Letter Cite This: ACS Macro Lett. 2019, 8, 1091−1095
pubs.acs.org/macroletters
Elastomer Reinforced with Innate Sulfur-Based Cross-Links as Ligands Xuhui Zhang,†,‡ Shuangjian Yu,† Zhenghai Tang,† and Baochun Guo*,† †
Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, People’s Republic of China ‡ Reliability Research and Analysis Center, No. 5 Electronics Institute of MIIT, Guangzhou, 510610, People’s Republic of China
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S Supporting Information *
ABSTRACT: Although the incorporation of sacrificial bonds into an elastomer is an effective way to provide a combination of high strength and high fracture toughness, this method normally involves complicated chemical processes. The coordination between metal ions and polysulfides has been documented. However, the potential of polysulfide structures in vulcanizates as ligands has long been neglected. Using innate sulfur-based cross-links, we show how weak and nonpolar elastomers achieve significant reinforcement without modification of the backbone. By simply soaking vulcanizates into solutions containing metal ions, dual ions are simultaneously introduced into the vulcanizate to generate coordinations with different bond strengths, resulting in an unprecedented high modulus. Overall, this work presents a universal yet high-efficiency reinforcing strategy to prepare high-performance elastomers without additional chemical modifications, which should promote comprehensive research and industrial application of sacrificial bond strategies for elastomers.
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coordination construction. Inspired by these facts, we propose a universal method for rubber reinforcement, namely, soaking vulcanizates in metal ion-containing solutions, to conveniently generate sulfide-metal coordination as sacrificial cross-links. Moreover, to strengthen the hierarchical energy dissipation mechanism, two kinds of sulfide-metal coordination with different bond strengths are simultaneously engineered to achieve optimized enhancement. As a proof of concept, a styrene−butadiene rubber (SBR) gum (Figure 1a) is compounded with a sulfur-based curing package and subjected to hot pressing to generate a vulcanizate (Figure 1b). Then, a vulcanizate with sulfide-metal coordination (Figure 1c) is achieved by soaking the vulcanizate in a tetrahydrofuran (THF) solution containing ferric chloride (FeCl3) and copper chloride (CuCl2). As shown, the covalent sulfur-based cross-links endow the vulcanizate with a threedimensional chemical network with good resilience. Moreover, metal ions coordinate with sulfur-based cross-links in the form of ion clusters (see transmission electron microscopy (TEM) images and energy dispersive spectroscopy (EDS) mapping, Figure S1) to generate reversible, sacrificial cross-links that can strengthen the vulcanizate. Because the reported polysulfidemetal coordination is interaction between small molecules and shows complicated ionic structures, such as [Cu6S17]−2, [Fe2S2(S5)2]2−,9 the elaborate coordination structures in the present work are difficult to identify due to the complicated
ubbers are irreplaceable in industry and daily life due to their unique resilience and large extensibility.1 The reinforcement of rubbers enables their practical utility. Adding nanofillers is the most common reinforcing method, but this method has the disadvantages of nanofiller dispersion/ aggregation, interfacial regulation, and difficult processing.2 Recently, reinforcing strategies based on hierarchical structures containing energy dissipation units, such as sacrificial double/ triple networks3,4 or reversible and sacrificial cross-links,5,6 have attracted great attention due to the high reinforcing efficiency and simultaneous improvement of the strength, modulus, and breaking strain. However, a sacrificial double/ triple network is generally achieved by polymerizing monomers into a cross-linked rubbery network, which involves complicated chemical reactions and intricate steps. Sacrificial and reversible cross-links require appropriate ligands in the network, leading to inevitable chemical modifications when introducing specific functional groups. Therefore, it is of great significance to construct sacrificial bonds in elastomers without involving chemical reactions while maintaining high reinforcing efficiency. Sulfur, the most common cross-linker for rubber products, can introduce poly-, di-, and monosulfide cross-links into the rubber network during cross-linking.7 Sulfide has been demonstrated to be an effective ligand to construct coordination bonds which is a popular interaction for serving as a sacrificial bond.8−10 In addition, it has been demonstrated that polysulfide is a versatile and fascinating polydentate ligand for cement metal ions or aggregates.9 Therefore, sulfur-based cross-links can be innate ligands in a vulcanizate for © XXXX American Chemical Society
Received: July 6, 2019 Accepted: August 14, 2019
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DOI: 10.1021/acsmacrolett.9b00512 ACS Macro Lett. 2019, 8, 1091−1095
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ACS Macro Letters
Figure 1. Proposed preparation procedure of SBR with sulfide-metal coordination: SBR gum (a) is first compounded with sulfur-based curing package and subjected to hot pressing to form a vulcanizate (b). Then, the vulcanizate with sulfide-metal ion coordination (c) is achieved by soaking the vulcanizate in a THF solution containing chlorates.
