Dual Cross-Linked Self-Healing and Recyclable Epoxidized Natural

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Dual Cross-linked Self-healing and Recyclable Epoxidized Natural Rubber Based on Multiple reversible effects Bo Cheng, Xun Lu, Jiahui Zhou, Rui Qin, and Yilin Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06437 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Sustainable Chemistry & Engineering

Dual Cross-linked Self-healing and Recyclable Epoxidized Natural Rubber Based on Multiple reversible effects Bo Cheng, Xun Lu*, Jiahui Zhou, Rui Qin, Yilin Yang, School of Materials of Science and Engineering, South China University of Technology, Guangzhou, 510640, China *E-mail:

[email protected]

Abstract: Generally, self-healing researches based on commercial rubber are of great significance in sustainable development by extending the lifetime of materials However, it is still a great challenge so far to prepare recyclable rubbers that combine excellent self-healing properties with good mechanical strength, and is also recyclable. Herein, we report the use of epoxidized natural rubber (ENR), a reactive polymer presenting dual functional groups (unsaturated double bonds and epoxy sites) available for cross-linking, to prepare a dual cross-linked self-healing ENR based on dynamic disulfide metathesis and thermo reversible hydrogen bonding. Specifically, different structures of aromatic disulfide compounds are introduced into the same system to promote the disulfide metathesis and thus improving the self-healing efficiency of the materials. As a result, the dual cross-linked ENR shows high strength (9.3±0.3MPa), high self-healing efficiency (up to 98%) and ideal recyclability. In addition, cyclic fatigue tensile test shows that the self-healing properties of the present material are not affected by the damage forms, whether it is complete fracture or cyclic fatigue damage. These outcomes are expected to promote the development of self-healing technology in the sustainable application of crosslinked rubber materials. Key words: self-healing rubber; dual cross-linked network; multiple reversible effects; fatigue damage repair; recycling

1

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Introduction Self-healing polymers have attracted significant attention because of their widespread applications in numerous fields, such as wearable electronics, coatings, flexible sensors and soft robotics.1,2 However, to the best of our knowledge, most self-healing structures are complicated and tailor-made, rather than commercial materials, and thus far from practical applications. Also, there are few studies on self-healing systems on the basis of commercially accessible materials.3,4 The main reason is that the molecular chain structures of commercial materials are fixed and it is difficult to introduce dynamic bonds such as DA reactions, hydrogen bonds, ionic bonds to construct self-healing systems through effective designs, especially for commercial rubber. However, compared to other specific synthetic systems, turning commercial rubber into self-healing materials has a real application prospect, which not only prolongs the service life of rubber materials, but also promotes the sustainable application of rubber through recycling.5 Most importantly, how to turn the commercial rubber into a self-healing material with both mechanical strength and high self-healing efficiency has become a major difficulty in the self-healing field.6 In order to obtain self-healing function, various dynamic bonds have been tried by researchers to introduce into the rubbers to construct reversible network, including dynamic covalent bonds and non-covalent bonds, such as Sciff-base bonds,7-9 ionic bonds,10-13 metal-ligand coordination.14 Among them, the disulfide metathesis is one of the few dynamic reversible reactions in which the conditions are mild and easy to implement.15,16 As one of the most widely used dynamic covalent bonds, the bond energy of the disulfide bonds is weaker than the carbon-carbon bonds, which makes them easier to cleavage and recombine.17-19 For the moment, model systems containing disulfide bonds have attracted considerable interest recently to produce self-healing materials under various stimuli like heat, light or redox agents. However, only a limited number of studies have focused on chemically cross-linked solvent-free polymer networks. Xiang et al.20 introduced CuCl2 into sulfur-vulcanized butadiene rubber (BR) to prepare a 2


