Toughening Elastomers Using a Mussel-Inspired Multiphase Design

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Toughening Elastomers Using Mussel-Inspired Multiphase Design Xuhui Zhang, Jun Liu, Zhiyu Zhang, Siwu Wu, Zhenghai Tang, Baochun Guo, and Liqun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08844 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Toughening Elastomers Using Mussel-Inspired Multiphase Design Xuhui Zhang,‡a Jun Liu,‡b Zhiyu Zhang,b Siwu Wu,a Zhenghai Tang,a Baochun Guo,*a Liqun Zhang*b ‡

a

Xuhui Zhang and Jun Liu contributed equally to this work* Department of Polymer Materials and Engineering, South China University of Technology,

Guangzhou, 510640, P. R. China. b

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

Technology, Beijing 100029, China KEYWORDS: elastomer, toughening, biomimetic, multiphase structure, coordination ABSTRACT: It is a challenge to simultaneously achieve high stretchability, high modulus and recoverability of polymers. Inspired by the multiphase structure of mussel byssus cuticles, we circumvent this dilemma by introducing deformable microphase-separated granule with rich coordination into a ductile rubber network. The granule can serve as additional cross-link to improve the modulus while the sacrificial, reversible coordination can dissociate and reconstruct continuously during stretching to dissipate energy. The elastomer with such bio-inspired multiphase structure exhibits over ten-fold increases in toughness compared to the original sample. We envision that this work offers a novel yet facile biomimetic way towards highperformance elastomers.

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Elastomers are of irreplaceable importance in fields of polymer materials thanks to the high extensibility and recoverability.

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However, due to the very limited mechanical strength,

elastomers are generally applied after reinforcement. Adding nanofillers is one of the most common way for reinforcing, but this method depends heavily on filler dispersion and interfacial regulation and requires high amount of fillers.2 In addition, highly filled elastomers exhibit severely deteriorated specific strength, which is very crucial in many applications, such as aeronautics and astronautics devices. In recent years, the incorporation of sacrificial bond has been an increasing applied strategy for elastomer reinforcement.3,

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The sacrificial bonds are

mainly weak reversible interactions, such as π-π interaction,5 hydrogen bonding,6 metal-ligand coordination7, 8 and ionic interaction,9 which preferentially break during stretching to dissipate energy and release the hidden length, resulting in toughened elastomers. This method is high efficient but limited to only polar elastomers. Alternatively, employing interpenetrating network as sacrificial networks also enables toughening weak elastomer by gradual mechanical rupture of sacrificial networks to dissipate energy.10, 11 This method requires at least one strong yet brittle network as sacrificial network and one flexible network as the main network.12 Noteworthily, all those methods generally grant the elastomers with a Young’s modulus no more than 10 MPa. As an obvious contrast, plastics possesses a high Young’s modulus but suffer from poor extensibility and recoverability. Therefore, rational structure design for elastomers with the combination of high modulus, high extensibility and recoverability are of great significance. Nature always endows us with ingenious solutions in materials designing or problem solving. Mussel byssus, which is renowned for the capability of fastening themselves to accessible surfaces of rocks based on the dopa-Fe3+ coordination,13 has imparted investigators great inspirations on the design of self-healing materials,14 adhesive,15 coatings,16 and novel


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actuators.17 In fact, the byssus cuticle of M. galloprovincialis, which possesses a stiffness four- to six-fold higher than that of the fibrous core while maintaining a high beak strain, is also interesting and inspiring.18, 19 The combination of high stiffness and extensibility of cuticle is mainly related to the presence of deformable microphase-separated granules with rich dopametal coordination. Those granules with a typical diameter of ~0.8 µm can arrest impinging force by microtear coalescence based on the capability of dissipating energy of the included coordination.20 This phenomenon gives an inspiring example in the design of materials with simultaneous high modulus and high extensibility. Inspired by this structure, Song et al. introduced deformable sulfonated styrene–ethylene/butylene–styrene triblock copolymer as seperated phase into polyvinyl alcohol. Significantly improved toughness of the PVA has been achieved.21 Nevertheless, the elastomers mimicking the structures of byssus cuticle have not been reported. Here, we develop a strong and tough elastomer in a solvent-free way via introducing deformable microphase-separated granule with rich epoxy-Fe3+ coordination into a ductile rubber network. The ‘rigid’ granule can serve as additional cross-links to increase the modulus while the sacrificial, reversible coordination in granule can dissociate and reconstruct continuously during stretching, resulting in energy dissipation and slippage of polymer chains in granule. Consequently, the developed elastomer exhibits strikingly improved toughness while maintaining the stretchability and recoverability. To achieve the microphase-separated structure similar to mussel byssus cuticle, two incompatible rubbers, butadiene styrene rubber (SBR) and epoxidized natural rubber (ENR) (Figure 1a), are mechanically compounded to form a microphase-separated structure (Figure 1b). Then FeCl3 is incorporated to generate granules with rich epoxy-Fe3+ coordination based on


