Mechanically Robust, Self-Healable, and Reprocessable Elastomers

May 9, 2019 - Zhenghai Tang* .... Department of Polymer Materials and Engineering, South China University of. Technology, Guangzhou 510640, P. R. Chin...
17 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Mechanically Robust, Self-Healable, and Reprocessable Elastomers Enabled by Dynamic Dual Cross-Links Yi Chen, Zhenghai Tang,* Yingjun Liu, Siwu Wu, and Baochun Guo* Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China

Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on May 10, 2019 at 00:50:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Covalent cross-linking of rubbers is essential for obtaining high resilience and environmental resistance but prevents healing and recycling. Integrating dynamic covalent bonds into cross-linked rubbers can resolve the trade-off between permanent cross-linking and plasticity. The state-ofthe-art elastomer-based dynamic covalent networks require either intricate molecular makeup or present poor mechanical properties. In this work, we demonstrate a simple way to prepare mechanically robust yet healable and recyclable elastomeric vitrimers by engineering dynamic dual crosslinks of boronic esters and coordination bonds into a commercial rubber. Specifically, epoxidized natural rubber is covalently cross-linked with a boronic ester cross-linker carrying dithiol through chemical reaction between epoxy and thiol groups. The covalently cross-linked networks are able to alter the topologies through boronic ester transesterifications, thereby conferring them with healing ability and reprocessability. In particular, the mechanical properties can be remarkably enhanced by introducing sacrificial metal−ligand coordination bonds into the networks without compromising the healing ability or reprocessability. the dissociative mechanism, which follows a “bond breaking− bond reforming” sequence,9 the associative pathway is particularly favorable in the design of polymeric materials for structural applications, since the number of cross-links is constant and network integrity is preserved during the exchange reactions due to a simultaneous “bond breaking− bond making” event.10,11 Recently, Leibler et al. introduced the concept of vitrimers based on an epoxy−acid system, which could alter the network topologies through catalytic transesterifications of β-hydroxy ester bonds.12 The viscosity of vitrimers exhibits an Arrhenius-like gradual variation, rather than an abrupt viscosity variation around the glass transition of thermoplastic polymers or a sol-to-gel transition in the networks having dissociative linkages, thus offering remarkable merits of a broad processing temperature range and flexible processing technologies.13,14 Inspired by the fascinating properties of vitrimers, a variety of exchangeable chemistries such as olefin metathesis,15 transalkylation,16,17 disulfide exchange,18,19 siloxane equilibration,20 boronic ester exchange,21,22 imine metathesis,23,24 transcarbamoylation,25 and silyl ether exchange26 have thus far been exploited to expand the diversity of vitrimers. Among these chemistries, boronate ester bonds are renowned for their dynamic and robust features and have been widely explored in solution-based systems for molecular sensors27 and self-healing hydrogels;28

1. INTRODUCTION Rubbers with unique elasticity are ubiquitous in daily life, having indispensable applications in tires and seals. The chemically cross-linked structure of rubbers renders them highly resilient and chemically resistant; however, it also makes them inherently difficult to be healed and recycled, which degrades their lifetime and causes serious environmental pollution. Approximately 30 million tons of scrapped rubbers are produced each year, most of which is incinerated for lowefficiency energy recovery and dumped in landfills.1,2 Although desulfurization of rubbers can be used to reclaim rubber by breaking cross-linking bonds, such a reclamation process, unfortunately, deteriorates the performance of recycled products due to the simultaneous scission of rubber backbones.3 Collectively, the outlets are down-cycled into very specific and less critical products. Therefore, it is highly desirable to seek a “Cradle to Cradle” approach to repair damage to extend the lifetime in practice and initiate a closed recycling loop at the end-of-life of rubber materials without compromising the performance. Integrating dynamic covalent bonds into chemically crosslinked networks provides a methodology to resolve the tradeoff between permanent cross-linking and plasticity.4,5 The dynamic covalent bonds can reversibly break and reform in response to external stimuli, which enables network rearrangements and consequently confers them with healing capability and reprocessability.6,7 Depending on the exchange mechanism, dynamic covalent bonds can function through dissociative and associative pathways.8 In striking contrast to © XXXX American Chemical Society

Received: March 1, 2019 Revised: April 28, 2019

A

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Routes for (a) synthesis of BDB and (b) preparation of the BEx networks by cross-linking ENR with BDB.

however, only limited studies have used them for “dry” polymers.22,29 Recently, the vitrimer concept has been implemented in rubber materials to impart them with the ability to be healed and recycled. For example, Cheng et al. designed and synthesized self-healing elastomers by incorporating dynamic hindered urea bonds as the building blocks into the networks.30 Stukenbroeker et al. reported a thermally malleable elastomer based on vinylogous urethane exchange by crosslinking an amino-functionalized polydimethylsiloxane with a bis-vinylogous urethane.31 However, most studies require either intricate molecular makeup or sophisticated modification of macromolecular chains.32−35 Aiming to achieve real applications, efforts have been made to prepare dynamic covalent networks from commercial elastomers without modifying their molecular structures.15,36 For instance, chemically cross-linked polybutadiene rubber could be imparted with malleability by incorporating specific catalysts to activate olefin metathesis15 and disulfide metathesis.37 Yet, the reported systems present poor mechanical properties. The inclusion of nanofillers can enhance the mechanical properties of vitrimers.38,39 Unfortunately, improvements in mechanical performance are achieved at the expense of dynamic properties because the chain mobility is restricted and network topology rearrangements are hindered by adding nanofillers.40,41 In this work, we synthesized a dithiol-bearing boronic ester and utilized it as a cross-linker for a commercial epoxidized natural rubber (ENR) based on the reaction between thiols and epoxy groups. Due to the dynamic features of boronic esters, the cross-linked networks are able to rearrange themselves and acquire healing capability and reprocessability. Besides, the prepared networks exhibit impressive mechanical properties, which can be further enhanced by incorporating the sacrificial metal−ligand coordination bonds without compromising their dynamic properties.