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disulfide-containing self-healing elastomer, which resulted in an improved healing efficiency (The material can recover about 80% of its performance after being repaired at 110 °C for 12 hours). But the poor mechanical strength (less than 4 MPa) limits its practical application. Norvez and his group17 introduced disulfide bonds into the ENR-25 by using dithiodibutyric acid (DTDB) as a crosslinker, which presented strong elastomeric properties and thermo-activated reprocessing ability. Actually, dynamic bonds can not only toughen and strengthen materials but also endow them with self-recoverability.21 Therefore, using the disulfide reversible chemistry in rubber seems to be a promising choice by which to combine self-healing ability and good mechanical behavior. Unfortunately, most reported self-healing rubber-based materials are generally faced with difficulties in balancing mechanical properties and self-healing performance. The lower disulfide bond dissociation energy (BDE), which is a reflection of the easiness to cleavage the S–S bond, can promote the disulfide metathesis reaction. When the phenyl ring of aromatic disulfide is substituted with an electron donating group, especially the amino group (NH2), the BDE of disulfide bonds can be greatly reduced, which means that the reversible recombination of disulfide bonds can be performed more easily.22 Actually, as a reactive polymer presenting a dual functionalities (dual bonds and epoxy sites) available for cross-linking, ENR can be effectively cured by carboxylic acids.23 Besides, ENR can also be cross-linked by amine compounds or aminosilanes via ring opening reaction of the epoxy groups with lower strength and modulus.24,25 In this regard, we designed a dual reversible cross-linking network in ENR to transform it into a recyclable rubber that combines good mechanical strength and high self-healing efficiency (Fig 1). Specifically, we used sulfur(S) to vulcanize the double bonds on the ENR molecular chain to construct a sulfur cross-linking network, and adjust the vulcanization system and the vulcanization process to make the disulfide bond content in the vulcanized rubber dominant.26 Next, for the active epoxy groups on the ENR, we innovatively envisage adopting a 2,2′-Dithiodibenzoic acid 3



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(DTSA)-based, 4,4′-dithiodianiline (DTDA)-assisted strategy for curing epoxy groups to introduce another disulfide cross-linking network, since using DTSA/DTDA simultaneously can not only ensure sufficient crosslinkage to obtain mechanical strength, but also introduce amino-substituted aromatic disulfide bonds with lower bond dissociation energy to accelerate the disulfide metathesis reaction, which be beneficial to the self-healing properties. The dual disulfide network maximizes the broad and uniform distribution of disulfide bonds within the network. Besides, the ring-opening reaction of the epoxy groups in ENR will produce a certain amount of hydroxyl groups, forming hydrogen bonding with the polar groups in the system. When the rubber sections are fully in contact with each other, since disulfide bonds can undergo metathesis reaction at a certain temperature, the hydrogen bonding effect is also thermally reversible, the cross-linked network of the cross-section is deconstructed, and the molecular chains can be slipped, diffused and re-entangled, in turn, a new cross-linking network is formed to firmly join the sections to complete the self-healing process. Combining these properties, this dual cross-linked ENR exhibits both high strength and ideally self-healing ability because of the readily conversion existed between a well cross-linked network and an uncrosslinked structure.27

Experimental section Materials ENR with an epoxidization degree of 50% was produced by the Agricultural Products Processing Research Institute, Chinese Academy of Tropical Agricultural Science, China. 2,2′-Dithiodibenzoic acid (DTSA), 1,2-Dimethylimidazole ( DMI), Di(2-ethylhexyl)phthalate (DOP), 4,4′-dithiodianiline (DTDA), 4,4′-Diaminodiphenyl ether (ODA), 4,4′-Oxybis(benzoic acid) (OBA), were supplied by Shanghai Aladdin Bio-Chem

Technology

Co.,

Ltd.

Rubber

additives

such

as

N-(Oxidiethylene)-2-benzothiazolyl sulfonamide (NOBS), zinc oxide (ZnO), stearic acid, basic magnesium carbonate (MgCO3), dicumyl peroxyide (DCP), triallyl isocyanurate (TAIC), antioxidant RD, sulfur (S) used in the study were supplied by 4


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Guangzhou Longsun Technology Co,. Ltd, China, and were of commercial grade. All reagents were used without further purification.

Preparation of dual cross-linked self-healing ENR material

Table 1 The details of basic vulcanization formulas Ingradients (phr*)

Disulfide-containing ENR-50 sample

Disulfide free ENR-50 sample

ENR-50

100

100

MgCO3

2

2

ZnO

5

5

Stearic acid

1

1

DOP

3

3

S

0.3

_

NS

0.5

_

DTSA

0.8

_

DTDA

0.8

_

DCP

_

0.3

TAIC

_

2

OBA

_

0.8

ODA

_

0.8

DMI

1

1

*Phr= parts per hunderd ENR compound were produced according to the formulas showed in Table 1. The raw ENR were first masticated on a two-roll mill for 5 or 6times at ambient temperature, then rubber additives were added and mixed evenly, after blending, samples were cured on the vulcanizing press under the pressure of 8 MPa for 20 min at 155 °C.