Figure 2. (a) Raman spectra of thiuram, CuCl2, and thiuram/CuCl2 and (b) Raman spectra of thiuram, FeCl3, and thiuram/FeCl3. (c) XPS S 2p spectra of thiuram, thiuram/CuCl2, and thiuram/FeCl3. (d) Tan δ−T curves of SS7 soaked in CuCl2/FeCl3/THF solution with different concentration ratios.
molecular structures of the vulcanizate and the self-assembled small ion clusters. Herein, the vulcanizate is prepared first to ensure the same covalent cross-linking before and after introducing metal ions, which can eliminate the effect of metal ions on curing behaviors. For clarity, all samples are referred to as SSx-Cuy/Fez, where x represents sulfur content (phr) and y and z represent the concentrations of CuCl2 and FeCl3 in the THF solution (mg/mL), respectively. To verify the existence of sulfide-metal coordination, model compounds thiuram/CuCl2 and thiuram/FeCl3 (the molar ratio between S and metal ions is 12:1) are prepared for spectral analysis. Because the reported sulfur source for sulfidemetal coordination is inorganic salt Na2Sx or K2Sx,10,11 herein, thiuram, a common rubber additive, is adopted as a sulfur source to simulate the sulfide cross-links in the vulcanizates. The interaction between the sulfide and metal ions is first demonstrated by Raman spectroscopy. As shown in Figure 2a, CuCl2 exhibits two peaks at approximately 105 and 210 cm−1, which are related to the long Cu−Cl bond and short Cu−Cl bond, respectively.12 After incorporating thiuram, those two peaks shift to higher wavenumbers. Similarly, with the incorporation of thiuram, peaks for the long Fe−Cl bond (111 cm−1) and short Fe−Cl bond in FeCl3 (335 cm−1) shift toward higher wavenumbers (Figure 2b).13 X-ray photoelectron spectroscopy (XPS) curves also verify the existence of sulfide-metal coordination. Upon the incorporation of metal ions, S 2p3/2 (161.49 eV) and S 2p1/2 (167.78 eV) peaks in the XPS S 2p spectra of the thiuram shift toward higher binding energies (Figure 2c), indicating an electron-deficient sulfur element. Correspondingly, peaks of Cu 2p3/2, Cu 2p1/2, Fe 2p3/2, and Fe 2p1/2 all shift toward lower binding energies (Figure S2), suggesting electron-rich metal ions and coordination between sulfur and metal ions.14 Interactions between sulfide cross-links and metal ions are further implied by tan δ−T curves (Figure 2d). As shown, compared with the neat SBR (SS7), the SBR with metal ions exhibits an obvious improved glass transition temperature (Tg) and decreased peak
value of tan δ, implying an enhanced constraint on the chain motion. The constraint in this system should be ascribed to the existence of sulfide-metal coordination. At the same total concentration of soaking solution, the sample with high FeCl3 concentration and low CuCl2 concentration (Table S1) exhibits a lower peak value, indicating that sulfide-Fe3+ coordination provides a stronger constraint on chain motion than sulfide-Cu2+ coordination. The effects of metal ions on the mechanical properties of SS7 are illustrated by typical stress−strain curves (Figure 3a). By introducing Cu2+ only, SS7-Cu4/Fe0 exhibits a very limited increase in the modulus, although the breaking strain and tensile strength are enhanced to a degree. In contrast, SS7 with Fe3+ only (SS7-Cu0/Fe4) shows an improved modulus but an obviously decreased breaking strain, resulting in little increase in the tensile strength. Interestingly, by simultaneously incorporating Cu2+ and Fe3+, corresponding samples exhibit a simultaneously increased modulus, tensile strength and toughness. Specifically, with 2.05 wt % Cu2+ and 1.98 wt % Fe3+, the