Enabling Design of Advanced Elastomer with Bioinspired Metal

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Enabling Design of Advanced Elastomer with Bioinspired Metal-Oxygen Coordination Xuhui Zhang, Zhenghai Tang, Baochun Guo, and Liqun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10881 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

Enabling Design of Advanced Elastomer with Bioinspired Metal-Oxygen Coordination Xuhui Zhang,† Zhenghai Tang,† Baochun Guo* , † and Liqun Zhang*, ‡ †

Department of Polymer Materials and Engineering, South China University of Technology,

Guangzhou 510640, China ‡

State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, China KEYWORDS biomimetic, metal-oxygen coordination, toughness, damping, triple-shape memory polymer

ABSTRACT It poses a huge challenge to expand application gallery of rubbers into advanced smart materials and achieve the reinforcement simultaneously. In the present work, inspired by the metal-ligand complexations of mussel byssus, ferric ion was introduced into an oxygenabundant rubber network to create additional metal-oxygen coordination cross-links. Such complexation has been revealed to be highly efficient in enhancing the strength and toughness of the rubbers. Significantly, such complexation also enables the functionalization of the rubber into highly damping or excellent multi-shape memory materials. We envision that the present work offers efficient yet facile way for creating advanced elastomers based on industrially available diene-based rubber.

ACS Paragon Plus Environment

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INTRODUCTION Rubbers are of irreplaceable importance in fields of seals, tires and shock absorbers,1-3 but their practical applications are greatly hampered due to the low modulus and strength. Besides, rubber-based materials suffer from bottleneck in expanding their applications into advanced smart materials. Therefore, rational designs are highly desired to achieve high strength and functions simultaneously. Inspiringly, mussels possess a stretchable byssus with high modulus as well as the capability of fastening themselves to accessible rocks,

4-7

which provides a typical

example in simultaneous reinforcement and functionalization. Both the high modulus and the unique adhesion capability have been demonstrated to be originated from the existence of reversible dopa-metal complexation.

8-9

Similarly, because of the existence of coordination

between Ca2+ and phosphoserine, the silks of caddisworms exhibit a combination of extensibility, high modulus and a unique adhesion to stones or plant matter.

10-11

Due to the reversibility and

high bond energy among noncovalent interactions, coordination can efficiently dissipate energy even after hundreds of extension cycles, which makes it a good choice to reinforce soft materials, such as hydrogel12 and rubber.13-15 Follow this concept, Tang et al. incorporated Zn2+ into commercially available butadiene−styrene−vinylpyridine rubber (VPR) to construct additional Zn2+-pyridine coordination in rubber network architecture, which resulted in a strikingly improved strength (increased by ~600%).13 What’s more, due to its responsiveness to stimuli, coordination is also promising in desinging of smart materials.16-18 By introducing alginate-Ca2+ complexes, Meng et al. fabricated an excellent shape memory hydrogel with capability of shape memory at the macro-/microscopic scales.19 Therefore, incorporating complexation seems to be a promising choice to simultaneously reinforce and functionalize rubbers. Unfortunately, most reported systems containing complexation involve intricate molecular makeup and processing. 20-


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In this regard, the construction of complexation via facile technologies is highly desired and

especially attractive. In this study inspired by mussel byssus, we introduced Fe3+, a metal ion possessing six coordination sites, into oxygen-abundant diene-based rubber network (epoxidized natural rubber, ENR) via a facile way to create metal-oxygen coordination cross-links. Such complexation has been revealed to be highly efficient in enhancing the strength and toughness of the rubber. Noticeably, such complexation enables additional new functions in these materials, such as widetemperature damping and excellent multi-shape memory effect. 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, Zhanjiang, China. Zinc acrylate (ZDA, chemically pure) was purchased from Alfa Aesar (China) Chemicals Co., Ltd. FeCl3.6H2O (analytically pure) was produced by Damao chemical reagent factory, Tianjin, China. Model Compounds Preparation. The ENR/FeCl3 model compounds were prepared for spectral analyses. ENR and FeCl3. 6H2O were dissolved in abundant chloroform and mixed in specific ratios (the molar ratios of epoxy to ferric are 9:1 and 3:1, respectively). After stirring for 6 h, the mixtures were poured into PTFE molds and dried. For sample with hexamethylenediamine, the hexamethylenediamine solution was added into the mixture of ENR/FeCl3 solution (molar ratio is 3:1). After stirring for 3 h, the mixture was poured into PTFE mold and dried. The dried samples were subjected to Raman, FTIR and XPS measurements. 0.1


