Bioinspired Engineering of Two Different Types of Sacrificial Bonds

Oct 31, 2016 - Bioinspired Engineering of Two Different Types of Sacrificial Bonds into Chemically Cross-Linked cis-1,4-Polyisoprene toward a High-Per...
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Bioinspired Engineering of Two Different Types of Sacrificial Bonds into Chemically Cross-Linked cis-1,4-Polyisoprene toward a HighPerformance Elastomer Jie Liu,† Sheng Wang,† Zhenghai Tang,† Jing Huang,† Baochun Guo,*,† and Guangsu Huang*,‡ †

Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China

‡

S Supporting Information *

ABSTRACT: The development of advanced elastomers with a combination of high strength, large extensibility, and excellent flex-cracking resistance is a huge challenge. In this contribution, we proposed a novel strategy to engineer a multinetwork by incorporating weaker sacrificial hydrogen bonds and stronger Zn-based units into a chemically cross-linked cis-1,4-polyisoprene network. The dynamic nature allows the sacrificial bonds to be ruptured and re-formed, resulting in high stretchability. During external loading, the sacrificial bonds rupture prior to fracture of the covalent network, thus dissipating energy efficiently and facilitating chain orientation to produce improved tensile modulus and fracture toughness as well as significant enhancement of flex-cracking resistance. We propose that the enhanced cracking resistance may originate from the energy dissipation and re-forming of sacrificial bonds, a new mechanism alternative to strain-induced crystallization. Overall, this concept provides unique inspiration for the design of advanced elastomers with excellent mechanical properties under both static and dynamic conditions.

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biomimetic strength and toughness.10−12 Sun et al. observed that polyampholytes that include randomly dispersed cationic and anionic repeat groups can form tough and viscoelastic hydrogels in which the strong bonds serve as permanent crosslinks imparting elasticity, and the weak bonds reversibly break and re-form, dissipating energy.13 Gong et al. discovered a general method for obtaining very strong hydrogels that exhibit a fracture strength as high as several tens of megapascals by inducing a double-network structure of various combinations of hydrophilic polymers with different structures and densities.14 Mozhdehi et al. developed a self-healing multiphase polymer in which a pervasive network of dynamic zinc−imidazole interactions was programmed in the soft matrix of a hard/ soft two-phase brush copolymer system and determined that the mechanical and dynamic properties of the materials could be tuned by varying a number of molecular parameters and the zinc/imidazole ratio.15 These results illustrate the effectiveness of sacrificial bonds in the enhancement of hydrogels. In our laboratory, we have incorporated sacrificial bonds or networks into elastomers with the goal of super strength and toughness. Tang et al. constructed reversible metal−ligand

INTRODUCTION Rubbers such as natural rubber (NR), styrene−butadiene rubber (SBR), and polybutadiene rubber (BR) are widely used in tires, seals, and shock absorbers because of their high entropy−elasticity.1,2 Most neat rubbers exhibit low tensile strength and insufficient fracture toughness, which severely hampers their practical applications. To solve this problem, various nanofillers such as carbon black (CB), silica, nanoclays, and novel nanocarbons have been incorporated to improve the strength and toughness of rubbers.3,4 However, rubber compounding causes significant constraints in terms of nanofiller dispersion or aggregation and interfacial regulation, resulting in deteriorated stretchability and processing difficulty. Therefore, many researchers have introduced effective energydissipation mechanisms to improve the mechanical performance of unfilled rubbers, such as slide-ring elastomer and dualnetwork heterogeneity rubber.5−7 Sacrificial bonds are a key structural factor determining the fascinating properties possessed by natural materials such as spider silk, mussel byssus, and bones. In these materials, the sacrificial bonds sustain a load under small deformation but preferentially rupture while the structural integrity is preserved, thus dissipating energy and conferring improved mechanical performance.8,9 Learning from nature, sacrificial bonds have been successfully incorporated into hydrogels to achieve © XXXX American Chemical Society

