. Therefore, the microscopic structural reconstruction in the healing region and the self-healing efficiency exhibit a nonlinear relationship. Finally, the complex nonlinear relationship was explored through machine learning. By evaluating the three features of beads, centers of mass of chains, and reversible interactions, it was found that the center of mass feature of chains is the most important, indicating that the diffusion of the entire chain into the healing region is the key factor determining the self-healing efficiency.

Fig. 22 (a) Constructed crack model; (b) healing efficiency after healing for 10 700τ at different temperatures; (c) normalized contents of beads, centers of mass of chains, and reversible interaction pairs after healing for 10 700τ at different temperatures; and (d) snapshots of polymer systems during the healing process.⁹⁷ Reproduced with permission from ref. 97, Copyright 2024, American Chemical Society.

5. Conclusion and perspective

The self-healing ability of polymer materials can prevent sudden fracture during service, thereby extending the service life of materials. In hydrogen-bonded self-healing polymers, the presence of hydrogen bonds is beneficial for the achievement of the self-healing behaviors. In addition, the existence of reversible bonds helps to achieve material recycling and reuse, which is beneficial for resource conservation, reducing pollution during disposal, and meeting the requirements of sustainable development.

This article reviews the improvement of both the mechanical properties and self-healing ability of polymer materials based on hydrogen bonding. Due to the relatively weak bonding energy of hydrogen bonds, introducing hydrogen bonds as physical crosslinks in polymers can decrease mechanical strength but provide good self-healing ability. When a large number of hydrogen bonds are introduced into polymers, the mechanical strength will be improved, but the chain mobility will be restricted, resulting in the reduction of the self-healing ability. Therefore, maintaining an appropriate amount of hydrogen bonds in polymers is the key to achieving a balance between mechanical strength and self-healing efficiency. However, introducing an appropriate content of hydrogen bonds may not necessarily meet the requirements for achieving mechanical properties. The mechanical properties can be ensured by changing the strength of hydrogen bonds, such as introducing multiple hydrogen bonds.

Then, in order to ensure that polymers have a high self-healing efficiency, it is necessary to ensure that polymer chains have sufficient mobility, so that they can quickly diffuse and interweave at the damaged interface, while hydrogen bonds can be reorganized and repaired. To ensure that polymer chains have sufficient mobility, the density of the hard domain structures cannot be too high, or the content of the hard segments of polymer chains cannot be too high. This requires the rational design of a chain structure, the control of the degree of phase separation, or the introduction of more flexible segments to increase the mobility of molecular chains. This allows polymer chains or chain segments to more easily diffuse into each other, thereby improving the healing rate and healing efficiency.

In addition, combining hydrogen bonds with other reversible interactions in a network is also a good strategy to simultaneously improve the mechanical strength and self-healing ability. The presence of hydrogen bonds not only facilitates rapid self-healing but also facilitates energy dissipation during fracture, while non-hydrogen bonding reversible interactions provide excellent mechanical strength and self-healing capabilities. The combination of these two reversible bonds enables the polymer to possess both good mechanical performance and self-healing ability simultaneously. Special topological structures will give polymers excellent performance. For example, comb-like structures allow the interpenetration of polymer side chains, which is beneficial for improving self-healing ability. Hyperbranched polymers have a large number of branching units and end groups, which still have high mobility at room temperature, and thus they exhibit rapid self-healing behaviors.

Finally, the healing mechanism of polymer self-healing was elucidated. When a polymer material is damaged, the hydrogen bonds or network structure at the damaged site are disrupted, the mobility of molecular chains is enhanced, and molecular chains can diffuse into the damaged region, thereby achieving the recovery of the damaged site. Furthermore, our group used MD simulations to reveal the microscopic mechanism of self-healing. When a polymer heals, polymer chains and segments diffuse into the crack region, and reversible interactions are reorganized, thereby realizing the healing of the crack. Moreover, through the method of machine learning, it was confirmed that the mobility of the molecular chains or chain segments plays a crucial role in the healing process, and the simulation results demonstrated that the diffusion of the entire chains into the healing region is the most critical factor determining the efficiency of self-healing.

In the future, more in-depth investigations are still needed in the research of hydrogen-bonded self-healing polymers. Firstly, microscopic mechanisms of self-healing polymers based on hydrogen bonds are not fully understood. Through molecular simulations, it is possible to directly observe microstructural changes at the molecular scale, and thus they can be used for the study of the self-healing mechanisms of hydrogen-bonded self-healing polymers.^82,212,218 Secondly, how to construct a quantitative correlation between the microstructure characteristics of hydrogen-bonded self-healing polymers and their self-healing efficiency and mechanical properties is also an interesting topic. Fortunately, machine learning or deep learning methods are effective tools for constructing quantitative relationships between the microstructures and the macroscopic physical properties of materials. We believe that machine learning or deep learning methods can be used to establish the relationship between the microstructure and the self-healing efficiency and mechanical properties of hydrogen bonded self-healing polymers. Thirdly, it is also important to design new molecular structures to achieve efficient self-healing processes of hydrogen-bonded self-healing polymers (high self-healing efficiency, high self-healing rate, and low self-healing temperature). We look forward to more and more new self-healing polymer materials being successfully developed and applied in the engineering field in the future.

Data availability

This review paper does not involve data. All data in this paper are obtained from the literature with permission.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We appreciate the financial support from the National Natural Science Foundation of China (no. 52173020 and 52073126) and Qing Lan Project of Jiangsu Province of China ([2022]29).

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