Robust, Stretchable and Self-Healable Supramolecular Elastomers

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Robust, Stretchable and Self-Healable Supramolecular Elastomers Synergistically Crosslinked by Hydrogen Bonds and Coordination Bonds Xianzhang Wu, Jinqing Wang, Jingxia Huang, and Shengrong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20303 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Robust,

Stretchable

and

Self-Healable

Supramolecular

Elastomers

Synergistically Crosslinked by Hydrogen Bonds and Coordination Bonds

Xianzhang Wua,b, Jinqing Wanga, b*, Jingxia Huanga,b, and Shengrong Yanga, b*

a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

b

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100049, China.

* Corresponding authors, [email protected] (J. Q. Wang); [email protected] (S. R. Yang) Fax: 0086-931-4968019 Tel: 0086-931-4968076

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Abstract Polymeric elastomers integrating high mechanical toughness and excellent self-healing ability can find attractive applications in electronic skin, soft robotics, and electrical devices. However, simultaneously enhancing the mechanical and self-healing properties of elastomers is still a great challenge because it’s difficult to balance the effects between strong and weak crosslinking bonds. Here, a novel self-healing elastomer is developed via a one-pot polycondensation reaction between bis(3-aminopropyl) terminated poly(dimethylsiloxane) and 2,4’-tolylene diisocyanate, followed being coordinated with Al(III) ions by metal-coordination bonds. In this elastomer system, the quadruple hydrogen bonds not only are able to achieve rapid re-formation after fracture, but also can dissipate strain energy as a weak dynamic bond, endowing the elastomer with excellent self-healing ability and high stretchability; while the treble Al-coordination bonds acting as a strong dynamic bond contribute to the robust molecular networks, leading to the significantly improved robustness and elasticity of self-healing elastomer. Owing to the accuracy design, the synthesized elastomer realizes all the desired properties, including high tensile stress (2.6 MPa), exceptional toughness (∼14.7 MJ m-3), high stretchability (∼1700%), and excellent self-healing ability (90%). The robust self-healing elastomer enables the easy fabrication of flexible electronic skin, which will open a new avenue for next-generation electrical devices. Keywords: elastomer, self-healing, dynamic bonds, crosslinking, mechanical properties

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1. Introduction As “soft and deformable” materials, polymeric elastomers with robust mechanical toughness and excellent self-healing ability are essential for smart flexible devices, especially for electronic skin.1-5 During the past few years, extensive efforts have been made to develop robust, durable, or self-healable elastomers.6-11 Unfortunately, it still remains a fundamental challenge to realize expected elastomers with combination of high mechanical strength (over 2.0 MPa) and excellent self-healing ability (over 90% for short period of time) because it’s difficult to balance the effects between strong and weak crosslinking bonds. Elastomers featured by high mechanical toughness are usually crosslinked by strong bonds, which can increase rigidity of polymer network and seriously limit self-healing ability of the elastomers. Weak crosslinking bonds produce the opposite effect. Currently, several kinds of self-healing elastomers with improved strength have been developed through incorporating extra networks in polymer networks.12-15 For instance, double-network hydrogels composed of the rigid and ductile polymer networks were synthesized, wherein the rigid network effectively dissipated energy while the ductile network endowed the hydrogel with excellent stretchability.16-18 One fatal shortcoming of this technique is that rupturing the rigid networks results in “irreversible” damage to the elastomer under large deformation, which seriously inhibits its self-healing ability. The recent strategy is employing the mixture with both strong and weak crosslinking hydrogen bonds to efficiently improve the strength and self-healing of elastomers, where the strong crosslinking bonds confer robustness while the weak bonds are able 3 / 34