acid and 4.01 g of 1-thioglycerol were dissolved in the mixed solvent of 80 mL of tetrahydrofuran and 0.1 mL of water, into which 5.0 g of magnesium sulfate was added. The reaction system was stirred at room temperature for 24 h, followed by filtration and concentration. The resultant solid was repeatedly washed with dichloromethane to yield the target compound (5.0 g, 88%). 1H NMR, 13C NMR and 11B NMR spectra of BDB are shown in Figures S1−S3, respectively. Mass spectrometry result indicates the molecular weight of 310.08, which is consistent with the theoretical value (310.06). 2.3. Preparation of Cross-Linked ENR Networks. The desired amounts of BDB cross-linker and DMAP catalyst were mixed with ENR on a two-roll mill. Then, the compounds underwent hot pressing at 160 °C for an optimum time measured using a vulcameter. The cured samples were maintained at 80 °C for 48 h for annealing purposes. The preparation route is shown in Figure 1b. The BDB contents were 1.0, 3.0, 5.0, 7.0, and 10.0 wt % relative to ENR, and the quantity of DMAP was 25.5 mol % relative to BDB. In this context, the abbreviation of BEx refers to ENR cross-linked with x wt % of BDB. A reference network without boronic ester bonds was prepared by cross-linking ENR with 1,3-propanedithiol (1.7 wt % relative to ENR; the mole number is the same as that of BDB in BE5) following similar preparation protocols. 2.4. Integration of Sacrificial Coordination Bonds into BE5 Network. The network containing Zn2+−O coordination bonds was prepared by adding ZnCl2 into uncured BE5 compound and compression molding at 160 °C for optimum time. The contents of ZnCl2 were 0.5, 1.0, and 1.5 wt % relative to ENR. The code of BE5− y represents BE5 with y wt % of ZnCl2. 2.5. Characterizations. NMR spectra were collected on an Inova spectrometer (500 MHz) in CDCl3. Mass spectrometry was performed on a Bruker maXis impact equipped with an electrospray ionization source operating in positive ion mode. Fourier transform infrared (FTIR) tests were carried out using a Bruker Vertex 70 FTIR spectrometer. Raman measurements were conducted on a LabRAM HR800 Raman spectrometer with a He−Ne ion laser (632.81 nm) source. Tensile tests were carried out on a U-CAN UT-2060 machine. Young’s modulus was obtained from the slopes of tensile curves from 1 to 5% strain. Cyclic tensile tests were performed by stretching the samples to 300% strain and then relaxing at room temperature for a certain time prior to the following cycle. Details for equilibrium swelling experiments are described in the Supporting Information. Dynamic mechanical analysis (DMA) and stress relaxation tests were carried out using a TA Q800 instrument. DMA experiments were conducted under a dynamic strain of 0.5% and a frequency of 1 Hz by heating the samples from −80 to 150 °C at 3 °C/min. Stress relaxation experiments were conducted by monitoring the stress decay at a constant strain of 3% after equilibrating at required temperatures for 10 min.

2. EXPERIMENTAL SECTION 2.1. Materials. ENR (epoxidation degree of 50%, Mn = 100 178, PDI = 2.196) was supplied by the Agricultural Products Processing Research Institute, China. 1-Thioglycerol (98%), 4-dimethylaminopyridine (DMAP, 99%), 1,4-phenylenediboronic acid (98%), 1,3propanedithiol (98%), and zinc chloride (ZnCl2, 99%) were purchased from Sigma-Aldrich. 2.2. Synthesis of 2,2′-(1,4-Phenylene)-bis[4-mercaptan1,3,2-dioxaborolane] (BDB). Dithiol-bearing boronic ester crosslinker was prepared by reacting 1,4-phenylenediboronic acid with 1thioglycerol (Figure 1a). Typically, 3.0 g of 1,4-phenylenediboronic B

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules For self-healing tests, the samples were cut into two separate pieces, which were then brought together by gentle pressing of the overlapped surfaces to allow healing (30% humidity). Recycling was performed by cutting the samples into small pieces and remolding under 10 MPa pressure at 160 °C for 1 h.

