Covalently Cross-Linked Elastomers with Self-Healing and Malleable

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Applications of Polymer, Composite, and Coating Materials

Covalently Cross-linked Elastomers with Self-Healing and Malleable Abilities Enabled by Boronic Ester Bonds Yi Chen, Zhenghai Tang, Xuhui Zhang, Yingjun Liu, Siwu Wu, and Baochun Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09863 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Applied Materials & Interfaces

Covalently Cross-linked Elastomers with Self-Healing and Malleable Abilities Enabled by Boronic Ester Bonds Yi Chen, Zhenghai Tang,* Xuhui Zhang, Yingjun Liu, Siwu Wu, Baochun Guo* Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China KEYWORDS: rubber, dynamic covalent bond, boronic ester bond, self-healing, reshape, recycling ABSTRACT: Covalently cross-linked rubbers are renowned for their high elasticity that play an indispensable role in various applications including tires, seals, medical implants. Development of self-healing and malleable rubbers is highly desirable as it allows for damage repair and reprocessibility to extend the lifetime and alleviate environmental pollution. Herein, we propose a facile approach to prepare permanently cross-linked yet self-healing and recyclable diene-rubber by programming dynamic boronic ester linkages into the network. The network is synthesized through one-pot thermally initiated thiol-ene “click” reaction between a novel dithiol-containing boronic ester cross-linker and commonly used styrene-butadiene rubber (SBR) without modifying the macromolecular structure. The resulted samples are covalently cross-linked and possess relatively high mechanical strength which can be readily tailored by varying boronic ester content. Owning to the transesterification of boronic ester bonds, the samples can alter network topologies, endowing the materials with

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self-healing ability and malleability.

1. INTRODUCTION Rubbers have been considered as strategically important materials, because they are extensively applied in tires, seals, medical implants and shock absorbers due to their high extensibility and capability of recovering the original shape after releasing stress. 1, 2

To acquire high elasticity and chemical resistance, covalent cross-linking of rubber

networks is essential.3 However, the irreversible cross-links make the cross-linked rubbers be incapable of self-healing upon injured or damaged in practice, which severely degrades their functionality and lifetime. In addition, it is inherently difficult to recycle and reprocess the end-of-life rubber products, posing an intractable obstacle encountered in covalently cross-linked molecular architectures. To date, bulk of the scrapped rubber products is incinerated for energy recovery in low grade form and dumped as landfills, resulting in serious environmental issue and resource waste.4, 5 Although thermal and mechanical treatments have been employed to break the sulfide-based cross-linking bonds in cross-linked networks for their recycling, such reclamation method is high energy consumption and is detrimental for the mechanical performance due to the inevitable scission of polymer backbone. Therefore, it is highly desirable to impart the permanently cross-linked rubbers with self-healing ability and malleability that allow for damage repair and recycling.

Recently, great efforts have been devoted to introducing elegant dynamic motifs (dynamic covalent bonds6-8 and noncovalent bonds9-12) into covalently cross-linked


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networks. These bonds are able to undergo reversible breaking and reformation upon external stimuli, thereby endowing the networks with dynamic properties including self-healing capability, malleability, and adaptability.13-17 In contrast to noncovalent bonds, reversible covalent bonds are particularly promising in the design of dynamic polymers towards structural applications because they typically possess high bond strength and stability.16, 18 Nowadays, quite a few exchangeable reactions including Diels-Alder chemistry,13, 19 imine exchange,20, 21 disulfide exchange,22 alkoxyamine chemistry,23 siloxane equilibration,24 transesterification,25 transamination,26, transcarbamoylation,

28, 29

27

transalkylation,30 and boronic ester exchange31-33 are

developed in the context of dynamic polymers, aiming to tuning their performance and broadening their application scopes. For examples, Lehn et al. synthesized a self-repairing polymer based on the reversibility of acylhydrazone bonds through condensation of siloxane-based dialdehyde and bisacylhydrazone.34 Cheng and co-workers reported that hindered urea bonds bearing bulky N-substituent could be incorporated as building blocks into polyureas and poly(urethane-urea)s, imparting them catalyst-free dynamic properties.35

Despite of these burgeoning progress, most studies focus on tailor-made polymers that have relatively poor mechanical properties and often require multistep preparation of starting functional polymer precursors as well as intricate molecular makeup, which are not suitable for practical applications. In particular, only a few examples of dynamic covalent bonds have been applied into commercial elastomers, especially for the most widely used diene-rubbers. For instance, self-healing ability