tensile strength and toughness of SS7-Cu1/Fe3 are 18.87 ± 1.25 MPa and 14.18 ± 0.67 MJ/m3, respectively, which are 6- and 5-fold that of SS7, respectively. Notably, the modulus (stress at 200% strain) of SS7-Cu1/Fe3 reaches 12.72 ± 0.47 MPa, which is unprecedented when compared with reported elastomers reinforced by sacrificial bonds or nanofillers.5,15−29 Figure 3b compares the moduli for reported elastomers reinforced by sacrificial bonds with that in this work. Herein, elastomers with a Tg higher than ambient temperature (20 °C) are excluded in the comparison because plastic behavior is dominant in those systems. Obviously, among the reported values, our work exhibits the highest modulus, although some other works exhibit a higher toughness due to a higher breaking strain. As the ion clusters can be regarded as nanofillers, the modulus of SBRs reinforced by nanofillers and that in our work are also compared, where the developed SBR also exhibits the highest modulus and a strikingly enhanced efficiency (Figure S3). Stress−strain curves 1092
DOI: 10.1021/acsmacrolett.9b00512 ACS Macro Lett. 2019, 8, 1091−1095
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Figure 3. (a) Typical stress−strain curves of SS7 soaked in CuCl2/FeCl3/THF solution with different concentration ratios, where the total concentration is 4 mg/mL. (b) Comparison of the moduli for reported elastomers reinforced by sacrificial bonds and this work. (c) Histogram of total and polysulfide-based Ve of SS3 and SS3-HA. (d) Stress−strain curves of SS3 and SS3-HA before and after soaking in CuCl2/FeCl3/THF solution.
of SBRs with different sulfur contents and chlorate concentrations are also examined. By increasing the sulfur content (from 1.7 to 7 phr) or solution concentration (from 2 to 6 mg/mL), the mechanical properties of the resulting elastomer increase monotonously (Figures S4 and S5). Due to the coexistence of mono-, di-, and polysulfide crosslinks in sulfur-cross-linked rubber, SBRs with the same sulfur content but different polysulfide cross-link content are prepared to investigate the main factor for the reinforcement. Figure 3c shows the total and polysulfide-based cross-linking density (Ve) of SS3 and SS3 with a high accelerator content (SS3-HA). As shown, although the total Ve of SS3 is lower than that of SS3-HA, the polysulfide-based Ve of SS3 is higher than that of SS3-HA. In Figure 3d, after being soaked in CuCl2/FeCl3/THF solution, SS3 exhibits a larger breaking strain and higher tensile strength than those for SS3-HA. Compared with SS3-HA, SS3 exhibits a larger Tg increment and a more obvious decrease in peak value after soaking (Figure S6), suggesting that the metal ions exhibit a stronger constraint on chain motion for SS3. Therefore, polysulfide cross-links are verified to play a key role in coordination construction and reinforcement. The mechanism for the reinforcement is investigeted next. Figure 4a depicts the hysteresis loop and loading−unloading curves for SS7 and SS7 with different CuCl2 and FeCl3 ratios. The hysteresis data of the samples were calculated (Figure S7). With the incorporation of metal ions, the hysteresis loops of the samples increased in size more than those of the ion-free sample, suggesting an energy dissipation capability for the
Figure 4. (a) Loading−unloading cycle curves. (b) Creep curves under a stress of 0.1 MPa. (c) Stress relaxation curves at a strain of 5% of SS7 and SS7 soaked in THF/FeCl3/CuCl2 solution with different FeCl3 and CuCl2 ratios. (d) Loading−unloading cycle curves of SS7Cu2/Fe2 with different waiting times.