5



Figure 1. Dual cross-linking of ENR-50: schematic representation of the disulfide containing network.

Characterization FT-IR was performed on a Bruker Vertex Fourier transform infrared spectrometer equipped with a Harrick-ATC-024 temperature controller with attenuated total reflectance (ATR) mode at room temperature or at a high temperature. The scan range is from 4000 to 450 cm-1 with are solution of 4 cm-1. For the reductive swelling experiments, tetrahydrofuran (THF) was preferred to toluene because of its miscibility with water. The samples (1.5 mm thickness discs of 5 mm diameter) were immersed in THF and their weight evolution was followed over time. When the equilibrium swelling was reached, a small excess of tri-n-butylphosphine (TBP) and a few drops of water were added under magnetic stirring. Optical microscope analysis was performed on a XSP-2CA optical microscope to observe the repair of the specimens at 50 and 100 magnifications. Dynamic mechanical analysis (DMA) was performed on a NETZSCH DMA 242C dynamic mechanical analyzer under the tension condition with a frequency of 1, 5, 10 and 16.666 Hz. The scanning temperature ranged from -100 to 150 °C at a heating rate of 3 °C/min. Stress relaxation experiments at different temperatures: 25 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C were performed with a INSTRON 5569 instrument. In stress relaxation, the samples were stretched to 300% strain at the strain rate of 250 min-1, and the constant strain 6


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was maintained to measure the relaxation of the stress for 60 min. The cyclic tensile tests were performed on a U-CAN UT-2060 instrument with an extension rate of 100 mm/min-1 at room temperature. In each cycle, the sample was stretched to a strain of 300% or 500% and then relaxed at room temperature for a certain waiting time (0, 60, 120, 300, 600, and 1800 s) prior to the subsequent loading process. After that, the sample was then heated at 120°Cfor 30 min and cooled at 25 °C for another 30 min to heal the dynamic bonds, followed by another loading–unloading cycle. The fatigue stretching test was carried out on a flexural tester model GT-7011-D, the sample was a dumbbell-shaped specimen of length × width= 75 × 4 mm. After the samples were stretched for 60,000 times, one set of samples were tested for mechanical properties immediately, and the other set were tested after healing at 120 °C for 6 h. Repeat the above process to test the ability of the material to be repaired multiple times. Tensile experiments were performed with a Zwick 1010 apparatus according to the GB/T 528-1998 standard at an extension rate of 100 mm/min-1 at 25 °C. Besides, for recycling experiments, the samples were reduced to a fine powder using a variable speed rotor mill, then kneaded the rubber powder on a Two-Roll-Mill, and finally heated at 155 °C for 30 min in a heating press. Self-healing properties The standard dumbbell-shaped specimens were cut with a sharp knife from the middle; then, the cut surfaces of broken samples were fully brought into contact at 120 °C under a small force (about 10 N),and kept for set times (30 min, 1 h, 3 h, 6 h, 12 h, and 24 h) before mechanical testing(Figure S1). To describe the healing effect, the factor of healing efficiency (η) is defined as the ratio of tensile strength of the healed sample (σhealed) to the original (σpristine), which was defined5,28 as follow:

σhealed

η= σ ×100% virgin Where σhealed and σpristine are the average tensile strengths of the healed samples and the pristine samples, respectively.

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Results and discussion Formation of dual cross-linked network based on dynamic disulfide bonds in ENR To verify our design concept, we investigated the self-healing properties of S/DTSA, S/DTDA and S/DTSA-DTDA cured ENR-50s in the same amount of S 0.3 phr and disulfide-containing crosslinkers 1.6 phr. As shown in Figure 2 and Table S1, due to the presence of the disulfide network, all samples of the three curing systems have self-healing properties. Among then, as described previously, the S/DTDA cured ENR-50 sample exhibits a lower mechanical strength of only 4.6±0.2 MPa due to the insufficient cross-linking density. S/DTSA cured ENR-50 sample has the highest strength of 10.8±0.8 MPa, since dicarboxylic acid salts are effective in curing ENR.29 The cross-linking density of the three samples in Figure 2b further confirmed this. However, to the best of our knowledge, an well cross-linked network not only restrict the free movement of the chains, but also hindered the dynamic exchange of disulfide bonds to some extent.18 Therefore, the S/DTSA vulcanized ENR-50 sample has the lowest healing efficiency and can only recover 64% of its original strength. It is particularly noteworthy that S/DTSA-DTDA cured ENR-50 sample has both good mechanical strength and excellent self-healing properties with tensile strength up to 9.3 ± 0.3 MPa and healing efficiency reaches 98%, while the DCP/OBA-ODA vulcanized disulfide-free ENR sample has a self-healing efficiency of only 22% (see Figure S3). Due to the existence of multiple reversible effects of dynamic disulfide metathesis and thermo reversible hydrogen bonding in rubber networks, ENR is endowed with excellent self-healing properties.