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and malleability were imparted into covalently cross-linked polybutadiene rubber by adding specific catalysts that enabled olefin metathesis36 or disulfide exchange.22 Thermo-reversible ethylene-propylene rubber was prepared by grafting furan group along rubber backbone and subsequently cross-linking with bismaleimide via Diels-Alder

coupling

reaction.37

We

previously

reported

that

epoxy

group-functionalized natural rubber was cross-linked with carboxyl group-containing nanoparticles, yielding exchangeable β-hydroxyl esters in the interphase that enabled network rearrangements via a catalytic transesterification reaction and conferred it the ability to be recycled.38 However, these trails require either the chemical modification of macromolecular chains or the addition of catalyst to access exchange reaction. Considering that only carbon-carbon double bonds can be used as active sites for chemical modification and cross-linking, it is not trivial to construct dynamic covalent bonds into diene-rubbers. In the present work, our basic philosophy is to develop a simple approach to prepare self-healing and malleable elastomers based on commercial diene-rubbers without modifying their molecular structures or adding catalyst, which is conducive to practical use.

Boronate ester bonds yielded from the condensation of boronic acids with diols are a kind of thermally stable and robust dynamic covalent bonds.39 Previous studies detailed the dynamic equilibrium between diols/boronic acids and boronate esters in solvents,

which

enabled

bond

rearrangement

through

reversible

hydrolysis/reformation and was utilized to prepare self-healing materials31,

40

and

molecular sensors39; however, such function mechanism is largely limited to solution


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based systems. Very recently, Leibler et al.32 and Guan et al.33 reported that boronic ester bonds could undergo associative exchange reactions to rearrange network topology in bulk polymers in the absence of solvents.

Herein, we report a straightforward approach to engineer reversible boronic ester bonds into a commercial styrene-butadiene rubber (SBR). For this purpose, a novel boronic ester bearing two thiol groups is synthesized and added into SBR by directly mechanical mixing. The boronic ester can serves as cross-linking site via thermally initiated thiol-ene “click” reaction between thiols of as-prepared boronic ester and pendent vinyl groups of SBR chains. Due to reversibility of boronic ester linkages, the covalently cross-linked samples are capable of relaxing stress and being self-healing and malleable. The main novelties for our present work include (1) the starting polymer is widely used in our daily life and does not require any chemical modification, (2) it is a one-pot and highly efficient way to introduce boronic ester bonds into the network via thiol-ene “click” reaction, (3) the resulted samples are covalently cross-linked and combine the features of high strength, self-healing ability and reprocessibility.

2. EXPERIMENTAL SECTION Materials. Commercial SBR (styrene content = 27 wt%, vinyl content = 40 wt%, 1H NMR spectrum in Figure S1) was obtained from Yanshan Petrochemical Co. Ltd., Sinopec, China. 1-Thioglycerol (98%), benzene-1,4-diboronic acid (98%) and 1,3-propanedithiol (98%) were supplied by Sigma Aldrich. Tetrahydrofuran, heptane



and magnesium sulfate were analytically pure and purchased from Tianjin Fuyu Fine Chemical Co. Ltd.

Synthesis of 2,2'-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane] (BDB). The cross-linker, a dithiol-containing boronic ester, was synthesized from the complexation between benzene-1,4-diboronic acid and 1-thioglycerol. Briefly, benzene-1,4-diboronic acid (3.0 g, 18.1 mmol) and 1-thioglycerol (4.01 g, 37.1 mmol) were dissolved in THF (80 mL) and water (0.1 mL), to which magnesium sulfate (5.0 g) was added. After stirring at room temperature for 24 h, the mixture was filtered and concentrated. Thereafter, the resulted solid was purified by repeatedly filtering and washing with abundant heptanes, and concentrating to obtain target compound as white solid (5.0 g, 88%) (Scheme 1a). The successful synthesis of BDB is explicitly confirmed by 1H NMR (Figure S2). 1

H-NMR (CDCl3, 500 MHz): BDB-δ 7.83 (s, 4H), 4.74 (m, 2H), 4.49 (dd, J=8 Hz, 7

Hz, 2H), 4.18 (dd, J=13 Hz, 5.5 Hz, 2H), 2.82 (dd, J=7.5 Hz, 5 Hz, 4H), 1.49 (t, J=7.5 Hz, 2H). Preparation of BDB-Crosslinked SBR Samples. A desired amount of BDB was directly blended with SBR on an open two-roll mill. The resulted compound was then compression molded at 160 oC for optimum curing time measured using a vulcameter. During hot press, thiol groups of BDB were coupled onto the pendent vinyl groups of SBR through thermally initiated thiol-ene “click” reaction, leading to the covalent cross-linking of SBR (Scheme 1b). The content of BDB was controlled to be 1.0, 2.0, 3.0, 4.0 and 5.0 phr. In the text, sample code of BSx represents BDB-crosslinked SBR ACS Paragon Plus Environment

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with x phr BDB.

