Extremely Stretchable, Self-Healable Elastomers with Tunable

Article Cite This: Chem. Mater. 2018, 30, 6026−6039

pubs.acs.org/cm

Extremely Stretchable, Self-Healable Elastomers with Tunable Mechanical Properties: Synthesis and Applications JianHua Xu,† Wei Chen,† Cheng Wang,† Ming Zheng,† ChenDi Ding,† Wei Jiang,‡ LingHua Tan,‡ and JiaJun Fu*,† †

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China National Special Superfine Powder Engineering Technology Research Center, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China

Chem. Mater. 2018.30:6026-6039. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/09/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Stretchable and autonomously self-healable elastomers with wide-ranging tunable mechanical properties have attracted increasing attention in various industries. To date, it continues to be a huge challenge to synthesize selfhealing elastomers integrating extreme stretchability, relatively high mechanical modulus, and autonomous and rapid selfhealing capability. Herein, we propose a novel covalent/ supramolecular hybrid construction strategy, in which the covalent cross-links are responsible for providing high modulus and elasticity, while supramolecular cross-links realize extreme stretchability and rapid self-healing under room temperature depending on the ultrafast exchange kinetics of metal−ligand motifs and multicoordination modes. The representative polyurea hybrid elastomer, CSHPPG-Zn-0.25, can be stretched more than 180× its original length with the highest Young’s modulus (1.78 ± 0.08 MPa) among reported ultrastretchable materials. CSH-PPG-Zn-0.25 can fully restore mechanical properties of completely cut samples within 3 h. Note that the healing process can take place under a low temperature of −20 °C and unaffected by surface aging and atmospheric moisture. Merely tailoring the molar ratio of metal/ligand actualizes wide-ranging tunability of mechanical and dynamic properties, such as Young’s modulus (from 1.71 ± 0.08 MPa to 5.56 ± 0.22 MPa), maximum tensile strength (from 0.32 ± 0.03 MPa to 4.42 ± 0.23 MPa), strain at break (from >18000% to 630 ± 27%), and storage modulus, ascribed to the increase of cross-linking density and formation of stiff ionic clusters. On the basis of the different material characteristics, two typical elastomers are employed, respectively, as flexible and self-healable conductor and self-healable automotive paint. Benefiting from the fantastic antiaging and low-temperature healing features of CSH-PPG-Zn-0.25, the prepared Ag-NWs/ CSH-PPG-Zn-0.25 conductor can even regain its conductive function below zero. CSH-PPG-Zn-0.50 material, meeting the strict mechanical requirements of automotive paints, is able to thoroughly eliminate the surface scratches and recover anticorrosion function in the local damaged region under the atmospheric environment.

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INTRODUCTION Synthetic self-healing polymeric materials, inspired by “accidental injury−spontaneous healing” for natural living systems, which can autonomously repair themselves and regain mechanical and functional properties upon local damage, have attracted considerable attention over the past decades due to their wide potential applications, including the following: wearable electronics, energy storage, protective coatings, healthcare materials, sensors, and so on.1−6 Incorporation of self-healing units into synthetic materials prolongs their service life, reduces maintenance costs, and ultimately avoids failure of materials stemming from internal microcracks.7−10 Hitherto, successful construction strategies for self-healing polymers that have been reported can be divided into the following two categories: chemo-mechanical self-healing and self-healing reactions.11,12Typically, the chemo-mechanical self-healing © 2018 American Chemical Society

strategy is encapsulation technology, in which the reactive microcapsules or hollow fibers containing reactive agents are embedded into a polymer matrix. These self-healing polymers, containing microcapsules or microvascular networks, are capable of closing cuts or cracks at the microscale via transporting or circulating encapsulated healing substances to the damaged area driven by capillary forces.13,14 With the advancement of microvascular techniques, self-healing polymers based on encapsulation technology have the strength to repair large-scale damage and address the weakness of limited healing times.14 However, these self-healing polymers focus more on the restoration of mechanical or barrier attribReceived: June 3, 2018 Revised: August 15, 2018 Published: August 16, 2018 6026

DOI: 10.1021/acs.chemmater.8b02320 Chem. Mater. 2018, 30, 6026−6039

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Chemistry of Materials

Figure 1. (a) Schematic illustration of the synthetic process of the CSH-PPG-Zn sample. (The inserted logo was reproduced with permission of Nanjing University of Science and Technology.) (b) 1H NMR of the CSH-PPG sample in CDCl3 at various concentrations. (c) FTIR spectra of CSH-PPG and CSH-PPG-Zn samples. (d) SEM image and corresponding EDX (Zn K) line scan along the dashed line of a cross-sectional view of the CSH-PPG-Zn sample. (e) Elemental mapping images of the selected region in part d confirming uniformity of the added Zn2+. (f) HAADFSTEM image of CSH-PPG-Zn sample. (g) DSC curves of CSH-PPG-Zn sample, the enlarged image inseted in part g clearly showing the Tg value.

utes;13−15 due to the inherent differences between healing substances and polymeric substrates, it is difficult to restore their original functionalities, which are diminished after structural damage and are greatly expected to be reacquired. Besides encapsulation technology, remote self-healing technology via electromagnetic radiation, magnetic fields, or NIR light is an another chemo-mechanical self-healing strategy that can restore simultaneously the mechanical strength and functionalities.11,16 Self-healing polymers based on different self-healing reactions in the polymer matrix have been engineered through the incorporation of dynamic covalent bonds (such as Diels− Alder reaction,17 disulfide,18 and dynamic boroxine bonds19) or supramolecular noncovalent bonds (such as hydrogen bonding,20 host−guest molecular recognition,21 π−π stacking,22 electrostatic interactions,23 and metal−ligand interactions24) into polymeric networks, and mend fractures via exchangeable dynamic reactions or reversible bond scission− re-formation. Intriguingly, some innate functionalities, including superhydrophobicity, corrosion resistance, antifouling

performance, electronic conductivity, etc., have been reported to be completely or partially resumed following structural recovery, showing competitive advantages over encapsulation self-healing strategies.25−29 However, most dynamic covalent bonds require input of external energy (heat, light or catalyst) to facilitate association/dissociation reversion.17−19,30,31 Considering the nondetectability of internal microcracks in some cases and the possibility of subsequent disastrous consequence, supramolecular polymers easily realize an autonomous selfhealing process under mild conditions according to the following two fundamental principles: (i) assurance of mobility of polymeric chains and (ii) selection of suitable supramolecular motifs. These principles are easy to meet for practical environmental needs and gain particular interest.7,9,20,24 Although great progress has been made in the development of supramolecular polymers with the room-temperature selfhealing feature, the resulting poor mechanical strength limits their application.32 Great efforts have been devoted to tackle 6027