Figure 2. (a) Self-healing properties; (b) cross-linking density of ENR-50 with different curing systems. 8


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The prerequisite for the dynamic essence of the dual cross-linked network is that reversible disulfide bonds are successfully introduced into the cross-linking molecular chains. Therefore, reductive swelling experiments were performed to confirm the existence of a large number of disulfide bonds in the dual cross-linked network of ENR-50. For comparison, we also prepared DCP/OBA-ODA dual cross-linked ENRs without disulfide bonds. A piece of rubber was swollen in THF until equilibrium and then

a

reductive

agent

tri-n-butylphosphine

(TBP)

was

added.

Since

Trialkylphosphines (R3P) are known to quantitatively reduce organic disulfides into thiols in the presence of water (equation (1)).17 (1)

Figure 3 Reductive swelling experiment (black: S/DTSA-DTDA cured ENR-50; blue: DCP/OBA-ODA cured ENR-50, The TBP was introduced after equilibration in pure THF.）

After equilibrium swelling in pure THF, an excess of TBP and water (to the theoretical number of disulfide bonds) was added and the additional swelling volume (if any) measured (Figure 3). After the additional of TBP, the two samples behave very differently as it contains disulfides or not. The swelling ratio of DCP/OBA-ODA(disulfides-free)

cured

sample

remain

stable

whereas

the

S/DTSA-DTDA (disulfides-containing) cured sample swell a lot, due to the reduction of disulfide cross-links into separate thiols.17 We checked that when TBP was not 9



added, the disulfides-containing dual networks behaved exactly like the traditional crosslinked rubber material. This confirms that the dynamic disulfide bonds are still present after curing process. The very large swelling of the disulfide-containing network weaken the material a lot. Under continuous stirring, the piece of rubber finally dissolves. This phenomenon indicates that the dual networks constructed by S/DTSA-DTDA mainly rely on reversible disulfide bonds. Although some irreverible monosulfide bonds and carbon-carbon bonds were introduced into the networks due to the S curing process in the meantime, but the content is negligible compared to the disulfide bonds. Therefore, when a large number of disulfide bonds in the network are reduced to thiols, the network structure is separated and collapsed, resulting in the final dissolution of the sample in the solvent. Detailed swelling process can be found in Figure S3. Besides, the existence of hydrogen bonding in the network can further promote the self-healing process, due to the hydrogen bonding between disulfide chains seems to be relevant to favor the contact among disulfide units, and it is crucial for the further disulfide exchange reaction to take place.22

Figure 4. FT-IR spectrums of Neat ENR-50, S/DTSA cured ENR-50 and S/DTSA/DTDA dual cured ENR-50

As shown in Figure 4, compared to neat ENR-50, the intensity of peak at 876 cm-1, which correspond to asymmetrical stretching vibration peak of epoxy groups30,31 was weakened after being cured by S/DTSA-DTDA. Meanwhile, the -C=O absorption peak at 1726cm-1of S/DTSA-DTDA cured-ENR-50 was significantly 10


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enhanced due to the esterification cross-linking of the hydroxy group (formed by the ring opening reaction of epoxy group) and the carboxyl group in DTSA. Beside, after vulcanization, the intensity of the -OH stretching vibration peak at 3456 cm-1 was significantly increased, and a new absorption peak appeared at 3388 cm-1, corresponding to the -N-H stretching vibration peak. All these show that S/DTSA-DTDA can effectively vulcanize ENR-50 and generates hydrogen bonding sites. To further illustrate the existence of thermally reversible hydrogen bonding in the network, FT-IR spectrums at different temperatures were conducted as follows.