For comparison, a control sample without boronic ester linkages was cross-linked with 1,3-propanedithiol (1.0 phr, the mole number is equal to that of BDB in BS3) according to the aforementioned preparation protocols.

Scheme 1. A schematic of (a) synthesis of BDB and (b) cross-linking of SBR with BDB via thermally initiated thiol-ene “click” reaction. Characterizations. 1H NMR spectra were recorded at 500 MHz on Inova spectrometer in CDCl3. Fourier transform infrared spectra (FTIR) were monitored on a Bruker Vertex 70 FTIR spectrometer with attenuated total reflectance mode. Cross-linking kinetics were determined at 160 oC on a U-CAN UR-2030 vulcameter. Mechanical properties were measured using a U-CAN UT-2060 instrument at room temperature with a 500 mm/min strain rate. Young’s moduli were determined from tensile curves with a strain from 1 to 5%. Cyclic tensile tests were conducted by stretching the specimen to 100% strain, followed by unloading process with a 100



mm/min strain rate. In each cycle interval, the sample was settled for 10 min prior to the subsequent cycle. The tests including dynamic mechanical analysis (DMA) and stress relaxation measurements were performed on a TA-Q800 DMA apparatus. DMA tests were conducted under a tensile mode with a dynamic strain of 0.5%. The temperature is increased from -80 to 150 °C at 3 °C/min and frequency is 5 Hz. For stress relaxation measurements, a constant strain 3% was applied and the stress decay was monitored over time. Sol fraction, swelling ratio and cross-link density were determined based on equilibrium swelling tests by immersing the cured samples in toluene.41 The detail procedures are presented in the Supporting Information. Self-healing experiments were performed by cutting film samples into two pieces with a razor blade and putting them together for healing at varied temperatures for different durations. Healing efficiencies were determined as the ratio of tensile strength or breaking strain of the healed samples to those of the original one. Recycling of the samples was carried out by cutting them into small chips and compression molding under 10 MPa at 160 °C for 1 h. Mechanical property values in self-healing and recycling tests were presented as the mean and standard deviation based on at least five trials.

3. RESULTS AND DISCUSSION Covalently Cross-Linking of SBR using BDB. Boronic ester-containing SBR networks are prepared according to Scheme 1b. BDB with two thiol groups is


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expected to be utilized as cross-linker for SBR through thermally initiated thiol-ene “click” reaction, which is very efficient and simple in material synthesis and modification.42 The coupling reaction between thiols of BDB and vinyl groups of SBR was revealed by FTIR spectra (Figure 1). In the FTIR spectrum of BDB, the absorptions at 2563 and 1219 cm-1 can be ascribed to the stretching vibrations of -SH and B-O, respectively. In the case of SBR, the absorption peaks at 1450, 967, 910 cm−1 can be ascribed to the bending vibrations of -CH2-, -CH=CH-, and >C=CH2, respectively. After mixing BDB with SBR but prior to curing, taking the compound with 5 phr BDB as an example, the uncured BS5 exhibits a superimposed spectrum of SBR and BDB. Compared with uncured BS5, the absorption associated with thiol groups completely disappears in cured BS5. Moreover, taking the absorption at 1450 cm−1 as a reference, the ratio of peak intensity between 910 and 1450 cm-1 (I(910/1450)) decreases in cured BS5 relative to uncured BS5, while that between 967 and 1450 cm-1 (I(967/1450)) is almost unchanged. These findings explicitly confirm that the coupling reaction take place between pendent vinyl groups and thiol groups while the carbon-carbon double bonds in SBR backbone do not take participate in the reaction.