Received: July 21, 2016 Revised: September 29, 2016

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package in a two-roll mill, during which Zn-based bonds were formed. The samples were then subjected to compression molding at 143 °C to generate the sulfide-based covalent cross-links. Therefore, the multinetwork in IR was constructed by sulfide covalent cross-links, hydrogen bonds, and Zn-based bonds. The optimal time for the compression molding was determined by a U-CAN UR-2030 (Taiwan) vulcameter (Figure S1). The sulfur-based curing package includes zinc oxide 5, stearic acid 1, N-cyclohexyl-2-benzothiazole 1.5, 2,2′-dibenzothiazole disulfide 0.5, and sulfur 1.5 (relative to 100 parts of IR or MAn-g-IR). The resulted rubbers were denominated ZnATAx, in which the molar ratio of ZnCl2 to ATA and MAn was x. To investigate the effect of hydrogen-bonding content, variable contents of ATA (ATA:MAn mole unit = 0.3:1, 0.5:1, 1:1, and 1.5:1) were reacted with MAn-g-IR. No ZnCl2 was added to these systems during the compounding process. After compression molding at 143 °C, the sulfide covalent cross-links were formed, and accordingly a dual-network IR was obtained. The rubbers were named ATA-x, where x indicates the molar ratio of ATA content to the molar ratio of the MAn group. In the present work, ATA-1 and ZnATA-0 represent the same sample. Sulfur-cured IR was used as the control sample. To examine the effect of sacrificial networks on fatigue resistance, CB-filled IR, ZnATA-0, and ZnATA-0.4 systems were used (coded as filled IR, filled ZnATA-0, and filled ZnATA-0.4, respectively) in which 60 phr CB (N330) and 8 phr aromatic oil were added relative to 100 phr rubber. For all samples, the rubber ingredients remained the same. Characterizations. The quantity of MAn grafted onto IR was determined by titrating the acidic groups derived from the anhydride functions.23,24 The precipitated MAn-g-IR (approximately 1 g) was first dissolved in 100 mL of toluene at 110 °C. Distilled water was then added to hydrolyze the anhydride function into carboxylic acid functional groups, and the solution was refluxed for another 2 h to complete the hydrolysis. The titration was conducted with 0.025 mol/ L potassium hydroxide (KOH) in methanol solution, and phenolphthalein in methanol was used as the indicator. When the titration endpoint was reached, another 2 mL of KOH/methanol was added to the solution, and the mixture was refluxed for 15 min. Finally, the system was back-titrated using HCl/isopropanol solution (0.02 mol/L). The grafted MAn content was calculated as shown in eq 1:

motifs in a chemically cross-linked architecture and obtained remarkably improved tensile strength and toughness without sacrificing extensibility.16 Zhang et al. incorporated a sacrificial network into a commercially available styrene−butadiene vulcanized rubber to form a double network and observed greatly improved fracture energy.17 As another type of dynamic bond, multiple hydrogen bonds can also act as sacrificial bonds and have been introduced into covalently cross-linked rubbers by other researchers. Gold et al. developed a new dual-network elastomer consisting of covalently cross-linked polybutadiene with a transient network formed by the association of urazole groups randomly attached to the polymer backbone. During stretching, the weaker hydrogen bonds formed by urazole groups sacrifice first and dissipate energy, conferring increased toughness.18 These examples clearly demonstrate the great potential of sacrificial bonds or networks in enhancing the mechanical performance of vulcanized rubbers. Inspired by the energy dissipation mechanism of sacrificial bonds, we describe here a novel approach that takes advantage of the versatility of sacrificial bonds to achieve an advanced rubber with high strength, stretchability, and excellent cracking resistance. Previous work involving sacrificial bonds for hydrogels and elastomers relied on either metal−ligand interactions or hydrogen bonds. To maximize the contribution of sacrificial bonds to mechanical properties, a combination of two types of sacrificial bonds with different binding energy is generally required. Specifically, we incorporated weaker hydrogen bonds and stronger metal-based units into a chemically cross-linked cis-1,4-polyisoprene (IR) network. For the first time, metal−triazole coordination, which has been widely used to prepare functional materials for luminescence, magnetism, catalysis, gas storage, and separation,19−21 was introduced into elastomers as dynamic bonds. The cooperative energydissipation effect of the sacrificial hydrogen bonds and metalbased units produced a combination of high stretchability and fracture energy. Significantly, we observed that this design leads to much improved flex-cracking resistance.

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grafted MAn content =

N1V1 − N2V2 × 98 × 100% 2w

(1)

where N1 and V1 are the concentration and volume of KOH/methanol solution, N2 and V2 are the concentration and volume of HCl/ isopropanol solution, and w is the weight of the MAn-g-IR sample. Fourier transform infrared (FTIR) spectra were collected on a Bruker Vertex 70 FTIR spectrometer. In situ FTIR measurements of uncured ATA-1 (excluding rubber additives, similarly hereinafter) were also carried out at a resolution of 1 cm−1 using a temperature chamber. To substantiate the interactions between the Zn ions and the triazole units, FTIR and 1H NMR of ATA and ATA/ZnCl2 mixture (molar ratio 2/1) were conducted. The 1H NMR spectra were recorded with a Varian NMR spectrometer. DMSO-d6 was used as the solvent. The loading−unloading cycles were performed on a U-CAN UT2060 (Taiwan) instrument with an extension rate of 100 mm/min at room temperature. In each cycle, the sample was stretched to a strain of 300% 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. The sample was then heated at 80 °C for 30 min and cooled at 25 °C for another 30 min to heal the dynamic network, followed by another loading−unloading cycle. The tensile strength was measured according to ISO standard 37-2005 with an extension rate of 500 mm/ min. The fracture energy (W) is defined as the area surrounded by the stress (σ)−strain (ε) curves and calculated using eq 2 as follows:

EXPERIMENTAL SECTION

Materials. IR (IR 2200, 98% cis-1,4 polyisoprene with Mn of 5 × 105 g/mol) was purchased from Zeonex (Japan). Maleic anhydride (MAn), 3-amino-1,2,4-triazole (ATA), and zinc chloride (ZnCl2) were obtained from Aladdin Chemicals. Carbon black N330 (CB) was provided by Tianjin Dolphin Carbon Black Co., Ltd. Other rubber ingredients were industrially available products and used as received. Preparation of MAn-g-IR. MAn-g-IR (MAn grafted IR) was prepared by a procedure modified from the literature.22 Typically, a mixture of 30.5 g of IR, 4.39 g of MAn (10% mol relative to the isoprene unit), 0.305 g of N-phenyl-N′-1,3-dimethylbutyl-p-phenylenediamine (6PPD, a kind of antioxidant, 1 wt % based on IR), 4.75 g of xylenes (10% mol relative to the isoprene unit), and 3.05 g of aromatic oil (10 wt % based on IR) was mixed at 45 rpm at 90 °C for 20 min in an internal mixer. The resulting mixture was further mixed at 45 rpm at 210 °C for 60 min to afford maleated IR. The compounded rubber mixture was then dissolved in toluene, and the solution was precipitated in acetonitrile. The precipitated polymer was washed thoroughly with acetonitrile and dried in vacuo at 50 °C for 12 h. The introduction ratio of MAn was determined to be 2.4 mol % based on the titration method. Construction of the Multinetwork in IR. A mixture of 30 g (0.01 mol of MAn unit) of MAn-g-IR, 0.84 g of ATA (ATA:MAn mole unit = 1:1), and 1.5 phr (parts per hundred rubbers) 6PPD was mixed at 50 rpm at 80 °C for 60 min in a mixer to incorporate multiple hydrogen-bonding units into IR. After that the rubbers were compounded with variable contents of ZnCl2 and sulfur-based curing

W=

ε = εmax

∫ε=0

σ dε

(2)

where ε is the strain and σ is the stress.25 The stress-relaxation experiments at 25 °C and creep experiments at 50 and 100 °C were performed with a TA DMA Q800 machine. In B

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Scheme 1. (a) Schematic Illustration for the Preparation of MAn-g-IR and ATA-Modified MAn-g-IR; (b) Proposed Model of the Multinetwork in IR

stress relaxation, the samples were stretched to 100% strain at the strain rate of 5 min−1, and the constant strain was maintained to measure the relaxation of the stress for 3600 s. In creep experiments, the stress of 0.5 MPa was applied to stretch the samples for 600 s. The strain sweeping measurement was conducted on a rheometer Anton Paar MCR302 (Austria) with a parallel plate−plate geometry at a constant angular frequency of 6.28 rad/s from 0.01% to 10% strain. The normal force was fixed as 1.6 N. Small-angle X-ray scattering (SAXS) measurements were carried out on a NanoSTAR instrument (Bruker-AXS, Germany) at room temperature. The wavelength of the X-ray was 0.154 nm. Dynamic mechanical analysis (DMA) was performed using a TA DMA Q800 with the preload force of 0.01 N. A tensile mode was adopted with a dynamic strain of 0.5%. The samples were scanned from −100 to 130 °C, and the frequency and heating rate were 1 Hz and 3 °C/min, respectively. Multifrequency tests were performed in the temperature scanning mode in the range of −80 to 100 °C, with a step of 10 °C. At each temperature the frequency sweeping was conducted from 0.5 to 50 Hz, and the strain amplitude was 0.3%. To construct master curves, isothermal spectra were processed in TA software (TTS Date Analysis) with −30 °C as the reference temperature. To measure the total cross-link density (Ve), the samples were swollen into toluene as the method described in refs 26 and 27. The IR−toluene interaction parameter is taken as 0.393.28 To determine the covalent cross-link density (Ve1), samples were swollen in toluene/dichloroacetic acid to destroy hydrogen bond and Zn-based bond cross-links.26,29,30 The cross-link density of dynamic bonds (Ve2) was calculated by subtracting Ve1 from Ve. Fatigue resistances for CB-filled systems were determined on a GT7011-D flexing resistance tester (Gotech, Taiwan) according to the ISO 132:1999 standard. According to the standard, the flex-cracking

resistance of vulcanizates is ranked at different levels, where grade I indicates that “needle-like points” (macroscopic number ≤10) form in the groove of the samples and grade III indicates that “needle-like points” extend to obvious cracks (one or more, with a crack length of 0.5−1 mm). Grade IV, grade V, and grade VI represent the longest cracks of 1−1.5, 1.5−3, and >3 mm, respectively.31 X-ray diffraction (XRD) patterns were obtained for prestretching samples at different strains (2θ = 10°−27°) using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm).