sulfide-metal coordination. In addition, with the same total concentration (4 mg/mL), the sample with a high FeCl3 fraction exhibited larger hysteresis loops than the sample with a high CuCl2 fraction, indicating a stronger capability for 1093
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preparation process of vulcanizates can be optimized to gradually approach practical applications. In summary, based on the innate ligand in sulfur-cured rubber, sulfide-metal coordinations are successfully constructed in nonpolar rubber to serve as sacrificial bonds. The sacrificial bonds are achieved by simply soaking vulcanizates in a solution containing metal ions, without any complicated chemical modifications. By introducing 2.05 wt % Cu2+ and 1.98 wt % Fe3+, the sample exhibits a 5-fold increase in tensile strength and a 4-fold increase in toughness. Notably, the modulus achieved is unprecedented when compared with reported elastomers reinforced by sacrificial bonds or nanofillers. Overall, this work presents a novel, generic, and high-efficiency method to prepare high-performance elastomers without any chemical modifications, which should promote comprehensive research and industrial application of sacrificial bond strategies to diene rubbers.
dissipating energy. In the creep experiments (Figure 4b), SS7 was stretched to a stable strain quickly and exhibited little creep, which is similar to the behavior of ideal elastomers. For SS7 with Fe3+ only, an obvious creep behavior was observed, which is ascribed to the slow dissociation of sulfide-Fe3+ coordination. In contrast, SS7 with Cu2+ only exhibited a behavior similar to ideal elastomers, indicating the quick dissociation of sulfide-Cu2+ coordination. Unexpectedly, SS7Cu2/Fe2 also exhibited little creep. This result is ascribed to the quick dissociation of sulfide-Cu2+ coordination, which preferentially consumes the low applied force (0.1 MPa) and maintains the stability of sulfide-Fe3+ coordination. The preferential rupture of the sulfide-metal coordination is demonstrated by stress relaxation curves. Generally, the relaxed stress in a cross-linked rubber network is ascribed to the elimination of weak cross-links, such as chain entanglements and reversible bonds (such as a hydrogen bond or coordination). As shown in Figure 4c, samples with metal ions relax more stress than the metal ion-free sample, which is mainly ascribed to the sacrifice of sulfide-metal coordination and indicates the preferential rupture of sulfide-metal coordination before covalent cross-links. In addition, the higher FeCl3 fraction provides an increased amount of relaxed stress, indicating a higher bond strength for the sulfide-Fe3+ coordination than for the sulfide-Cu2+ coordination. The recoverable nature of sulfide-metal coordination is revealed by the loading−unloading cycle curves with different waiting times (Figure 4d). As shown, the first loading−unloading cycle curve of SS7-Cu2/Fe2 exhibits a hysteresis loop due to energy dissipation arising from the sacrifice of sulfide-metal coordination. For the subsequent cycle with no waiting time, a smaller hysteresis loop is observed due to the limited reconstruction of broken coordination. Notably, after waiting 300 s at room temperature, the new loading−unloading cycle almost overlapped with the original one, indicating the quick reconstruction of the sulfide-metal coordination at room temperature. Besides, increasing metal ions concentration results in enlarged hysteresis loops and increased relaxed stress of SS7, confirming that metal ions is benefit to coordination construction and energy dissipation (Figures S8 and S9). Based on the above discussion, we can summarize the reinforcing mechanism as follows. On the one hand, the formed coordination can act as additional physical cross-links to improve the modulus; on the other hand, when the sample containing Cu2+ and Fe3+ is subjected to an external force, the sulfide-Cu2+ coordination first breaks due to the quick dissociation, followed by the sacrifice of sulfide-Fe3+ coordination. The sequential rupture of sulfide-Cu2+ and sulfide-Fe3+ coordination can continuously absorb impacting energy, suppress stress concentration and promote the orientation of the chain segment while maintaining the integrity of the sample. Upon increasing the external force, covalent cross-links rupture, and the sample fails. As a consequence, the sample can simultaneously achieve high modulus, high strength, and a large extensibility. This method is a potentially universal strategy, as most industrial rubber products are cross-linked by sulfur. Although the method cannot be applied to industrial rubbers directly due to the use of a solvent and the possible negative effect of aging from the metal ions, the purpose of this work is to reveal the strong reinforcement effect of sulfide-metal coordination in rubbers. In the following investigations, the recipe and
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00512.