Figure 5. FT-IR spectrums of S/DTSA-DTDA curedENR-50 samples at different temperatures

As shown in Figure 5, a stretching vibration absorption peak of hydroxyl group was appeared between 3300 and 3340 cm-1 and the position shifts with temperature. The peak was 3456 cm-1 at room temperature and shifted to 3461cm-1 at 60°. When the temperature rises to 100°C, the peak shifted further to 3375cm-1, and almost returned to the position before heating when the temperature drops back to room temperature. In fact, when the temperature rises, the hydrogen bonds inside the material dissociate, and the hydroxyl groups change from bound to free, so the peak position blue shifts toward the highwavenumber.5,22 As the temperature drops back to room temperature, hydrogen bonding were reformed between the free hydroxyl 11



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groups and the polar epoxy groups as well as carbonyl groups in network, shown in the spectrum as the peak of the hydroxyl groups red shift to the position before heating. All these results indicate the existence of thermally reversible hydrogen bonding in the network. When the cracked samples are re-spliced together and heat-treated, the dissociation and recombination of the hydrogen bonds between the broken molecular chains can further promote the binding between the fracture surfaces and contribute to the self-healing behavior. The dynamic disulfide metathesis coupled with thermally reversible hydrogen bonding gives the material excellent self-healing properties. According to the FT-IR spectrums, the hydrogen bonding state in the S/DTSA-DTDA cured ENR-50 is shown as Figure 6.

Figure 6.Schemic illustration of the hydrogen bonds present in S/DTSA-DTDA cured ENR-50

The effect of the amount of vulcanizing agents on self-healing performance After

comparing

different

vulcanization

systems,

the

S/DTSA-DTDA

curedENR-50 exhibited excellent self-healing properties. Therefore, we further investigated the effect of the amount of curing agents on the self-healing performance of ENR-50 (Figure 7 and Table S2, S3). A series of ENR-50 samples were prepared with different amounts of vulcanizing agents. As shown in Figure 7a, the tensile strength of the material improves gradually with the increasing S dosage, but the self-healing properties gradually deteriorated due to the increased cross-linking 12


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density of the network. Besides, during S curing process, a certain proportion of C-S bonds and a small amount of C-C bonds were generated, and increased with the amount of S increasing, which increase the number of permanent cross-linking points in network, resulting in a rise in material strength and a decrease in self-healing performance. Similarly, increasing the amount of DTSA and DTDA also results in an increase of cross-linking density, the molecular chains of the network are bound by more cross-linked points, making it difficult to relax and diffuse. Even though more dynamic disulfide bonds and hydrogen bonds were introduced into network at the same time, the self-healing properties of the material shows a decreasing trend. When the curing system is S-0.3/DTSA-0.8/DTDA-0.8 phr, the ENR has both good tensile strength and excellent self-healing properties, with the pristine sample strength reaches9.3±0.3 MPa and the self-healing efficiency up to 98%.

Figure 7. Self-healing properties of ENR-50 with different amount of vulcanizing agents: (a) DTSA-0.8/DTDA-0.8/S-variable (phr); (b) S-0.3/DTSA-DTDA-variable (phr)

The effect of healing time on self-healing performance

Figure 8.Mechanical properties and healing efficiency of dual cross-linked ENR-50 as a function of healing time 13



We also investigated the effect of healing time on the self-healing properties of S/DTSA-DTDA cured ENR-50 samples (Figure 8). The results show that the healing efficiency of the material increases by prolonging healing times. After 24 hours, the healed sample exhibits a tensile strength of 9.1±0.2 MPa, corresponding to a healing efficiency of 98%, which can be considered quite a remarkable result for an effectively cross-linked rubber. When the healing time was further extended to 48h, the healing efficiency decreased slightly, which may be due to the inevitable thermo-oxidative aging of the material when it was exposed to the air atmosphere at 120 °C for a long time, and thus affecting the performance of the material. In this regard, the 24-hour healing time is considered to be the best and energy-saving choice. Besides, the optical microscope was used to observe the fracture surfaces of samples before and after healing (Figure 9). As can be seen from the Figure 9a-9d, after the spliced specimen was repaired at 120° C for 24 h, the crack at the fracture surface was healed under the dual function of the disulfide metathesis reaction and the thermoreversible hydrogen bonding,20 leaving only a faint trace.