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RESULTS AND DISCUSSION

Incorporation of Hydrogen Bonds and Zn-Based Units into IR. To introduce a hydrogen-bonding moiety into IR, the active hydrogen compound ATA was adopted to react with MAn-g-IR to form amide triazole−carboxylic acid groups. Each group was capable of interacting with other groups via hydrogen bonding, which has been demonstrated previously to proceed effectively.22 Then, ZnCl2 was added to form Zn-based units (Scheme 1). The formation of hydrogen bonds and Zn-based units was confirmed by FTIR analysis. As shown in Figure 1, compared with IR, MAn-g-IR exhibited characteristic peaks of a saturated acid anhydride at 1860 and 1780 cm−1, which were attributed to symmetric and asymmetric CO stretching, respectively. These features indicate the presence of grafted succinic anhydride on the backbone of IR.22,32 The addition of ATA to MAn-g-IR was also identified. As shown in ATA-modified C

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Figure 1. FTIR spectra of IR, MAn-g-IR, and ATA-modified MAn-gIR. Figure 3. Evolution of FTIR spectra of ZnATA-x with increasing zinc content.

MAn-g-IR, the absorption peaks of anhydride carbonyl groups at 1860 and 1780 cm−1 were weakened, and the characteristic absorption peak of carboxylic acid groups at 1726 cm−1 emerged, indicating successful reaction of ATA to MAn-gIR.22,33,34 Simultaneously, new absorption peaks at 1598 and 1519 cm−1 ascribed to NCH and C−NC stretching in the triazole ring were observed.20,21,35 These amide triazole− carboxylic acid units of rubber chains can form hydrogenbonded architectures that act as physical cross-links, as depicted in Scheme 1b.22,36 The temperature-denpendent FTIR spectra are shown in Figure 2. With increasing temperature, the characteristic

triazole complexation, the peak around 1519 cm−1 assigning to C−NC stretching also shifts to 1549 cm−1, further indicating the formation of Zn2+−triazole coordination.39 The results of FTIR and 1H NMR spectra of ATA and ATA/ZnCl2 model compound further substantiate the coordination nature between the Zn ions and the triazole units (Figure S2). Because of the equimolar ATA/MAn ratio in ATA-1, a part of carboxylic acid moieties survive in the chain (Scheme 1a), and Zn ions may also interact with the remaining carboxylic acid units. According to the reference, when ionic bond is formed between metal and carboxylic acid units, characteristic peaks in FTIR generally shift to lower frequencies, typically from around 1730 to 1640 cm−1.40 As shown in Figure 3b, with increasing ZnCl2 content, the peak around 1644 cm−1 is intensified, indicating the formation of zinc−carboxylate bonds. Therefore, it can be concluded that Zn-based bonds of ZnATAx samples are composed of Zn−triazole coordination and Zn− carboxyl units (Scheme 1b). Overall, the above results definitely demonstrate the successful incorporation of hydrogen bonds and zinc-based bonds into IR. Dynamic Features of Sacrificial Multiple Hydrogen Bonds and Zn-Based Bonds. The multiple hydrogen-bond moiety (amide triazole−carboxylic acid unit) generated by the addition reaction of ATA with acid anhydride might form a hydrogen-bonding structure producing a relatively strong physical cross-linking moiety.22,36 To confirm the existence of hydrogen bonding as sacrificial bonds, cyclic tensile tests were performed with stretching to a predefined 300% strain (Figure 4). As expected, low hysteresis was observed for the sulfurvulcanized IR in the first cycle (Figure 4a), which is reasonable for an unfilled rubber with sulfide-based cross-links only. By contrast, taking sample ATA-1 as an example (Figure 4b), significant hysteresis was observed in the first loading− unloading cycle. The hysteresis behavior indicates the dissipation of energy during stretching, which is associated with the rupture of the hydrogen-bonding network. In the unloading process, ATA-1 quickly recovers because the elastic contraction is dominant over the sacrificial hydrogen bonds, and it cannot return to its original length and leave a notable residual strain. When the sample is allowed to relax at room temperature after loading, the residual strain gradually decreases. However, the recovery process is very slow, and the stress−strain curve does not completely recover to the original loading curve after a relatively long wait time (1800 s). The wait-time dependence of the residual strain indicates that the recovery process involves competition between the

Figure 2. Evolution of FTIR spectrum of uncured ATA-1 with increasing temperature.

absorption peak of carboxylic acid shifts from 1726 to 1733 cm−1,22 indicating gradual breakage of hydrogen bonds. At the same time, the CO stretching of amide group shifts from 1664 to 1658 cm−1,37 and the (NCH) stretching of the triazole ring shifts from 1598 to 1584 cm−1.20 These temperature-dependent peak shifting provides convincing evidence to the formation of hydrogen bonds. Triazole units can function as ligands for metal−ligand coordination when they interact with metal salts.19,20,38 In the present work, metal−ligand coordination was introduced by adding different contents of ZnCl 2 to the hydrogenincorporated systems. To verify the coordination formation between Zn ions and triazole moieties of the chains, FTIR spectra of the samples with increasing zinc content are compared (Figure 3a). In ZnATA-0 (ZnCl2 free system), the absorption peak at 1598 cm−1 is assigned to NCH stretching of triazole ring.20 After introducing Zn2+, this peak is apparently broadened, more obvious at higher Zn2+ content (ZnATA-0.4, as indicated by the blue arrow), which is due to NCH stretching upon complexation.20 Meanwhile, upon Zn2+− D

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Figure 4. Tensile loading−unloading curves of (a) IR and (b) ATA-1. (c) Dependence of W2/W1 and ΔW for ATA-x rubbers on the ATA/MAn molar ratio. The dashed lines in black and red represent W2/W1 and ΔW of IR, respectively.