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the elective affinity of FeCl3 to ENR (Figure 1c). Subsequently, sulfur-based curing package is added into the mixture and the resulted mixture is subjected to hot-pressing to crosslink elastomer to form

Figure 1. Schematic diagram of the preparation process of the biomimetic structure: (a) Chemical structures of SBR and ENR; (b) Microphase-separated ENR phases; (c) Microphaseseparated structure with granules rich in epoxy-Fe3+ coordination; (d) Iron-containing multiphase network.

iron-containing multiphase network (Iron-MN, Figure 1d). Notably, both SBR and ENR can be cured by sulfur, resulting in the generation of links at the interface, which is beneficial to stress transferring. It is necessary to emphasize that the ENR/FeCl3 compound is hard to well disperse in SBR due to the high cohesive energy rising from plentiful coordination interaction between


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ENR and FeCl3, so it is rational to blend ENR with SBR firstly. For comparison, the iron-free multiphase network (Iron-free MN) is also prepared by curing the SBR/ENR in the absence of FeCl3. The microphase-separated structures of Iron-free MN and Iron-MN are directly confirmed by TEM observations (Figure 2a and 2b). The bright disperse phase is the ENR phase while the gray

Figure 2. Evidences for the bioinspired microphase-separated structure: HRTEM image of Ironfree MN (a) and Iron-MN (b); (c) Histogram of iron mass fraction of selected areas; Temperature dependence of tan δ (d) and storage modulus (e) of Iron-MN and the control samples.

continuous phase relates to the stained SBR phase. The black phase in the images shouldbe ascribed to the phase with rich zinc oxide. Noticeably, some extra dark gray dots with a typical


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diameter of ~20 nm are observed in ENR phase of Figure 2b. To clarify the ingredients of those dark gray particles, we performed the EDX scans for different phases in Iron-MN and collected the mass fraction of C, O, Fe and Zn (Table S2). The histogram of iron mass fraction (Figure 2c) indicates that Fe element mainly locates in area 3, the ENR phase with abundant gray dots, implying that those dots are the clusters of Fe3+. The migration of Fe3+ into ENR phase should be ascribed to the coordination interaction between Fe3+ and epoxides on ENR chains. The microphase-separated structure is further verified by the appearance of two peaks in the tan δ-T curve of Iron-MN (Figure 2d). Compared with Iron-free homogeneous network (Iron-free HN) and Iron-free MN, we can ascribe the peak around -20 oC to the glass transition of SBR while the peak around 60 oC relates to the ENR phase with rich coordination. By tuning the content of ENR or Fe3+, the Tg belonging to ENR phase can be continuously manipulated while the peak relating to SBR remains unchanged (Figure S1), indicating Fe3+ is essentially located in ENR phase. The Tg shift of iron-rich ENR phase should be ascribed to the generated Fe3+-epoxy coordination which can enhance the constraints on segmental motion and has been demonstrated in our previous work.22 SEM images (Figure S2) further confirm the microphase-separated structure of Iron-MN and results of EDX scans (Table S3) verify the selective distribution of Fe3+ into ENR phase. The modulus of Iron-MN at room temperature is significantly higher than that of Iron-free MN (up to about one order of magnitude, Figure 2e and Figure S3), indicating that the iron-rich phase behaves as the hard region to reinforce the elastomer. Tensile tests (Figure 3a) and calculated fracture toughness (the work to failure, Figure 3b) reflect the striking effects of the ‘rigid’ granule on improving the modulus, tensile strength and toughness of the elastomer. Iron-free HN exhibits a Young’s modulus of 3.7 MPa, a tensile strength of 1.8 MPa and a toughness of 2.4 MJ/m3 (Table S4). When FeCl3 is introduced only,


