Integrating Sacrificial Bonds into Dynamic Covalent Networks toward

Letter Cite This: ACS Macro Lett. 2019, 8, 193−199

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Integrating Sacrificial Bonds into Dynamic Covalent Networks toward Mechanically Robust and Malleable Elastomers Yingjun Liu, Zhenghai Tang,* Siwu Wu, and Baochun Guo* Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China

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S Supporting Information *

ABSTRACT: Vitrimers are a class of covalently cross-linked polymers that have drawn great attention due to their fascinating properties such as malleability and reprocessability. The state of art approach to improve their mechanical properties is the addition of fillers, which, however, greatly restricts the chain mobility and impedes network topology rearrangement, thereby deteriorating the dynamic properties of vitrimer composites. Here, we demonstrate that the integration of sacrificial bonds into a vitrimeric network can remarkably enhance the overall mechanical properties while facilitating network rearrangement. Specifically, commercially available epoxidized natural rubber is covalently cross-linked with sebacic acid and simultaneously grafted with N-acetylglycine (NAg) through the chemical reaction between epoxy and carboxyl groups, generating exchangeable β-hydroxyl esters and introducing amide functionalities into the networks. The hydrogen bonds arising from amide functionalities act in a sacrificial and reversible manner, that is, preferentially break prior to the covalent framework and undergo reversible breaking and reforming to dissipate mechanical energy under external load, which leads to a rarely achieved combination of high strength, modulus, and toughness. The topology rearrangement of the cross-linked networks can be accomplished through transesterification reactions at high temperatures, which is accelerated with the increase of grafting NAg amount due to the dissociation of transient hydrogen bonds and increase of the ester concentration in the system.

R

number of cross-links keeps invariable and the networks are insoluble at all times due to the associative nature of exchange reactions. Besides, the viscosity of vitrimers exhibits Arrheniuslike gradual variations with respect to temperatures, in contrast to an abrupt viscosity change around glass transition of thermoplastic or sol-to-gel transition in the network with dissociative bonds, offering unique advantages of broad processing temperature windows and flexible processing methods.5 Several exchangeable chemistries such as olefin metathesis,6 transalkylation,7 disulfide exchange,8 imine exchange,9 siloxane equilibration,10 boronic ester exchange,11 transcarbamoylation,12 and silyl ether exchange13 have thus far been exploited in the context of vitrimers. Very recently, vitrimer concept has been implemented into rubbers to impart them with dynamic properties.14 For example, Guan and co-workers incorporated Grubbs’ Ru catalyst into covalently cross-linked polybutadiene to make it malleable through olefin metathesis.6 We previously prepared permanently cross-linked yet recyclable styrene−butadiene rubber by incorporating exchangeable boronic ester bonds into the network.15 To improve the mechanical properties of

ubbers are renowned for high elasticity, which are indispensable in tires and seals.1 However, the covalent cross-linking of rubbers is essential to acquire the necessary mechanical properties, high resilience, and environmental resistance, which makes their recycling inherently difficult. Currently, the scrapped rubbers are mainly underwent incineration for low-efficiency energy recovery, dumping as menial landfills, and desulfurization for very specific and less critical applications.2 Collectively, the outlets obtained from these recycling methods are “down cycled”. Therefore, it is of great significance to explore an alternative way to initiate a real recycling loop. To address this dilemma, an increasingly growing interest has been focused on introducing dynamic covalent bonds into thermosetting polymer materials. Dynamic covalent bonds are able to reversibly break and reform under external stimuli, which enables network rearrangement and endows them with dynamic properties such as self-healing ability and malleability.3 Representative examples to prepare thermally adaptive networks are based on reversible Diels−Alder reaction. However, the retro-Diels−Alder reaction causes a gel-to-sol transition and loss of network integrity at elevated temperatures, which is unfavorable for structural applications.2a Recently, Leibler et al. pioneered the concept of vitrimers based on epoxy-acid system, which could rearrange network topology through a catalytic transesterification reaction.4 The © XXXX American Chemical Society

Received: January 7, 2019 Accepted: January 30, 2019

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DOI: 10.1021/acsmacrolett.9b00012 ACS Macro Lett. 2019, 8, 193−199