Figure 5. (a) Tensile loading−unloading curves of ZnATA-0.2. (b) Dependence of W2/W1 and ΔW for ZnATA-x rubbers at various Zn2+/ATA molar ratios. (c) Normalized residual strain vs wait time for ZnATA-0 and ZnATA-0.2.

The hysteresis ratio between the second cycle and the first cycle (W2/W1) is indicative of the energy dissipation of the sacrificial bonds, where W1, the dissipating energy of the first loading−unloading cycle, is defined as the area surrounded by the first loading and unloading stress−strain curve, and W2, the dissipating energy of the second loading−unloading cycle with the waiting time of 0 s, is defined as the area surrounded by the second cycle. A lower W2/W1 value represents higher energy dissipation. The dependence of W2/W1 on ATA content is shown in Figure 4c. With increasing hydrogen bond content (Figure S4a), W2/W1 exhibits a monotonic declining trend, indicating increasing hysteresis. As a more intuitive approach,

elasticity of the covalent network and the strength of the temporarily re-formed hydrogen bonds. At room temperature, the residual elastic contraction becomes weak, and the reformed hydrogen bonds slow the recovery of the primary chain to its equilibrium state. The slow process probably also includes the reorganization of the re-formed bonds. After heating at 80 °C, the hydrogen bonds retarding the recovery of the covalent network dissociated, and nearly complete self-recovery, without residual strain, was observed (as the purple line in Figure 4b).13,16 The samples with other content of hydrogen bonds exhibit similar hysteresis behavior (Figure S3). E

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Figure 6. Strain sweeping measurement at a constant frequency of 6.28 rad/s.

the hysteresis difference ΔW (W1 − W2) is also depicted, revealing the gradual enlargement of the hysteresis brought about by the rupture of more hydrogen bonds. From the comparison of stress−strain curves of the samples during the first cycle (Figure S5a), it is observed that the tensile modulus consistently increases with increasing content of hydrogen bonds. For instance, the modulus at 300% strain for ATA-1.5 is about 4-fold compared with that for ATA-0.3, indicating the significant effect of the hydrogen bonds on the improvement in machnical properties. The reversibility of Zn-based bonds was also illustrated by cyclic tensile tests. Typical loading−unloading curves for sample ZnATA-0.2 are shown in Figure 5a. The hysteresis performance of the samples with other content of Zn-based bonds has also been measured (Figure S6). A significantly larger hysteresis comparing with that of ZnATA-0 (Figure 4b) was observed in the first loading−unloading cycle. Because essential hydrogen bonds will be retained after the incorporation of Zn-based bonds, the energy dissipation related to the hysteresis originates from the fracture of both hydrogen bonds and Zn-based units. After relaxing under room temperature, the residual strain gradually decreased with increasing wait time. During heating at 80 °C for 30 min, the re-formed hydrogen bonds and Zn-based interaction broke at the elevated temperature, and the restriction on the covalent network to its equilibrium state was diminished, accelerating the recovery process. When the relaxation of the covalent network is finished, the hydrogen bonding and Zn-based bonds healed upon cooling, contributing to the recovered hysteresis. One may also compare the modulus of the samples with changing content of Zn-based bonds. As the content of Zn-based bonds increases, the tensile modulus also consistently increases, indicating the significant effect of Zn-based bonds on the improvement in machnical properties (Figure S5b). The dependences of W2/W1 and ΔW on ZnCl2 content are presented in Figure 5b. Increasing the ZnCl2 content leads to higher dissipative energy due to the increased content of Znbased dynamic bonds (Figure S4b). With the incorporation of a 0.4 molar ratio of ZnCl2 (sample ZnATA-0.4), an approximately 2.5-fold increase in ΔW compared with that of ZnATA0 was achieved. To examine the recovery rate of hydrogen bonding and Znbased bonds qualitatively, the normalized residual strain dependences of the wait time for ZnATA-0 and ZnATA-0.2 were compared (Figure 5c). Compared with ZnATA-0, the residual strain in the multinetwork ZnATA-0.2 recovers significantly faster at room temperature. During stretching, the elastic contraction is dominant at large deformation and the re-formed dynamic bonds would slow down the recovery of the primary chain to its equilibrium state when the external force is