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g ENR or ENR/FeCl3 mixture was dissolved in 100 mL choloroform to prepare samples for UVvis spectroscopy. Elastomeric Samples Preparation. The ENR/FeCl3 master batch was firstly prepared by solution mixing. Then designed content of master batch, ENR and ZDA were mixed on an open two-roll mill, followed by hot-pressing at 160 oC with the optimum curing time. Specially, the SMP samples (with higher ZDA content) were hot-pressing at 160 oC for 1 h. The formulations of all the samples are listed in Table S1. To ease the description, all samples are named as EZxFy, where E, Z, F represent ENR, ZDA and FeCl3, respectively and x, y represent the content of ZDA and FeCl3 (parts per 100 parts of gum), respectively. Characterization. Raman spectra were collected on a LabRAM HR 800 Raman spectrometer with a HeeNe ion laser (632.81 nm) as the source. Before the measurement, the instrument is calculated with a glass slide. Spectra were generally accumulated over a spectral range of 2000~100 cm-1 with a full-length shot (100 µm). The integration time was 15 s and the accumulation number was 2. The collected data were normalized directly with an internal peak at 1667 cm-1, which was ascribed to the stretching vibration of C=C

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. UV-Vis spectroscopy was

performed on a TU-1810 UV-Vis Spectrophotometer at room temperature. The scan range is from 200 to 600 nm with a resolution of 1 nm. XPS analysis was carried out on a Kratos Axis Ultra DLD equipped with a Al Ka radiation source (1486.6 eV). FTIR was performed on a Bruker Vertex Fourier transform infrared spectrometer with attenuated total reflectance (ATR) mode at room temperature. The scan range is from 2000 to 600 cm-1 with a resolution of 4 cm-1. The collected data were normalized to the intensity of the peak around 1378 cm-1, which corresponded to the bending vibration of methyl group.

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Tensile tests of samples at room

temperature were measured using a Goetch instrument with extension rate of 500 mm/min. At


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least five specimens have been tested and the average data were adopted. The loading-unloading cycles were performed by a Goetch instrument with extension rate of 100 mm/min at a predefined strain of 100%. Dynamic mechanical analysis (DMA) was performed on a TA Q800 dynamic mechanical analyzer (USA) under the tension condition with a frequency of 1 Hz. The scanning temperature ranged from -40 to 140 °C at a heating rate of 3 °C/min. The above mentioned hysteresis testing and DMA measurements were repeated with at least three specimens. The fracture toughness (G) is calculated by the equation (1),

G = σdε

(1)

, where σ is the stress (MPa), ε is the strain. Shape memory behaviors were characterized by the thermo-mechanical cycle experiments and performed on a TA Q800 instrument under controlled force mode. Considering the distinguishable Tg values, the first deforming temperature (Td1) and the second deforming temperature (Td2) are designed as (Tend +15) oC and (Ton + 5) oC, where Ton an Tend are the onset and end of the glass transition, respectively. Prior to deformation, sample with dimension of 8 × 4 × 0.6 mm3 was heated to Td1 and equilibrated for 3 min. Then the sample was deformed to the first setting strain (ε ), followed by cooling the sample to Td2 and releasing the stress to obtain the first fixed strain (ε ). Then the sample was further deformed to the second setting strain (ε ) at Td2, followed by cooling and unloading to obtain the second fixed strain (ε ). Upon heating to the first recovery temperature (Tr1) and keeping for 25 min, the sample recovered to the first temporary strain (ε ). As the temperature was further heated to the second recovery temperature


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(Tr2), the sample was recovered to the original strain (ε ). Tr1 and Tr2 are equal to Td2 and Td1, respectively. The heating and cooling rates were both 5 oC/min. The shape memory tests were repeated with at least three specimens. Fixing ratio (R ) and recovery ratio (R ) for triple-SMP are defined as Equation (2) and (3): R (X → Y) = 100% × (ε − ε )/(ε − ε )