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fabricated and used as self-healable automotive paint. The mechanical properties of this kind of paint are all compliant with the industrial standard. More importantly, it can thoroughly eliminate surface scratches under atmospheric environment and recover anticorrosion function without input of external energy.

the contradiction between mechanical robustness and chain mobility for dynamic healing,20,33 which also reminds us to pay close attention to mechanical properties before subjecting materials to practical applications. Guan and co-workers proposed the hard−soft multiphase design, in which hard phases (polystyrene, poly methyl methacrylate, etc.) with high Tg values provide mechanical stiffness/strength, while soft phases (polymethacrylate, etc.) with low Tg values and elegant supramolecular motifs are responsible for room-temperature self-healing behavior.20 Recently, another promising approach is to assemble covalent/supramolecular hybrid polymeric networks, with incorporation of appropriate covalent crosslinkers that enhance overall mechanical properties and do not compromise self-healing capability.34,35 Nowadays, the flourishing exploitation of supramolecular self-healing polymers greatly broadens scopes available to engineering applications.25−29 The self-healing capability, particularly recovery of the ability of special functionalities as the crucial factor that needs to be guaranteed, at the same time, is of significance for the adjustment of mechanical properties of polymers for meeting the criteria for various industrial uses. For example, the utilization of flexible self-healable conductors not only demands a quick recovery of electrical conductivity after fracture, but also emphasizes an ability to accommodate large strain and stable electrical conductivity during deformation.36 As another example of self-healable automotive paint in different working environments, a simultaneous combination of superior mechanical properties, satisfactory anticorrosion performance, as well as self-diagnosis/self-healing functions is urgently demanded.37 Diverse applications have different requirements for material characteristics; therefore, the design and facile preparation of autonomous self-healing polymers with tunable mechanical properties for multipurpose uses are innovative and still full of challenges. The investigations of mussel-inspired iron-catechol materials offer the optional route to build self-healing polymers by making use of dynamic and reversible coordination bonds.38,39 The adjustable metal−ligand binding constants and kinetical volatility are convenient to equilibrate mechanical properties and the macroscopic dynamic nature of the resultant selfhealing polymers.24,25,33,40 Herein, we report a series of polyurea hybrid elastomers, containing covalent cross-links and supramolecular metal−ligand cross-links, by simply regulating the metal/ligand ratio. Through a reasonable control ratio between covalent and supramolecular cross-links, an extremely stretchable, autonomous, and rapid self-healing polyurea hybrid elastomer was readily synthesized. The polyurea hybrid elastomer can be stretched more than 180× its original length with the highest reported Young’s modulus (1.71 ± 0.08 MPa) among ultrastretchable materials. The covalent cross-links provide robustness and elasticity, while the ultrafast exchange kinetics of supramolecular cross-links are responsible for the extreme stretchability. Furthermore, this material exhibits a fast self-healing capability. After the material is cut in half, all the mechanical properties can be fully restored at room temperature for only 3 h. Combined with antiaging and low-temperature healing characteristics, the polyurea hybrid elastomer demonstrates a bright application prospect in flexible and self-healable conductors. On the basis of this structure, only varying the metal/ligands ratio actualizes wideranging tunability of mechanical properties. Another hybrid polyurea elastomer, integrating balanced stiffness, strength, toughness, and autonomous self-healing capability, was

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RESULTS AND DISCUSSION Design, Synthesis, and Characterization of Polyurea Hybrid Elastomer. Our design strategy to fabricate covalent/ supramolecular hybrid polyurea elastomers primarily comprises two fundamental principles: (i) The incorporation of covalent cross-links into polymeric networks is to improve mechanical robustness. (ii) The utilization of dynamic supramolecular metal−ligand cross-links is to impart polymeric networks with a self-healing capability under room temperature. The whole synthetic route is illustrated in Figure 1a. First, tolylene 2,4diisocyanate terminated polypropylene glycol (PPG-NCO), 1(3-aminopropyl) imidazole (IMD-NH2), and trimethylolpropane tris[poly(propylene glycol), amine terminated] ether (PPG-NH2) were gently mixed and reacted at 60 °C with a fixed 3:3:1 stoichiometric molar ratio (PPG-NCO/IMD-NH2/ PPG-NH2) for 6 h to afford a viscous liquid. The reaction was monitored by Fourier transform infrared spectroscopy (FTIR), with the disappearance of stretching bands at 2319 cm−1 corresponding to isocyanate groups and the appearance of two new peaks at 1690 and 1650 cm−1 assigned to stretching vibrations for CO in urea groups and NCN in imidazole rings,26,41 respectively, which establish the complete polycondensation reaction and formation of the permanent covalent network, carrying imidazole ligands. It should be noted that the atmospheric water vapor is likely to act as the extra chain extender during the reaction process (CSH-PPG, Figure S1, Supporting Information).42 In addition, with increasing concentration in solution, the more and more apparent downfield shift of the NMR signal of NHC NH atoms confirms the existence of intermolecular hydrogen bonding from urea groups (Figure 1b).3,43 In a second step, a zinc(II) salt (zinc trifluoromethanesulfonate, zinc(CF3SO3)2) chosen as the metal source due to its good solubility as well as high mobility of counterions,33 CF3SO3− in the solid states, was added to introduce supramolecular metal− ligand cross-links, keeping the molar ratio between Zn2+ and imidazole group at 1:4. After being cured under vacuum to remove solvent, the resultant polyurea hybrid elastomer appearing as a transparent, colorless, freestanding film was obtained (CSH-PPG-Zn, Figure S2). Both covalent cross-links and supramolecular noncovalent cross-links play pivotal roles in the polymer molding process. As a comparison, in the absence of PPG-NH2 as covalent cross-linkers, this only resulted in a viscous fluid with no appreciable mechanical properties, as evidenced by the dynamic mechanical tests in which the loss modulus is higher than the storage modulus over the tested frequency (25 °C, Figure S3 and Movie S1). After addition of Zn(CF3SO3)2, the storage modulus (25 °C, 1 Hz) substantially increased from 1383 Pa (CSH-PPG) to 494 632 Pa (CSH-PPG-Zn), preliminarily suggesting the formation of supramolecular cross-links (Figure S4, SI). In order to further examine the interactions between Zn2+ and imidazole, attenuated total reflectance-FTIR (ATR-FTIR) experiments were conducted and the results are shown in Figure 1c. An absorption peak at 1650 cm−1, assigned to the characteristic vibration of NCN in the imidazole ring, was 6028