removed. According to the literature, the binding energy of metal−ligand coordinative bonds is several orders higher than that of hydrogen bonds.41,42 Since the principle of “strong means slow” has been reported by others, the re-forming of Znbased bonds should be slower than hydrogen bonds.43,44 Therefore, the restriction on the recovery in ZnATA-0.2 is weaker than ZnATA-0, contributing to the faster strain recovery in ZnATA-0.2. The above evidence indicates that the multinetwork structure is composed of a sulfur covalent network, a weaker sacrificial hydrogen bond network, and a stronger sacrificial Zn-based network. The rheological measurements were obtained by measuring the storage modulus (G′) and the loss modulus (G″) at various strains. As shown in Figure 6, at smaller strains, G′ values for all samples remain almost constant, and the initial G′ values for ZnATA-0 and ZnATA-0.2 are higher than that of IR, implying the contributions from hydrogen-bonding and Zn-based networks. With increasing strain, G′ decreases for all samples at larger strain because of the disentanglement and rupture of the dynamic bonds. Because of the probable enlarged localized strain, the faster dissociation/reassociation of multiple hydrogen bonds may occur. The rearrangement of the hydrogen bonds and the slightly decreased covalent cross-linking density lead to the observed larger G″ and tan δ for ZnATA-0. Because of the relatively stronger binding energy of Zn-based bonds compared with that of hydrogen bonds, the dissociation of Znbased units may be restricted at smaller strains, and consequently G″ of ZnATA-0.2 is smaller than ZnATA-0. However, at larger strain amplitude, the disssociation and reassociation of Zn-based bonds may also occur, leading to increased G″ and tan δ for ZnATA-0.2. To further illustrate the different binding energy of hydrogen bonds and Zn-based units as dynamic bonds, stress relaxation analysis was performed at 25 °C (Figure 7). All samples were quickly subjected to a strain of 100%, and then the strain was maintained for 3600 s. The characteristic relaxation times are determined and compared (Supporting Information). Compared with IR, ZnATA-0 released the applied force rapidly, providing strong evidence that the hydrogen bonds undergo dissociation under loading. Compared with ZnATA-0, the relaxation process in ZnATA-0.2 was slower because of the restriction derived from the stronger Zn-based bonds. After a significant drop, the force for ZnATA-0 and ZnATA-0.2 remained nearly unchanged with time due to the load retention from the covalently cross-linked network.15 The stronger binding energy of Zn-based units than that of hydrogen bonds can be also corroboratively evidenced by creep behaviors (Figure S7). Effects of the Multinetwork on Mechanical Performance. The dependence of the stress−strain profiles on ATA F

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the strand is able to relax locally, which dissipates energy and prevents the local concentration of stress, facilitating the chain orientation. Therefore, a larger elastic modulus and fracture energy (Figure 9) are achieved. The dynamic nature of the sacrificial hydrogen bonds and Zn-based bonds helps to sustain the high stretchability of the rubber. When more than 0.2 molar ratio of ZnCl2 is incorporated, reduced extensibility and fracture energy are observed (Figure 9), which may be due to the presence of possible inhomogeneities at higher zinc content. The SAXS profile of ZnATA-0.2 (Figure S8) shows a scattering shoulder peak, implying that there is a little electron density fluctuation at the nanometer scale. This phenomenon should be attributed to the slight phase separation of some ionic bonds as it has been previously demonstrated that the coordination does not generate phase separation.16 However, since the actual content of ZnCl2 is very low in the present system (less than 1 part relative to 100 parts gum rubber), the improved properties should be mainly attributed to the dynamic feature of the bonds, instead of the possibly formed clusters. In order to disclose the definite contribution of the Zn2+− triazole coordination bonds to the mechanical property, an additional experiment on the sample without residue carboxylic acid groups was supplemented. In this experiment, the ATA/ MAn ratio was adjusted to 2/1, and theoretically the resulted sample (ATA-2) does not contain carboxylic groups anymore. Upon addition of ZnCl2 (Zn/ATA molar ratio of 0.5/1), the mechanical property of the sample is definitely higher than that for the control sample, indicating the contribution of coordination to the mechaical performance (Figure S9). To probe the effect of sacrificial networks on thermomechanical behavior, the temperature dependences of the storage modulus (E′) and tan δ for the dual-network ATA-x and multinetwork ZnATA-x are presented in Figures 10 and 11, respectively. E′ at 25 °C as a function of ATA content is depicted in Figure S10a. As shown in Figure 10a, with the incorporation of hydrogen bonds, the values of E′ for ATA-1 and ATA-1.5 are higher than that for IR when the temperature is sufficiently low (less than 50 °C). At higher temperatures, because of the heat-induced dissociation of hydrogen bonds, the moduli of the samples with dual networks start to decrease. Strikingly, E′ for IR remains nearly constant in the rubbery region. The dissociation of hydrogen bonds is also implicated by an upward trend of tan δ near 50 °C (Figure 10b). The glass transition temperature (Tg) determined by the peak value of tan δ gradually shifts to a higher temperature with increasing ATA content, suggesting a restriction of segmental relaxation by the hydrogen bonds.