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Scheme 1. (a) Preparation of Dynamic Covalent Networks with Sacrificial H-Bonds; (b) Schematic Illustration of the Energy Dissipation Mechanism for ENR-xN with NAg through Reversible Breaking and Reformation of H-Bonds under Deformation

vitrimers, various nanofillers have been added.16 It is particularly critical for rubber-based vitrimers as most neat rubbers suffer from poor mechanical performance. However, the inclusion of nanofillers greatly restricts chain mobility and retards network topology rearrangements, which consequently deteriorate the dynamic properties of vitrimer composites.14b,16b Moreover, other challenges such as filler dispersion/aggregation, interfacial regulation, and processing difficulty imposed by nanofillers should also be taken into considerations.17 The incorporation of noncovalent sacrificial bonds has been demonstrated as a promising methodology to enhance mechanical properties of biological materials.18 On molecular level, the noncovalent sacrificial bonds can break prior to covalent bonds and undergo reversible bond breaking and reforming events, which provide an efficient energy-dissipating mechanism for reinforcement.19 Learning from nature, many trails have been reported to transfer sacrificial bond principles into man-made polymers, aiming to achieve biomimetic strength and toughness.19a For example, following titin’s modular architecture, Kushner et al. engineered quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidone (UPy) motif into polymer networks, in which the reversibly unfolding of UPy dimers dissipated enormous energies and conferred them with high toughness.20 Thus, we envision that the integration of noncovalent sacrificial bonds into dynamic covalent networks can in principle provide an elegant methodology to improve the mechanical properties. In this work, we present a conceptual method to prepare chemically cross-linked yet recyclable, mechanically strong rubber vitrimers by combining sacrificial H-bonds and exchangeable β-hydroxyl ester linkages into rubber networks. To realize this goal, commercially available epoxidized natural rubber (ENR) is cross-linked with sebacic acid (SA) and

simultaneously grafted with N-acetylglycine (NAg) through the chemical reaction between epoxy and carboxyl groups, generating exchangeable β-hydroxyl ester-based cross-links and introducing amide functionalities into the networks (Scheme 1a). DMI acts as accelerator to promote the reaction between epoxy and carboxyl groups (Figure S1), and TBD acts as catalyst to accelerate transesterifications. Details of sample preparation and characterizations are described in the Supporting Information. In the context, sample code of ENR-xN refers to cross-linked ENR with x wt% of NAg relative to ENR. The cross-linked networks can alter their topologies and release stress through transesterification at high temperatures, imparting them with reprocessability. Meanwhile, the pendent amide functionalities are capable of forming hydrogen bonds (H-bonds) and function as sacrificial bonds to drastically improve the mechanical properties while retaining the dynamic properties of the resulting vitrimers. The chemical cross-linking of ENR with SA and grafting of NAg onto ENR chains can be explicitly evidenced using FTIR measurement. As shown in Figure S2, the absorptions at 3494 and 1724 cm−1 in uncured ENR-0N are attributed to the stretching vibrations of OH and CO in carboxyl groups of SA, respectively. After curing, these two peaks shift to 3468 and 1733 cm−1 in cured ENR-0N, respectively, which are due to the conversion of carboxyl groups into β-hydroxyl esters.4 In addition, the absorption for epoxy groups at 876 cm−1 decreases after curing. These observations confirm the reaction of carboxyls with epoxy groups and generation of β-hydroxyl esters. In the case of cured ENR-xN samples with NAg, the characteristic peaks at 1662 and 1550 cm−1 correspond to C O stretching (amide I) and CN stretching with contributions of NH bending (amide II) in amide functionalities, respectively (Figure 1a).21 With the increase of NAg loadings, the absorptions related to ester groups and amide function194

DOI: 10.1021/acsmacrolett.9b00012 ACS Macro Lett. 2019, 8, 193−199

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Figure 1. (a) FTIR spectra for ENR-xN with various NAg loadings. (b) Variable temperature FTIR spectra for ENR-6N in the range of 1500−1590 cm−1 from 30 to 120° with a temperature increment of 10°. The spectra are normalized by using the absorption intensity of −CH3 at 2963 cm−1 as an internal reference.