to dissipate energy.19 However, it is difficult to further enhance the strength of the self-healing elastomers because the hydrogen bonds are weak dynamic bonds. Therefore, the stronger crosslinking bonds are needed to enable decent mechanical properties. Coordination bond is attractive cross-linker in supramolecular chemistry, attributing to its dynamic nature and high bonding strength comparable to covalent bond.20-25 For example, Thanneeru, et al. introduced coordination bonds of metal ions into polymer networks and obtained significantly enhanced toughness.26 However, these polymers still hold unsatisfactory ductility and self-healing because of the too strong metal-ligand interactions; furthermore, they were sensitive to fracture or notch. In addition, because polyurethane networks can provide desirable dynamic bonding sites through polyurethane and polyurea components, they were introduced into polymer matrix to synthesize robust and self-healable elastomers.27-30 And these elastomers showed outstanding mechanical properties and self-healing ability. Cao, et al. designed an Ag+ crosslinked moldable polymer that exhibited remarkable mechanical properties: toughness of 84.7 MJ m-3, strength of 24.0 MPa and strain of 600%.31 Nevertheless, since covalent bonds, well known to be irreversible after fracture, were also used to crosslink molecular networks, thereby the polymer showed poor self-healing after mechanical damage. To this end, Zhang et al. introduced both metal-coordinated bonds and hydrogen bonds into polymer networks to tune the mechanical and self-healing properties of polymers,32 and the resulting polymer had partly achieved a balance between mechanical strength and self-healing 4 / 34


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ability. However, the sextuple metal-coordinated bond is typically very strong, which severely suppresses molecular chain mobility, leading to the moderate self-healing efficiency (90% for 48 h); meanwhile, the contribution of hydrogen bonds to the self-healing ability of polymers is greatly limited due to the low hydrogen bonding sites. Therefore, it is desirable to develop a new synthesis strategy of polymers with well-defined weak and strong crosslinking bonds in order to enhance the mechanical strength and self-healing ability synchronously. Herein, inspired by crosslinking mechanism from weak and strong bonds, we design and prepare a novel polymeric elastomer, wherein the stronger treble Al-coordination bonds and weaker quadruple hydrogen bonds are introduced into polymeric networks to tune the mechanical and self-healing properties of elastomer. The quadruple hydrogen bonds not only are able to achieve rapid re-formation after fracture, but also can dissipate strain energy as a weak dynamic bond, endowing the elastomer with excellent self-healing ability and high stretchability; while the treble Al-coordination bonds acting as a strong dynamic bond contribute to the robust molecular networks, leading to the significantly improved robustness and elasticity of self-healing elastomer. Owing to accuracy design, the synthesized elastomer realizes all the desired properties, mainly including high tensile stress of 2.6 MPa, exceptional toughness of ∼14.7 MJ m-3, high stretchability of ∼1700%, and high self-healing efficiency of 90%. Based on the elastomer-supported gold (Au) thin film, a stretchable smart skin (tensile sensing) is subsequently developed, which can generate an electric signal in response to a physical deformation for lateral tensile sensing. In addition, it 5 / 34



also exhibits a high stretchability of 1250% and autonomic self-healing ability. Owing to inheriting the notch-insensitivity of the resulted elastomer, the smart skin is observed to realize a high stretchability of 900% with a notch. It is believed that this synthesis strategy also can be applicable to other systems and open the avenue to robust, self-healable and highly stretchable supramolecular elastomers for potential applications in smart robots, soft electronics and artificial intelligence. 2. Results and discussion The parent elastomer of PDMS-TDI was synthesized via a one-pot polycondensation reaction between PDMS and TDI. Subsequently, the PDMS-TDI oligomers (Figure 1a) is used as binding motifs to coordinate Al(III) ions that further act as cross-linkers to construct robust polymer networks, whose structure is shown in Figure 1b. Our design is inspired by the synergistic effects between hydrogen bonds and coordination bonds, which possesses three features: 1) the quadruple hydrogen bonds are able to dissipate strain energy as one kind of weak dynamic bond, endowing the polymer with high stretchability and excellent notch-insensitivity; 2) the treble Al-coordination bonds acting as one kind of strong dynamic bonds contribute to the robust molecular networks; 3) the dynamic natures of hydrogen bond and coordination bond enable the elastomer to possess the outstanding self-healing ability. Specifically, Al(III) is chosen for the coordination because it is known to be colorless and large trivalent ions, thusly making it feasible to coordinate with urea groups in PDMS-TDI to form a transparent and robust elastomer (Figure S1).

