Self-Healing Natural Rubber with Tailorable Mechanical Properties

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Self-Healing Natural Rubber with Tailorable Mechanical Properties Based on Ionic Supramolecular Hybrid Network Chuanhui Xu, Liming Cao, Xunhui Huang, Yukun Chen, Baofeng Lin, and Lihua Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09997 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Self-Healing Natural Rubber with Tailorable Mechanical Properties Based on Ionic Supramolecular Hybrid Network Chuanhui Xu1,2#, Liming Cao 2#, Xunhui Huang 2, Yukun Chen2,* Baofeng Lin1, Lihua Fu1 1

School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China

2

The Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (South

China University of Technology), Guangzhou, 510640, China

Corresponding Author: Yukun Chen [email protected] #The first two authors contributed equally to this paper. ABSTRACT: In most cases, the strength of self-healing supramolecular rubber based on non-covalent bonds is in the order of KPa, which is a challenge for their further applications. Incorporation of conventional fillers can effectively enhance the strength of rubbers, but usually accompanied by a sacrifice of self-healing capability due to that the filler system is independent of the reversible supramolecular network. In the present work, in-situ reaction of methacrylic acid (MAA) and excess zinc oxide (ZnO) was realized in natural rubber (NR). Ionic crosslinks in NR matrix was obtained by limiting the covalent crosslinking of NR molecules and allowing the in-situ polymerization of MAA/ZnO. Because of the natural affinity between Zn2+ ion-rich domains and ZnO, the residual nano ZnO participated in formation of a reversible ionic supramolecular hybrid network, thus having little obstructions on the reconstruction of ionic crosslinks. Meanwhile, the well dispersed residual ZnO could tailor the mechanical properties of NR by changing the MAA/ZnO molar ratios. The present study thus provides a simple method to fabricate a new self-healing NR with tailorable mechanical properties which may have more potential applications. KEYWORDS: ionic supramolecular hybrid network; NR; self-heal; reinforce; mechanical properties

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1. INTRODUCTION There is considerable interest in elastomeric materials which can sense and repair physical damage by themselves1,2. This magical characteristic is called self-healing capability3, and these materials have broad potential applications in sensors4, smart actuators5, artificial skin6 and so on, where the self-healing property is necessary. Unfortunately, conventional vulcanized rubbers exhibit no self-healing behaviors due to the formation of a nonreversible covalently crosslinked network7. To obtain the self-healing capability, introducing dynamic bonds to construct a reversible rubber network is a feasible approach. The reversible bonds, including dynamic covalent bonds8-17, such as Diels-Alder bonds 9,10, disulfide bonds11-15, schiff base bonds16,17 and non-covalent bonds, such as hydrogen bonding18-20, ionic bonding21,22, metal-ligand interaction23,24, molecular interdiffusion25-27, can serve as crosslinking points which have association or dissociation spontaneous behaviors at ambient or specific conditions to realize the damage-repair process. Before healing the mechanical damage, diffusion of polymer chains across the fracture surfaces is important to regenerate reversible associations26,27. Therefore, excellent chain mobility is critical for the self-healing process. This requires high molecule flexibility to provide sufficient chain mobility even in a crosslink network. A reversible supramolecular network28-34 is quite suitable for designing self-healing rubbers. Unfortunately, without covalent crosslinks, self-healing rubbers based on non-covalent supramolecular network usually exhibit poor mechanical performances18-24. In most cases, the tensile strength of self-healing supramolecular rubber is in the order of KPa (lower than 1MPa), which is a challenge for their further applications. To overcome this disadvantage, many prospective approaches have been reported. Zhang et al.35,36 used effective catalysts to trigger the healing in sulfur crosslinked rubbers at accurate temperatures. The mechanical properties could be restored once the temperature stimulus was stopped. Tournilhac and Michaud37 changed the ratios of hydrogen-bonds/covalent-bonds to tailor the mechanical performance of a supramolecular network. However, the self-healing capability was inevitably decreased by increasing chemical crosslinks. To design a reprocessable thermoplastic elastomer (TPE) having a two-phase structure is a more useful method to gain high mechanical properties38. However, this technical solution somewhat deviates from the topic of non-covalent supramolecular network in self-healing rubbers. It is well known that incorporation of nano-fillers is the most effective and convenient approach to enhance the strength of rubbers. However, in most times the self-healing property of a filler-reinforced supramolecular network deteriorates significantly39-41. This is because the filler system is usually independent of the reversible supramolecular network whose recovery is hindered or blocked by filler nano-particles. We think that, if the filler nano-particles themselves are able to participate in constructing a reversible supramolecular hybrid network, then the negative effects of nano-fillers on self-healing process will be minimized. Based on this idea, we focus on the self-healing ionic supramolecular network which is reported by us recently42. Our previous supramolecular network was constituted by the formation of ionic crosslinks in natural rubber (NR) via the polymerization of zinc dimethacrylate (ZDMA)42,43. The material exhibited self-healing behavior, but the best tensile strength was only ~0.63MPa which was a shortcoming for its practical application. In this paper, we proposed a facile method to create self-healable ionic supramolecular network with improved mechanical properties. Our strategy was based on an in-situ reaction of excess zinc oxide (ZnO) and methacrylic acid (MAA) in NR. The 2