Figure 2. (a) Representative stress−strain curves for ENR-xN. (b) Modulus and toughness for ENR-xN.

a trade-off between modulus and extensibility because a ductile network is able to endure large deformations but has shallow stress responses, while a constrained network exhibits increased modulus but readily fails with small deformations.19a Herein, a rare combination of these improvements may be because the H-bonds preferentially rupture and undergo reversible breaking and rebuilding, by which an enormous amount of mechanical energy is dissipated and the material integrity is survived. To gain insights into the energydissipating mechanism, cyclic tensile tests are performed. ENR-0N exhibits negligible hysteresis loop (area surrounded by loading−unloading curves), and hysteresis energy is increased with increasing NAg loading (i.e., H-bonds content; Figure S4). Typically, in the loading−unloading curves of ENR-6N, an obvious hysteresis loop is observed in the first cycle, suggesting enormous energy dissipation due to the rupture of H-bonds during stretching. Whereas, the hysteresis loop becomes much smaller in the second cycle (Figure 3a). This may be because parts of ruptured H-bonds cannot reform in the time scale of individual cycle, and thus fewer H-bonds can participate in energy dissipation in the successive cycle. After storing the sample at 80° for 1 min, the loading− unloading curves overlap with the first cycle, demonstrating that the covalent framework is remained and the H-bonds are almost completely reformed. These findings confirm that the H-bonds behaves in a sacrificial and reversible manner to dissipate energy during stretching while the covalent crosslinks impart the material with elasticity, as schematically illustrated in Scheme 1b. Considering the finite lifetime of H-bonds, their contribution to the network modulus is expected to be strain ratedependence.23 Stress−strain curves for ENR-6N at different strain rates are displayed in Figure 3b. The modulus of ENR6N gradually increases as the strain rate increases. Presumably, the fraction of unrelaxed H-bonds at shorter time scales (i.e., higher strain rates) is larger than that at longer time scales (i.e.,

alities consistently increase while that for epoxy groups conversely decreases, indicating that NAg is grafted onto ENR chains through β-hydroxyl ester linkages. After immersing ENR-xN chips in tetrahydrofuran/ethanol (V:V, 3:1) and stirring them for 3 days, the FTIR test shows that no characteristic absorptions related to NAg are observed in the mixture solvent, indicating that almost all NAg is covalently incorporated into the polymer network. To verify the formation of H-bonds between amide functionalities in cured ENR-xN samples, variable temperature FTIR was carried out. Taking ENR-6N as an example, the absorption for NH bending systematically shifts from 1550 to 1535 cm−1 when the temperature increases from 30 to 120° (Figure 1b) because the H-bonds arising from amide bonds are weakened at elevated temperatures.22 DSC results show that the glass transition temperature for ENR-xN is gradually increased with the increase of NAg loading (Figure S3). This is because the segment mobility is restricted by grafting NAg and introducing H-bonds. Uniaxial tensile curves for ENR-xN are displayed in Figure 2a, and the mechanical properties are tabulated in Table S1. ENR-0N exhibits a relatively weak mechanical property with a stress at 300% strain (hereafter referred to as modulus) of 2.4 MPa and ultimate strength of 5.6 MPa. It is evident that the addition of NAg leads to remarkable enhancements on the modulus, ultimate strength, and toughness, without the sacrifice of stretchability (Figure 2b). For example, when compared to ENR-0N, the modulus and ultimate strength of ENR-8N reach 7.1 and 28.0 MPa, which are improved by about 200% and 400%, respectively. In addition, the breaking strain of ENR-8N is slightly increased, and the toughness that is determined as the area surrounded by stress−strain curve is improved by 300% (Table S1). It is of significant importance, but often challenging, to simultaneously enhance the modulus, strength, and ductility of polymer materials. Generally, there is 195

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Figure 3. (a) Loading−unloading cycle for ENR-6N. (b) Stress−strain curves for ENR-6N at different strain rates. (c) Reduced stress f * vs λ−1 for ENR-6N at various strain rates and ENR-0N at a strain rate of 0.4 s−1. (d) Time dependence of f p* under uniaxial deformation for ENR-6N at various strain rates.

reversible associations in double cross-linking networks consisting of covalent and physical bonds.27,28 Due to the formation of β-hydroxyl ester linkages in the cross-linking and grafting processes, we envision that ENR-xN can rearrange the network topologies via transesterifications at an elevated temperature. Figure 4a showcases the strain/ recovery profiles for ENR-6N at 50 and 180 °C. At 50 °C, ENR-6N is elongated upon applying external force and recovers with the release of force. When the temperature increases to 180 °C, ENR-6N shows a viscous deformation following by an initial thermal expansion and elastic deformation. Such discrepancy in the viscoelastic behaviors is because the network topologies are quenched due to the depressed transesterification reactions at a low temperature, while the transesterifications are activated to enable network rearrangements at a high temperature. Figure 4b is the creep curves of ENR-xN at 150°. After an elastic response, the strain for ENR-xN is increased linearly with time, revealing that the networks can flow at a high temperature although they are covalently cross-linked. Interestingly, the slope of strain-time curves is increased with NAg loading, namely, the sample having a higher NAg grafting amount exhibits a faster network rearrangement. Furthermore, the stress relaxation results show that ENR-xN samples can substantially relax stress with time at 180°, and the sample with a higher NAg grafting content exhibits a faster relaxation rate and shorter relaxation time (Figure S6). In previously reported dynamic covalent networks, the main way to enhance their mechanical properties is the addition of nanofillers, which, however, greatly restricts chain mobility and hinders topology rearrangements.16b,c In the study of Legrand et al., the characteristic relaxation time τ* (the time required to relax 63% of the original stress) of the epoxy-based vitrimer composite with 25 wt % silica is 9× longer than that of the neat vitrimer.16b In our work, the integration of sacrificial Hbonds not only significantly improves the mechanical properties, but also facilitates the network rearrangement. Two