(2)

R (Y → X) = 100% × (ε − ε )/(ε − ε )

(3)

, where X and Y denote two different shapes, respectively; ε represents the setting strain under load, ε is the fixed strain after cooling and load removal, and ε is the strain after recovery. RESULTS AND DISCUSSION Figure 1a is the proposed structure map of the designed material. ENR is cured by ZDA to construct the permanent network;

26-27

At the same time, the coordination between Fe3+ and

epoxy group acts as the physical cross-links dispersing in the permanent network. Specifically, ZDA-cured ENR network acts as the permanent cross-links, granting the vulcanizates with elasticity; Meanwhile Fe3+-O coordination physical cross-links intersperse in the permanent network, improving the crosslink density. Besides, the coordination bonds behave in a sacrificial manner by reversibly breaking and reconstructing to dissipate energy and improve the strength and toughness (Figure 1b). As the curing reaction between ENR and ZDA have been verified,26 the spectral analyses were performed on ENR/FeCl3 model mixtures to demonstrate the formation of Fe3+-O coordination. As shown in the Raman spectra (Figure 2a), the peaks at 1020, 1445 and 1667 cm-1 of neat ENR are ascribed to the stretching vibration of C-O, bending vibration of methylene and stretching vibration of C=C, respectively. 24, 28 For ENR/FeCl3


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Figure 1. (a) ENR networks with combination of permanent cross-links and Fe3+-O coordination physical cross-links; (b) Fe3+-O coordination bond can reversibly break and re-construct during loading/unloading test; (c) Coordination bond can fix temporary shape upon cooling while it can dissociate and release the temporary shape upon heating.

Figure 2. Spectral evidences for coordination between Fe3+ and epoxy group: (a) Raman spectra; (b) UV-Vis spectra; (c) FTIR spectra and (d) XPS O 1s spectra of neat ENR and ENR/FeCl3 mixtures.


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mixture (EF-3:1), even though the baseline is lifted due to the existence of fluorescence and noises, one can still find a new peak around 330 cm-1, which is attributed to the coordination between Fe3+ and oxygen.29 Then hexamethylenediamine, a metal chelator, was added into the coordinated sample (EFH-3:1:3) to capture Fe3+ and suppress the formation of Fe3+-O coordination. The disappearance of the peak around 330 cm-1 in the spectrum of EFH-3:1:3 should be ascribed to the chelation between Fe3+ and hexamethylenediamine, which verifies the formation of Fe-O coordination in ENR/FeCl3 sample. The UV-Vis spectra of neat ENR, FeCl3 and ENR/FeCl3 mixture are compared in Figure 2b. For neat ENR, there is no peak in the range of 300-500 nm, while a strong peak at 339 nm for neat FeCl3 is detected. When FeCl3 is incorporated into ENR, the single peak at 339 nm is replaced by two peaks at 319 nm and 360 nm, which is attributed to ligand-to-metal charge transfer and hence indicates the existence of Fe3+-O coordination.29-30 The association between Fe3+ and ENR is further substantiated by FTIR spectra (Figure 2c). The peak at 876 cm-1 and 1256 cm-1 are related to asymmetric and symmetric stretching vibration of epoxy ring respectively.26, 31 Upon the addition of Fe3+, the two peaks shift to 857 cm-1 and 1242 cm-1 respectively, indicating the association between Fe3+ and epoxy group. Besides, tetrahydrofuran ring and alcohol are two inevitable by-products for ENR with high epoxy content due to the ring-opening reaction of epoxy groups.

32-33

The peaks in

region from 1060 to 1150 cm-1 of ENR spectra are related to stretching vibration of C-O-C, 34-35 and the peak around 1068 cm-1 has been demonstrated to be the stretching vibration of tetrahydrofuran ring.