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Figure 2. (a) Photographs of a CSH-PPG-Zn sample before stretching (left) and showing high stretchability at >18 000% stretching (right) without fracture. (b) Raman spectra of CSH-PPG and CSH-PPG-Zn samples. XPS spectra of the CSH-PPG-Zn sample: (c) high resolution N 1s spectra and (d) high resolution O 1s spectra. (e) 1H NMR spectra of the CSH-PPG-Zn sample with increasing Zn2+ content. (f) Tensile curves of the CSH-PPG-Zn sample at a deformation rate of 100 mm min−1. (g) Enlarged tensile curves of CSH-PPG-Zn sample collected from part f.

observed in a spectrum of CSH-PPG sample.41 Upon coordination with Zn2+, this characteristic peak shifted to a high wavenumber, 1668 cm−1, revealing the Zn2+/imidazole coordination complexes. Cross-sectional line-scan and mapping images shown in Figure 1d,e manifest the uniform distribution of Zn2+ within a polymeric network. No Zn2+ aggregates were detected in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1f) even at the nanoscale level (sensitive for high atomic number). The differential scanning calorimetry (DSC) curve shows that the glass-transition temperature (Tg) of the CSH-PPG-Zn film is about −50.1 °C (Figure 1g), far below room temperature, and no melting point is found in the DSC curve (−80 to 150 °C), demonstrating the amorphous structure, which is further corroborated by X-ray diffraction (Figure S5). The thermal stability of the film was assessed by thermal gravimetric analysis (TGA). As shown in Figure S6, the initial decomposition temperature is up to 250 °C, exhibiting good thermal stability. Investigation of Mechanical Properties of Polyurea Hybrid Elastomer. In order to characterize mechanical properties of CSH-PPG-Zn, we cut the CSH-PPG-Zn film,

fabricated by a solvent evaporation method, into a rectangular shape with a tensile size of 5 mm (L) × 10 mm (W) × 1 mm (T) and quantify its stress−strain behavior using a uniaxial tensile testing machine with a 100 N load cell; these measurements were performed in air, at room temperature (20 °C). In a typical test, a CSH-PPG-Zn sample was stretched at a deformation rate of 100 mm min−1. It is of great interest to note that the CSH-PPG-Zn sample exhibits extreme stretchability and can be stretched to >180 times its original length without fracture (limited by the measurement range of the machine, Figure 2a, Movie S2). Curiously, at a strain of 18 000%, the CSH-PPG-Zn sample could also be violently shaken (Movie S3), implying the potential for further stretching. Recently, Bao and co-workers reported the synthesis of a highly stretchable elastomer using an Fe(III)2,6-pyridinedicarboxamide complex with different bond strengths, which could be stretched over 100× its initial length (note: tensile rate was 10 mm min−1), and the Young’s modulus and maximal strength were 0.09 ± 0.01 MPa and 0.03 ± 0.01 MPa, respectively.24 For comparison, the CSH-PPG-Zn sample possesses not only higher stretchability, but also larger Young’s modulus and maximal strength, up to 1.71 ± 0.08 6029


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Figure 3. (a) CSH-PPG-Zn sample was subjected to cyclic tensile test of varying maximum stretching. (b) Consecutive cyclic loading−unloading curves of CSH-PPG-Zn sample with 5 min intervals and a maximum strain of 50%. (c) After the first loading−unloading cycle up to 1000% (gray curve), the sample was allowed to recover under different conditions and was reloaded again. (d) Consecutive cyclic loading−unloading curves with gradual strain increase up to 4000% for the CSH-PPG-Zn sample; the dashed lines are the loading−unloading curve with 4000% strain for the same film. (e) Schematic illustration of the proposed mechanism for the strain recovery; coordinate bonds re-form at their original position at small strain, whereas they migrate and reconnect to adjacently accessible sites at large strain, causing residual strain.

MPa and 0.32 ± 0.03 MPa, respectively. To the best of our knowledge, the stretchability and Young’s modulus are evaluated to be the highest reported so far for polymeric materials owing to the extremely stretchable property.24,28,44−49 Attempting to clarify the reasons for the combination of extreme stretchability and superior mechanical strength, we first carried out the systematic study on the designed supramolecular motifs, which have been believed to account for the high toughness or stretchability via bond breakage and re-formation in many previous literature studies.24 According to our original design, the Zn2+/imidazole coordination complex, which has fast exchange kinetics and a relatively small binding constant,33,40 was employed as a key component for spontaneous self-healing, and the molar ratio of Zn2+ to imidazole ligand was controlled as 1:4 to consciously assemble tetradentate bridged theoretically stable complexes.40 However, UV−vis titrations of two synthesized compounds, 1butyl imidazole (BIMD, only imidazole moiety is in BIMD) and urea, N-butyl-N′-[3-(1H-imidazol-1-yl) propyl] (UBIMD, imidazole, and urea moieties are present in UBIMD), with Zn(CF3SO3)2 are apparent when imidazole and urea ligands coexist; Zn2+ simultaneously coordinates with two ligands and thus alters the original 1:4 Zn2+/imidazole stoichiometry (Figures S7−S10). Considering the proven urea-induced hydrogen bonds in CSH-PPG networks, it is necessary to make out the influence of Zn(CF3SO3)2 on structures of supramolecular cross-links. As a result, not only the imidazole moiety but also the urea moiety participate in the coordination process, as evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) of the CSH-PPG-Zn sample. Compared with CSH-PPG, the new characteristic peaks at 256 and 316 cm−1, respectively assigned to stretching

vibrations of Zn2+−N and Zn2+−O bonds,50,51 appeared in the Raman spectrum of CSH-PPG-Zn (Figure 2b). Meanwhile, as shown in Figure 2c,d, the Zn2+−N (399.6 eV, high resolution N 1s spectrum) and Zn2+−O (532.2 eV, high resolution O 1s spectrum) could be easily distinguished from the deconvoluted peaks in the XPS spectra of CSH-PPG-Zn, in agreement with Raman results, providing strong evidence for the multicoordination modes.52 1H NMR titrations were performed to further elucidate the coordination modes for Zn(CF3SO3)2 and CSH-PPG in CDCl3 (Figure 2e and Figure S11). The clear downfield shifts of the H1,2,3 protons of the imidazole moiety were observed and can be explained by a paramagnetic shielding effect induced by the Zn2+ bound imidazole ligands.53 At the same time, the relatively small shifts of H4,5 protons were also found particularly for urea signals (Figure 2e). It is worth noting that the proton signals of imidazole and urea moieties continued to downshift even when the molar ratio of Zn2+ to imidazole exceeded 1:4. Obviously, as a result of the existence of the urea moiety, substantial uncoordinated free imidazole ligands are present in the CSH-PPG-Zn network, which increases the exchange kinetics of the Zn2+/imidazole complex on the basis of the metal center with the preferred exchange mechanism proposed by Guan and co-workers.40 It is believed that the accelerated exchange kinetics of the Zn2+/ imidazole complex and the enormous additional urea coordination sites caused by the multicoordination modes are the two crucial factors for the unprecedented stretching behavior. Afterward, we carefully analyzed the stress−strain curve, which can be divided into four distinct regions as shown in Figure 2f,g. For region I (purple line), at small strain (ε < 13%), the stress significantly increased with strain in a linear manner, reflecting typical elastic deformation deriving from the 6030


