Ultra-Tough, Strong, and Defect-Tolerant Elastomers with Self-Healing

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Applications of Polymer, Composite, and Coating Materials

Ultra-tough, strong and defect-tolerant elastomers with self-healing and intelligent-responsive abilities Yong Zhu, Qiaoqiao Shen, Laiyun Wei, Xuan Fu, Cheng Huang, Yiqiao Zhu, Lijuan Zhao, Guangsu Huang, and Jinrong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11041 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Applied Materials & Interfaces

Ultra-tough, strong and defect-tolerant elastomers with self-healing and intelligent-responsive abilities Yong Zhu,† Qiaoqiao Shen,† Laiyun Wei,† Xuan Fu,† Cheng Huang,† Yiqiao Zhu,‡ Lijuan Zhao,‡ Guangsu Huang,*,† and Jinrong Wu*,† † State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China ‡ College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, China

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ABSTRACT

Mechanical strength, toughness and defect tolerance are usually exclusive in most artificial materials. Herein, inspired by many biomateials that overcome this trade-off by integrating soft and hard ingredients through elaborate structur-al designs, we report a facile latex-assembly method to fabricate ultra-tough, strong and defect-tolerant elastomers. The elastomers are featured by a microscopic inverse opal-mimetic rigid skeleton of dynamically crosslinked chitosan and a continuous soft matrix of vulcanized natural rubber. Such structural design enables the load-bearing capability, sacrificial property and self-healing ability of the skeleton, the stress redistribution and extensibility of the matrix as well as the stiffness variation between hard and soft ingredients, thereby imparting the elastomers with outstanding mechanical strength and defect tolerance, as well as extremely high toughness of 122 KJ m-2 which is even higher than that of the current state-of-the-art titanium alloys. Moreover, the elastomers show prominent humidity sensitivity due to the hydrophilic nature of chitosan skeleton. Harnessing these advantages, we fabricate a walking robot trig-gered by humidity variation and shoes that are able to regulate temperature and humidity. The concept of designing rigid sacrificial skeleton within soft continuous matrix on microscale is quite general, enabling the development of highperformance and intelligent materials for emerging applications.

KEYWORDS：Tough, defect-tolerant, self-healing, humidity-responsive, elastomer


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INTRODUCTION Soft materials with the ability to deform easily and reversibly hold great promise for applications in broad fields, from wearable electronics and soft robotics to medical devices, automotive engineering and aerospace exploration.1-8 Any applications, however, require the soft material to possess high toughness, strength and even defect-tolerance to ensure the load-bearing ability and long-term durability of the materials. Nevertheless, these properties are mutually exclusive in most synthetic materials. In sharp contrast, nature has created various strong and tough biological materials with exceptional fracture resistance during the long eons of evolution and natural selection. Significantly, many of these biomaterials are hybrid materials created by fusing soft and hard ingredients with complementary properties through unique structural designs on multiscales, realizing attractive combinations of mechanical performance and functionality which are inaccessible by their separate components alone. For example, stiff biological tissues including nacre, bone, and sea sponge exoskeletons involve highly sophisticated hierarchical structures consisting of alternating hard ceramic and soft organic components with dimensions spanning from nanoscale to macroscale;9-12 such structures lead to fracture toughness far exceeding what could be expected from their main mineral phase, yet without sacrificing the stiffness and strength of the materials. Coincidentally, soft biological tissues, such as muscle, skin and tendon, also exhibit similar hierarchical designs by embedding relatively hard and aligned collagen fibers in soft organic matrix across multiple length scales.13,14 In both cases, the “hard” components in the hybrid materials provide mechanical strength to resist deformation, while the “soft” ingredients confer toughness to resist fracture. Moreover, the stiffness variation between alternating soft and hard structures can erect various obstacles in the interfaces to significantly hinder crack propagation by deflecting cracks.15 More


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importantly, these biological tissues usually show self-healing and stimuli-responsive properties, in addition to the high mechanical strength, toughness and defect tolerance. While artificial materials may possess one or two of these proper-ties, a combination of all these mechanical and intelligent properties in a material has not been realized. To date, extensive research efforts have been made to explore artificial materials with high tensile strength and toughness by mimicking the structural design strategies of bio-materials. Nacre-like brick-mortar structures and lamellar structures have been widely adopted to relieve the intrinsic conflict between strength and toughness in synthetic soft gels and composites.16-21 However, these materials are tough and strong only in the direction parallel to the layers, while soft and weak in the vertical direction because of the discontinuous structure in this direction. Furthermore, unlike natural materials, these materials usually do not show stimuli-responsive and self-healing abilities. Basically, stimuli-responsive soft materials can be designed in two ways. One relies on the introduction of reversible associations (such as reversible covalent bonding,22 metalligand coordination,23 hydrogen bonding and ionic interactions.24,25) and functional moieties (such as azobenzene groups and spiropyrans.26-27) to impart the soft materials with certain stimuli-responsive properties. In particular, the reversible associations can concurrently enable the self-healing ability of the materials. Nevertheless, soft materials obtained in this way often suffer from poor mechanical performances due to the low bond energy of reversible associations and instability of functional moieties. Another widely used strategy to fabricate stimuli-responsive soft materials is incorporating functional additives such as nanostructured carbon, metals and low melting-point alloys into soft polymeric matrix through special structural designs such as multilayer,28 core-shell and three-dimension networks,29,30 realizing electric conductivity, shape memory, and/or self-healing. However, these methods are