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polymerization of in-situ MAA/ZnO brought massive ionic crosslinks, and, the ratio of MAA/ZnO could be changed so that to tailor the mechanical properties of NR. Since the residual nano ZnO participated in constructing the new ionic supramolecular hybrid network, it served as reinforcer for the NR/MAA/ZnO compounds and had only little obstructions on the reconstructing of the ionic supramolecular hybrid network. As expected, the resultant material exhibited excellent self-healing ability as well as improved mechanical properties. We believe the new self-healing material with tailorable mechanical properties has potential applications in sensors, smart actuators or others. EXPERIMENTAL SECTION 2.1 Materials. Natural rubber (NR SMR CV60) was from Guangzhou rubber industry research institute (China). ZnO (analytical pure), with an average size about 50nm, was purchased from XiLong Chemical Company (Guangzhou, China). MAA (analytical pure) was purchased from Sinopharm Chemical Reagent Co. Ltd (China). Dicumyl peroxide (DCP) was purchased from Guangzhou Xingang Chemic factory (China). 2.2 Preparation of NR/MAA/ZnO compounds. ZnO was first added into NR to achieve a uniform dispersion in a two-roller mill. Then, MAA was carefully added into NR/ZnO compound by using an injector. The temperature of rollers was kept at 40~45°C as possible to help the full in-situ reaction of ZnO and MAA. At last, DCP was added. The total mastication time of every sample was strictly limited within 10min. 2.3 Characterizations. Before taking the vulcanization measurement, the masticated NR/MAA/ZnO compounds were stored in a vacuum environment for 24h. Then, the cure-curves were recorded in a Rotorless Rheometer (UR-2010SD, U-CAN Dynatex Inc.) at 140°C, amplitude 0.5° and frequency 1.67 Hz. A FT-IR Spectrometer (Bruker Tensor 27, Germany) was used to record FT-IR spectra. Test conditions: 32 scans, 4 cm-1, attenuated total reflectance (ATR) model. SEM observation was done in a Merlin SEM (Zeiss, Germany). The specimens were treated by gold sputtering. Optical micrographs were recorded on a DMS-756TR (Shanghai Yanfeng Precision Instrument Co.,LTD, China). Tensile performance was done on a universal stretcher (UT-2080, U-CAN Dynatex Inc.). Consecutive cyclic tensile performances were done from ε= 50% to ε=100% to ε=150% to ε=200%, the tensile speed was 50 mm/min. The TEM observation were done on a transmission electron microscope (JEM-100CX II, Japan), 100 kV accelerating voltage. Before observation, the sample was cryomicrotomed into about 100 nm thin sections by using a Leica EMUC6. The ionic crosslink density, covalent crosslink densities and total crosslink density of the NR/MMA/ZnO compounds were determined by equilibrium swelling experiments and more details can be found in our previous report42. 2. RESULTS AND DISCUSSION 3.1 Key approach for preparation of ionic supramolecular hybrid network