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Figure 4. (a) Tensile curves of CSH-PPG-Zn-x (x = 0.25, 0.33, 0.50, 1.00) samples. (b) Summary of the mechanical properties of CSH-PPG-Zn-x (x = 0.25, 0.33, 0.50, 1.00) samples. (c) Storage modulus versus temperature curves of CSH-PPG-Zn-x (x = 0.25, 0.33, 0.50, 1.00) samples. (d) Photographs of CSH-PPG-Zn-1.00 sample (100 mm (L) × 10 mm (W) × 1 mm (T)) sustaining a weight of 4475 g without rupture. (e) SAXS diffraction of the CSH-PPG-Zn-0.25 and CSH-PPG-Zn-1.00 samples. The inset picture shows the distinct two-dimensional grazing-incidence small-angle X-ray scattering (2D-GISAXS) pattern of the CSH-PPG-Zn-1.00 sample. (f) Schematic illustration of the formation of ionic clusters; the increased content of Zn2+ and counterions in the CSH-PPG-Zn matrix causes the original coordinate complex to pack together, leading to the formation of ionic clusters. The corresponding distance of clusters is 4.9 nm.

covalent networks. Compared with the pure supramolecular cross-linked polymeric networks, the incorporation of appropriate covalent cross-links is the key step for enhanced mechanical strength and does not compromise stretchability if properly designed. For region II (red line), a clear yield point emerged at a strain of ≈65%, and a following strain-softening region represents the beginning of the disassociation of reversible coordination bonds and the partial movement of polymeric chains. For region III (orange line), as the strain was increased to 860%, the transient strain-softening region reversely transformed to a strain-hardening region and produced the maximum tensile strength, 0.32 ± 0.03 MPa at a strain of 6000%. In this region, the partially broken hybrid network was continuously pulled apart, and the polymer chains were rearranged along the stretching direction, leading to a self-reinforcing effect. Notably, we ruled out the possibility of strain-induced crystallization for enhancement of stress because polymer crystallization behavior was not detected during this stage of stretching (860% < ε < 6000%) via X-ray diffraction (XRD) and differential thermal analysis (DSC) measurements (Figure S12). For region IV (pink line), as the strain exceeded 6000%, the polymeric chains experienced the large slippage, in which the supramolecular cross-links were extensively ruptured, as reflected by the gradual decrease of stress. However, the broken Zn2+/imidazole-urea complexes can rapidly re-form depending on the special internal design: (i) Fast exchange kinetics of the Zn2+/imidazole coordination complex are the primary guarantee for re-establishment of the network, and uncoordinated imidazole ligands further accelerate this exchange rate. (ii) Massive amounts of free urea ligands, especially for neighboring urea ligands, close to imidazole ligands, provide sufficient coordination sites for unsaturated metals ions to regenerate dynamic coordination bonds, striving to maintain the supramolecular networks. The

influence of deformation rates on the tensile property was also used to estimate the dynamic nature of supramolecular crosslinks. The tensile experiments for CSH-PPG-Zn were tested at deformation rates of 600 mm min−1 and recorded using a camera (Movie S4). Generally speaking, with the increase of tensile rate, the strain at the break of elastomers will decrease significantly because of limited recombination rates.24,46 Surprisingly, even at a tensile rate of 600 mm min−1, the CSH-PPG-Zn sample did not fracture at a strain of 18 000%, which is convincing evidence for ultrafast kinetics of the coordination complex within hybrid networks. Rapid reconstruction of supramolecular networks redistributes the accumulated local stress and makes CSH-PPG-Zn withstand large deformation.34 To further investigate the viscoelastic properties of CSHPPG-Zn, the cyclic tensile tests were performed. As shown in Figure 3a, when the samples were stretched to the presetting strains, the pronounced hystereses were observed in all the cycles, indicating that Zn2+/imidazole-urea coordination bonds, serving as sacrificial bonds, can effectively dissipate energy.25,39 At small applied strains (ε < 50%), the notable residual strains originating from the dissociation of metal/ ligand bonds decreased to zero in a short relaxation time (5 min) (Figure 3b). This result demonstrates that the partially broken coordinate bonds are capable of re-forming after rupture, keeping the hybrid network intact. At high applied strain (ε = 1000%), the sample could not return back to its original length and leave a notable residual strain (≈120%) after the immediate second loading. Interestingly, unlike the previously reported polymeric elastomers,54 the two conventional approaches, including allowing longer relaxation time (20 °C, 12 h) or adopting a heating method (60 °C, 12 h), could hardly help samples to decrease the residue strain (Figure 3c), which initially verifies that the permanent residual 6031


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Figure 5. (a) Optical microscope images demonstrating the self-healing capability of the CSH-PPG-Zn-0.25 film. (b) Photographs of the CSHPPG-Zn-0.25 film: (I) Sample was cut in the middle section (20 mm (L) × 10 mm (W) × 1 mm (T)), and then the cut pieces were put in contact with each other, and healed for 10 min at room temperature (II). Panels III and IV show the stretching process to a strain of ≈2800%. (c) Tensile curves of the CSH-PPG-Zn-0.25 film at various healing times at room temperature. (d) Comparison of the CSH-PPG-Zn-0.25 film with previously reported highly stretchable and self-healable polymers, in terms of healing time, healing temperature, and maximum tensile strength. (e) Apparent shear strength of CSH-PPG-Zn-0.25 film measured by multiple lap shear tests at room temperature. (f) Tensile curves of the CSH-PPG-Zn-0.25 film healed at −20 and 0 °C for 12 h, and the tensile curves of the film healed at room temperature for 6 h after surface aging for 12 h. Inset shows fluorescence microscopic images confirming the diffusion and entanglement of polymer chains, leading to the self-healing behavior of the CSHPPG-Zn-0.25 sample at low temperature. Scale bar = 100 μm. (g) Tensile curves of the CSH-PPG-Zn-0.50 film at various healing times under room temperature. (h) CSH-PPG-Zn-0.50 film healed at room temperature for 12 h can be largely stretched and hold a weight of 500 g without rupture. (i) Stress−relaxation curves of CSH-PPG-Zn-0.25 film at 20 °C, and CSH-PPG-Zn-1.00 film at 20 and 50 °C.