Figure 1. FTIR spectra BDB, SBR, uncured BS5 and cured BS5. The accessible thiol-ene reaction is believed to enable the covalent cross-linking of SBR with BDB through boronic linkages. Equilibrium swelling experiments show that the as-prepared BS samples cannot be soluble in their good solvent (Figure S3), indicating the covalently cross-linked nature of the networks. The cross-link density, Ve, of BS samples is consistently increased with increasing BDB content (Figure 2a), revealing that a denser network is formed. Consequently, the sol fraction and swelling ratio are monotonously decreased with BDB loading. Cross-linking kinetics for BS compounds were followed by measuring the torque at 160 oC in a rheometer (Figure 2b). After an initial decrease in the torque due to heating-induced softening, a sudden increase in the torque is observed, which is originated from the cross-linking of SBR with BDB. As the curing proceeds, the maximum torque is observed within 10 min, suggesting that the reaction proceeds vigorously and is accomplished in a relatively short time. In addition, the maximum torque value of BS samples is monotonously increased with BDB content, which also


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reflects the increase of cross-link density. It is consistent with the results of equilibrium swelling tests.

Figure 2. (a) Cross-link density, sol fraction and swelling ratio of BS samples; (b) Cross-linking kinetics of BS samples at 160 oC in a rheometer.

Mechanical Properties and Malleability of BS Samples. Figure 3a displays typical tensile curves of BS samples, and their Young's modulus, tensile strength and breaking strain are tabulated in Table S1. It can be found that BS samples exhibit Young’s modulus of 1.86-2.43 MPa and breaking stain higher than 200%, and the strain entirely recovers after stretching (Figure S4), showing typical features of permanently cross-linked rubber materials. When compared to most self-healing polymers, BS samples exhibit relatively higher strength (Figure 3b, Table S2). Moreover, the mechanical properties of BS samples can be readily varied by changing BDB content. With the increase of BDB content, the Young’s modulus and ultimate tensile strength are enhanced while the breaking strain is decreased, which can be explained by the increase of cross-link density.

According to DMA results, as BDB loading increases, the storage modulus consistently enhances and the glass transition temperature taken at tan δ peak value ACS Paragon Plus Environment


slightly increases (Figure S5 and S6). These observations are likely because the mobility of rubber chains is reduced due to increased cross-link density. Tan δ peak value can be used as an indicator for the elasticity of material, and a lower value implies a higher elasticity. Figure S5 shows that the tan δ peak value gradually decreases with increasing BDB amount, indicating that the elasticity of the samples increases with cross-link density. Besides, the storage modulus at rubbery state is constant, suggesting that the network maintains the structural integrity at higher temperatures. This can be explained by the fact that boronic ester transesterification is associative mechanism in essence,32,

33

namely, the bond breaking and new bond

formation concurrently occur and thus the numbers of chemical bonds keep unchanged during topology rearrangement. It is distinctly different from that observed in dissociative mechanism such as Diels-Alder reaction, in which the retro Diels-Alder process leads to a loss of structural integrity.4

Figure 3. (a) Stress-strain curves of BS samples; (b) Comparison on tensile strength and breaking strain of our prepared BS samples with other reported self-healing polymers containing different reversible bonds. PDMS (poly(dimethyl siloxane)) with metal-ligand bond,11, 43 bis-imine and hydrogen bond,31 π-π interaction,44 disulfide


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bond,45 and boronic ester bond;46 PU (polyurethane) with disulfide bond,47 diselenide bond,48 and carbon-carbon bond;49 PB (polybutadiene) with carbon-carbon double bond,36 and ionic bond;50 PSBE (polysulfide-based elastomer) with disulfide bond;51-53 PEGDMMA (poly(ethyleneglycol)dimethacrylate) with alkoxyamine bond;54

PCO

(polycyclooctene)

with

boronic

ester

bond;33

HBN-GO

(amine-terminated oligomer cross-linked with graphene oxide) with hydrogen bond;55 PTEG (poly(tetramethylene ether)glycol) with hydrogen bond;56 LCE (liquid crystalline elastomer) with hydrogen bond;57 PA (polyacrylate) with imine bond.58

Due to the reversibility of boronic ester linkages, the as-prepared BS samples are expected

to

relax

stress

and

rearrange

network

through

boronic

ester

transesterification reaction. As shown in Figure 4a, BS samples significantly release stress with time at 80 °C, suggesting that the networks can flow at evaluated temperatures although they are covalently cross-linked. In striking contrast, the control sample that does not contain boronic ester linkages cannot release stress even at 150 oC (Figure S7), which provides convincing evidence that boronic ester plays a key role in network rearrangement in BS samples. In addition, BS samples with higher BDB content exhibit much slower relaxation rate and longer characteristic relaxation time (determined at 1/e of the normalized relaxation stress). Two reasons may account for this phenomenon. On one hand, more exchanges are necessary to ensure topology rearrangement in the samples with higher cross-link densities. On the other hand, an increase in cross-link density results in a more constrained mobility of rubber chains, which consequently reduces the possibility of chain movement for