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Figure 1. Schematic illustrations of the approach for preparation of ionic supramolecular hybrid network Our strategy is based on in-situ formation of ZDMA by mixing MAA and excess ZnO in NR. Due to the high reactivity between MAA and ZnO, they are readily reacted to form ZDMA during NR mastication44,45. The theoretical equivalent amount of in-situ ZDMA was fixed at 40phr in per 100phr NR with DCP fixed at 1phr. According to the molecular formula of ZDMA that one Zn2+ combined two –OOC-C(CH3)=CH2, the theoretical ideal molar ratio of MAA:ZnO was 2:1. We selected a series of MAA/ZnO molar ratios of 2:1, 2:1.1, 2:1.2, 2:1.3 and 2:1.4 to make sure the ZnO was excess. Therefore, the corresponding weights of MAA/ZnO in 100g NR were 29.3g/13.8g, 29.3g/15.2g, 29.3g/16.6g, 29.3g/17.9g and 29.3g/19.3g, respectively. According to our previous study42, the polymerization of ZDMA could occur before the formation of NR covalently crosslinked network under a carefully controlled condition. The polymerization of in-situ MAA/ZnO generated mass ion pairs in polymerized ZDMA. Due to the powerful electrostatic interactions46, ion pairs constituted ion multiplets and further ion clusters which were able to restrict the mobility of adjacent NR chains, consequently forming an ionic supramolecular network. Note that the in-situ ZDMA came from the reaction of MAA and excess ZnO, thus the residual ZnO was well dispersed with in-situ ZDMA in the NR matrix. During polymerization, nano-ZnO participates in the formation of a new ionic supramolecular hybrid network. Because of the natural affinity between Zn2+ ion-rich domains and nano-ZnO43, the residual ZnO was part of the ionic hybrid network which did not obstruct the reconstruction of ionic supramolecular hybrid network. Meanwhile, the well dispersed residual ZnO played another important role——“reinforcer” to enhance the mechanical performance of NR. As a result, the nature of reconstruction of ionic crosslinks fulfilled the self-healing behavior and, the changed molar ratio of MAA/ZnO would tailor the mechanical properties of NR. The schematic illustrations of above idea are illustrated in Figure 1. 3.2 In-situ formation of ZDMA

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Figure 2. (a) FTIR spectra of neat NR, MAA and NR/MAA/ZnO compounds with various MAA/ZnO ratios: a-2:0.8, b-2:0.9, c-2:1, d-2:1.1 and e-2:1.2; (b) XRD curves As shown in Figure 2a, the neutralization of MAA and ZnO in the NR compounds was confirmed by FTIR analysis. MAA was in form of dimer and showed characteristic absorption peaks at 1698 (C=O) and 1637 cm-1 (C-O) 44. Thus the absorption peak at ~1700 cm-1 represented the –COOH in un-reacted MAA42. It showed a reduced tendency with increasing ZnO concentration. Note that the characteristic absorption peak of –COOH was still observed when MAA/ZnO was the ideal ratio of 2:1 (spectra c), which suggested that the MAA did not react completely with the ZnO. Further increasing the ZnO dosage could solve this problem. As seen, when the MAA/ZnO molar ratio was increased to 2:1.1 (spectra d) and 2:1.2 (spectra e), the characteristic absorption peak of –COOH had disappeared. This strongly suggests that the MAA and its corresponding ZnO has been converted into in-situ ZDMA whose yield quantity was determined by the amount of MAA. To further prove the formation of in situ ZDMA, the X-ray diffraction curves of ZnO, ZDMA and NR/MAA/ZnO compounds are showed in Figure 2b. The ZnO curve shows the typical characteristic peaks at approximately 2θ=32.1°, 34.5°, 36.4°, 47.6° and 56.8°, while the ZDMA shows at approximately 2θ=10.6° and 11.7°. As seen from Figure 2b, all of the NR/MAA/ZnO compounds show typical crystalline patterns in the range from 2θ=10° to 28° which belong to ZDMA crystals, indicating that MAA and ZnO reacted and formed into ZDMA during the rubber mastication process. When the MAA/ZnO molar ratio goes beyond 2:1.1, the typical characteristic peaks of ZnO crystal were detected, confirming the existence of residual ZnO in the rubber compounds. As shown in Figure S1, the residual ZnO brought the NR/MAA/ZnO compounds a white appearance. 3.3 Formation of ionic supramolecular hybrid network