(Figure S13). In fact, the residue strain is associated with the competition between the contraction of the elastic chain and the strength of the temporary restriction of supramolecular networks. The multicoordination modes brought about supramolecular cross-links with different structures, such as Zn2+/imidazole, Zn2+/urea, and Zn2+/imidazole-urea complexes. Upon stretching, the rupture and re-formation of supramolecular cross-links are kept in dynamic equilibrium. The instantaneously uncoordinated Zn2+ centers preferentially bound adjacent accessible ligands to constitute the new supramolecular cross-link.38 Due to the inhomogeneous random distribution of the two ligands, the structures and the strength of supramolecular cross-links are totally different from the original one, which is the reason for the residue strain after the cyclic tensile test at large strains (Figure 3e).

strain cannot be ascribed to the unrecoverable noncovalent bonds. The consecutive cyclic tensile tests, in which the CSHPPG-Zn sample was subjected to a cycle of loading and unloading with different presetting strain and without a rest of each interval, were carried out to support our hypothesis. As shown in Figure 3d, during the first loading−unloading cycle at strain of 400%, the yield point appeared (ε = 130%) as expected. After the immediate second loading−unloading cycle at strain of 1000%, the yield point for this cycle reappeared at a strain of 226%, lower than the maximum strain (400%) of the first cycle. The similar phenomenon also occurred in the third and fourth cycles. These imply that during each loading− unloading cycle, the continuously broken noncovalent bonds rapidly re-formed before the beginning of the next loading− unloading cycle.44,45 The consecutive loading−unloading test with a constant strain of 1000% also supported this conclusion 6032


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between zinc-rich metal−ligand cross-linked domains (Figure 4f). The increased content of Zn2+ as well as counterions in the solid state facilitate the formation of ionic clusters, which makes the originally scattered supramolecular cross-links with small binding constants pack together, require higher dissociation energies to separate out, and thus increase the mechanical strength. Self-Healing Behaviors of CSH-PPG-Zn-x Films. Benefiting from the fast exchange kinetics of the Zn2+/ imidazole complex and low Tg values, CSH-PPG-Zn-x elastomeric materials are also expected to have outstanding self-healing capabilities at room temperature. As a typical example, the self-healing behavior of the CSH-PPG-Zn-0.25 film was initially evaluated. We first investigated the scratch recovery ability using an optical microscope. The artificial scratch that is 1 mm long and 20 μm wide was made on the freestanding film surface; after only being kept at room temperature for 3 h, the scratch completely disappeared, without leaving any minor scars (Figure 5a). The scratch recovery process that featured a fast recovery speed, no remnant trace, and no assistance from external energy is superior to that of the previously reported self-healing elastomers.24,56,57 The restoration ability of mechanical properties of the full cut films is subsequently studied using a tensile testing machine. We cut the CSH-PPG-Zn-0.25 film into two separate pieces with a razor blade and then gently put them together to allow healing at room temperature without any intervention. In order to clearly distinguish the separate pieces, one of the pieces was stained with rhodamine B. Notably, when the healing process lasted only 10 min, the rejoined film already could be easily stretched to a strain of 2800% (Figure 5b). Stress−strain curves of rejoined films at different healing times are shown in Figure 5c. The mechanical healing efficiency is defined as the proportion of the restored toughness relative to the original toughness for comprehensively considering restoration of tensile stress and strain at break. As shown in Figure 5c, a longer healing time leads to a higher healing efficiency as expected. Remarkably, the CSHPPG-Zn-0.25 film is able to completely restore all mechanical properties, including Young’s modulus, tensile strength, and strain, within 3 h, demonstrating an excellent self-healing capability (Movie S5). Among the reported ultrastretchable elastomers with self-healing functionality, the CSH-PPG-Zn0.25 film has impressive advantages in stretchability and maximum tensile strength (Figure 5d).24,45−49 Since the fractured region of the CSH-PPG-Zn-0.25 film was severely deformed, the repeated self-healing capability cannot be tested by reconnecting the fractured interfaces. In view of this, lap shear tests were employed to determine the repeated healing effects39 (Figure S15). The CSH-PPG-Zn-0.25 film (25.4 mm × 1.25 mm) was bonded to the two aluminum alloy plates as depicted in the inset image of Figure 5e. Then, the adhered alloy plates were pulled by a tensile testing machine at a deformation rate of 1 mm min−1. The shear strength of the CSH-PPG-Zn-0.25 film on aluminum alloy plates was 0.089 MPa. After bringing two torn films, which were all still stuck to the plates (Figure S16), into contact at room temperature for 3 h, the shear strength was measured again and did not obviously decrease (0.085 MPa). After the end of fifth repeated healing, the shear strength still remained unchanged, clarifying that the CSH-PPG-Zn-0.25 film is able to repeatedly heal the same damaged regions. Considering the ultralow Tg value (−50.1 °C), we endeavored to assess the self-healing behaviors at low

Apparently, the CSH-PPG-Zn sample manifests astoundingly stretchable and relatively high mechanical strength among reported ultrastretchable materials; on the basis of this molecular framework, we successfully synthesized a series of highly stretchable materials with tunable mechanical properties and denoted these as CSH-PPG-Zn-x (x represents the molar ratio of Zn2+ to imidazole, and CSH-PPG-Zn equates to CSH-PPG-Zn-0.25) via simply varying the Zn2+ to imidazole molar ratio. The typical stress−strain curves of CSHPPG-Zn-x samples, generated at room temperature with a deformation rate of 100 mm min−1, are shown in Figure 4a, and a complete summary of mechanical properties for CSHPPG-Zn-x samples is included in Figure 4b. With an increase in the molar ratio of Zn2+ to imidazole going from 0.25 to 1.00, the Young’s modulus of CSH-PPG-Zn-x samples increases from 1.71 ± 0.08 to 5.56 ± 0.22 MPa, and the maximum tensile strength increased from 0.32 ± 0.03 to 4.42 ± 0.23 MPa, whereas the strain at the break decreased from >18 000% to 630% ± 27%. Only through the increase of the content of Zn(CF3SO3)2 did the mechanical properties of CSH-PPG-Znx undergo tremendous changes, from an extremely stretchable, mechanically soft material to a moderately stretchable, mechanically strong material (Figure 4b,c). It was found that the maximum tensile strength of the CSH-PPG-Zn-1.00 sample increased 14 times as compared with that of the CSH-PPG-Zn-0.25 sample and was able to lift a weight of 4.475 kg without tearing, 6400× its own weight (Figure 4d). Wide-ranging mechanical tunability arouses our interest and motivates us to understand reasons for transformation. The DSC curves of four samples, CSH-PPG-Zn-x (x = 0.25, 0.33, 0.5, 1.0), are shown in Figure S14a. With the increase of Zn2+, the Tg values increased from −50.1 to −44.7 °C, revealing that the mobility of polymer chains was suppressed to a certain extent. However, no obvious crystallization peak was observed in all samples up to 150 °C. In combination with the XRD results (Figure S14b), we exclude the possibility of crystallization-induced mechanical reinforcement. In addition, the HAADF-STEM image of CSH-PPG-Zn-1.00 also shows no Zn precipitations are formed within the hybrid network, similar to the situation for CSH-PPG-Zn-0.25, eliminating the possibility of nanofillers filling the induced mechanical reinforcement (Figure S14c). It is plausible that the improvements of the mechanical strength of CSH-PPG-Zn-x (x = 0.25, 0.33, 0.5, 1.0) are related to the increase in cross-linking density. With the increase of Zn2+ content in the polymer network, the cross-linking density gradually increased, which led to the increase of antideformability of the CSH-PPG-Zn-x polymer network (Table S1). As a result, the mechanical strength of the CSH-PPG-Zn-x material gradually increased, whereas the strain at the break continuously reduced. Indeed, when the molar ratio of Zn2+ to imidazole was up to 1:1, the cross-linking density of CSH-PPG-Zn-1.0 reached 4.31 ± 0.21 × 10−4 mol/cm−3, which is much higher than that of the CSHPPG-Zn-0.25 sample (1.78 ± 0.09 × 10−4 mol/cm−3). Thus, the CSH-PPG-Zn-1.0 sample possesses the highest tensile strength, up to 5.56 MPa. In addition, the formation of ionic clusters of CSH-PPG-Zn-1.00 is the other factor contributing to high mechanical strength.55 This assumption is supported by the small-angle X-ray scattering (SAXS). As shown in Figure 4e, the CSH-PPG-Zn-0.25 sample does not exhibit any scattering peaks, whereas the CSH-PPG-Zn-1.0 sample shows a signal at Q = 1.29 nm−1 and the corresponding distance is calculated as 4.9 nm, stating the correlation distance (r) 6033