MPa and from 1.60 to 14.63 MPa, respectively, and the elongation at break decreases from 811 to 475%. This is because an increase in the BDB content results in a more constrained network and a restricted chain mobility due to increased cross-linking density. The mechanical properties of the BEx series are significantly higher than those of the previously reported self-healing polymers (healing temperature ranging from 60 to 160 °C) (Figure 2b and Table S2). The origin of the impressive mechanical properties of the BEx samples may be due to the strain-induced crystallization behavior of the ENR matrix and the robust features of boronic ester bonds. According to the DMA results (Figure 2c), the storage modulus (E′) over the entire measured temperature range consistently increases, and the glass transition temperature (Tg) is gradually elevated with the increase in the BDB content, which are related to the formation of constrained networks. Additionally, the tan δ peak value consistently decreases with the BDB content, demonstrating an increase in the elasticity of BEx. It should be pointed out that E′ is almost constant and that no additional loss peak is observed above Tg, indicating that the number of chemical bonds remains unchanged and that the network retains its structural integrity at elevated temperatures. This is because the boronic ester transesterification follows an associative exchange pathway: the original cross-link will be disconnected only when a new one is formed. Considering the dynamic nature of boronic ester bonds, we envisage that the BEx networks can release stress and alter the topologies through transesterifications of boronic esters. Figure S8 shows the stress relaxation results for the BEx series at 160 °C. All of the samples can substantially release stress, implying that the networks can rearrange themselves at high temperatures. Comparatively, the reference network without boronic ester bonds exhibits limited stress relaxation due to intermolecular entanglements (Figure S9), providing convincing evidence that boronic ester bond is critical to the network rearrangement in the BEx networks. Obviously, the samples with higher cross-linking density release stress much slower and exhibit longer relaxation times (τ*, the time for releasing 63% of the initial stress). This is likely because more exchanges are needed to accomplish network rearrangements in the samples with higher cross-linking density. Moreover, an increased cross-linking density results in a more restricted chain mobility, which slows the diffusion of reactive groups and reduces the opportunities for the available sites to seek bond exchanges, thereby hindering bond reshuffling and depressing exchange reactions. Figure S10 shows that the stress relaxation rate is increased with temperature, which is because the thermoactivated boronic ester transesterifications are accelerated and the chain mobility is promoted at elevated temperatures.13 The relaxation times exhibit an Arrhenius dependence on temperature, providing evidence that the boronic ester transesterifications follow an associative pathway in the BEx networks (Figure S11). Accordingly, the activation energy is determined to be 26.2 kJ/mol, which approximates to the reported value for small-molecule boronic ester transesterifications.29 3.3. Healing Capability and Reprocessability of the BEx Samples. As revealed above, the bond reshuffling and network rearrangement can take place through boronic ester transesterifications, and thus the BEx samples are expected to

3. RESULTS AND DISCUSSION 3.1. Chemical Cross-Linking of ENR. The chemical cross-linking of ENR proceeds based on the chemical reaction between thiols of BDB and epoxy groups of ENR, which can be explicitly confirmed by FTIR spectra. Compared with the uncured sample and taking BE5 as an example, the absorption related to −SH completely disappeared after curing, demonstrating the chemical reaction between thiols and epoxy groups (Figure S4). The covalently cross-linked molecular architecture of the prepared BEx samples can be further evidenced by the fact that they are insoluble in organic solvents such as toluene (Figure S5). The cross-linking process can be monitored by measuring the torque value of the BEx compounds at 160 °C on a rheometer. As shown in Figure S6, the torque value first increases and then levels off with time. The increased torque is on account of the cross-linking of ENR. In addition, the maximum torque value consistently increases with BDB loading, which is indicative of an increase in the cross-linking density. Such conclusions can also be drawn from equilibrium swelling tests. As the BDB content increases, the cross-linking density of the BEx samples consistently increases, while the sol fraction and swelling ratio inversely decrease (Figure S7). 3.2. Mechanical Properties and Malleability of BEx. The representative tensile curves of BEx are shown in Figure 2a, and the mechanical properties are tabulated in Table S1. It is evident that the mechanical properties can be easily tuned by varying the BDB contents. Typically, increasing the BDB content leads to an enhancement in the Young’s modulus and the tensile strength and a reduction in the elongation at break. For example, compared to those of BE1, the Young’s modulus and the tensile strength of BE10 increase from 0.85 to 4.45

Figure 2. (a) Tensile curves for BEx series. (b) Ultimate strength and elongation at break of the previously reported self-healing polymer systems and the prepared BEx series. Poly(dimethyl siloxane) (PDMS),42−47 polyacrylate (PA),48,49 polysulfide (PSU),50,51 poly(propylene glycol) (PPG),52 olefin elastomer (OLE),36,37,53,54 and natural rubber (NR).55−57 (c) Dependence of E′ and tan δ versus temperature for the BEx series. C