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Figure 3. (a) Curing-curves under 140°C; (b) crosslink densities of NR with MAA/ZnO=2:1.4; (c) photos of equilibrium swelling experiments and (d) photos of toluene-swollen gels and the schematic of the crosslinked networks in gels The cure curves of the NR/MAA/ZnO compounds were showed in Figure 3a. As seen, the torque values at the initial curing times were quite low and increased very slowly. The 30s-torque values of the NR with MAA/ZnO=2:1, 2:1.1, 2:1.2, 2:1.3 and 2:1.4 were 0.55, 0.71, 0.92, 1.02 and 2.19 dNm, respectively. Even the 60s-torque values of them were only increased to 0.86, 1.11, 1.44, 1.67 and 2.85 dNm, respectively. The above low torque values indicated that the crosslinking was controlled in a very small scale within 60s, which provided the possibility to construct a supramolecular network at this curing stage47. Note that the torque value was enhanced with the increased ZnO molar ratio that revealed the effectual reinforcement of the residual ZnO on the network. The appearances of 60s-cured NR/MAA/ZnO compounds are showed in Figure S2. The ionic supramolecular hybrid network in NR/MAA/ZnO compounds was confirmed from the equilibrium swelling experiments. Since the ionic crosslinks were come from the electrostatic associations of Zn2+ ion pairs, some polar molecules e.g. chloroacetic acid44-46, were able to destroy them. We show the ionic, covalent and total crosslink densities of the NR with MAA/ZnO=2:1.4 in Figure 3b and the corresponding photos of equilibrium swelling experiments in Figure 3c. The NR/MAA/ZnO compound cured for 60s was swollen in toluene. This strongly suggested that a network structure had indeed been formed which against the dissolution of NR. However, the swollen gel was dissolved in the mixture of toluene/chloroacetic acid, suggesting that the network structure was dominated by ionic crosslinks. Because the covalently crosslinked NR network was not formed at this time, the ionic crosslink density was equal to total crosslink 6

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density. However, the calculated “n” of 60s-cured sample was only 0.48×10-4 mol/cm3, which was a quite low value compared with a fully cured rubber. Such a low crosslink density ensured the sufficient chain mobility in the network, which benefited the diffusion of NR chains to realize the self-healing behavior. Similar to our previous study42, the 180s-cured gel was partial dissolved in the mixture of toluene/chloroacetic acid. It was clearly seen some insoluble fragments precipitated on the bottom. Obviously, partial continuous covalent network was formed at this time. When the curing time reached 300s, a continuous covalently crosslinked network was formed. The sample was not dissolved after its ionic crosslinks were cut off. Then, the covalent and ionic crosslink densities were determined to be 0.61×10-4 mol/cm3 and 0.89×10-4 mol/cm3, respectively. Note that the ionic crosslink density was larger than the covalent crosslink density, which suggests that ionic crosslinks occupied a dominant position in the network. To further confirm the ionic supramolecular network, we did a TEM observation on the 60s-cured NR with MAA/ZnO=2:1. As shown in Figure S3, the whole NR matrix was full of dark ion-rich domains 48,49. The photographs of the toluene-swollen gels are provided in Figure 3d to give an intuitive visual perception of the reinforcement of the excess ZnO. It was clearly seen that the strength of ionic supramolecular network was very weak. The gel with MAA/ZnO=2:1 was so soft and fragile that it was cracked into several fragments when it was pinched out of the solvent. As for the sample with MAA/ZnO=2:1.4, it showed a strengthened shape-keeping due to the reinforcement of residual ZnO. At 300s, the continuous covalently crosslinked network endowed the gel with dramatically improved mechanical property. The schematic diagram of the network structures are also showed in Figure 3d. Because of the residual ZnO, the network structure in the 60s-cured gel was a strengthened ionic supramolecular hybrid network. Figure 4 shows the ionic crosslink density of 60s-cured NR with various MAA/ZnO molar ratios. The residual ZnO contributed additional physical absorptions to the ionic supramolecular hybrid network. As a result, the crosslink density was increased from 0.45 to 0.8910-4 mol/cm3 as the MAA/ZnO ratio changed from 2:1 to 2:1.4, reflecting that the supramolecular hybrid network was enhanced. Considering that the dosage of MAA was constant in all of the samples, the increased crosslink density should be reasonable containing physical adsorptions from ZnO. Several covalent crosslinking points might be existed in the network, but they were not connected to be a continuous covalent network42.