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Figure 6. (a) Schematic illustration of the preparation process of the flexible and self-healable conductor. (b) Photograph of the flexible and selfhealable conductor with the sheet resistance of 0.35 Ω sq−1. (c) Top view SEM images of the as-fabricated flexible and self-healable conductor. (d) Demonstration of the healing process with a white LED in series with a flexible and self-healable conductor. (e) I−V curves of a series circuit, in which the flexible and self-healable conductor was connected to a 20 Ω resistor, measured using the original conductor with the conductor undergoing multiple cutting−healing cycles.

healing process in practical applications, and this can be ascribed to the following three aspects: (i) Zn2+/imidazoleurea coordination bonds are unaffected by the atmospheric moisture or oxidation. (ii) The high mobility of polymer chain at room temperature is the prerequisite for achieving the antiaging effect, which facilitates a surface approach and provides the opportunity for intimate contact of metal−ligand complexes being at different interfaces. (iii) The ultrafast exchange rate of the Zn2+/imidazole complex plays a decisive role in the antiaging process. We employed ATR-FTIR to monitor molecular events during the cutting−healing process (Figure S18a).58,59 It was found that the ATR-FTIR spectra of the original area and the damaged area overlapped; in particular, the peak intensity and position of Zn2+/imidazole coordination bonds (1650 cm−1) were almost unchanged, indicating the instantaneous completion of the surface rearrangement process. The dynamic association and dissociation of supramolecular cross-links occurring in the fractured interfaces enable the structural restoration (Figure S18b). The self-healing behaviors of CSH-PPG-Zn-x (x = 0.33, 0.50, and 1.00) films were subsequently studied. CSH-PPGZn-0.33 and CSH-PPG-Zn-0.50 films all exhibit moderate mechanical recovery ability at room temperature; the healing times needed for full recovery are appropriately 6 and 12 h, respectively, slower than that of the CSH-PPG-Zn-0.25 film (Figure 5g and Figure S19a). This phenomenon is likely due to the increase of cross-linking density of the CSH-PPG-Zn-x film (Table S1). The higher the cross-linking density is, the worse the dynamics of the polymer network. Different from the CSHPPG-Zn-0.25 film featured with extreme stretchability and rapid self-healing capability, the CSH-PPG-Zn-0.50 film presents the more balanced performances between mechanical strength (Young’s modulus, 4.46 ± 0.15 MPa; tensile strength, 2.26 ± 0.12 MPa), toughness (40.83 MJ m−3), and spontaneous room temperature self-healing. After healing for

temperature. As shown in Figure 5f, after bring two fractured surfaces into contact at −20 °C and waiting for 12 h, a recovered strain at break of 6880% and a healing efficiency of 42.31% were obtained. Raising the healing temperature to 0 °C for 12 h gave a high healing efficiency of 89.77%, which verifies the feasibility of executing a normal healing mission at low temperature for the CSH-PPG-Zn-0.25 material. It can be inferred that the Tg value, far below room temperature, guarantees the sufficient mobility of polymer chains at low temperature, which gives rise to the low-temperature healing function. This assumption is also proved by the fluorescence microscopy images (Figure 5f). After being in contact at 0 °C for 12 h, a clear merging area, overlapping red with green luminescence, appeared, which can be attributed to the diffusion or entanglement of polymer chains across damaged interfaces. Another attractive characteristic of self-healing in CSH-PPG-Zn-0.25 film is its antiaging ability. For a majority of self-healing polymers, especially for hydrogen-bonding polymers, long-term separation results in the deactivation of the functional groups on fractured interfaces due to their sensitivity to atmospheric moisture or oxidation, and the selfhealing functionality gradually diminishes with the prolongation of separation time.9,20 Curiously, the CSH-PPG-Zn-0.25 film displayed superior antiaging performances. When the freshly cut surfaces were exposed to the atmosphere for 12 h, and then brought into contact for another 6 h at room temperature, the healing efficiency astonishingly reached 91.44%, and the extreme stretchability was nearly restored. In addition, the unfractured surfaces could be pushed together to achieve a self-healing performance. As depicted in Figure S17, the CSH-PPG-Zn-0.25 film was cut into two pieces, and the separated pieces were reversed. The unfractured surfaces were then held together for 12 h at room temperature, and the healing efficiency was measured as high as 88.98%. The antiaging characteristic is of great importance for the self6034