lower strain rates), and thereby, their contribution to the material modulus is increased with strain rate. To better visualize the function mechanism of transient sacrificial H-bonds to ENR-xN, the cross-linked networks are understood by plotting the reduced stress (f *) against the reciprocal of extension ratio (λ) using the well-known Mooney equation24 f * = σ /(λ − λ−2) = 2C1 + 2C2λ−1

where σ is stress and 2C1 and 2C2 are constants (Figure 3c). For ideal networks of Gaussian chains, the f * is identified as shear modulus and independent of λ.25 Herein, f * of ENR-0N is nearly invariable at small λ (λ−1 > 0.3), suggesting that without H-bonds, its uniaxial deformation can be well described by the rubber elasticity model (affine theory). However, the positive slopes of Mooney plot are observed for ENR-6N at small λ, which can be explained by the strain softening of the material due to breakage of weak H-bonds. Moreover, with the increase of strain rate, the slope values are consistently increased, indicating that the relaxation of Hbonds is time dependent. At large λ (λ−1 < 0.3), the Mooney plots of both ENR-0N and ENR-6N exhibit negative slope, which is related to the strain hardening caused by the straininduced crystallization of ENR.26 Furthermore, by plotting f * against stretching time t, all the curves overlap except for the strain stiffening in longer time regimes (Figure S5).27 The f (t)* is a sum contributions of chemical cross-links (fc*) and physical cross-links (f p*). After subtracting the contribution of chemical cross-links (here is 0.6 MPa taken from the value of ENR-0N), a master curve of f p (t)* (=f (t)* − fc*) versus time t can be obtained (Figure 3d). The f p* value in the overlapped region is irrelevant to λ and depends only on time, indicating that strain has no effect on the breakage rate of H-bonds in the networks. The slope of the overlapped region is −0.5, which is well consistent with the associative Rouse mode concerning relaxation behavior of 196

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number of NAg in ENR-6N) into ENR-0N. The stress relaxation curve for ENR-0N/Ne is almost overlapped with that for ENR-0N, indicating that amide functionalities do not have catalytic effect on transesterifications (Figure S8). The effect of temperatures on stress relaxation of ENR-xN is shown in Figures 4c and S9. It can be seen that the relaxation rate is increased when the temperature is elevated, which can be explained by the fact that the thermo-activated transesterification reactions are accelerated with temperature. In addition, the relaxation time follows an Arrhenius law (Figure S10). Accordingly,the activation energies for all ENR-xN samples are calculated to be about 110 kJ/mol, which are within the range of reported values for β-hydroxyl ester-based vitrimer networks (80−120 kJ/mol).4,30 As revealed above, ENR-xN can alter their topologies through transesterifications, which make their recycling possible. To demonstrate the reprocessability, the samples after tensile test are cut into small chips and then remolded. After being remolded at 180 °C for 5 min, new coherent samples are obtained (Figure 5a). The recycled samples can

Figure 4. (a) Strain/recovery profiles for ENR-6N at different temperatures. Stress was alternated between 0.1 MPa for 60 min and 0 MPa for 10 min in each cycle. (b) Creep test curves for ENR-xN with a 0.1 MPa stress at 150°. (c) Stress relaxation curves for ENR-6N at different temperatures with 2% strain.