Figure 4. Ionic crosslink density of 60s-cured NR with various MAA/ZnO molar ratios 3.4 Enhanced mechanical properties

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Figure 5. SEM images of NR with MAA/ZnO ratios of (a) 2:1, (b) 2:1.2 and (c) 2:1.4 The dispersion of residual ZnO was observed by SEM on the cross-sectional surface of NR/MAA/ZnO compounds. As shown in Figure 5, the residual ZnO nanoparticles show a uniform dispersion. Additionally, the nano ZnO was well-embedded in the NR matrix, which is predominately attributed to the natural affinity between Zn2+ ion-rich domains and nano ZnO43. The graft in-situ polymerized ZDMA42,48,49 also promoted the compatibility between ZnO nano-particles and the NR chains.

Figure 6. Typical tensile behaviors of the NR/MAA/ZnO compounds cured for 60s The tensile behaviors of the NR/MAA/ZnO compounds are shown in Figure 6. A 60s-cured NR with 40phr ZDMA compound was used as reference here. In this reference sample, the ZDMA was directly mixed into NR matrix. Their accurate data of the tensile properties are given in table 1. The reinforce effect of in-situ MAA/ZnO (~0.6MPa) was better than that of ZDMA (~0.4MPa). More important, the mechanical properties of the NR compounds could be tailored by the MAA/ZnO ratios. A significant improved mechanical strength of the NR compound was achieved by changing the MAA/ZnO molar ratios from 2:1 to 2:1.4. For example, the tensile strength of NR with MAA/ZnO=2:1.4 increased to 1.89MPa which was almost 3 times of that of the NR with MAA/ZnO =2:1 (0.64MPa). Although the elongation at break showed a slight reduce from 488% to 435%, it still maintained at high level above 400%. Table 1 The data of mechanical properties of samples Samples Tensile strength (MPa) Elongation (%)

ZDMA

MAA/ZnO molar ratios

0.54 488

2:1

2:1.1

2:1.2

2:1.3

2:1.4

0.64 478

0.95 473

1.71 442

1.53 436

1.89 435

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3.5 Self-healing behavior

Figure 7. Optical microscopy images of NR/MAA/ZnO compound (MAA/ZnO=2:1.4) cured for 60s: (a) self-healed for 0min; (b) self-healed for 20min; (c) inside of the healing position; (d) inside of the healing position self-healed for 20min. The self-healing behavior of the 60s-cured NR/MAA/ZnO compound was observed through optical microscopy. The cut two fragments were able to well adhere to each other when self-healed at room temperature for a certain time. Figure 7a and b show the optical images of the cut/resealed surface after self-healing for 0 and 20 min at room temperature. Although the cut surface was remained after healing for 20 min, an obvious shrinkage mark around cut line was clearly observed, which was similar to the autonomic healing of skin incision wounds. The healing status inside the cut/resealed position was also observed, as shown in Figure 7c and d. The cut line between the two fracture surfaces only remained a negligible trace, exhibiting good self-reparability.

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Figure 8. Photographs of the healed samples under strong twisting and stretching: (a) 60s-cured NR with MAA/ZnO=2:1; (b) 60s-cured NR with MAA/ZnO=2:1.4 To give a more intuitive recovery result, we show the photographs of the healed samples under strong twisting and stretching. The neat NR showed a poor healing result in Figure S4 due to no revisable ionic network in it. As for the NR/MAA/ZnO compounds, we selected the 60s-cured NR with MAA/ZnO=2:1 and 2:1.4 shown in Figure 8a and b, respectively, to illustrate their excellent self-healing behaviors. After being self-healed for 20 min, both of the healed samples were able to undergo a large elongation without broken. The diffusion of NR chains brought reconstruction of ionic associations and resulted in the rearrangement of a new ionic supramolecular network at the cut/resealed position. Note that the white appearance and the excellent self-healing behavior in Figure 8b, this strongly suggests that the residual ZnO did not stop the self-repairing of the ionic supramolecular network. In fact, the stretching of NR with MAA/ZnO=2:1.4 needed much more power than the one with MAA/ZnO=2:1. The enhanced mechanical property of the NR/MAA/ZnO compound with excellent self-healing performance highlighted its potential for practical applications, as compared to poor mechanical strength of most self-healing soft materials. Further increasing the residual ZnO concentration resulted in a further improved mechanical strength, but inevitably accompanied by a sacrifice of self-healing capability (see Figure S5).