Figure 7. Stress relaxation of IR, ZnATA-0, and ZnATA-0.2 at 100% strain.

content is depicted in Figure 8a. IR exhibits typical straininduced crystallization (SIC) when the strain is stretched to a strain greater than the threshold value (about 500%) corresponding to the onset of crystallization. During stretching, the crystals which are aligned with the stretching direction will be formed, contributing to improved mechanical properties.45−47 As the incorporation of hydrogen bonds, the tensile modulus is significantly improved. This improvement can be interpreted as an increase in the energy dissipation caused by the rupture of hydrogen bonds. In the dual network, the elastomeric architecture is held together by dual cross-links consisting of a covalent network and hydrogen bonding. During stretching, the hydrogen bonds act as sacrificial units by reversibly rupturing and re-forming to dissipate energy, which also helps to render high stretchability and contributes to the increased fracture energy (Figure 8b). The preferential rupture and dynamic features of the sacrificial hydrogen bonds “smooth out” the stress distribution and prevent local concentrations of stress, which facilitates chain sliding and orientation when force is applied, thus improving the strength of the rubber.48,49 When a 1.5 molar ratio of ATA was added, the fracture energy is improved to more than twice that of IR (as indicated by the dashed blue line). The stress−strain curves of the ZnATA-x system at various ZnCl2 contents are presented in Figure 9a. The tensile modulus increased with ZnCl2 content, indicating the significant contribution from Zn-based bonds. The proposed mechanism of energy dissipation in the multinetwork IR system is shown in Scheme 2. During stretching, the sacrificial hydrogen-bonding network breaks first because of its weaker bond strength. At larger strains, the Zn-based bonds subsequently rupture. By releasing the transient cross-links, excess stress is released, and

Figure 8. (a) Stress−strain curves of hydrogen-bond-incorporated IR dual-network systems of ATA-x. (b) Fracture energy of the rubber at various ATA/MAn molar ratios for ATA-x. The blue dashed line represents the fracture energy of IR. G

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Figure 9. (a) Stress−strain curves of multinetwork IR systems with various Zn2+ contents. (b) Fracture energy of the multinetwork rubber with various Zn2+/ATA contents.

Scheme 2. Proposed Mechanism of Cross-Link Rupture in the Multinetwork IR System during Stretching

Figure 10. Dependence of E′ (a) and tan δ (b) on temperature for the ATA-x system.

Figure 11. Dependence of E′ (a) and tan δ (b) on temperature for the ZnATA-x system.

Figure 11a shows the effects of the multinetwork on the thermomechanical properties of the rubber. Although the introduction of Zn-based units leads to a limited improvement of E′ at lower temperatures, the modulus of the multiple network at higher temperatures definitely increases with increasing Zn-based bond content (Figure 11a and Figure S10b). Consequently, this additional improvement should be attributed to the contribution of Zn-based bonds. Because of

the higher binding energy of Zn-based bonds compared with hydrogen bonds, a portion of the Zn-based units can remain intact at high temperatures (>50 °C), contributing to enhanced E′. However, the effect of Zn-based bonds on Tg is minimal, as shown in Figure 11b, which may be due to its lower content. Multifrequency experiments were performed to evaluate the frequency-dependent behavior. The Tg gradually shifts to lower frequency with the incorporation of hydrogen bonds and ZnH

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cycles of grade I in filled ZnATA-0.4 tripled compared with those of the filled IR and increased to more than 7.5-fold compared to filled ZnATA-0. Previous studies have extensively sought to elucidate the mechanism underlying the improved cracking resistance during the deformation of NR and IR. Cavitation occurs in the bulk material at small deformations, and the number of preexisting microvoids per unit volume strongly correlates with the mechanical performance of elastomers.50,51 Le Cam et al. investigated fatigue-crack growth in a CB-filled IR under relaxing and loading conditions and observed that cavitation induced by decohesion between zinc oxide and the rubber matrix was the major fatigue damage.52 SIC has also been regarded as a key factor in cracking resistance performance in NR. Le Cam et al. investigated the mechanism of fatigue crack growth in NR subjected to severe relaxing and loading conditions and found that the resultant high crystallinity at the crack tip halted crack growth in the plane perpendicular to the loading direction.53 The effect of SIC on cracking resistance in NR has also been verified by others.31,54 Saintier et al. reported a cumulative process of crystallization at the crack tip of NR under a no-relaxation cyclic loading condition and stated that the crack branching from the crystallization was the main reason for the reinforcement of crack-growth resistance.55 Yan et al. prepared NR composites containing graphene (GE) and observed that GE had an opposite effect on the crack growth resistance of NR. At lower fatigue strains, the inclusion of GE accelerated crack growth because of the greater number of cavities generated by the incorporation of GE into the rubber matrix. At higher strains, SIC occurred, and the crystallites that formed at the crack tip hampered crack growth and resulted in crack branching, enabling significantly greater energy dissipation and improved crack-growth resistance.56 Nearly all of these studies have attributed the enhanced cracking resistance to SIC, but the cracking mechanisms remain controversial.