reasons may account for this accelerated effect. On one hand, H-bonds are dissociated at high temperatures and thus do not exert restrictions on chain mobility. On the other hand, the grafting of NAg onto ENR that is conducted through the reaction between epoxy and carboxyl groups can increase the concentration of β-hydroxyl esters in the systems, which may provide more opportunities for bond exchange reactions. To verify this assumption, a control sample ENR-H with the same amount of ester as that in ENR-6N but without H-bonds was prepared by replacing NAg with the same molar number of hexanoic acid. Stress relaxation rate of ENR-H is nearly identical to that of ENR- 6N and much faster than that of ENR-0N, further confirming that the accelerated transesterifications are due to the increase in ester concentration (Figure S7). In the study of Hillmyer et al., they also reported that hydroxyl-terminated star-shaped polylactide-based vitrimers exhibited a faster relaxation rate, which was explained by the fact that a higher concentration of ester groups in the system facilitated the transesterifications.29 Moreover, to exclude the catalytic effect of amide bonds on transesterifications, another control sample ENR-0N/Ne was prepared by adding N-ethylacetamide (equal to the molar

Figure 5. (a) Photos demonstrating the recycling of ENR-xN. The scale bar represents 1 cm. (b) Recovery ratio of mechanical properties for recycled ENR-xN. (c) Stress−strain curves for ENR-4N after multiple generations of recycling.

recover most of the original mechanical properties, as displayed in Figures 5b and S11. It should be noted that a more obvious decrease in the ultimate stress of recycled ENR-8N is likely because its stress sharply increases at large strain and thus a similar loss of strain results in a remarkable decrease in stress. More strikingly, the mechanical properties are well maintained 197

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(2) (a) Trovatti, E.; Lacerda, T. M.; Carvalho, A. J. F.; Gandini, A. Recycling Tires? Reversible Crosslinking of Poly(butadiene). Adv. Mater. 2015, 27 (13), 2242−2245. (b) Jiang, K.; Shi, J.; Ge, Y.; Zou, R.; Yao, P.; Li, X.; Zhang, L. Complete Devulcanization of SulfurCured Butyl Rubber by Using Supercritical Carbon Dioxide. J. Appl. Polym. Sci. 2013, 127 (4), 2397−2406. (3) (a) Zou, W.; Dong, J.; Luo, Y.; Zhao, Q.; Xie, T. Dynamic Covalent Polymer Networks: from Old Chemistry to Modern Day Innovations. Adv. Mater. 2017, 29 (14), 1606100. (b) Jin, Y.; Yu, C.; Denman, R. J.; Zhang, W. Recent Advances in Dynamic Covalent Chemistry. Chem. Soc. Rev. 2013, 42 (16), 6634−6654. (4) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. SilicaLike Malleable Materials from Permanent Organic Networks. Science 2011, 334 (6058), 965−968. (5) Capelot, M.; Montarnal, D.; Tournilhac, F.; Leibler, L. MetalCatalyzed Transesterification for Healing and Assembling of Thermosets. J. Am. Chem. Soc. 2012, 134 (18), 7664−7667. (6) Lu, Y. X.; Guan, Z. Olefin Metathesis for Effective Polymer Healing via Dynamic Exchange of Strong Carbon-Carbon Double Bonds. J. Am. Chem. Soc. 2012, 134 (34), 14226−14231. (7) Obadia, M. M.; Mudraboyina, B. P.; Serghei, A.; Montarnal, D.; Drockenmuller, E. Reprocessing and Recycling of Highly CrossLinked Ion-Conducting Networks through Transalkylation Exchanges of C-N Bonds. J. Am. Chem. Soc. 2015, 137 (18), 6078−6083. (8) Xiang, H. P.; Qian, H. J.; Lu, Z. Y.; Rong, M. Z.; Zhang, M. Q. Crack Healing and Reclaiming of Vulcanized Rubber by Triggering the Rearrangement of Inherent Sulfur Crosslinked Networks. Green Chem. 2015, 17 (8), 4315−4325. (9) Taynton, P.; Yu, K.; Shoemaker, R. K.; Jin, Y.; Qi, H. J.; Zhang, W. Heat- or Water-Driven Malleability in a Highly Recyclable Covalent Network Polymer. Adv. Mater. 2014, 26 (23), 3938−3942. (10) Zheng, P.; McCarthy, T. J. A Surprise from 1954: Siloxane Equilibration is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism. J. Am. Chem. Soc. 2012, 134 (4), 2024−2027. (11) Röttger, M.; Domenech, T.; Weegen, R. v. d.; Breuillac, A.; Nicolaÿ, R.; Leibler, L. High-Performance Vitrimers from Commodity Thermoplastics through Dioxaborolane Metathesis. Science 2017, 356 (6333), 62−65. (12) Zheng, N.; Fang, Z.; Zou, W.; Zhao, Q.; Xie, T. Thermoset Shape-Memory Polyurethane with Intrinsic Plasticity. Angew. Chem., Int. Ed. 2016, 55 (38), 11421−11425. (13) Wu, S.; Yang, Z.; Fang, S.; Tang, Z.; Liu, F.; Guo, B. Malleable Organic/Inorganic Thermosetting Hybrids Enabled by Exchangeable Silyl Ether Interfaces. J. Mater. Chem. A 2019, 7, 1459. (14) (a) Imbernon, L.; Norvez, S.; Leibler, L. Stress Relaxation and Self-Adhesion of Rubbers with Exchangeable Links. Macromolecules 2016, 49, 2172−2178. (b) Liu, Y.; Tang, Z.; Chen, Y.; Zhang, C.; Guo, B. Engineering of beta-Hydroxyl Esters into ElastomerNanoparticle Interface toward Malleable, Robust, and Reprocessable Vitrimer Composites. ACS Appl. Mater. Interfaces 2018, 10 (3), 2992−3001. (c) Zhang, H.; Wang, D.; Liu, W.; Li, P.; Liu, J.; Liu, C.; Zhang, J.; Zhao, N.; Xu, J. Recyclable Polybutadiene Elastomer Based on Dynamic Imine Bond. J. Polym. Sci., Part A: Polym. Chem. 2017, 55 (12), 2011−2018. (15) Chen, Y.; Tang, Z.; Zhang, X.; Liu, Y.; Wu, S.; Guo, B. Covalently Cross-Linked Elastomers with Self-Healing and Malleable Abilities Enabled by Boronic Ester Bonds. ACS Appl. Mater. Interfaces 2018, 10 (28), 24224−24231. (16) (a) Yang, Z.; Wang, Q.; Wang, T. Dual-Triggered and Thermally Reconfigurable Shape Memory Graphene-Vitrimer Composites. ACS Appl. Mater. Interfaces 2016, 8 (33), 21691−21699. (b) Legrand, A.; Soulie-Ziakovic, C. Silica-Epoxy Vitrimer Nanocomposites. Macromolecules 2016, 49 (16), 5893−5902. (c) Yu, K.; Shi, Q.; Dunn, M. L.; Wang, T.; Qi, H. J. Carbon Fiber Reinforced Thermoset Composite with Near 100% Recyclability. Adv. Funct. Mater. 2016, 26 (33), 6098−6106. (d) Ruiz de Luzuriaga, A.; Martin, R.; Markaide, N.; Rekondo, A.; Cabañero, G.; Rodríguez, J.; Odriozola, I. Epoxy Resin with Exchangeable Disulfide Crosslinks