Figure 9. SEM images of 20min-healed position under stretching state: (a) MAA/ZnO=2:1; (b) MAA/ZnO=2:1.1;(c) MAA/ZnO=2:1.2; (d) MAA/ZnO=2:1.3 and (e) MAA/ZnO=2:1.4 Figure 9 shows the cut/healed position of the NR/MAA/ZnO compounds cure for 60s under stretching state (strain=50%). As clearly seen, no new cracks were observed at the 20min-healed position, suggesting that the residual ZnO did not influence the self-healing effect of the NR/MAA/ZnO compounds. The cut two fragments had a well healing and only remained some wrinkle traces at the adhered cut lines. This agrees with the result of the optical microscopy observations.

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Figure 10. Typical tensile behaviors of the original and the 20min-healed samples: (a) MAA/ZnO=2:1; (b) MAA/ZnO=2:1.2; (c) MAA/ZnO=2:1.4; (d) snapshots of the healed NR with MAA/ZnO=2:1.4 during stretching The healing performance was also evidenced in the tensile measurements of the healed dumbbell-shaped specimens. Typical tensile behaviors of the virginal and self-healed samples with MAA/ZnO=2:1, 2:1.2 and 2:1.4 are showed in Figure 10a, b and c, respectively. The healed samples almost fully regained their original strength after 20 min healing at room temperature. The healing efficiencies of the samples were all as high as above 96%, e.g., the healing efficiencies of tensile strength were 96%(MAA/ZnO=2:1), 97%(MAA/ZnO=2:1.2) and 100%(MAA/ZnO=2:1.4) and the healing efficiencies of elongation were 97%(MAA/ZnO=2:1), 99%(MAA/ZnO=2:1.2) and 96%(MAA/ZnO=2:1.4). The snapshots of the healed NR with MAA/ZnO=2:1.4 during stretching are showed in Figure 10d to exhibit the considerable regained mechanical property. The cut/resealed position easily sustaining a large strain over 300% confirmed the considerable healing effect. 3.6 Reconstruction of new ionic supramolecular network and diffusion of NR chains

Figure 11. (a) Tensile behaviors of the NR with MAA/ZnO=2:1.3 healed at different temperatures for 3min; (b) the corresponding healing efficiencies for tensile strength and elongation. 11

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The reconstruction of ionic associations are essential mechanism for the self-healing behavior of the ionic supramolecular hybrid network in NR/MAA/ZnO compounds, which can be confirmed by the stress-strain measurements of the specimens healed at different temperatures. As shown in Figure 11a, the healing effect was decreased with the increased healing temperature which was similar to our previous study42. The 23°C-healed sample reached a recovered tensile strength of 1.11MPa and strain of 380%, while the 80°C-healed sample failed at a strength of only 0.76MPa and strain of 330%. Their corresponding healing efficiencies are showed in Figure 11b. It is well known that the ionic associations were quite weak at higher temperature due to that the electrostatic interaction of ion pairs was dramatically deteriorated at elevated temperatures46,50. Although a higher temperature facilitated the diffusion of NR chains across the cut surface, the seriously deteriorated ionic associations were unable to construct powerful new ionic crosslinks during healing. This further confirmed that the reconstruction of new ionic supramolecular hybrid network played the dominate role in the self-healing process.

Figure 12. Cyclic tensile behaviors of the samples with various MAA/ZnO ratios: (a) cured for 60s and (b) cured for 300s To further confirm the reconstruction of new ionic supramolecular network, a cyclic tensile experiment was conducted. As shown in Figure 12a, the cyclic stretching of the NR/MAA/ZnO compounds left a high permanent deformation set (εr)51, which was attributed to rupture and simultaneously reconstruction of new ionic associations during deformation. Without the binding effect of covalent crosslink network, the neonatal ionic crosslinks could easily restricted NR chains in the instantaneous network, which hindered the recovery of network structure. The neat NR was also provided here to compare with the 60s-cured NR/MAA/ZnO compounds. As seen, the neat NR showed a considerable recovery of the residual strain, e.g εr=72% for the 4th cycle (εmax=200%). As for the NR with MAA/ZnO=2:1.1, it showed a large εr=176% due to the reconstruction of new ionic supramolecular network. This suggested that the ionic supramolecular network was powerful in the sample, whose reconstruction at the cut surfaces was the essential mechanism for the self-healing behavior. It was worthwhile to mention that the NR with MAA/ZnO=2:1.4 showed a slight reduced εr=160%, compared with the one with MAA/ZnO=2:1.1. The slight decrease in residual strain might be attributed to the increased physical absorption which was originated from the residual nano-ZnO. The physical absorption acted as physical crosslinking points to facilitate the recovery of NR chains after deformation. When the curing time increased to 300s, the powerful NR continuous covalently crosslinked network had been formed. As a result, the covalent network of NR provided strong elastic force which resulted in a significantly reduced εr=58% for all of the samples. This also suggested that 12