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the improvements in mechanical properties is negligible. When ENR is added only to form a microphase-separated structure, the improvements in mechanical properties are still limited. Strikingly, by engineering the biomimetic coordination-rich granules into the elastomer, the Young’s modulus, tensile strength and toughness of Iron-MN are improved by ~3, ~7 and ~11 times when compared to Iron-free HN. Importantly, the modulus and toughness of elastomers can be conveniently manipulated by varying the ENR or FeCl3 concentrations (Figure S4, S5, and

Figure 3. Typical stress-strain curves (a) and histogram of work to failure (b) of Iron-MN and control samples; (c) Recovery of Iron-MN after heating.


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Figure 4. (a) Loading-unloading cycles of Iron-MN samples with increasing Fe3+ dose; (b) Sequential loading-unloading cycles of Iron-MN with incremental strain; (c) Stress relaxation profiles of Iron-MN and control samples; (d) Strain sweeping of Iron-Free MN and Iron-MN at a constant frequency of 1 HZ; (e) Schematic diagram of the toughening mechanism of the biomimetic microphase-separated structure during stretching. Table S5, S6). The Young’s modulus can further reach 44.4 MPa (~12 times of Iron-free HN) by increasing granule content. Noticeably, due to the selective distribution of Fe3+ into ENR, so only a few FeCl3 (no more than 1.8 wt%) can strinkingly improve the modulus of disperse phase and the mechanical properties of resulted elastomer. Besides, the elastomer with simultaneous high modulus and toughness also possess capability of recovering its mechanical properties after stretching (Figure 3c). After waiting at 90 oC for 1 min, the samples can almost fully recover the original mechanical properties, including modulus and strength, indicating the quick reconstruction of coordination structures at high temperature. Alternative, after waiting at room


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temperature for 6 h, the sample can also recover the original properties with a compromise (a residual strain of ~2% and a Young’s modulus of 71% of the original one), implying the slow reconstruction of coordination at room temperature. To expound the mechanism of toughening, loading-unloading cycles with no time interval are conducted. One can clearly find that increasing Fe3+ concentration results in a continuously increment in hysteresis areas, indicating the capability of dissipating energy of coordination in the granules (Figure 4a). In the tensile cycles with incremental strains, Iron-MN (Figure 4b) exhibits a much larger hysteresis area at each cycle than Iron-Free MN (Figure S6), indicating continuous energy dissipation during stretching. In stress relaxation experiments, compared with contrast samples, Iron-MN releases the applied force much faster and shows a more significant drop of force (Figure 4c), implying that the coordination bonds undergo dissociation under loading. In the strain sweeping (Figure 4d), the higher initial G’ of Iron-MN corroborates that the coordination-rich granule can serve as extra cross-link to improve the modulus while the quicker decrease of G’ at a smaller strain verifies the preferential rupture of the coordination prior to covalent cross-link. To further analyze the reinforcing mechanism, the classical coarsegrained molecular dynamics simulations (CGMDSs) are adopted. The deformation process monitored by snapshots corroborates the deformability of coordination-rich granule, and the granule becomes more deformed at the large strain (Figure S7), indicating the dissociation of Fe3+-O coordination during stretching. Simulated stress-strain curves (Figure S8) indicate that the tensile strength of Iron-MN increases continuously with the increment of Fe3+ content, which is consistent with the experimental results. In conclusion, we can ascribe the toughening mechanism of the bioinspired microphase-separated structure to three points. On one hand, the formed ‘rigid’ granules can act as additional cross-link to improve the modulus. Besides, when