even after multiple generations of recycling, demonstrating the excellent recyclability (Figure 5c). In addition, FTIR spectra and cross-linking densities of the recycled samples are nearly identical to that of the original ones, indicating that there is no significant change in network functionalities during the recycling process (Figures S12 and S13). In summary, we demonstrate a conceptual approach to improve the overall mechanical performance of dynamic covalent networks while accelerating their dynamic properties by incorporating sacrificial H-bonds into the networks. Commercially available ENR is covalently cross-linked with SA and simultaneously grafted with NAg through the chemical reaction between epoxy and carboxyl groups, generating exchangeable β-hydroxyl esters along the rubber chains and introducing amide functionalities into the networks. The Hbonds arising from amide functionalities break prior to the covalent framework and undergo reversibly bond breaking and reforming under external load, which dissipates enormous energy and leads to a rare combination of high strength, modulus, and extensibility. The cross-linked networks can alter their topologies and relax stress via transesterifications at high temperatures, which confer them with reprocessability. Due to the dissociation of H-bonds at high temperatures and increase of ester concentration, the network rearrangement rate is accelerated with the grafting of NAg. We envisage that this work may open up new avenues to design other vitrimers with an integration of enhanced mechanical properties and accelerated dynamic properties by varying types of dynamic covalent chemistries and sacrificial bonds.

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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/acsmacrolett.9b00012. Experimental details, supporting table, and figures (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Baochun Guo: 0000-0002-4734-1895 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 (51790503, 51703064, and 51673065).

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REFERENCES

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DOI: 10.1021/acsmacrolett.9b00012 ACS Macro Lett. 2019, 8, 193−199

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DOI: 10.1021/acsmacrolett.9b00012 ACS Macro Lett. 2019, 8, 193−199