14


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Figure 9. Optical microscope images of ENR specimen before (a, c) and after healing (b, d)

The presence of disulfide metathesis reactions and hydrogen bonding enable network to rearrange dynamically To combine autonomous and reversible healing properties in material, a weak bonding strength is generally required together with a low glass-transition temperature (Tg) to enable polymer mobility.32 Therefore, dynamic mechanical analysis (DMA) were performed to obtain the Tg of materials. Besides, Arrhenius plots of the loss factor-temperature (tanδ-T) curves at different frequencies allowed us to estimate the apparent activation energy (Ea) for the slippage of polymer chains according to the equation(2) proposed by Muller and Huff (Figure 10b).33

E  R

dInf dLogf  2.303R d (1 / T ) d (1 / T )

（2）

ΔE: Activation energy；R: The gas constant；f: Frequency；T: Absolute temperature As shown in Figure 10a, S/DTSA-DTDA cured ENR-50 has the lowest Tg of -17.3°C.The lower Tg of material facilitates the diffusion of polymer chains after the deconstruction of the cross-linked network, and thus contributes to improve the self-healing properties. In addition, it also has the highest tanδ value of 2.326, which is obviously higher than that of the DCP/OBA-ODA cured sample without disulfide bonds. In general, the higher tanδ value of the material, the more energy dissipated under dynamic deformation.34When the material is subjected to an external force under heating, the disulfide bonds in the crosslinked network are activated for disulfide exchange reaction and thus consuming energy continuously. On the other hand, the presence of hydrogen bonding in rubber networks increases the interaction between polymer chains, which can enhance the internal fiction of rubber chains during movement. Besides, the tensile and relaxation of the sample during the DMA test will lead to the cleavage and recombination of hydrogen bonds between chains, and thus resulting in energy dissipation.35,36 The lower the Ea value, the lower the energy barrier needed to be overcome by the molecular segmental motion, the 15



stronger the movement ability of the chain segments.33

Figure10. DMA curves of different curing systems of ENR-50: (a) tanδ-T curves; (b) Arrhenius plots based on tanδ-T curves in different frequencies; (c) Apparent activation energy (Ea) for the slippage of polymer chains

In order to verify the reversibility of dynamic disulfide bonds and hydrogen bonds during stretching process, tensile-recovery tests were performed (Figure 11) as follow. Each sample was stretched to a strain of 500% and then was recovered by releasing the stress. Since the strain is very close to the fracture strain of the material, partial crosslink will be ruptured during stretching, which can be proved by the obvious tensile strength decreasing of the material after stretching. Clearly, significant hysteresis (area surrounded by tensile-recovery curves) are observed in the first loading-unloading cycle (Figure 11a and 11b), which indicates the energy dissipation from the rupture of cross-link under large deformation. For the cycle followed immediately, a much smaller hysteresis was found due to the breakage of partial 16


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crosslinks. When the samples are allowed to relax at room temperature after loading, the hysteresis of both samples gradually increased as prolonging the waiting time. However, the recovery process are quit slow due to the remaining crosslinks will limit the recovery of the primary chain to its equilibrium state.37,38 Even if under a relatively long waiting time (1800 s), both stress-strain curves of samples does not recover obviously. Notably, after heating at 120°C for 1800s and cooling down at room temperature, the loading-unloading curve is nearly overlapped with the first cycle, suggesting the reversibility essence of the dual crosslinked sample based on dynamic disulfide bonds and hydrogen bonds.4,20 (Figure 11c). In obvious contrast (Figure 11b and 11c), DCP/OBA-ODA cured ENR-50 sample without disulfide bonds shows a much smaller stress recovery due to the irreversibility of the C-C crosslinks. Since heating treatment can only accelerate the rubber chains return to the equilibrium state, but can not restore the irreversible fracture crosslinks.39

17



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Figure11.Tensile loading-unloading curves of (a) S/DTSA-DTDA cured ENR-50 and (b) DCP/OBA-ODA cured ENR-50. (c) W and Stress recovery of samples on different waiting times