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

can be reformed across the fracture surfaces in the sample with lower cross-linking density. As a comparison, the reference network without boronic ester linkages essentially cannot heal (Figure S14). Previous studies reported that the boronic ester-based polymers could be readily healed at room temperature by wetting the cut surfaces or healing them in high-humidity environment, since the boronic esters can be hydrolyzed into boronic acids and diols in the presence of water.22,58 Herein, we also tested the self-healing properties at room temperature by dabbing water onto the cut surfaces. While the healing efficiency was nearly identical to that obtained without the addition of water. This may be because the highly hydrophobic rubber matrix retards the infiltration of water to prevent the hydrolysis of boronic esters, as evidenced by the fact that no weight increase is observed when the samples are immersed in water for several days. Due to the associative nature of boronic ester transesterifications, the viscosity of the BEx networks follows an Arrhenius-like gradual variation, which enables them to be reshaped and reprocessed in a solid state. As a proof of concept, geometrically complex objects can be readily accessed by arbitrarily bending strip-shaped samples and allowing them to release stress at high temperatures without the use of a mold or accurate temperature control (Figure 4a). The newly formed samples are stable and maintain high elasticity even at 200 °C, revealing that the samples adapt to the new permanent shapes.

have the ability to be self-healed and recycled (Figure S12). The self-healing experiments were conducted by cutting the samples into two separate pieces, which were brought together for healing. To make the cut region more distinguishable, one piece was stained with black dye. As vividly shown in Figure 3a, after healing at 80 °C for 1 h, the healed sample can endure a large strain due to the re-established linkages across the interfaces.

Figure 3. (a) Photographs demonstrating the healing of BE5; tensile curves of BE5 healed (b) at different temperatures for 24 h and (c) at 80 °C for different times; and (d) healing efficiencies of the BEx samples healed at 80 °C for 24 h.

Healing efficiency, defined as the ratio of the mechanical properties of the healed sample to those of the original, is obtained from tensile measurements. The tensile curves of BE5 after healing at different temperatures for 24 h are shown in Figure 3b. In particular, when healed at room temperature for 24 h, the sample can still regain an elongation at break of 400% and a tensile strength of 2.52 MPa. Although the healing efficiencies are relatively low, the recovered properties can be comparable to the original properties of most of the reported self-healing polymers, which are very soft to ensure sufficient diffusion of polymer chains (Table S2). When the healing temperature increases to 80 °C, the mechanical properties of the healed BE5 are completely recovered. An increase in healing temperature leads to a higher healing efficiency, which is because the accelerated boronic ester transesterifications and faster chain mobility facilitate the healing process at elevated temperatures. Figure 3c shows the effects of healing time on the restoration of the mechanical performance of BE5 at 80 °C. As expected, the healing efficiencies are increased by prolonging the healing time. Healed BE5 can recover approximately 30% of the initial mechanical properties after 1 h and 98% after 24 h. The tensile curves of other BEx samples healed at different temperatures and for varied time periods are displayed in Figure S13. Figure 3d compares the healing efficiencies of BEx after healing at 80 °C for 24 h. All BEx samples exhibit excellent healing efficiencies above 85%, and the highest healing efficiencies are observed for BE3 and BE5 with moderate cross-linking densities. This is because the wetting and diffusion of the rubber chains in the sample having higher cross-linking density are hindered at the interface as a result of restricted chain mobility, while less amount of covalent bonds

Figure 4. Photographs demonstrating (a) the reshaping and (b) recycling of BE5. The scale bar represents 1 cm. (c) Recovery ratios of the mechanical properties for the recycled BEx samples. (d) Tensile curves of BE3 after multiple cycles of recycling.

To verify the reprocessability, the BEx samples were cut into small chips and then hot-pressed at 160 °C for 1 h. New coherent and homogeneous samples were obtained (Figure 4b). The recovery ratios of the mechanical properties for the recycled samples are shown in Figures 4c and S15. It can be seen that most of the mechanical properties are restored after reprocessing. For example, the recovery ratios of tensile strength, Young’s modulus, and elongation at break for BE5 are 70, 120, and 90%, respectively. In addition, the samples are capable of being repeatedly reprocessed due to the robust feature of boronic ester bonds (Figure 4d). It should be noted that the Young’s modulus of the recycled sample is slightly increased. Considering that the BEx samples are thermally stable during reprocessing (Figures S16 and S17), the increased modulus may be due to side reactions, such as the D

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. (a) ENRx−y networks with dual cross-links of dynamic boronic ester bonds and noncovalent Zn2+−O coordination bonds. (b) Reversible breaking and reforming of Zn2+−O coordination bonds during loading−unloading tests.