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strength. Actually, the Ag-NWs layer firmly adheres to the CSH-PPG-Zn-0.25 film substrate and is hard to peel off. As expected, the sheet resistance of the as-prepared Ag-NWs/ CSH-PPG-Zn-0.25 conductor is as low as 0.35 Ω sq−1. In particular, when the Ag-NWs/CSH-PPG-Zn-0.25 conductor was bent 1000 times at a bending angle of 90°, the sheet resistance only increased by ca. 7.0% compared to that of the pristine film (Figure S22), and the sheet resistance can be totally restored through heating at 30 °C for 1 h (Figure S22). In order to evaluate electrical healing upon damage, the AgNWs/CSH-PPG-Zn-0.25 conductor was connected to a lightemitting diode (LED) and a power source (3 V). As shown in Figure 6d, when the conductor was cut with a knife, the lit LED immediately went off. However, the LED relit with almost the same brightness after bringing the two bifurcated conductors contact for only 10 s. The damage−repair cycles in the same damaged region did not evidently alter electrical conductivity. Even after the 20th cycle, the electrical conductivity was still in a low range, as shown in I−V curves (Figure 6e). Such restoration of electrical conductivity mainly resulted from the rapid self-healing of the underlying CSHPPG-Zn-0.25 film, facilitating the reconnection of the separated Ag-NWs layer.64 This typical healing imparting mechanism was further confirmed by SEM results of the cut and healed conductor (Figure S23), in which the broken AgNMs layer is finely reconnected again, accompanied by the self-healing process of the underlying film. The electrical conductivity of the healed conductor was barely changed when it was bent or twisted, which is similar to the pristine film (Figure 6d). More importantly, the antiaging and lowtemperature healing characteristics endow the Ag-NWs/ CSH-PPG-Zn-0.25 conductor with possible use in special application environments. The cut conductors (aging time: 12 h) were able to light the LED immediately upon the gentle touch of the random aged surface (Figures S24 and S25 and Movie S6). In addition, the electrical conductivity was able to be recovered even upon healing at low temperature (Figure S26). To the best of our knowledge, this is the first report of a self-healable conductor capable of resisting surface aging and healing below zero. Compared with the previously reported self-healable conductors which choose hydrogen bonded crosslinked polymers as the conductor substrate, our CSH-PPG-Zn0.25 elastomer is more suitable for preparing flexible and selfhealable conductors in practical use. Self-Healable Automotive Paint. Self-healable automotive paint, which is able to actively repair a minor scratch and thus greatly reduce the maintenance cost of automotive paint, represents the development trend of multifunctional automotive paint.65 Currently available commercial self-healable automotive paints accomplish the automatic repair dependent on local ultraviolet light irradiation or heating treatment.66 It is still a challenge to fabricate autonomously and rapidly selfhealable automotive paint without additional energy input. Not only that, the automobile is faced with complex changeable application environments, which need strong and reliable mechanical properties of automotive paint to protect the underlying metal substrates. The combination of autonomous self-healing capability in the atmospheric environment and desirable mechanical properties raises higher requirements for the design of automotive paint. As is stated above, CSH-PPGZn-0.50 material possesses a combination of balanced mechanical properties and room-temperature self-healing capability, which has the potential to be used as qualified

12 h at room temperature, the rejoined CSH-PPG-Zn-0.50 film could be largely stretched and could bear a weight of 500 g (Figure 5h). Different material properties lay the foundation for their different potential applications. Herein, one puzzling problem is that although Tg values of CSH-PPG-Zn-1.00 and CSH-PPG-Zn-0.25 films are all far below room temperature, the self-healing capability of the CSH-PPG-Zn-1.00 film was substantially worse than that of the CSH-PPG-Zn-0.25 film at room temperature, and the healing efficiency was only 20.76% after 24 h healing. Further prolonging the healing time did not improve the healing efficiency (Figure S19b). However, healing at 50 °C promotes the self-healing process (10 h for 100% recovery of mechanical properties). The enormous differences in self-healing at room temperature can be explained by the stress−relaxation experiments. The normalized stress−relaxation curves of CSH-PPG-Zn-0.25 (20 °C), CSH-PPG-Zn-1.00 (20 °C), and CSH-PPG-Zn-1.00 (50 °C) under 50% strains are shown in Figure 5i. Relaxation time, τ*, defined as the time for stress relaxation modulus to reach 37% of its initial value, was marked in the curves. The τ* value of CSH-PPG-Zn-0.25 at 20 °C is 0.38 min; in comparison, the τ* value of CSH-PPG-Zn-1.00 is beyond the monitoring time (>2 h) at 20 °C (Figure S20). The τ* value is regarded to be an important index for measuring the dynamic reconfiguration of the networks. The shorter the τ* value is, the better the ability to accommodate deformation by restructuring the supramolecular network is, which is also linked to excellent selfhealing performances. The fast exchange rate of the Zn2+/ imidazole coordination complex is responsible for the low τ* value of the CSH-PPG-Zn-0.25 film. With regard to CSHPPG-Zn-1.00, the stress−relaxation is on very slow time scales at room temperature, so it is likely that the originally dynamic nature of metal/ligand complex is restrained by the formation of ionic clusters. Merely relying on the mobility of polymer chain cannot attain the satisfactory self-healing effect. The temperature dependent τ* values of the CSH-PPG-Zn-1.00 film were plotted and then fitted by the Arrhenius equation (Figure S21).60 The apparent activation energy (Ea) was calculated as 95.6 kJ mol−1, meaning that external energy is needed for dissociating ionic clusters and regaining the capability for network rearrangement, consistent with the τ* value of CSH-PPG-Zn-1.00 at 50 °C (4.13 min, at the same level of CSH-PPG-Zn-0.25 at 20 °C). Flexible and Self-Healable Conductor. Flexible and selfhealable conductors, which are crucial elements for fabrication of next-generation electronics, have become a central issue in conductive materials.61,62 The tolerance to large mechanical deformation and the mechanical/conductivity resilience after being mechanically damaged are two priorities and influence their service life and durability.36,61 Having shown extreme stretchability, acceptable mechanical strength, and multifunctional self-healing characteristics, the utility of CSH-PPG-Zn0.25 material is very suitable for preparation of flexible and selfhealable conductors. As illustrated in Figure 6a, the flexible and self-healable conductor was fabricated by spreading Ag nanowires (Ag-NWs) onto the surface of the CSH- PPG-Zn0.25 film via vacuum filtration method.63 Ag-NWs (length, 10−20 μm; diameter, 100 nm) randomly stack on the surface of a CSH-PPG-Zn-0.25 film in a staggered manner, forming a high density of wire-to-wire junctions, which is conducive to high conductivity for flexible and self-healable conductors (Figure 6b,c). As mentioned in the lap shear experiments, the CSH-PPG-Zn-0.25 film has already shown strong adhesion 6035


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Figure 7. (a) Schematic illustration of the preparation process of self-healable automotive paint. (b) Flexibility, hardness, adhesion strength, and impact strength of the prepared CSH-PPG-Zn-0.50 automotive paint. (c) 3D laser images and (d) the corresponding depth profiles of the scratched CSH-PPG-Zn-0.50 automotive paint before and after healing for 12 h at room temperature. Scale bar, 500 μm. (e) Optical microscope images demonstrating the self-healing capability of CSH-PPG-Zn-0.50 paint under continuous sunshine; the visible scratch could be totally eliminated after healing for 2 h under sunshine. (f) Evolution of SVET current density maps for CSH-PPG-Zn-0.50 automotive paint: (I) beginning process, anodic current density is 12.85 μA cm−2, cathodic current density is −5.74 μA cm−2; (II) 2 h, anodic current density is 4.3 μA cm−2, cathodic current density is −1.75 μA cm−2; and (III) 6 h, corrosive signals almost vanished.