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the diffusion of NR chains was restricted and the reconstruction of new ionic associations at the cut surfaces were largely reduced, which decreased the self-healing capability.

Figure 13. (a) Healing efficiencies of 60s-cured NR/MAA/ZnO compounds self-healed at room temperature for different time; (b) Healing efficiencies of 60s-cured NR with MAA/ZnO=2:1.4 under an assistance force Obviously, the diffusion of NR chains was important for the self-healing behavior due to that the diffusion of NR chains brought more contacts of ionic associations. Figure 13a shows the healing efficiencies of the NR/MAA/ZnO compounds self-healed at room temperature for different time. As expected, sufficient time for the diffusion of NR chains helped to reconstruct more ionic crosslinks at the cut/resealed surfaces. For example, the healing efficiency of NR with MAA/ZnO=2:1 was about 69% at 1min and further increased to 98% at 5min. However, the physical adsorptions of the residual ZnO had a disadvantage effect on the mobility of the NR chains, which delayed the healing time. As see, the healing efficiency of NR with MAA/ZnO=2:1.4 was only 53% at 1min and further increased to 76% at 5min. Although the healing process was delayed, the sample was still able to be completely healed after 15min. To resolve this disadvantage, an assistance force was applied to press the contacted surface of the two pieces, as shown in the inset in Figure 13b. The assistance force enhanced the contact of NR chains at the cut/resealed surfaces, consequently promoting the reconstruction of the ionic associations. As seen, the self-healed NR with MAA/ZnO=2:1.4 at room temperature for 1min had a 53% healing efficiency while it gained a 90% healing efficiency under an assistance force at 1min. Under the press force, the sample obtain an obvious accelerated healing process that it gained a completely healing at 4~5min. Although the manual assistance force was not quantitative, the above experimental results confirmed the excellent healing capability of the NR/MAA/ZnO compounds with excess ZnO, as well as the enhanced mechanical properties. The schematic for rebuilding new ionic crosslinks from the NR chains diffusion is proposed in Figure 14.

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Figure 14. Schematic for rebuilding new ionic crosslinks from the NR chains diffusion 3. CONCLUSIONS In-situ reaction of MAA and excess ZnO formed ZDMA in NR. A carefully controlled vulcanization permitted the polymerization of in-situ MAA/ZnO to generate massive ionic crosslinks in NR, while the formation of covalent crosslinks was suppressed. Due to the natural affinity between Zn2+ ion-rich domains and nano-ZnO, the residual ZnO participated in the formation of a new ionic supramolecular hybrid network. Meanwhile, by changing the MAA/ZnO molar ratio, the well dispersed residual ZnO played another important role——“reinforcer” to tailor the mechanical properties of NR. The tensile strength of NR with MAA/ZnO=2:1.4 increased to 1.89MPa which was almost 3 times of the NR with MAA/ZnO =2:1 (0.64MPa). The reconstruction of new ionic supramolecular hybrid network was the essential for self-healing behavior, where NR chains diffusion promoted the self-healing process. Higher temperature was favorable for the diffusion of NR chains, but dramatically deteriorated the ionic associations. The physical adsorptions of the residual ZnO slightly restricted the NR chains, which delayed the whole self-healing process. The NR with MAA/ZnO=2:1.4 showed a healing efficiency of 53% at 1min and it only increased to 76% at 5min. Although the healing process was delayed, the sample was still able to be completely healed after 15min, showing the good self-healing property. The tailorable mechanical property of the NR/MAA/ZnO compound with excellent self-healing performance highlighted its potential for more practical applications. ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of Guangxi Province (2016GXNSFAA380145) and the Project Sponsored by the Scientific Research Foundation of GuangXi University (Grant No.XTZ140787).