based units (Figure S11), suggesting the restriction effect of segmental relaxation, which is also consistent with the results presented in Figures 10 and 11. Contribution of the Multinetwork to Fatigue Resistance. Because rubbers usually undergo a large variety of dynamic loading conditions in real service, fatigue resistance, such as the cracking resistance property, has always been a primary concern to ensure the safety and reliability of various applications. To investigate the influence of sacrificial networks on the fatigue properties of rubber, flex-cracking tests were conducted on IR, ZnATA-0, and ZnATA-0.4 filled with carbon black. As shown in Figure 12, the sacrificial hydrogen-bonding

Figure 12. Cracking resistance of filled-IR, filled-ZnATA-0, and filledZnATA-0.4.

and Zn-based bonds had opposing effects on flex-cracking resistance. With the incorporation of the hydrogen-bonding network (sample filled ZnATA-0), even the flex-cracking resistance was somewhat deteriorated compared with the filled IR. Surprisingly, the introduction of the Zn-based bonds improved the flex-cracking resistance significantly. When incorporating a small amount of Zn2+ (mole ratio of 0.4), the

Figure 13. XRD patterns of (a) IR, (b) ZnATA-0, and (c) ZnATA-0.2 at selected strains. I

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To explore the source of the significantly improved fatigue resistance in the multinetwork, the effect of the sacrificial networks on SIC was first investigated. The linear diffraction profiles taken from the XRD measurements of IR, ZnATA-0, and ZnATA-0.2 at typical strains are provided in Figure 13. According to the literature, two diffraction peaks should be observed near 2θ = 14° (200 plane) and 2θ = 21° (120 plane) as the IR crystal forms.57 In the present work, as the strain increased, IR began to exhibit two weak peaks at approximately 2θ = 14° and 21° when the strain is 500% (Figure 13a). Further increasing the strain to 550% intensified both of these peaks. However, in ZnATA-0 and ZnATA-0.2, only a sharp peak at 2θ = 13.5° was observed (Figure 13b,c), which is assigned as the slight residue crystals of zinc maleate.58 These results definitely demonstrate that SIC is not promoted effectively by hydrogen bonds or Zn-based bonds, suggesting a different mechanism of improved flex-cracking resistance compared to earlier reports. In the present work, after incorporating hydrogen bonds, filled ZnATA-0 is assumed to generate more cavities because of the rupture of the hydrogen bonds near the crack tip under cyclic loading.59 During the flex-cracking tests, the cavity membranes tear, resulting in cavity growth and eventual coalescence, destruction of the crack tip, and propagation of the crack perpendicular to the loading direction.52 Because of the stronger strength of the Zn-based bonds, the incorporation of Zn-based units is believed to increase the energy necessary to break the sacrificial bonds at the crack tip. During the flexcracking tests, the hydrogen bonds broke first, followed by rupture of Zn-based units. Both of these ruptures dissipate energy. Furthermore, because of the fast dynamics, re-forming of Zn-based dynamic bonds and hydrogen bonds at the cracking tips may also be possible and may hinder tearing of the cavity membranes, thus contributing to the significantly improved flex-cracking resistance. It should be emphasized that the comprehensive and accurate physical picture of the fatigue mechanism needs further investigations in the future. However, our findings provide significant inspiration for the development of elastomers with high fatigue resistance.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01576. Curing curves, hysterisis curves, cross-link densities, creep behavior, SAXS profiles, tan δ master curves of ATA-x and ZnATA-x; FTIR and NMR of model compound; dependences of modulus as a function of ATA or zinc concentration (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.G). *E-mail: [email protected] (G.H). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2015CB654703), National Natural Science Foundation of China (51673065, 51333003, U1462116 and 51473050), and Natural Science Foundation of Guangdong Province (2014A030310435 and 2014A030311051).

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REFERENCES

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CONCLUSIONS In summary, we have developed advanced elastomers with a combination of high strength, large extensibility, and excellent flex-cracking resistance. This goal was accomplished by constructing a multinetwork by incorporating weaker sacrificial hydrogen bonding and stronger sacrificial Zn-based bonds into a chemically cross-linked IR network. The dynamic nature of the sacrificial hydrogen bonding and Zn-based bonds was substantiated. The transient bonds are ruptured during stretching and re-form with the aid of heat upon unloading. Upon loading, the hydrogen bonds break first, followed by rupture of the Zn-based bonds, which dissipates energy and confers significantly improved tensile modulus and fracture toughness. Although the introduction of hydrogen bonding led to deteriorated flex-cracking resistance, the flex-cracking resistance in the filled multinetwork was drastically improved. Because SIC in IR was not effectively promoted by hydrogen bonds or Zn-based bonds in the present work, the enhanced fatigue resistance is mostly attributable to the energy dissipation and the possible re-forming of sacrificial bonds. We envision that the design concepts of a multinetwork presented here will open up a significant avenue to the preparation of advanced elastomers with excellent mechanical properties under both static and dynamic conditions. J

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