self-healable automotive paint. As depicted in Figure 7a, CSHPPG-Zn-0.50 paint was coated on the surface of the aluminum alloy 2024 (AA2024) by a drop-casting method, and the thickness of the paint was about 135 μm (Figure S27). The basic paint parameters are listed in Figure 7b. Flexibility, hardness, adhesion strength, and impact strength are all in accordance with the industrial standard (QC/T 484-1999). When CSH-PPG-Zn-0.50 paint suffered from a mechanical scratch (118.9 μm wide, 70.5 μm deep), a laser microscope was used to observe the morphological change of the scratch. After being placed in an atmospheric environment for 12 h, the

scratch was fully mended in both horizontal and vertical directions (Figure 7c,d). This result indicates that CSH-PPGZn-0.50 paint can overcome the strong interactions between the paint and the AA2024 substrate, which restrict the mobility of polymer chains, and implement the scratch elimination function. Strong sunlight would help to increase the dynamic exchange kinetics of coordination bonds in the polymer network, promoting the interdiffusion process of the fractured polymer interface, and thus accelerating the speed of scratch elimination (Figure 7e and Figure S28). Taking into account anticorrosion function of automotive paint, the recovery of 6036


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strength, toughness, and self-healing capability, meeting the restricted requirements of automotive paints. CSH-PPG-Zn0.50 paint can autonomously eliminate the surface scratches in the atmospheric environment and regain anticorrosion performance of the locally damaged region via structural recovery.

anticorrosion function was also emphasized by scanning vibration electrode technology (SVET).67 The evolutions of corrosion current density around artificial scratches (diameter 200 μm, deep to surface of AA2024) under aggressive species (H2O, Cl−, O2) attack are shown in Figure 7f. In the beginning stage, the anodic/cathodic corrosive current densities were found surrounding the artificial scratches, suggesting that aggressive species reached the surface of AA2024 and initiated the local corrosion (Figure 7fI). However, the local corrosion did not spread rapidly with the passing of time; on the contrary, the anodic/cathodic activities were effectively inhibited after 2 h (Figure 7fII). At the end of the monitoring (6 h), the corrosive signals almost vanished (Figure 7fIII), demonstrating that, with the structural restoration of CSHPPG-Zn-0.50 paint, the channels for aggressive species were thoroughly blocked, the aggressive species were separated from the AA2024 substrates, and eventually the local anticorrosion function returned. It is well accepted that the water can weaken the self-healing efficiency of hydrogen bonded polymer.20 However, the coordinated bond cross-linked polymer is different.39 The self-healing process of CSH-PPG-Zn-0.50 paint is not sensitive to moisture. To some extent, water can even promote the dynamic exchange process of coordinate bonds.39 Therefore, CSH-PPG-Zn-0.50 paint can repair scratches on its surface at room temperature in the presence of salt water. Given these findings, we can conclude that, except for encapsulation technology, the self-healing paint integrating with certain strength and anticorrosion capability can also be constructed by use of supramolecular chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02320. Movie S1 showing the supramolecular polymer without covalent cross-links showing the viscous fluid behavior with no appreciable mechanical properties (AVI) Movie S2 showing a CSH-PPG-Zn-0.25 sample showing stretching up to 180× its original length without rupture at a deformation rate of 100 mm min−1 (AVI) Movie S3 showing that a highly stretched CSH-PPG-Zn0.25 specimen with a strain of 18 000% could be dramatically shaken without rupture (AVI) Movie S4 showing a CSH-PPG-Zn-0.25 sample showing stretching up to 180× its original length without rupture at a deformation rate of 600 mm min−1 (AVI) Movie S5 showing that the completely healed CSHPPG-Zn-0.25 specimen could be stretched to 180× its original length without rupture at a deformation rate of 100 mm min−1 (AVI) Movie S6 of Ag-NMs/CSH-PPG-Zn-0.25 conductor showing that connecting the unfractured surface of the damaged conductor could restore the electrical conductivity to relight the LED (AVI) Materials and methods, and characterization information including FTIR, rheological testing, XRD, TG, 1H NMR, UV−vis, cyclic loading−unloading, lap shear test, selfhealing test,stress relaxation experiment, conductivity testing, and summary of cross-linking density. (PDF)

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CONCLUSION In summary, the extremely stretchable (>180×), self-healable polyurea hybrid elastomer, CSH-PPG-Zn-0.25, with the highest Young’s modulus among reported ultrastretchable materials, was successfully fabricated via rationally introducing permanent covalent cross-links and dynamic supramolecular coordination cross-links into the hybrid network. The covalent cross-links endow CSH-PPG-Zn-0.25 with enhanced stiffness and elasticity. The fast exchange kinetics of the Zn2+/imidazole complex, uncoordinated imidazole ligands, and massive free urea ligands are the key points in the design of supramolecular cross-links. The uncoordinated imidazole ligands accelerate originally fast exchange kinetics of the Zn2+/imidazole complex, and the free urea ligands facilitate multicoordination modes, which make coordination bonds readily dissociate and associate during stretching or between two cut surfaces, and are the foundation of extreme stretchability and rapid selfhealing. This novel construction strategy provides an alternative avenue for fabricating extremely stretchable materials with a rapid self-healing function under room temperature. Simply increasing the molar ratio of Zn2+ to imidazole actualizes the wide-ranging tunability of mechanical properties, which can be ascribed to the formation of ionic clusters, and render application possibilities in different fields. After deposition of the Ag-NWs layer, Ag-NWs/CSH-PPG-Zn0.25 material, acting as the flexible and self-healable conductor, offers a broad application prospect. The Ag-NWs/CSH-PPGZn-0.25 conductor not only sustains large deformation, but also exhibits rapid recovery of electrical conductivity. The special healing characteristics, including low-temperature healing and antiaging ability, greatly expand the working environments of the flexible conductor. CSH-PPG-Zn-0.50 material demonstrates the equilibrium between mechanical

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

JiaJun Fu: 0000-0002-8542-9556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (Grants 51672133 and U1737105); the National Science Foundation of Jiangsu Province (Grant No. BK20161496); Fundamental Research Funds for the Central University, Grant 30915012207; the QingLan Project, Jiangsu Province, China; and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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Extremely Stretchable, Self-Healable Elastomers with Tunable

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