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usually cumbersome, and their reinforcing and toughening effects are limited. Therefore, progress in this field depends on developing a pioneering and alternative strategy to endow synthetic soft materials with a combination of robust mechanical properties, prominent intelligent-response and self-healing abilities. Herein, we develop a facile latex assembly method to fabricate a class of defect-tolerant, strong and tough soft materials (DTST-SM) with humidity-responsive and self-healing abilities. The DTST-SM consists of a microscopic three-dimensional inverse opal-mimetic skeleton of glutaraldehyde crosslinked chitosan and a continuous matrix of sulfur vulcanized natural rubber. The as-designed rigid skeleton can serve as sacrificial skeleton that preferentially rupture upon deformation to dissipate a large amount of energy and self-heal after heat treatment due to reversible Schiff-base crosslinks, while the soft rubber matrix guarantees extensibility and allows stress redistribution; moreover, the stiffness variation between the skeleton and matrix confers the ability to deflect cracks and hinder crack propagation. As a result, the DTST-SM is extraordinarily tough, strong and defect-tolerant. More intriguingly, the DTST-SM is extremely humidity-responsive due to the presence of hydrophilic chitosan skeleton. Harnessing these properties, a soft robot with walking ability and shoes that are able to regulate temperature and humidity are developed to illustrate the great potential of DTST-SM in practical applications.

RESULTS AND DISCUSSION Preparation of the DTST-SM To construct the interconnected inverse opal-mimetic skeleton in the soft material, a latex assembly method is developed (Figure. 1a). In this method, NR latex is prevulcanized at 70 ℃ for 1 hour with the presence of homogeneously dispersed sulfur and other additives in advance,


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Figure 1. Fabrication and morphology of DTST-SM. (a) Schematic illustrations for preparation of DTST-SM with inverse opal-mimetic skeleton using a conceptually simple latex assembly method. NR latex is pre-vulcanized, stabilized and acidified in advance. (b) SEM images for the morphology evolution of chitosan particles in DTST-SM before ( ⅰ ) and after （ ⅱ ） treating with glutaraldehyde. (c) AFM images for the morphology evolution of NR latex particles in DTST-SM before (ⅰ) and after（ⅱ）post-vulcanizing. (d) FTIR spectra of chitosan (CS), NR, DTST-SM before and after treated with glutaraldehyde. resulting in lightly crosslinked latex particles. The light crosslinking is able to prevent chitosan from entering the latex particles and allow the latex particles to maintain their spherical shape in subsequent processing. The lightly crosslinked latex is then stabilized with a surfactant (Peregal O) and acidified with 36 wt % acetic acid to a pH of 4, as directly mixing alkaline NR latex and


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acid chitosan solution would otherwise cause coagulation. After this treatment, the acidified latex is mixed with chitosan solution under vigorous stirring to form a stable homogeneous mixture. The mixture is casted in a petri dish and dried under ambient condition, allowing the latex particles to co-assemble with the chitosan molecules to form a composite film. During this process, the latex particles come into contact with each other as water evaporates, while the chitosan molecules are pushed into the interstitial spaces of the rubber particles. Scanning electron microscope (SEM) image in Figure 1b (ⅰ) and atomic force microscope (AFM) image in Figure 1c (ⅰ) reveal that the chitosan could self-assemble into uniform nanoparticles with an average size of around 60 nm (Figure S1, Supporting Information), which are aligned along the boundaries of rubber particles, benefiting from the intra- and inter-molecular hydrogen bonding of chitosan molecules.31 This morphology allows the rubber molecules to diffuse across the chitosan particles and merge into a continuous matrix, since the rubber molecules are lightly crosslinked and thus have relatively high chain mobility. Subsequently, the composite film is immersed in glutaraldehyde solution to crosslink the chitosan molecules. It is surprising to notice that the chitosan particles are interconnected to form a continuous inverse opal-mimetic skeleton upon this treatment, as shown in the SEM image of Figure 1b (ⅱ). Such a phenomenon suggests that the chitosan molecules in the composite film are mobilized in the presence of water, leading to the interconnection between chitosan particles. This interconnection is solidified as the chitosan molecules are crosslinked by glutaraldehyde through Schiff based reaction and results in a strong skeleton (Figure S2, Supporting Information). Fourier transform infrared spectroscopy (FTIR) measurements provide concrete evidence for the successful incorporation and crosslinking of chitosan. The composite film shows a characteristic peak of -NH2 groups at 1561 cm-1 before the crosslinking of chitosan (Figure 1d), which indicates the presence of chitosan in