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Figure 1. Molecular structures of (a) PDMS-TDI and (b) PDMS-TDI-Al. (c) Schematic illustration of Al (III) coordinated PDMS-TDI elastomer, highlighting the PDMS-TDI-Al coordination bonds and the hydrogen bonds. (d) UV-vis spectra of PDMS-TDI (black), AlCl3 solution (red), and PDMS-TDI-Al-3 (blue). The spectra of the PDMS-TDI and PDMS-TDI-Al-3 were measured in THF, while the AlCl3 solution was recorded in H2O. (e) FT-IR spectra of different samples. (f) Element detection of the PDMS-TDI-Al-3. 1H NMR demonstrated the successful preparation of bulk PDMS-TDI, as indicated by the presence of characteristic peaks of TDI and PDMS segments in polymeric backbones (Figure S2). Moreover, elemental mapping analysis of energy dispersive spectroscopy (EDS) (Figure 1f) reveals that Al(III) ions are successfully 7 / 34



incorporated and uniformly dispersed in the supramolecular networks (Figure S3). Apparent aggregates of Al(III) ions are not detected by a high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) even at the nanoscale level (Figure S4).33,34 To identify the existence of coordination bonds, UV-vis spectroscopy results of self-healing elastomers are shown in Figure 1d. Apparently, PDMS-TDI presents a negligible absorbance in UV-vis curve, whereas AlCl3 solution only displays a weak absorption peak centered at ∼251 nm, assigning to the n→π* of carbonyl groups.35,36 In contrast, PDMS-TDI-Al-3 shows the evident absorption at 249 nm, attributing to the Al-N/O (urea groups) charge-transfer band.37,38 Furthermore, FT-IR is also used to corroborate the presence of coordination-bonding interactions in the polymer networks (Figure 1e). For the PDMS-TDI, the peaks at 852 and 1028 cm-1 can be assigned to the absorption of Si-O of H2N-PDMS-NH2.39 Upon introduction of Al(III) ions, two new peaks appearing at 1495 and 3652 cm-1 can be assigned to the absorptions of Al-N and Al-O, respectively.40,41 More importantly, Al(III) ions with 3s2 outer shell electrons have the ligand-field stabilization effects, which can form octahedral geometry with urea groups.42 Therefore, with the increase of AlCl3 molar ratios in PDMS-TDI, the intensity of Al-O increases. It is confirmed that Al(III) ions successfully coordinate with urea groups of PDMS-TDI, which is in good agreement with that obtained from UV-vis spectroscopy. The mechanical properties of the elastomer vary greatly depending on the AlCl3 molar ratios. When the molar ratio is improved, the tensile stress and Young’s modulus of the PDMS-TDI-Al increase while the fracture strain decreases (Figure 2b 8 / 34


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and e). This result indicates that the incorporation of various amounts of coordination bonds is effective in systematically tuning the mechanical strength. Moreover, Al(III) ions can also form coordination bonds with Si-O in the PDMS molecular chain, which can strengthen polymeric networks, further improving the robustness and elasticity of the self-healing polymer.43 To obtain high mechanical strength of PDMS-TDI-Al, a moderate crosslinking density of coordination bonds is needed. Moreover, the formation of the robust molecular network in the PDMS-TDI-Al is driven by the synergistic effect between weak hydrogen bonds and the strong coordination bonds. When stretched, the supramolecular network effectively dissipates strain energy through the rupture of weaker hydrogen bonds while the stronger coordination bonds are probably sufficient to maintain the high tensile stress. Most remarkably, the PDMS-TDI-Al-3 can be stretched to 1700% at a loading rate of 20 mm min-1 (Figure 2a, b), which is 17 times longer than its original length. In comparison, other self-healing polymers and tough hydrogels, such as DPC31 and PEDOT44 suffered from poor stretchabilities of 750% and 470%, respectively (Table S1). Moreover, none of them can achieve tensile stress more than 2 MPa, implying much worse mechanical properties than our elastomer (Figure 2f). The effects of coordination bonds on mechanical properties of the PDMS-TDI-Al have been identified by the synthesis of the bulk PDMS-TDI which only consists of crosslinked hydrogen bonds. The tensile stress of the PDMS-TDI film is 0.62 MPa while its stretchability can reach to 2700% (Figure 2c). In contrast, the tensile stress of the PDMS-TDI-Al-3 film is greatly increased up to 2.64 MPa, which indicates that the crosslinking strength of 9 / 34