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subjected to external force, the sacrificial, reversible coordination can dissociate and reconstruct continuously during stretching, resulting in continuous energy dissipation and deformation of granule (Figure 4e). What’s more, just like granule in the cuticle of mussel byssus, the ‘rigid’ granules can arrest impinging force to prevent stress concentration, which is beneficial to the stretching of rubber chains. Importantly, this method possesses the university. By constructing this bioinspired MN, the Young’s modulus, tensile strength and toughness of polybutadiene rubber (BR) are increased by ~1.4, ~3.1 and ~11 times respectively, when compared with neat BR (Figure S9a). Although neat cis-polyisoprene (IR) possesses a high tensile strength due to the strain-induced crystallization, its Young’s modulus and work to strain (for instance 500%) are also increased by ~2.6 and ~1.9 times respectively when MN is introduced (Figure S9b). Besides, when other metal ions (such as Zn2+, Ni2+, Co2+ and La3+) are introduced to be a substitute of Fe3+, resulted elastomers also exhibit a strikingly increase in, Young’s modulus, tensile strength and toughness (Figure S10). In addition, the glass transition temperature and peak intensity of the separated phase can be conveniently tuned, rendering the elastomer shape memory effect. The large deformability and high fixity/recovery ratio of Iron-MN-3 are demonstrated (Figure S11). Although mechanical mixing is hard to generate a well-defined structure as delicate as solvent-involving methods, we definitely achieve submicron hard regions with rich coordination, which is highly consistent with the structure of mussel byssus cuticle. More importantly, this method is more environmentally friendly and more potential in practical applications due to the avoidance of intricate syntheses and organic solvent. Besides, different from sacrificial bonds requires all polar matrices, this bioinspired multiphase method requires a little polar rubber and can be applied to any nonpolar elastomers.


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In summary, we have developed a tough and strong elastomer in a solvent-free way via introducing mussel-inspired microphase-separated granule with rich epoxy-Fe3+ coordination into a ductile rubber network. The ‘rigid’ granule can serve as additional cross-link to improve the modulus while it is deformable when subjected to external force via the rupturereconstruction of coordination to dissipate energy. Consequently, elastomers with such biomimetic structure exhibit a combination of high modulus, high stretchability and recoverability. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. Experimental details, Data of EDX scans, details of mechanical properties, DMA profiles and stress-strain curves with various Fe3+ content or ENR content, SEM images of MN, and results of CGMDSs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS


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This work was supported by the National Basic Research Program of China (2015CB654703) and National Natural Science Foundation of China (51473050, 51320105012, 51521062). REFERENCES (1) He, S. J.; Wang, Y. Q.; Wu, Y. P.; Wu, X. H.; Lu, Y. L.; Zhang, L. Q., Preparation, Structure, Performance, Industrialisation and Application of Advanced Rubber/clay Nanocomposites Based on Latex Compounding Method. Plast. Rubber Compos. 2010, 39, 33-42. (2) Domun, N.; Hadavinia, H.; Zhang, T.; Sainsbury, T.; Liaghat, G. H.; Vahid, S., Improving the Fracture Toughness and the Strength of Epoxy Using Nanomaterials - a Review of the Current Status. Nanoscale 2015, 7, 10294-10329. (3) Tang, Z. H.; Huang, J.; Guo, B. C.; Zhang, L. Q.; Liu, F., Bioinspired Engineering of Sacrificial Metal-Ligand Bonds into Elastomers with Supramechanical Performance and Adaptive Recovery. Macromolecules 2016, 49, 1781-1789. (4) Nabavi, S. S.; Harrington, M. J.; Fratzl, P.; Hartmann, M. A., Influence of Sacrificial Bonds on the Mechanical Behaviour of Polymer Chains. Bioinspir. Biomim. Nan. 2014, 3, 139-145. (5) Chen, Z. X.; Lu, H. B., Constructing Sacrificial Bonds and Hidden Lengths for Ductile Graphene/polyurethane Elastomers with Improved Strength and Toughness. J. Mater. Chem. 2012, 22, 12479-12490. (6) Hayashi, M.; Matsushima, S.; Noro, A.; Matsushita, Y., Mechanical Property Enhancement of ABA Block Copolymer-Based Elastomers by Incorporating Transient Cross-Links into Soft Middle Block. Macromolecules 2015, 48, 421-431. (7) Filippidi, E.; Cristiani, T. R.; Eisenbach, C. D.; Waite, J. H.; Israelachvili, J. N.; Ahn, B. K.; Valentine, M. T., Toughening Elastomers Using Mussel-inspired Iron-catechol Complexes. Science 2017, 358, 502-505.


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Toughening Elastomers Using a Mussel-Inspired Multiphase Design

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