Why do S/DTSA-DTDA dual-crosslinked ENR-50 samples have such high healing efficiency, and can readily recover from fracture under heating? We considered that its healing properties are dominated by the dynamic reversible essence of the dual crosslinked networks in the material, which allows the crosslinked network to be deconstructed and rearranged during heating and cooling process. The deconstruction of the crosslinked network makes it possible to diffuse and entangle between the molecular chains, and thus leading to the interpenetration of polymer chains at the fractured portions upon heating, thereby causing the cracked cross-links to re-form the network structure under the combined action of the disulfide exchange reaction and the thermo reversible hydrogen bonds.37 Therefore, the thermal reversibility of dual-crosslinked network of the material was further investigated by variable

temperature

experiments.40,41 S/DTSA

stress-relaxation

cured

and

DCP/OBA-ODA cured ENR-50 samples were presented for comparison. As shown in the Figure 12a and 12b, the relaxation behavior of the S/DTSA and S/DTSA-DTDA cured ENR-50 samples was the same as that of the conventional cross-linked polymer materials at room temperature, showing a slow relaxation rate due to the stable cross-links of the material at low temperature. And then, the samples exhibited an accelerated stress-relaxation rate with the increasing temperature. As the temperature was further increased above 120 °C, the stress relaxation curves of the two were reduced rapidly in the initial stage, and then the stress continually relaxes to zero, which contrasted distinctly with DCP/OBA-ODA cured ENR-50 sample without disulfide bonds at 130 °C (Figure 12c and 12d). Obviously, the stress relaxation behavior of dual cross-linked ENR-50 with disulfide bonds at higher temperature (above 120 °C) was related to the disulfide metathesis reaction. It suggests that the internal dual-cross-linked network is no longer permanently bonded and allows for rearrangement to reach less stretched state due to disulfide metathesis.42 In the DCP/OBA-ODA curing system, the network has no possibility of rearrangement as it 18


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does not contain disulfide bonds. In addition, the dependence of the relaxation times on temperature is plotted in Figure 12d using the Arrhenius equation (equation (3)).

τ(𝑡 ) = τ0𝑒𝐸𝑎 /𝑅𝑇

(3)

It can be observed that the relaxation time of the cross-linked networks fits well with the Arrhenius law and the corresponding activation energy (Ea) of S/DTSA- and S/DTSA-DTDA cured ENR-50 were calculated as (88.02 and 63.93 KJ/mol, respectively) from the slope. Due to the DTSA-based, DTDA assisted cross-linking strategy, the lower bond dissociation energy (BDE) of disulfide bonds and more thermally reversible hydrogen bonds in the network gives the rubber of better dynamic reversible properties, which is of benefit to the heat-induced self-healing ability, and thus the S/DTSA-DTDA cured ENR-50 sample has a lower relaxation Ea than S/DTSA cured one.

19



Figure 12.Stress-relaxation curves of (a) S/DTSA cured ENR-50; (b) S/DTSA/DTDA cured ENR-50 and (c) DCP/OBA-ODA cured ENR-50 measured at different temperatures; Stress-relaxation curves of (d) S/DTSA, S/DTSA-DTDA and DCP/OBA-ODA cured ENR-50 measured at 130 °C; (e) Linear fitting of the relaxation times by Arrhenius equation

The dynamically reversible network can promote excellent self-healing behavior and enable recycling of the rubber It is well known that in practical applications, fatigue failure is a major cause of shortening the service life of materials. Since microcracks generated inside the material are not easily noticeable, it is difficult to detect and make an effective remedy in time until the material breaks.27 If the material is given self-healing capabilities, self-healing process can be performed spontaneously when cracks occur inside the material, and thus the fracture failure caused by the expansion of the microcracks can be effectively avoided. Therefore, in particular, we further investigate the self-healing properties of the material through cyclic fatigue stretching test (Figure 13). It can be seen that after stretching 60,000 times under fixed strain (about 250%), the strength of the sample decreased from 9.3±0.3 MPa to 3.1±0.2 MPa. After healing at 120 °C for 6h, the fatigue damaged samples were almost completely recovered its original strength, and the self-healing efficiency can reach to 98%. Repeated stretching-healing process results show that the self-healing 20


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performance of the material is reproducible. After three stretching-repair processes, the sample can still recover 87% of the original strength, indicating that the material has excellent fatigue damage healing capabilities and can be self-repaired multiple times. In contrast, the DCP/OBA-ODA cured ENR samples exhibited lower healing efficiency like conventional cross-linked rubber due to its irreversible cross-linking network (Figure S4). For lack of a repair mechanism inside, the sample can only rely on the entanglement between the broken molecular chains under heat to restore a little performance. Combined with the results of the above fracture repair test and cyclic fatigue tensile repair test, the self-healing properties of the material are not affected by the damage forms (whether complete fracture or fatigue damage).