hydrogen bonds and coordination bonds are significantly different. In specific, the hydrogen bond, as one kind of weak dynamic bond, is able to dissipate the strain energy, while the coordination bond acting one kind of strong dynamic bond can enhance the mechanical strength of the PDMS-TDI-Al. The PDMS-TDI-Al can dissipate mechanical energy effectively by hydrogen bond breaking under strain. As shown in Figure S5b and c, the cyclic stress-strain curves display prominent hysteresis loops, in which its area represents the energy dissipated per unit volume from the weak hydrogen bond breakage (Figure 2h). When the PDMS-TDI-Al-3 film is stretched with small tensile strain (20-60%), the mostly unbroken hydrogen bonds and coordination bonds drive the polymer networks maintaining the original state and the hysteresis is insignificant (Figure S5a). If larger strain (1130%) is applied on the same sample, the hysteresis loop of second cycle remarkably decreases (Figure S5b). However, the second stress-strain curve of the elastomer film is almost overlapping to that of the initial cycle after resting interval of 60 min (Figure S5c). These results reveal that the mechanism of strain energy dissipation is mainly governed by the breakage of hydrogen bonds. Moreover, the strain energy dissipation also can be confirmed by the stretching-speed-dependent tensile behavior. With increasing the stretching rate from 10 to 80 mm/min, the tensile stress of elastomer film increases significantly but the stretchability decreases (Figure 2g). Upon tension, most of hydrogen bonds and partial coordination bonds are able to reform as soon as possible after breakage. A very similar tensile behavior is also observed for quadruple H-bonding and ionically crosslinked polymer.48,49 It is worth 10 / 34


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mentioning that the PDMS-TDI-Al-3 film achieves high notch-insensitive stretching (1200%) due to the strain energy dissipation (Figure 2d), which is only observed in a few polymers. 19

Figure 2. Mechanical properties of the PDMS-TDI-Al elastomer films. (a) Photographs of the PDMS-TDI-Al-3 elastomer films before and after stretching to 1700%. (b) Stress-strain curves of the elastomer films for PDMS-TDI (red) and PDMS-TDI-Al (black) with a sample thickness of 0.5 mm, a width of 5 mm, and a length of 30 mm at a deformation rate of 20 mm min-1. (c) Stress-strain curves for PDMS-TDI-Al series with different molar ratios of TDI to Al(III), deformation rate: 20 mm min-1. (d) Photographs of the notched-PDMS-TDI-Al-3 elastomer film before 11 / 34



(left) and after being stretched (right) to 900%. (e) Al(III) ions molar ratios dependence of PDMS-TDI-Al-3 on fracture strain and maximum stress. (f) A comparison of our PDMS-TDI-Al with several tough polymers and hydrogels in the recent literatures. (g) Stress-strain curve of the PDMS-TDI-Al-3 elastomer film stretched at different rates for a sample with the thickness of 0.5 mm, width of 5 mm, and length of 30 mm. (h) Schematic illustration of the proposed mechanism for highly stretchable PDMS-TDI-Al.