32-33

Therefore, the peak around 1110 cm-1 should be ascribed to the

tetrahydrofuran ring associated with hydrogen bonds. When ENR solution is mixed with FeCl3, the hydrogen interaction is disrupted by Fe3+-O coordination, resulting in merging of the two peaks for tetrahydrofuran ring into one peak. The results of X-ray photoelectron spectroscopy


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(XPS) provide additional implication for Fe3+-O coordination (Figure 2d). For neat ENR, only one dominant peak with binding energy of 531.2 eV is observed in O 1s spectra, corresponding to C-O-C or C-O-H.36-37 When Fe3+ is added, additional small peak with binding energy of 532.0 eV emerges. This peak corresponds to more electron-deficient oxygen36 and hence implies the successful coordination between Fe3+ and oxygen.38 All those spectral results convincingly verify the existence of Fe3+-O coordination in ENR/FeCl3 mixtures. The typical stress-strain curves of EZ2Fy series are depicted in Figure 3a and the detailed data are summarized in Table S2. As a control sample, EZ2F0 exhibits a tensile strength of 5.15 MPa, a Young’s modulus of 1.58 MPa and a tensile modulus (modulus at 100% strain) of 0.72 MPa. With the inclusion of 3 phr FeCl3 (EZ2F3), the tensile strength, Young’s modulus and tensile modulus are increased by about 2, 47 and 11 times, respectively. Such reinforcing efficiency is spectacular, which is very hard to achieve by nanoreinforcements of rubbers. The fracture toughness also increases monotonously with increasing FeCl3 content (Figure 3b). Compared with EZ2F0, the fracture toughness of EZ2F3 is almost three-folded. The strikingly improvements in tensile strength, modulus and toughness should be ascribed to the incorporation of Fe3+. When ZDA content is designed as the variable, the modulus and tensile strength of composites also increase obviously due to the increased crosslink density of permanent network (Figure S1 and Table S3). The coordination still works even at high ZDA content, which is reflected by the consistently improved modulus and decreased strain with the incorporation of Fe3+ (Figure S2). It is necessary to clarify that, when high ZDA content is incorporated, there are two kinds coordination between ferric and oxygen, namely Fe3+-epoxy group coordination and the one between Fe3+ and carbonyl in ZDA. The coordination between Fe3+ and carbonyls have been well documented.

29, 39

For samples with 2 phr ZDA, the carbonyl content is negligible


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(ncarbonyl/nepoxy = 0.015). Both kinds of coordinations serve as reversible physical cross-links and are responsible for the improved modulus and enhanced constraint on segmental relaxation.

Figure 3. (a) Typical stress-strain curves and (b) Calculated fracture toughness of EZ2Fy series.

Figure 4. (a) Tensile-recovery curves of EZ2Fy series; (b) Calculated W1 value of EZ2Fy series as a function of Fe3+ content, where W1 refers to the hysteresis area surrounded by the tensilerecovery curves. (c) Tensile-recovery cycles of EZ2F0.5. After the first cycle, the original sample (black line) is stretched after different time intervals; (d) The dependence of residual strain and hysteresis ratio on waiting time for EZ2F0.5, where hysteresis ratio is determined by the hysteresis area ratio of the followed cycles to the first cycle.


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To verify the sacrificial and reversible nature of Fe3+-O coordination during stretching, tensilerecovery tests were performed. Each sample was stretched to a strain of 100% and then was recovered by releasing the stress. Figure 4a exhibits several typical tensile-recovery curves of EZ2Fy series. Clearly, with the increment of Fe3+ content, the sample exhibits an obviously improved modulus and hysteresis (area surrounded by tensile-recovery curves, W1). Because the strain is far less than the fracture strain of the sample, so the rupture of permanent cross-links is negligible and hence the increases in modulus and W1 are mainly ascribed to Fe3+-O coordination, which is sacrificed before the rupture of permanent cross-links. In general, W1 value reflects the dissipated energy during the tensile-recovery process. Larger W1 value means more energy is dissipated. Figure 4b depicts increasing trend of W1 value with increasing Fe3+ content, indicating that more energy is dissipated due to increasing Fe-O bonds content. Then the tensile-recovery cycles with different waiting time for EZ2F0.5 were performed (Figure 4c). After the first cycle, an obvious hysteresis occurs, corresponding to energy dissipated by the breakage of Fe-O bond and unrelaxed segments. For the cycle followed immediately, a much smaller hysteresis is found. As parts of broken Fe-O bonds don’t have enough time to be reconstructed, less Fe-O bonds participate in energy dissipation during the second cycle. As the waiting time prolongs, namely more recovery time is supplied, the hysteresis gradually approaches to the first cycle. Notably, after heating at 80 oC for 1 min, the loading-unloading curve is almost overlapped with the first cycle, suggesting the reversibility essence of coordination and the elasticity essence of the sample. As an obvious contrast (Figure S3), EZ2F0 shows a much smaller hysteresis than EZ2F0.5 due to the absence of coordination. The dependences of calculated hysteresis ratio and residual strain on waiting time are shown in Figure 4d, where hysteresis ratio is determined by the hysteresis area ratio of the followed