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Supporting Information. Figure S1. The appearances of samples, the white appearance of the NR with higher MAA/ZnO ratios suggested the existence of residual ZnO. In additional, the hardness of NR/MAA/ZnO compounds was increased with more of residual ZnO. Figure S2. The appearances of 60s-cured NR/MAA/ZnO compounds. Some bubbles appeared in the NR with MAA/ZnO=2:1, which was originated from the un-reacted MAA. When the MAA/ZnO exceeded 2:1.1, the bubbles disappeared, which suggested the MAA had been converted to be ZDMA. The changes of the appearances of 60s-cured NR/MAA/ZnO compounds agreed well with the FTIR and XRD results. Figure S3. TEM images of the 60s-cured NR with MAA/ZnO=2:1 at different magnifications: (a) X10000; (b) X20000. Figure S4. Neat NR could not self-heal due to no revisable ionic network in it: healing time 20min, 23°C. Figure S5. The sample in Figure S4 is the 60s-cured NR with MAA/ZnO=2:1.5. The residual ZnO concentration was quite high at this time, which formed a filler-filler network. The strength and hardness of the sample were significantly increased, but, the self-healing capability was lost. REFERENCES (1) Cordier, P.; Tournilhac, F.; Soulié-Ziakovic, C.; Leibler, L. Selfhealing and Thermoreversible Rubber from Supramolecular Assembly. Nature 2008, 451, 977-980. (2) Li, C. H.; Wang, C.; Keplinger, C.; Zuo, J. L.; Jin, L.; Sun, Y.; Zheng, P.; Cao, Y.; Lissel, F.; Linder, C.; You, X. Z.; Bao, Z. A. A highly Stretchable Autonomous Self-Healing Elastomer. Nat. Chem. 2016, 8, 619-625. (3) Taylor, D. L.; Panhuis, M. I. H. Self-Healing Hydrogels, Adv. Mater. 2016, 28, 9060-9093. (4) Liu, S.; Lin, Y.; Wei, Y.; Chen, S.; Zhu, J.; Liu, L. A High Performance Self-Healing Strain Sensor with Synergetic Networks of Poly(ɛ-caprolactone) Microspheres, Graphene and Silver Nanowires. Compos. Sci. Technol. 2017, 146, 110-118 (5) Zhan, Y.; Meng, Y.; Li, Y. Electric Heating Behavior of Flexible Graphene/Natural Rubber Conductor with Self-Healing Conductive Network. Mater. Lett. 2017, 192, 115-118 (6) Goh, M.; Hwang, Y.; Tae, G. Epidermal Growth Factor Loaded Heparin-Based Hydrogel Sheet for Skin Wound Healing. Carbohydr. Polym. 2016, 147, 251-260. (7) Hernandez, M.; Bernal, M. M.; Grande, A. M.; Zhong, N.; van der Zwaag, S.; Garcia, S. J. Effect of Graphene Content on the Restoration of Mechanical, Electrical and Thermal Functionalities of a Self-Healing Natural Rubber. Smart Mater. Struct. 2017, 26, 085010 (8) Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S. Room-Temperature Self-Healing Polymers Based on Dynamic-Covalent Boronic Esters. Macromolecules 2015, 48, 2098-2106. (9) Kuang, X.; Liu, G.; Dong, X.; Liu, X.; Xu, J.; Wang, D. Facile Fabrication of Fast Recyclable and Multiple Self-Healing Epoxy Materials Through Diels-Alder Adduct Cross-Linker. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2094-2103. (10) Wu, S.; Li, J.; Zhang, G.; Yao, Y.; Li, G.; Sun, R.; Wong, C. Ultrafast Self-Healing Nanocomposites via Infrared Laser and Their Application in Flexible Electronics. ACS Appl. Mater. Interfaces 2017, 9, 3040-3049 (11) Guo, R.; Su, Q.; Zhang, J.; Dong, A.; Lin, C.; Zhang, J. Facile Access to Multisensitive and Self-Healing Hydrogels with Reversible and Dynamic Boronic Ester and Disulfide Linkages. 15

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