To further get insights into the role of hydrogen bonds, the PDMS-IP-Al consisted of weaker hydrogen bonds (dual H-bonding) and coordination bonds is synthesized (Figure S6b). Compared to PDMS-TDI-Al crosslinked by quadruple H-bonding (Figure S6a) and coordination bonds, the weaker hydrogen bonding makes the PDMS-IP-Al softer and better stretchability (Figure 3a), indicating that the hydrogen bond plays a key role in improving mechanical property of PDMS-IDI-Al system. Moreover, the PDMS-IP-Al shows the larger hysteresis loops than PDMS-IDI-Al in the stress-strain curves (Figure S5d). These results suggest that the fracture of hydrogen bonds during mechanical strain is responsible for the energy dissipation. Similar mechanism was also confirmed for H-bonding crosslinked polymers. 49,50

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Figure 3. Self-healing properties of the PDMS-TDI-Al elastomer film. (a) Stress-strain curves of the elastomer films for PDMS-TDI-Al-3 (red) and PDMS-IP-Al-3 (black) with a sample thickness of 0.5 mm, a width of 5 mm, and a length of 30 mm at a deformation rate of 20 mm min-1. (b) Optical microscope images of the damaged and healed PDMS-TDI-Al-3 film. (c) Optical photos of the PDMS-TDI-Al-3 film after being cut, healed for 36 h (top), and stretched (bottom). (d) Stress-strain curves of original and healed PDMS-IP-Al-3 for different periods of time from 6 to 12 h and then to 36 h under ambient conditions (25 °C). (e-f) SEM images of the cut PDMS-TDI-Al-3 film after healing. (g) Stress-strain curves of 13 / 34



PDMS-IP-Al-3 healed in different environments. The dynamic natures of coordination bonds and hydrogen bonds confer the PDMS-TDI-Al autonomous self-healing capability at room temperature. As shown in Figure 3b and Figure S7, the scar on the damaged PDMS-TDI-Al-3 film almost completely disappears after 24 h of healing under ambient conditions. Next, the elastomer film was cut into two separate pieces and immediately attached back without applied stress (Figure 3c). Its stretchable capability is restored to 700% after being healed for 6 h. When the healing time is further prolonged to 36 h, higher stretchable capacity (1600%) is observed with a high self-healing efficiency of 90% which is calculated by the integral area in tensile curve after healing divided by that of the uncut samples (Figure 3d). Such high self-healing efficiency for our elastomer is super than those of other polymers or hydrogels (Table S2). Remarkably, the PDMS-TDI-Al samples of PDMS-TDI-Al-2 and PDMS-TDI-Al-1 with lower AlCl3 molar ratios are able to achieve 91% and 87% healing efficiencies after being healed for 36 h (Figure S8a, b). The incremental self-healing efficiency for PDMS-TDI-Al-2 is ascribed to the gradual decrease in coordination bonds ratio, which reduces the rigidity of the molecular network, thusly impelling fast molecular mobility at room temperature and allowing rapid reconstitution of damaged polymer.51,52 For PDMS-TDI-Al-1, the supramolecular network embeds very few coordination bonding sites, in which its dynamic coordinating ability can help stabilize the complex structure,53,54 thereby resulting in the decrease of the self-healing efficiency. The incision morphology of the healed elastomer film is subsequently observed by SEM. 14 / 34


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It is seen that the healing incision of the PDMS-TDI-Al-3 film suffers obviously bent upward (Figure 3e, f), and the incision edge appears “wrinkle” (Figure S9a). In contrast, the healing incision of the PDMS-TDI-Al-1 is more “smooth” (Figure S9b). Such finding is very different from previous reports, where the healing incisions generally bend downward.31 It is ascribed to the fact that the dynamic coordination bonds acting as “pull-power” make the polymer network to aggregate into small domains due to its strong bond energy, thereby leading to the formations of the “bent upward” and “wrinkle” morphologies. The environment significantly affects the self-healing properties of elastomer. Therefore, the self-healing capability of the PDMS-TDI-Al-3 film in different aqueous solutions was investigated. Compared with the self-healing in the air, the self-healing efficiencies of the PDMS-TDI-Al-3 film in other aqueous solutions decrease to different extents (Figure 3g and Figure S10). For example, the PDMS-TDI-Al-3 film displays self-healing efficiencies of 86% in water and 85% in KOH solution (pH=13.2), while only 82% level is observed in H2SO4 solution (pH=2.1). The state of Al(III) ions during the coordination process can be represented by the following chemical formula: Al3+ + [H2N-PDMS-NH2]n-