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Figure 5. Tan δ curves of EZ2Fy series (a) and EZ20Fy series (b) as a function of temperature; (c) Sandwiched damping materials with ingredients of EZ2F0 and EZ2F3. cycles to the first cycle and the residual strain is defined as the starting strain of the next cycle. The better the recovery capability is, the smaller the residual strain is; And the higher the hysteresis ratio is, the better the reversibility of Fe-O bonds is. As the waiting time is prolonged, the residual strain decreases sharply to a rather small value (~2.5%) while the hysteresis ratio increases from 0.68 to 0.93, indicating the reversibility and recovery capability of the coordination. So it is believed that Fe3+-O coordination behaves in a sacrificial and reversible manner. The effect of Fe3+-O coordination on the viscoelasticity is revealed in Figure 5a and 5b. The incorporated Fe3+ exhibits the capability of tuning the glass transition temperature (Tg) consistently. For samples with 2 phr ZDA, the Tg increases from 7 oC to 37 oC by incorporating just 3 phr FeCl3. For samples with 20 phr ZDA, the Tg also increases by ~20 oC with inclusion of 4 phr FeCl3. Besides, the peak values of tan δ decrease consistently with increasing FeCl3 content. The results are reasonable due to that the coordinations improve the crosslink density and hence enhance the constraints on segmental motion. Impressively, all samples exhibited broad glass transitions and the peak temperatures are tunable, which makes such materials potentially useful


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in the fabrication of functional elastomers, such as wide-temperature damping and multi-shape memory polymers (multi-SMPs). As for damping materials, a wide temperature range around room temperature is necessary. 4041

Due to the consistently tunable Tg, the damping range in this system can be enhanced by

transition merging with multilayered structure. Specifically the damping sample was prepared by stacking and hot pressing of sheets with different Tg values. As a proof of concept, EZ2F0 and EZ2F3 were first pressed into sheets at 60 oC and then stacked in a sandwich structure with EZ2F3 as the core. The stacked sample was then hot-pressed to prepare cross-linked sample. Figure 5c depicts the tan δ curves of the sandwiched damping sample. The integrated interphases merge the two curves into a much wider curve. Impressively, the temperature range with tan δ value higher than 0.3 reaches ~60 oC. Therefore, the present work offers an efficient method for the preparation of highly damping materials by sandwiched structure with metalligand coordination. Samples in Figure 5a and 5b exhibit wide transitions and high glass transition temperatures, potentially fulfilling the requirement of SMP with mechanism of vitrification.42 What’s more, the responsiveness of coordination to heat, namely dissociation at elevated temperature and reconstruction at low temperature, can serve as another shape fixing mechanism (Figure 1c). As reported, polymers with wide glass transition are known as potential candidates for multiSMPs.43 However, most reported triple-SMPs suffer from the unsatisfying Rf at medium temperature.44-46 In the present system, the incorporated complexation, which can serve as additional mechanism for shape fixing, is anticipated to solve the problem effectively. Specifically, we studied the triple-shape memory effect of EZ20Fy series. The original shape S0


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Figure 6. (a) Triple-shape memory performance of EZ20F0. (b) Triple-shape memory performance of EZ20F4 recovered at (Td2+2) oC and (Td1+20) oC; (c) The shape evolution of EZ20F4. S0, the original shape after being heated at 100 oC; S1, the shape fixed at 43 oC after being deformed at 100 oC; S2, the shape fixed at 0 oC after being deformed at 43 oC; S1r, the shape after recovering at 43 oC; S0r, the shape after recovering at 100 oC. Table 1. Summarized parameters for triple-shape memory properties Sample

Td1, o C

Td2, o C

Rf(S0→S1), %

Rf(S1→S2), %

Rr(S2→S1), %

Rr(S1→S0), %

EZ20F0

90.0

26.0±1

70.4±3.0

97.5±0.5

88.0±5.2

89.8±1.7

EZ20F2

100.0

36.0±1

83.4±3.3

97.0±0.8

86.3±2.2

85.8±3.6

EZ20F4

110.0

42.0±1

90.3±2.7

97.2±0.6

81.2±0.4

84.5±2.8

EZ20F4*

110.0

42.0±1

90.8±3.5

97.8±0.7

99.7±3.8

87.6±3.9

o

Asterisk donates the sample is recovered at (Td2+2) C and (Td1+20) oC.