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seeking available site for bond exchanges, thus retarding exchange reaction.59

Shown Figure 4b is the effect of temperature on stress relaxation behavior of BS3. It is obvious that relaxation rate is increased with temperature. This is because the relaxation process is essentially controlled by the thermo-activated boronic ester transesterification reaction, which is accelerated at a higher temperature. Moreover, the relaxation time shows an Arrhenius-like temperature dependence, which further indicates the associative exchange mechanism of boronic ester transesterification reaction (Figure S8). Accordingly, the activation energies, Ea, for all BS samples are calculated from the slope and summarized in Table 1, which are close to the value reported by Leibler et al. for small-molecule boronic ester transesterification.32 The Ea value of BS samples increases with BDB content, which can be explained by the diffusion-limiting topology in the cross-linked networks. In a more densely cross-linked network, the chain mobility and reactive group diffusion are more restricted, which consequently hinder bond reshuffling and increase activation energy. Table 1. Relaxation time at 80°C and activation energy of BS samples. Sample Relaxation time at 80 °C (min) Ea (kJ/mol)

BS1

BS2

BS3

BS4

BS5

14.5

18.3

23.8

24.7

30.2

7.7

8.0

8.3

10.4

13.8


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Figure 4. (a) Stress relaxation curves of BS samples at 80 oC; (b) Stress relaxation curves of BS3 at different temperatures.

Self-healing and Reprocessing of BS Samples. Owing to the transesterification reaction of boronic ester linkages, the network rearrangement and bond reshuffling should take place, and covalent bonding can be re-established across the interfaces of the fractured surfaces, as schematically illustrated in Figure 5. As a result, we envision that BS samples can acquire self-healability and reprocessability through boronic ester bond exchange-induced network rearrangement.

Figure 5. Network rearrangement of BS samples via boronic ester transesterification.

The self-healing ability was examined by cutting film samples into two pieces with a blade and then putting them together for healing under different conditions. To obtain a better observation, one piece of the sample was colored into black. As vividly demonstrated in Figure 6a, when the fractured surfaces are returned into contact for 1



h, the sample can sustain a large strain due to the reformation of covalent bonding across the interfaces. The self-healing efficiencies were quantified by tensile tests. Figure 6b displays the representative stress-strain curves of BS3 specimen healed at 80 oC for different times. It is evident that the healing efficiencies for tensile strength and breaking strain are increased by prolonging healing times. At 80 oC, BS3 specimen can be healed to ∼30% for 1 h and reaches ~90% for 24 h of its original mechanical properties prior to cutting. In particular, the elastic regions of the healed sample are almost the same as those of the original one irrespective of healing times. Figure 6c demonstrates the effect of temperatures on the recovery of mechanical properties of BS3 specimen. The healing efficiency is markedly improved with increasing temperature, which is because the boronic ester transesterification is accelerated at a higher temperature, thus facilitating the healing process. Specially, after being healed at room temperature, BS3 can still recover a tensile strength of 1.2 MPa, ~60% of its original value. Although this healing efficiency is lower, the absolute magnitude of the recovered strength is comparable to the pristine strength of most self-healing elastomers that are very soft to ensure sufficient polymer chain mobility (Figure 3b and Table S2). When healed at 80 oC for 24 h, BS samples are found to recover above 80% of the tensile strength and breaking strain. It is noted that the healing efficiencies do not monotonously change with cross-link densities, and a highest healing capability is observed in BS3 (Figure S9). In general, wetting and diffusion of polymer chains at the interface are hindered in the samples with a higher cross-link density due to


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restricted chain mobility, which consequently results in poorer healing efficiency. However, a higher cross-link density indicates that a higher amount of covalent linkages can be re-established across the interface via transesterification, thus offering a higher healing effect. These two contradictory factors lead to the results that the highest healing effect is obtained in the sample with moderate cross-link density. In the control sample without boronic ester bonds, essentially no healing effect is detected after being healed at 80 °C for 24 h (Figure 6d). It provides complementary evidence that the boronic ester transesterification plays the critical role in self-healing process.

Figure 6. (a) Photo of BS3 before and after healing, and the healed film after stretching; Typical stress-strain curves of BS3 (b) healed at 80 °C for various times



and (c) healed at various temperatures for 24 h; and (d) Typical stress-strain curves of control sample healed at 80 °C for 24 h, the inset is enlargement of the front part of the curves (up to 20% strain).