Al[H2N-PDMS-NH2]n-3

(1)

The formula (1) clearly demonstrates that upon the increase of pH levels, the reaction equilibrium will move to the left (Figure S11), leading to the dissociation of the coordination bonds. Therefore, the self-healing efficiency in KOH aqueous solution is weaker than the healed samples in the water and air. Interestingly, the 15 / 34



PDMS-TDI-Al-3 film in H2SO4 solution displays the lowest self-healing efficiency and tensile strength (Figure 3g), which attributes to the fact that the decrease of pH level prevents hydrogen bonding from reformation after break.55 This suggests that hydrogen bonds play a leading role in the self-healing process of elastomers.

Figure 4. (a) SEM images of the Au film electrode with microcracks. (b) A schematic demonstration of the stretching process involving the flexible electrode. (c) Stress-strain curves of the polymer films for PDMS-TDI-Al-3 (black), original (red) and healing electrode (blue) with a sample thickness of 0.5 mm, a width of 5 mm, and 16 / 34


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a length of 30 mm at a deformation rate of 20 mm min-1. (d) Photographs of a notched strain electrode on relaxed and stretching station. (e) The change of relative resistance of electrode under different strains. (f) A durability test of strain electrode under a repeated stretching and release of 40%. (g) Photographs of electrode on relaxed and stretching station. (h) Photographs of the healing process for the electrode with an LED in series with a self-healing electrode.

In view of the practical applications of self-healing elastomer in electronic skin, we hope that the PDMS-TDI-Al possesses shape recovery capability after extreme compression. As shown in Figure S12, the deformed PDMS-TDI-Al almost totally recovers its initial shape after a resting period of 6 h. This may be attributable to the relatively soft supramolecular chains in our polymer networks, in which the “moving” of the soft chain contributes to the outstanding shape recovery capability of PDMS-TDI-Al. Considering these outstanding performances, the PDMS-TDI-Al film was employed to fabricate a flexible electrode, in which PDMS-TDI-Al-3 was used as the supporting layer while an Au film coated onto its surface was applied as a conductive layer. Similar Au film was also fabricated in previous reports,45,56 but its nanofractures can develop into interconnected blocks under the strain that prevent the resistance from changing with the strain. Here, the microcracks on the Au surface layer were formed by using sandpaper as the template under prestretch (50%) (Figure 4a). Subsequently, another PDMS-TDI-Al-3 film coated onto the Au film was used as the 17 / 34



encapsulation layer (Figure S13). When the flexible electrode is stretched laterally, the size of the microcracks increases, resulting in the reduction of conductive channels and an increased resistance. Upon releasing the stress, the resistance will be restored to the initial value (Figure 4b). Interestedly, the flexible electrode exhibits excellent stretchable properties, notch-insensitive stretching (Figure 4d) and autonomous self-healing ability. Importantly, this electrode possesses a high stretchability of 1500% (Figure 4c, g) and variable resistance (Figure 4e). Moreover, the electrode also displays decent durability with 60% strain over 100 cycles (Figure 4f). Next, the flexible electrode is cut into two separate pieces and immediately attached back. After being healed for 36 h at the room temperature, the hydrogen bonds and coordination bonds are re-crosslinked among the contacted interfaces, and the conductivity is recovered completely (Figure 4h). Moreover, the healed electrode shows a 1200% fracture strain (Figure 4c). Based on this well-defined working mechanism and attractive features, the flexible electrode is applied as a novel strain sensor to monitor the diverse ranges of human motions in real time. The strain sensitivity (S) of the flexible electrode can be defined as S = (ΔR/Roff)/ε, where ΔR is the relative resistance change, Roff denotes the electrode resistance at the relaxed state, and ε denotes the tensile strain. As illustrated in Figure 5a, ΔR/Roff increases linearly in the first strain region (ε40%. The different sensing regions are attributed to the viscoelastic behavior of the elastic supporting substrate. Figure 5b shows the detection of the index finger 18 / 34