formation of ether bonds by the homopolymerization of residual epoxy groups during reprocessing.59 3.4. Integration of Sacrificial Bonds into the Networks. The incorporation of noncovalent sacrificial bonds has been demonstrated as a promising methodology to enhance the mechanical properties of biological materials.60,61 At molecular level, the noncovalent sacrificial bonds are able to withstand load at small deformations but break prior to covalent bonds and undergo reversible bond breakage/ reformation when a load is applied, which provides an efficient energy-dissipating mechanism for reinforcement.62,63 Learning from nature, intense efforts have been devoted to transferring sacrificial bond principles into man-made polymers.64,65 Herein, aiming to further enhance the mechanical properties of the BEx samples, we engineered noncovalent Zn2+−O coordination bonds into the BEx networks by introducing ZnCl2 during the compounding process. The formation of coordination between Zn2+ and epoxy groups is convincingly evidenced by FTIR (Figure S18) and Raman (Figure S19) spectra of the ENR-ZnCl2 model compound. Therefore, the resulting BEx−y networks are held together by dual cross-links of boronic ester linkages and Zn2+−O coordination bonds, as schematically illustrated in Figure 5a. Specifically, the dynamic covalent cross-links of boronic esters impart elasticity, and the noncovalent Zn2+−O coordination bonds act as sacrificial cross-links that can reversibly break and reform to dissipate energy. To verify the sacrificial and reversible nature of Zn2+−O coordination bonds, cyclic tensile tests were performed on the BE5−y series. As shown in Figure 6a, compared with BE5−0, BE5−y series with Zn2+−O coordination bonds exhibit larger hysteresis (area surrounded by loading−unloading curves), indicating enormous energy dissipation due to the rupture of coordination bonds during stretching. The higher the Zn2+−O coordination bond content, the larger is the amount of dissipated energy. As demonstrated in Figure 6b, taking BE5− 1.0 as an example, the hysteresis loop is much smaller in the successive cycle. This is possibly because parts of the ruptured coordination bonds cannot fully reform in the time scale of an individual cycle; thus, fewer coordination bonds can participate in energy dissipation in the following cycles. When the samples are allowed to relax for a certain time period prior to subsequent cycle, the loading curves gradually recover to the first one as the waiting time increases. After storing the sample at 80 °C for 5 min, the loading−unloading curves completely overlap with the first cycle, which is because the dynamic of the coordination bonds is accelerated at elevated temperature, leading to the complete reformation of coordination bonds.

Figure 6. (a) Loading−unloading cycles for BE5−y with different ZnCl2 contents. (b) Loading−unloading cycles for BE5−1. Prior to the subsequent cycle, the samples are allowed to relax at room temperature for different time intervals and at 80 °C for 5 min. (c) Tensile curves of BE5−y with various ZnCl2 contents. (d) Comparison of the tensile curves of the original, healed, and recycled BE5−1.0 samples.

These findings demonstrate that the coordination bonds behave in a sacrificial and reversible manner to dissipate energy during stretching, while the covalent framework survives and imparts elasticity (Figure 5b). The tensile curves of the BE5−y series are displayed in Figure 6c. The incorporation of coordination bonds gives rise to a slight increase in the ultimate strength and remarkable improvements in the Young’s modulus and tensile modulus (stress at 100% strain). For example, the Young’s modulus and tensile modulus of BE5−1.5 are increased by about 400 and 250%, respectively, when compared with those of BE5−0 due to the contribution of coordination bonds to the stiffness. Interestingly, the mechanical properties are improved without compromising the dynamic properties, such as the self-healing capability and reprocessability. The stress relaxation curves for the BE5−y series with different ZnCl2 contents are almost overlapped with that of BE5−0 (Figure S20). The healed and recycled BE5−y series can recover most of the mechanical properties of the original one (Figure 6d), and their healing efficiencies upon healing and recovery ratios upon recycling are nearly identical to those of BE5−0 (Figures S21 and S22). Generally, there is an inherent compromise between the dynamic properties and the mechanical properties due to the E

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

■

different requirements of chain mobility for these two demands. Herein, such a dilemma is resolved by incorporating sacrificial noncovalent bonds into dynamic covalent networks. The coordination bonds can enhance the mechanical properties of the covalent networks at service temperature, while they dissociate at elevated temperatures and thus exert little restriction on chain mobility to affect the dynamic properties.

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.T.). *E-mail: [email protected] (B.G.). ORCID

Baochun Guo: 0000-0002-4734-1895 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS We demonstrate a simple approach to prepare mechanically robust, self-healing, and recyclable elastomeric vitrimers by programming dynamic dual cross-links of boronic ester bonds and Zn2+−O coordination into a commercial ENR. The ENR is covalently cross-linked with BDB based on the chemical reaction between thiols and epoxy groups. The prepared networks exhibit impressive mechanical properties, which can be easily tuned by changing the boronic ester cross-linker contents. Due to boronic ester transesterifications, the networks can relax the stress and alter the topologies, enabling them to heal, reshape, and recycle. The networks can restore more than 85% of their original mechanical properties after healing at 80 °C for 24 h, and they retain most of the mechanical performance even after multiple generations of recycling events. The integration of sacrificial Zn2+−O coordination bonds into the networks leads to remarkable improvements on the modulus without compromising the selfhealing capability and reprocessability because the noncovalent coordination bonds contribute to the stiffness at service temperature and dissociate at elevated temperatures where network rearrangement occurs. We envisage this work offers a straightforward approach to prepare elastomeric vitrimers with an integration of excellent self-healing and reprocessing capabilities and impressive mechanical performance for real applications.

■

■

ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (51825303), the National Natural Science Foundation of China (51790503, 51703064, and 51673065), and the Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2016TQ03C734).