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Experimental details, mass fractions of metal ions, TEM and EDS mapping of SS7-Cuy/Fez series, XPS Cu 2p and Fe 2p spectra, comparison of SBR reinforced by nanofillers and sulfide-metal coordination, stress−strain curves of SSx-Cu2/Fe2 series and SS7-Cuy/Fez series, Tan δ−T profiles of SS3-Cu2/Fe2 and SS3-HA-Cu2/ Fe2, histogram of hysteresis areas of SS7-Cuy/Fez (the sum of y and z is 4 mg/mL) series, loading−unloading cycles, and relaxation curves of SS7-Cuy/Fez (y is equal to z) series (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Baochun Guo: 0000-0002-4734-1895 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (51825303) and National Natural Science Foundation of China (51673065, 51790503, and 51703064).
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REFERENCES
(1) Stockelhuber, K. W.; Das, A.; Kluppel, M. Designing of Elastomer Nanocomposites: From Theory to Applications. Polymer 2017, 275, 129−145. (2) Domun, N.; Hadavinia, H.; Zhang, T.; Sainsbury, T.; Liaghat, G. H.; Vahid, S. Improving the Fracture Toughness and the Strength of Epoxy Using Nanomaterials - a Review of the Current Status. Nanoscale 2015, 7, 10294−10329.
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(23) Hayashi, M.; Noro, A.; Matsushita, Y. Highly Extensible Supramolecular Elastomers with Large Stress Generation Capability Originating from Multiple Hydrogen Bonds on the Long Soft Network Strands. Macromol. Rapid Commun. 2016, 37, 678−684. (24) Hayashi, M.; Matsushima, S.; Noro, A.; Matsushita, Y. Mechanical Property Enhancement of ABA Block Copolymer-Based Elastomers by Incorporating Transient Cross-Links into Soft Middle Block. Macromolecules 2015, 48, 421−431. (25) Liu, J.; Wang, S.; Tang, Z. H.; Huang, J.; Guo, B. C.; Huang, G. S. Bioinspired Engineering of Two Different Types of Sacrificial Bonds into Chemically Cross-Linked cis-1,4-Polyisoprene toward a High-Performance Elastomer. Macromolecules 2016, 49, 8593−8604. (26) Zhang, X. H.; Liu, J.; Zhang, Z. Y.; Wu, S. W.; Tang, Z. H.; Guo, B. C.; Zhang, L. Q. Toughening Elastomers Using a MusselInspired Multiphase Design. ACS Appl. Mater. Interfaces 2018, 10, 23485−23489. (27) Li, C. H.; Wang, C.; Keplinger, C.; Zuo, J. L.; Jin, L.; Sun, Y.; Zheng, P.; Cao, Y.; Lissel, F.; Linder, C.; You, X. Z.; Bao, Z. A. A highly Stretchable Autonomous Self-healing Elastomer. Nat. Chem. 2016, 8, 618−624. (28) Tang, Z. H.; Huang, J.; Guo, B. C.; Zhang, L. Q.; Liu, F. Bioinspired Engineering of Sacrificial Metal-Ligand Bonds into Elastomers with Supramechanical Performance and Adaptive Recovery[J]. Macromolecules 2016, 49, 1781−1789. (29) Neal, J. A.; Mozhdehi, D.; Guan, Z. B. Enhancing Mechanical Performance of a Covalent Self-Healing Material by Sacrificial Noncovalent Bonds. J. Am. Chem. Soc. 2015, 137, 4846−4850.