It is inherently difficult to recycle or reshape permanently cross-linked polymers, while reshaping and recycling thermoplastics are also not trivial because macroscopic flow occurs as a result of a sudden viscosity decrease around glass transition. Herein, the boronic ester undergoes associative transesterification reaction in the network, which enables the network rearrangement and results in a gradual Arrhenius-like viscosity dependence, thus imparting the network the ability to be reshaped and reprocessed in solid state. As proof of concepts, complex shapes are obtained from strip-shaped BS samples by successive twists/bends and stress relaxation at 80 °C (Figure 7a). The newly obtained complex shapes are stable when settled at 200 °C and maintain high elasticity. It should be noted that it is not necessary to use molds or accurately control temperature to keep structural integrity, which offers significant opportunities to access geometrically complex objects.

When BS samples are cut into small pieces, they can be remolded into a new coherent and smooth samples (Figure 7b). As shown in Figure 7c and Figure S10, mechanical properties of the recycled samples are well restored after being remolded at 160 oC for 1 h irrespective of the cross-link density. Typically, the recycled BS-4 exhibit recovery ratio of 102%, 96%, and 87% for ultimate strength, Young’ modulus, and breaking strain, respectively. More strikingly, the samples can be repeatedly recycled due to the robust characteristic of the dynamic boronic ester bonds. After ACS Paragon Plus Environment

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multiple recycling cycles, the tensile curves of the samples are almost overlapped (Figure 7d). As for the control sample without boronic ester bonds, it can’t be molded into a smooth and uniform sample anymore after being cutting into small pieces, confirm that boronic ester transesterification reaction plays the critical role in the reprocessing of BS samples. It should be noted that although non-chemically crosslinked SBR can completely recover the mechanical properties after being reprocessed and healed (Figure S11), it has no practical application value due to the lack of elasticity and environmental resistance. By TGA tests, BS samples exhibit good thermal stability with onset degradation temperature above 300 °C, and the weight loss is negligible after keeping them at 160 °C for 2 h (Figure S12 and S13), suggesting that BS samples can withstand high temperature and are stable when being tested and reprocessed.

Figure 7. (a) Optical images demonstrating the malleability and elasticity of BS3; (b)



Optical images of thermal recycling ability of BS samples; (c) Mechanical property recovery ratio of recycled BS samples; (d) Typical tensile curves of BS4 after multiple cycles of recycle. The scale bar in (a) and (b) represents 1 cm.

4. CONCLUSIONS

We present a straightforward way to prepare permanently cross-linked yet self-healing and recycle elastomers based on commonly used diene rubber by engineering dynamic reversible boronate ester bond into the network. The network is yielded via one-pot thiol-ene “click” reaction between SBR and dithiol-functionalized boronic ester, which has been verified by FTIR, rheological test and swelling experiment. Mechanical performance of the obtained samples can be readily varied by changing boronic este loading. Owning to the dynamic exchange of boronic ester bond, the covalently cross-linked rubber networks can rearrange their topology, allowing them be healed, reshaped and recyclable. As a consequence, the samples can recover their original mechanical properties in large extent after healing or recycling, and they are able to be wrought to access geometrically complex shapes. Due to the simple preparation process and largely available diene rubbers, we envisage the present work providing a facile yet efficient way to develop elastomers with a combination of permanent cross-linking, relatively high mechanical properties, healing capability, and malleability towards practical applications.

ASSOCIATED CONTENT


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Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. 1

H NMR spectra of pure SBR and BDB; photo of BS3 in toluene for 72 hours;

loading-unloading curves of BS3 under 100% strain at room temperature; DMA results of BS samples; stress relaxation for control sample at 80 oC and 150 oC; fitting of relaxation time to temperature; healing efficiency of BS samples after healing at 80 o

C for 24 h; stress-strain curves of original and recycled BS1, BS2, BS3 and BS5;

Comparison on the stress-strain curves of non-chemically crosslinked SBR after being reprocessed and healed; TGA curves of BS samples; isothermal TGA curve of BS4 under nitrogen atmosphere at 160

o

C; mechanical properties of BS samples;

comparison on the mechanical properties between reported self-healing materials and BS samples.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2015CB654703), National Natural Science Foundation of China (51790503,



51703064 and 51673065), and Open Research Fund of Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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Table of Contents


Covalently Cross-Linked Elastomers with Self-Healing and Malleable

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