■

REFERENCES

(1) Imbernon, L.; Norvez, S. From landfilling to vitrimer chemistry in rubber life cycle. Eur. Polym. J. 2016, 82, 347−376. (2) Polgar, L. M.; Duin, M. V.; Broekhuis, A. A.; Picchioni, F. Use of Diels−Alder chemistry for thermoreversible cross-linking of rubbers: the next step toward recycling of rubber products? Macromolecules 2015, 48, 7096−7105. (3) Fang, Y.; Zhan, M.; Wang, Y. The status of recycling of waste rubber. Mater. Design 2001, 22, 123−128. (4) Kloxin, C. J.; Bowman, C. N. Covalent adaptable networks: smart, reconfigurable and responsive network systems. Chem. Soc. Rev. 2013, 42, 7161−7173. (5) Jin, Y. H.; Yu, C.; Denman, R. J.; Zhang, W. Recent advances in dynamic covalent chemistry. Chem. Soc. Rev. 2013, 42, 6634−6654. (6) Roy, N.; Bruchmann, B.; Lehn, J. M. Dynamers: dynamic polymers as self-healing materials. Chem. Soc. Rev. 2015, 44, 3786− 3807. (7) Zou, W. K.; Dong, J. T.; Luo, Y. W.; Zhao, Q.; Xie, T. Dynamic covalent polymer networks: from old chemistry to modern day innovations. Adv. Mater. 2017, 29, No. 1606100. (8) Denissen, W.; Winne, J. M.; Prez, F. E. D. Vitrimers: permanent organic networks with glass-like fluidity. Chem. Sci. 2016, 7, 30−38. (9) Berto, P.; Pointet, A.; Coz, C. L.; Grelier, S.; Peruch, F. Recyclable Telechelic Cross-Linked Polybutadiene Based on Reversible Diels−Alder Chemistry. Macromolecules 2018, 51, 651−659. (10) Scott, T. F.; Schneider, A. D.; Cook, W. D.; Bowman, C. N.; et al. Photoinduced plasticity in cross-linked polymers. Science 2005, 308, 1615−1617. (11) Amamoto, Y.; Kamada, J.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units. Angew. Chem., Int. Edit. 2011, 50, 1660−1663. (12) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silicalike malleable materials from permanent organic networks. Science 2011, 334, 965−968. (13) Capelot, M.; Unterlass, M. M.; Tournilhac, F.; Leibler, L. Catalytic control of the vitrimer glass transition. ACS Macro Lett. 2012, 1, 789−792. (14) Smallenburg, F.; Leibler, L.; Sciortino, F. Patchy particle model for vitrimers. Phys. Rev. Lett. 2013, 111, No. 188002. (15) Lu, Y. X.; Guan, Z. B. Olefin metathesis for effective polymer healing via dynamic exchange of strong carbon-carbon double bonds. J. Am. Chem. Soc. 2012, 134, 14226−14231. (16) Hendriks, B.; Waelkens, J.; Winne, J. M.; Du Prez, F. E. Poly(thioether) vitrimers via transalkylation of trialkylsulfonium salts. ACS Macro Lett. 2017, 6, 930−934. (17) Obadia, M. M.; Mudraboyina, B. P.; Serghei, A.; Montarnal, D.; Drockenmuller, E. Reprocessing and recycling of highly cross-linked

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00419. 1

H NMR, 13H NMR, and 11B NMR spectra of crosslinker; FTIR spectra of BDB, neat ENR, uncured BE5 compound, and cured BE5 sample; photographs of BE5 swollen in toluene; cross-linking kinetics curves of BEx samples; cross-linking density, sol fraction and swelling ratio for BEx series; stress relaxation curves of BEx series at 160 °C; stress relaxation curves of BE5 and control sample; fitting of relaxation time to temperature; schematic of network rearrangement; tensile curves of the BEx series healed at different temperatures and for varied times; tensile curves of reference sample heated at 80 °C for 24 h; tensile curves of the recycled BEx series; thermogravimetric analyzer results for BEx samples; FTIR and Raman spectra of ENR-ZnCl2 model compound; healing efficiency and recovery ratios of the BE5−y series; stress relaxation curves for the BE5−y series at 160 °C; mechanical performance of the BEx series; comparison of the mechanical performances between other self-healing systems and the BEx series (PDF) F