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was deformed at Td1 and fixed at Td2 to achieve the first temporary shape S1, which was further deformed to the second temporary shape S2 at Td2 and fixed at low temperature. Upon reheating to Tr1 yielded the recovered first temporary shape S1r, which was further recovered to the original shape S0r when the temperature was heated to Tr2. As a control sample, the Rf(S0→S1) of EZ20F0 is only ~70 % (Figure 6a and Table 1), which is due to the undesired creep and retraction of polymer chains when the external force is removed. In striking contrast, samples with FeCl3 exhibit a greatly improved Rf(S0→S1) (Figure S4, S5 and Table 1). For example, the Rf(S0→S1) of EZ20F4 are as high as 90 %, which is higher than most reported triple-SMPs (Table S4). The greatly improved Rf is due to that the dissociated complexation at Td1 is reconstructed as the temperature is re-cooled to Td2, limiting the retraction and relaxation of segments. As to shape recoveries, when compared with EZ20F0, the samples containing Fe3+ exhibit a slightly compromised Rr due to the enhanced limit on segmental motion. However, the problem is easy to be conquered by slightly tuning up the recovery temperature. The thermo-mechanical cycle of EZ20F4 with slightly improved recovery temperature is shown in Figure 6b. As Tr1 is increased to (Td2 + 2) oC, the Rr(S2→S1) is improved greatly to ~100% while Rf remains high value (90 %). Figure 6c demonstrates the visual shape evolution of EZ20F4. The temporary shapes (S1 and S2) are very similar to the designed shapes, suggesting high fixities. In addition, the recovered shapes (S1r and S0r) are almost identical to S1 and S0, demonstrating excellent triple-shape recovery ability. By incorporating Fe3+-O coordination, low Rf at Td2, the notorious predicament of multi-SMPs, has been effectively solved based on the dissociation and reconstruction of coordination at different temperatures; Meanwhile, the triggering temperatures for such tripleSMP are tunable by adjusting ferric content.


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It is necessary to emphasize that ENR is just a typical example of rubbers containing abundant oxygen which is used as a proof of the concept. As all epoxidized or hydroxylated rubbers are capable of forming coordination with metal ions, the adopted method in the present paper is also applicable to other functionalized diene rubbers, such as epoxidized diene rubber

47-50

and

hydroxylated diene rubbers 51-53. CONCLUSIONS Inspired by mussel byssus, ferric ion was introduced into oxygen-abundant commercially available rubber network to create additional metal-oxygen coordination cross-links. Such complexations have been revealed to be highly efficient in enhancing the strength and toughness of the rubbers. Significantly, the continuous shifting of glass transition enables design of highly damping material with multilayer concept. Such physical cross-links also serve as additional mechanism for fixing the temporary shape, resulting in an excellent and tunable multi-shape memory effect. We envision that the present work offers efficient yet facile way for creating advanced elastomers based on industrially available diene-based rubber. ASSOCIATED CONTENT Supporting Information. The detailed mechanical properties of EZ2Fy and EZxF1, stress-strain curves of EZ20Fy series, tension-recovery cycles of EZ2F0, and thermo-mechanical cycles of EZ20F2, EZ20F4 are listed in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * B. Guo. E-mail: [email protected]


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* L. Zhang. E-mail: [email protected] Funding Sources The National Basic Research Program of China (2015CB654703) and National Natural Science Foundation of China (51473050, 51320105012, 51521062 and U1462116). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (2015CB654703) and National Natural Science Fundation of China (51473050, 51320105012, 51521062 and U1462116). REFERENCES 1.

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Enabling Design of Advanced Elastomer with Bioinspired Metal

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