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activities with different bending frequencies. The strain sensor is attached onto opisthenar and stretched as the finger bends. The relative resistance fluctuates periodically with finger activities, and the signal frequency increases with improving the finger bending frequency. Such findings indicate that this strain sensor possesses the highly sensitive sensing performance. To further demonstrate its sensing advantage, the strain sensor is attached onto a human wrist to recognize wrist action. As illustrated in Figure 5c, bending the wrist can be accurately detected by monitoring the relative change of the ΔR/Roff value (Movie S1). Besides, by integrating the strain sensor on the elbow joint, the electrical signals produced by the joint bending can also be precisely recorded in real time (Figure 5d). We envision that the flexible electrode featured by excellent stretchable properties and autonomous self-healing ability is a promising candidate to assemble the smart sensor for healthcare diagnosis and sports monitoring.

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Figure 5. Sensing performances of the flexible electrode. (a) Normalized relative resistance as a function of stretchable strain and the corresponding linear fittings. (b) Detection of finger folding with different frequencies in real time. The relative resistance changes were demonstrated by opisthenar (c) and elbow (d) folding, respectively. Inset photographs show opisthenar and elbow folding with different angles in the test.

3. Conclusions In summary, we successfully prepared supramolecular elastomers, wherein the stronger coordination bonds and weaker hydrogen bonds are introduced into the polymeric networks to tune the mechanical properties and self-healing ability of elastomer. Compared to other self-healing polymers, our elastomers possess the 20 / 34


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following features and advantages: (1) the quadruple hydrogen bonds not only are able to achieve rapid re-formation after fracture, but also can dissipate strain energy as the weak dynamic bond, endowing the elastomer with excellent self-healing ability and high stretchability; (2) the treble Al-coordination bonds acting as the strong dynamic bond contribute to the robust molecular networks, leading to the significantly improved robustness and elasticity of the self-healing elastomer. Owing to the reasonable design, the synthesized elastomers realize all the desired properties, including high tensile stress of 2.6 MPa, exceptional toughness of∼14.7 MJ m-3, high stretchability of∼1700%, and excellent self-healing ability. Additionally, a stretchable strain sensor (tensile sensing) has been successfully assembled based on the elastomer-supported Au thin film, which exhibits a high strain sensitivity of 2.63, a high stretchability of 1500% and excellent notch-insensitity. With these promising characters, the strain sensor can be used to precisely monitor a diverse range of human motions in real time. We believe that this crosslinking strategy with weak and strong dynamic bonds provides a positive exploration to improve self-healing and mechanical properties of many other polymer systems. Furthermore, our multi-functional self-healing elastomers are promising for versatile applications, including future robots, health monitoring platforms and artificial skins.

4. Experimental section Materials: 2,4’-tolylene diisocyanate (TDI) was purchased from Shanghai Sanyou 21 / 34



Reagent

Co.

Ltd.

Bis(3-aminopropyl)

terminated

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poly(dimethylsiloxane)