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules ion-conducting networks through transalkylation exchanges of C-N bonds. J. Am. Chem. Soc. 2015, 137, 6078−6083. (18) Lei, Z. Q.; Xiang, H. P.; Yuan, Y. J.; Rong, M. Z.; Zhang, M. Q. Room-temperature self-healable and remoldable cross-linked polymer based on the dynamic exchange of disulfide bonds. Chem. Mater. 2014, 26, 2038−2046. (19) Yang, W. J.; Tao, X.; Zhao, T.; Weng, L.; Kang, E.-T.; Wang, L. Antifouling and antibacterial hydrogel coatings with self-healing properties based on a dynamic disulfide exchange reaction. Polym. Chem. 2015, 6, 7027−7035. (20) Zheng, P. W.; 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, 2024−2027. (21) Cromwell, O. R.; Chung, J.; Guan, Z. Malleable and self-healing covalent polymer networks through tunable dynamic boronic ester bonds. J. Am. Chem. Soc. 2015, 137, 6492−6495. (22) Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S. Roomtemperature self-healing polymers based on dynamic-covalent boronic esters. Macromolecules 2015, 48, 2098−2106. (23) Taynton, P.; Ni, H. G.; Zhu, C. P.; Yu, K.; Loob, S.; Jin, Y. H.; Qi, H. J.; Zhang, W. Repairable woven carbon fiber composites with full recyclability enabled by malleable polyimine networks. Adv. Mater. 2016, 28, 2904−2909. (24) Sims, M. B.; Patel, K. Y.; Bhatta, M.; Mukherjee, S.; Sumerlin, B. S. Harnessing imine diversity to tune hyperbranched polymer degradation. Macromolecules 2018, 51, 356−363. (25) Zheng, N.; Fang, Z. Z.; Zou, W. K.; Zhao, Q.; Xie, T. Thermoset shape-memory polyurethane with intrinsic plasticity enabled by transcarbamoylation. Angew. Chem., Int. Ed. 2016, 55, 11421−11425. (26) Nishimura, Y.; Chung, J.; Muradyan, H.; Guan, Z. B. Silyl ether as a robust and thermally stable dynamic covalent motif for malleable polymer design. J. Am. Chem. Soc. 2017, 139, 14881−14884. (27) Bull, S. D.; Davidson, M. G.; Jm, V. D. E.; Fossey, J. S.; Jenkins, A. T.; Jiang, Y. B.; Kubo, Y.; Marken, F.; Sakurai, K.; Zhao, J.; et al. Exploiting the reversible covalent bonding of boronic acids: recognition, sensing, and assembly. Acc. Chem. Res. 2013, 46, 312− 326. (28) Deng, C. C.; Brooks, W. L. A.; Abboud, K. A.; Sumerlin, B. S. Boronic acid-based hydrogels undergo self-healing at neutral and acidic pH. ACS Macro Lett. 2015, 4, 220−224. (29) Röttger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.; Nicolaÿ, R.; Leibler, L. High-performance vitrimers from commodity thermoplastics through dioxaborolane metathesis. Science 2017, 356, 62−65. (30) Ying, H. Z.; Zhang, Y. F.; Cheng, J. J. Dynamic urea bond for the design of reversible and self-healing polymers. Nat. Commun. 2014, 5, No. 3218. (31) Stukenbroeker, T.; Wang, W.; Winne, J. M.; Du Prez, F. E.; Nicolay, R.; Leibler, L. Polydimethylsiloxane quenchable vitrimers. Polym. Chem. 2017, 8, 6590−6593. (32) Pan, X. Q.; Driessen, F.; Zhu, X. L.; Du Prez, F. E. Selenolactone as a building block toward dynamic diselenidecontaining polymer architectures with controllable topology. ACS Macro Lett. 2017, 6, 89−92. (33) Wu, J. R.; Cai, L. H.; Weitz, D. Tough self-healing elastomers by molecular enforced integration of covalent and reversible networks. Adv. Mater. 2017, 29, No. 1702616. (34) Kim, S. M.; Jeon, H.; Shin, S. H.; Park, S. A.; Jegal, J.; Hwang, S. Y.; Oh, D. X.; Park, J. Superior toughness and fast self-healing at room temperature engineered by transparent elastomers. Adv. Mater. 2018, 30, No. 1705145. (35) Ji, S. B.; Cao, W.; Yu, Y.; Xu, H. P. Visible-light-induced selfhealing diselenide-containing polyurethane elastomer. Adv. Mater. 2015, 27, 7740−7745. (36) Chen, Y.; Tang, Z. H.; Zhang, X. H.; Liu, Y. J.; Wu, S. W.; Guo, B. C. Covalently cross-linked elastomers with self-healing and malleable abilities enabled by boronic ester bonds. ACS Appl. Mater. Interfaces 2018, 10, 24224−24231.