Figure 13. (a) Fatigue stretching process and (b) stress-strain curves before and after healing

Since the disulfide bonds in the vulcanized network of the ENR can undergo an disulfide exchange reaction under temperature stimulation,17,20,43-45 and the hydrogen 21



bonding is also thermally reversible,7,11 the cracked crosslinks can be reformed during the dynamic rearrangement of the network, and thus the ruptured sample can recover its original performance. Even if the sample is broken into particles, in principle, the dynamic crosslinked network can be reformed under heating when the cross sections of rubber particles can be sufficiently contacted, which provides a possibility for recycling and reprocessing of this crosslinked rubber. Based on this, we further explored the recyclability of the self-healing rubber (Figure14). It can be seen that, as we have envisaged, S/DTSA-DTDA cured ENR has good recycling and reprocessing ability, and the rubber can recover 78% of its performance after being broken into particles and re-vulcanized into sheets. As the recycling times increases, the mechanical properties of recycled rubber show a downward trend. After the second pulverization-kneading-molding cycle, the rubber material can maintain about 66% of mechanical properties, and about 43% for the third pulverization-kneading-molding cycle. The mechanical performance of recycled materials decreases seriously with the recovery times can be attributed to two factors. On the one hand, in order to allow full contact between the interfaces of crushed rubber particles, a kneading treatment was carried out on a Two-Roll-Mill, therefore the mechanical shearing will lead to partial chains break and the molecular weight will decrease, thus resulting in the mechanical strength degradation with the recovery times.46,47 On the other hand, the crushed particles need to be vulcanized again at high temperature under pressure to obtain the rubber sheets. Actually, long time high temperature action will lead to the thermal degradation of polymer chains, which will affect the mechanical properties of the sample.48-50

22


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Figure 14.Typical recycling of S/DTSA-DTDA cured ENR. The sheeted rubber (a) is firstly crushed into fragments; (b)which are then pulverized into small particles (c); The ENR particles are molded at 155°C,leading to reclaimed rubber sheet; (d) Typical tensile stress-strain curves of the recycled and virgin rubber samples

Conclusion The dual cross-linked self-healing ENR, which based on dynamic disulfide metathesis and thermally reversible hydrogen bonding, was successfully prepared through special designed vulcanization system. The material prepared with relatively simple protocol presented a maximum self-healing efficiency of 98% with tensile strength up to 9.3±0.3 MPa, which is a much improved value in self-healing rubbers compared to previous reported materials. And the self-healing performance of this vulcanized rubber are not affected by the damage forms (whether complete fracture or fatigue damage). After self-healing at 120 °C for 6h from 60,000 times of fatigue stretching, the healed sample can almost recover its original strength and can be repaired multiple times. Besides, the vulcanized rubber also has the recycling ability, and the sample can recover its 78% performance after simple pulverization-molding process, which may effectively extend the lifespan of rubber products and achieve the 23



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reuse of materials. The present work is expected to provide a solution for preparing a self-healing rubber material with good mechanical strength and high self-healing properties, which is difficult to achieve in most cross-linked self-healing materials reported so far. The strategy of transforming commercial rubbers into reusable self-healing materials by constructing a dual dynamic reversible cross-linking network is also hoped to provide assistance in the development of new technologies for sustainable applications of cross-linked rubber materials.

Associated content Supporting Information Figure S1, photo graphic repairing process of sample. Figure S2, reductive swelling experiment process. Figure S3, Tensile stress-strain curves of DCP/OBA-ODA cured ENR-50 sample. Figure S4, Fatigue stretching stress-strain curves before and after healing of DCP/OBA-ODA cured ENR-50.Table S1-S3, Specific mechanical properties data. Conflicts and of interest The authors declare no competing financial interest. Acknowledgement This study was funded by National Natural Science Foundation of China (51873065)

and

Natural

Science

Foundation

of

Guangdong

Province

(2017A030313277). References:

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TOC/Abstract Graphic (For Table of Contents Use Only)

Dual cross-linking network based on multiple dynamic bonds gives rubber excellent self-healing performance and recyclability, thus promoting the development

of

self-healing

technology

for

sustainable

cross-linked rubbers.

29


application

of

Dual Cross-Linked Self-Healing and Recyclable Epoxidized Natural

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