(H2N-PDMS-NH2, abbreviated as PDMS, Mn = 5000-7000) was purchased from ShinEtsu. Trimethylamine, tetrahydrofuran, aluminium chloride (AlCl3·6H2O) and chloroform were purchased from Sinopharm Chemical Reagent Co. Ltd. Au paint was purchased from Boerrui electronic technology Co., Ltd. All chemicals were used as received without further purification. Preparation of PDMS-TDI gel: Typically, 8g PDMS and 1.5 mL trimethylamine were mixed in 25 mL chloroform. Subsequently, the mixture was cooled to 0 °C by an ice bath and stirred for 1 h under nitrogen atmosphere. Then, a chloroform solution of 1g TDI was added dropwise. After being stirred for additional 1 h, the viscous composite gel of PDMS-TDI was then obtained, whose average molecular weights of Mw and Mn determined by gel permeation chromatography (GPC) are 105,000 and 66,000 (Đ = 1.5), respectively. Preparation of PDMS-TDI-Al films: 4.1g (0.017 mol) AlCl3·6H2O was dissolved into THF (5 mL) to obtain solution 1, which was then poured into another solution of PDMS-TDI (8 g) in THF (10 mL). With stirring at 60 ºC for 24 h, a viscous gel was obtained. The gel was poured onto the octadecyltrichlorosilane (OTS)-treated planar cells and then dried at room temperature for 4 h, followed by baking in vacuum oven at 80 °C for 12 h to remove the solvent completely. Finally, elastomer films were peeled off, and cut into certain dimensions. The resulted film was coded as PDMS-TDI-Al-3. Other PDMS-TDI-Al-x elastomer films with molar ratios of Al(III) to TDI ranging from 1 to 4 were also fabricated by tuning the concentration of the 22 / 34


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Al(III). Fabrication of sensing electrode: An elastomer film with a width of 5 mm, a thickness of 0.5 mm, and a length 10 of mm was firstly prepared. The Au paint was coated onto the surface of the elastomer film and dried at room temperature for 1 h. The microcracks were formed on the Au surface layer by using sandpaper as the template under prestretch (500%). Then, the resulted film connected with copper wire to form electrodes while conducting tape and copper wire was attached to the Au layer and then another polymer film with 0.2 mm thickness was subsequently put on coating film as an encapsulation layer. After bonding at 60 °C for 2 h, sensing electrode was obtained with robust interface. Characterizations. The mechanical properties of the elastomers were evaluated at room temperature using an AGS-X at a 20 mm/min. Tensile test was performed on the rectangular-shape specimens with a width of 5 mm, a thickness of 0.5 mm and a length of 30 mm. All the tests were repeated three times. For self-healing measurements, the elastomer films were cut into halves using blades and then the two separate halves were put in contact immediately without applied stress. The healed elastomer films were stretched to calculate the healing efficiency (defined as the strain at break ratio between the healed film and original film). Fourier transform spectrometer (FT-IR) spectra of the polymers were measured using a Nicolet 170SX reflection absorption infrared spectrometer. Scanning electron microscopy (SEM) inspections were taken on a JSM-5601LV field emission system. Scanning transmission electron microscopy (STEM) observation was performed in a high angle 23 / 34



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annular dark-field STEM (HAADF-STEM) configuration to check for any clustering of aluminium atoms. Ultraviolet-visible (UV-Vis) spectra were recorded using a Shimazu

2600

spectrophotometer.

Gel

permeation

chromatography

(GPC)

experiments were performed on a Malvern VE2001 GPC solvent/sample Module with a DAWN multiangle laser light scattering (MALLS) detector and an Optilab TrEX differential refractometer. 1H NMR spectra were recorded on a Bruker Avance III HD 400 spectrometer by using the mixture of CDCl3 and MeOD as the solvent at room temperature. The real-time electrical signals of the strain sensors were recorded by the source meter Model 2450.

Supporting Information Optical photo of PDMS-TDA-Al-3 elastomer; Proton NMR spectrum of PDMS-TDI; Element detection and elemental mappings of energy dispersive spectrometer (EDS) for the PDMS-TDI-Al-3; Stress-strain curves of PDMS-TDA-Al elastomer film; Simulated dimer models of TDI-TDI and IP-IP; Optical microscope images of damaged and healed PDMS-TDI-Al-3 film; SEM images of the cut PDMS-TDI-Al-3 film; Fabrication process of the flexible electrode; A summary for self-healing ability and mechanical properties of recently reported materials. The real-time video for finger bending detection using the flexible electrode (AVI)

Acknowledgements We gratefully acknowledge the funding support from the National Natural 24 / 34


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Science Foundation of China (Grant Nos. 51675514 and 51575507). Conflict of Interest The authors declare no conflict of interest

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Table of Contents graphic

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Robust, Stretchable and Self-Healable Supramolecular Elastomers

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