(37) 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, 4315−4325. (38) Yang, Z.; Wang, Q.; Wang, T. Dual-triggered and thermally reconfigurable shape memory graphene-vitrimer composites. ACS Appl. Mater. Interfaces 2016, 8, 21691−21699. (39) Yang, Y.; Pei, Z.; Zhang, X.; Tao, L.; Wei, Y.; Ji, Y. Carbon nanotube−vitrimer composite for facile and efficient photo-welding of epoxy. Chem. Sci. 2014, 5, 3486−3492. (40) Legrand, A.; Souliéziakovic, C. Silica−epoxy vitrimer nanocomposites. Macromolecules 2016, 49, 5893−5902. (41) Liu, Y. J.; Tang, Z. H.; Chen, Y.; Zhang, C. F.; Guo, B. C. Engineering of beta-hydroxyl esters into elastomer-nanoparticle interface toward malleable, robust, and reprocessable vitrimer composites. ACS Appl. Mater. Interfaces 2018, 10, 2992−3001. (42) Xu, W.; Huang, L. B.; Hao, J. Fully self-healing and shapetailorable triboelectric nanogenerators based on healable polymer and magnetic-assisted electrode. Nano Energy 2017, 40, 399−407. (43) Mei, J. F.; Jia, X. Y.; Lai, J. C.; Sun, Y.; Li, C. H.; Wu, J. H.; Cao, Y.; You, X. Z.; Bao, Z. A highly stretchable and autonomous selfhealing polymer based on combination of Pt···Pt and π-π interactions. Macromol. Rapid Commun. 2016, 37, 1667−1675. (44) Zhao, J.; Xu, R.; Luo, G.; Wu, J.; Xia, H. A self-healing, remoldable and biocompatible crosslinked polysiloxane elastomer. J. Mater. Chem. B 2016, 4, 982−989. (45) Liu, L.; Liang, S.; Huang, Y.; Hu, C.; Yang, J. A stretchable polysiloxane elastomer with self-healing capacity at room temperature and solvatochromic properties. Chem. Commun. 2017, 53, 12088− 12091. (46) Jia, X. Y.; Mei, J. F.; Lai, J. C.; Li, C. H.; You, X. Z. A highly stretchable polymer that can be thermally healed at mild temperature. Macromol. Rapid Commun. 2016, 37, 952−956. (47) Jia, X. Y.; Mei, J. F.; Lai, J. C.; Li, C. H.; You, X. Z. A selfhealing PDMS polymer with solvatochromic properties. Chem. Commun. 2015, 51, 8928−8930. (48) Zhang, B.; Digby, Z. A.; Flum, J. A.; Chakma, P.; Saul, J. M.; Sparks, J. L.; Konkolewicz, D. Dynamic Thiol−Michael chemistry for thermoresponsive rehealable and malleable networks. Macromolecules 2016, 49, 6871−6878. (49) Hohlbein, N.; Shaaban, A.; Bras, A. R.; Pyckhout-Hintzen, W.; Schmidt, A. M. Self-healing dynamic bond-based rubbers: understanding the mechanisms in ionomeric elastomer model systems. Phys. Chem. Chem. Phys. 2015, 17, 21005−21017. (50) Gao, W.; Bie, M.; Liu, F.; Chang, P.; Quan, Y. Self-Healable and reprocessable polysulfide sealants prepared from liquid polysulfide oligomer and epoxy resin. ACS Appl. Mater. Interfaces 2017, 9, 15798−15808. (51) Canadell, J.; Goossens, H.; Klumperman, B. Self-Healing materials based on disulfide links. Macromolecules 2011, 44, 2536− 2541. (52) Liu, J.; Liu, Y.; Wang, Y.; Zhu, J.; Yu, J.; Hu, Z. Disulfide bonds and metal-ligand co-crosslinked network with improved mechanical and self-healing properties. Mater. Today Commun. 2017, 13, 282− 289. (53) Liu, X.; Lu, C.; Wu, X.; Zhang, X. Self-healing strain sensors based on nanostructured supramolecular conductive elastomers. J. Mater. Chem. A 2017, 5, 9824−9832. (54) Kuang, X.; Liu, G.; Dong, X.; Wang, D. Enhancement of mechanical and self-healing performance in multiwall carbon nanotube/rubber composites via Diels-Alder bonding. Macromol. Mater. Eng. 2016, 301, 535−541. (55) Hernández, M.; Grande, A. M.; Dierkes, W.; Bijleveld, J.; van der Zwaag, S.; García, S. J. Turning vulcanized natural rubber into a self-healing polymer: effect of the disulfide/polysulfide ratio. ACS Sustainable Chem. Eng. 2016, 4, 5776−5784. (56) Xu, C.; Cao, L.; Huang, X.; Chen, Y.; Lin, B.; Fu, L. Self-healing natural rubber with tailorable mechanical properties based on ionic G

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules supramolecular hybrid network. ACS Appl. Mater. Interfaces 2017, 9, 29363−29373. (57) Xu, C.; Cao, L.; Lin, B.; Liang, X.; Chen, Y. Design of selfhealing supramolecular rubbers by introducing ionic cross-links into natural rubber via a controlled vulcanization. ACS Appl. Mater. Interfaces 2016, 8, 17728−17737. (58) Bao, C.; Jiang, Y. J.; Zhang, H.; Lu, X.; Sun, J. Roomtemperature self-healing and recyclable tough polymer composites using nitrogen-coordinated boroxines. Adv. Funct. Mater. 2018, 28, No. 1800560. (59) Tang, Z.; Liu, Y.; Guo, B.; Zhang, L. Malleable, mechanically strong, and adaptive elastomers enabled by interfacial exchangeable bonds. Macromolecules 2017, 50, 7584−7592. (60) Sun, T. L.; Kurokawa, T.; Kuroda, S.; Bin Ihsan, A.; Akasaki, T.; Sato, K.; Haque, M. A.; Nakajima, T.; Gong, J. P. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat. Mater. 2013, 12, 932−937. (61) 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.; Nan, B. Z. A highly stretchable autonomous self-healing elastomer. Nat. Chem. 2016, 8, 618−624. (62) Chung, J.; Kushner, A. M.; Weisman, A. C.; Guan, Z. Direct correlation of single-molecule properties with bulk mechanical performance for the biomimetic design of polymers. Nat. Mater. 2014, 13, 1055−1062. (63) Nabavi, S. S.; Harrington, M. J.; Fratzl, P.; Hartmann, M. A. Influence of sacrificial bonds on the mechanical behaviour of polymer chains. Bioinspired, Biomimetic Nanobiomater. 2014, 3, 139−145. (64) Zhou, X.; Guo, B.; Zhang, L.; Hu, G. H. Progress in bioinspired sacrificial bonds in artificial polymeric materials. Chem. Soc. Rev. 2017, 46, 6301−6329. (65) 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.

H

DOI: 10.1021/acs.macromol.9b00419 Macromolecules XXXX, XXX, XXX−XXX