(3) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. DoubleNetwork Hydrogels with Extremely High Mechanical Strength. Adv. Mater. 2003, 15, 1155−1157. (4) Ducrot, E.; Chen, Y. L.; Bulters, M.; Sijbesma, R. P.; Creton, C. Toughening Elastomers with Sacrificial Bonds and Watching Them Break. Science 2014, 344, 186−189. (5) Luo, M. C.; Zeng, J.; Fu, X.; Huang, G. S.; Wu, J. R. Toughening Diene Elastomers by Strong Hydrogen Bond Interactions. Polymer 2016, 106, 21−28. (6) Filippidi, E.; Cristiani, T. R.; Eisenbach, C. D.; Waite, J. H.; Israelachvili, J. N.; Ahn, B. K.; Valentine, M. T. Toughening Elastomers Using Mussel-Inspired Iron-Catechol Complexes. Science 2017, 358, 502−505. (7) Kruzelak, J.; Sykora, R.; Hudec, I. Sulphur and Peroxide Vulcanisation of Rubber Compounds - Overview. Chem. Pap. 2016, 70, 1533−1555. (8) Golding, R. M.; Tennant, W. C. An E.S.R. Study of a Copper (II) Complex of Tetraethylthiuramdisulphide. Mol. Phys. 1972, 24, 301−308. (9) Müller, A.; Diemann, E. Polysulfide Complexes of Metals. Adv. Inorg. Chem. 1987, 31, 89−122. (10) Draganjac, M.; Rauchfuss, T. B. Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands. Angew. Chem., Int. Ed. Engl. 1985, 24, 742−757. (11) Ma, S. L.; Huang, L.; Ma, L. J.; Shim, Y.; Islam, S. M.; Wang, P. L.; Zhao, L. D.; Wang, S. C.; Sun, G. B.; Yang, X. J.; Kanatzidis, M. G. Efficient Uranium Capture by Polysulfide/Layered Double Hydroxide Composites. J. Am. Chem. Soc. 2015, 137, 3670−3677. (12) Medeiros, F. E. O.; Araujo, B. S.; Ayala, A. P. Raman Spectroscopy Investigation of the Thermal Stability of The Multiferroic CuCl2 and Its Hydrated Form. Vib. Spectrosc. 2018, 99, 1−6. (13) Marston, A. L.; Bush, S. F. Raman Spectral Investigation of the Complex Species of Ferric Chloride in Concentrated Aqueous Solution. Appl. Spectrosc. 1972, 26, 579−584. (14) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Investigation of Multiplet Splitting of Fe 2p XPS Spectra and Bonding in Iron Compounds. Surf. Interface Anal. 2004, 36, 1564−1574. (15) Hou, J. B.; Zhang, X. Q.; Wu, D.; Feng, J. F.; Ke, D.; Li, B. J.; Zhang, S. Tough Self-Healing Elastomers Based on the Host-Guest Interaction of Polycyclodextrin. ACS Appl. Mater. Interfaces 2019, 11, 12105−12113. (16) Liu, Y. J.; Tang, Z. H.; Wu, S. W.; Guo, B. C. Integrating Sacrificial Bonds into Dynamic Covalent Networks toward Mechanically Robust and Malleable Elastomers. ACS Macro Lett. 2019, 8, 193−199. (17) Huang, J.; Zhang, L. J.; Tang, Z. H.; Wu, S. W.; Ning, N. Y.; Sun, H. B.; Guo, B. C. Bioinspired Design of a Robust Elastomer with Adaptive Recovery via Triazolinedione Click Chemistry. Macromol. Rapid Commun. 2017, 38, 1−7. (18) Zhang, H.; Chen, Y. J.; Lin, Y. J.; Fang, X. L.; Xu, Y. Z.; Ruan, Y. H.; Weng, W. G. Spiropyran as a Mechanochromic Probe in Dual Cross-Linked Elastomers. Macromolecules 2014, 47, 6783−6790. (19) Comi, M.; Lligadas, G.; Ronda, J. C.; Galia, M.; Cadiz, V. Adaptive Bio-based Polyurethane Elastomers Engineered by Ionic Hydrogen Bonding Interactions. Eur. Polym. J. 2017, 91, 408−419. (20) Xia, X. X.; Wu, Z. J.; Wang, W. J.; Shangguan, Y. G.; Zheng, Q. A Facile and Environmentally Friendly Approach to Fabricate Hybrid Crosslinked Nitrile Butadiene Rubber with Comprehensively Improved Mechanical Performances by Incorporating Sacrificial Ionic Bonds. Polymer 2019, 161, 55−63. (21) Sun, H. B.; Liu, X. Y.; Yu, B.; Feng, Z. B.; Ning, N. Y.; Hu, G. H.; Tian, M.; Zhang, L. Q. Simultaneously Improved Dielectric and Mechanical Properties of Silicone Elastomer by Designing a Dual Crosslinking Network. Polym. Chem. 2019, 10, 633−645. (22) You, Y.; Huang, W. Y.; Zhang, A. Q.; Lin, Y. L. A Facile and Controllable Synthesis of Dual-Crosslinked Elastomers Based on Linear Bifunctional Polydimethylsiloxane Oligomers. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3760−3768. 1095
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