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the composite film. After treating with glutaraldehyde, the absorption peak at 1561 cm-1 becomes less prominent, while the peak intensity at 1652 cm-1 shows an obvious increase, which indicates the conversion of

-NH2 groups into imine bonds (C=N) through the reaction between

glutaraldehyde and chitosan molecules. In addition, to increase the mechanical properties of the rubber matrix, the composite film is further post-vulcanized at 130 ℃ for 30 minutes. This process not only increases the crosslinking density inside the rubber particles, but also leads to interparticles crosslinking.32 As a result, the rubber matrix forms a continuous network throughout the whole sample, as shown in the AFM image in Figure 1c ( ⅱ ). Finally, an elastomer with soft NR matrix reinforced with the rigid chitosan skeleton is obtained. We fabricate a series of elastomers with such structure by systematically changing the chitosan weight fraction. The elastomers are denoted as DTST-SMx, where the subscribe x represents x phr (parts per hundred parts of gum) chitosan. The successful preparation of DTST-SM with various chitosan contents is proved by FTIR measurements in Figure S3 (Supporting Information). To illustrate the superiority of DTST-SM, we also fabricate a series of control samples with dispersed chitosan, denoted as NR/CSx, whose morphology is shown in Figure S4 (Supporting Information) and the details of the fabricating process can be found in the Experimenal Section. Fracture process and energy dissipation of chitosan skeleton in DTST-SM We can envision that the rigid skeleton in DTST-SM, featured by dynamic covalent crosslinking and small fracture strain, could serve as sacrificial skeleton that rupture preferentially upon deformation and dissipate energy. To prove this, cyclic tensile tests are carried out and the results are compared with that of neat crosslinked NR and NR/CSx, as shown


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Figure 2. Energy dissipation and morphology evolution of chitosan skeleton during stretching. (a) Representative cyclic stress-strain curves of unfilled NR, NR/CS5, DTST-SM5. (b) Energy dissipation efficiency of unfilled NR, NR/CS5, DTST-SM5. LSCM images for the evolution of the chitosan skeleton in DTST-SM5 under ε = 0 (c), ε = 0.5 (d), ε = 1.5 (e), and ε = 3.5 (f). (g) Cyclic stress-strain curves of DTST-SM with different chitosan contents. Tensile-recovery curves of DTST-SM5 in successive cycles with different time intervals at room temperature (RT)


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(h) and at 90 ℃ (i) after the first cycle. (j) Schematic illustration of the damage-healing process of the inverse inverse opal-mimetic skeleton. in Figure 2. Upon cyclic loading, crosslinked NR is quite elastic with small energy dissipation, as its cyclic loops show very small hysteresis and temporary residual strain (Figure 2a). By contrast, introducing the chitosan skeleton significantly increases the energy dissipation, which is manifested by the prominent hysteresis loops of DTST-SM5. However, if chitosan is dispersed, the hysteresis becomes relatively low even the sample has the same amount of chitosan, indicating that the skeleton structure is more efficient in dissipating energy than the randomly dispersed structure. Quantitatively, we define the efficiency of energy dissipation as the ratio between the integrated area of the hysteresis loop and that under the loading curve. Clearly, the efficiency of energy dissipation for DTST-SM5 with inverse opal-mimetic skeleton is remarkably high (about 70%) and moreover independent of strain (Figure 2b), which is a characteristic feature of typical tough materials;33 while the efficiency of energy dissipation for crosslinked NR and NR/CS5 with dispersed chitosan is much lower. To unravel the reason for this significant difference in energy-dissipating ability, chitosan molecules are labeled by a cyanine dye (Cy-5), and laser scanning confocal microscopy (LSCM) is used to monitor the morphology evolution of chitosan skeleton with increasing strain (ε). Primarily, it clearly reveals a well-ordered inverse inverse opal-mimetic skeleton in DTST-SM5 when ε=0 (Figure 2c), which is highly consistent with the morphology observed in the SEM image of Figure 1c (ⅱ). This interconnected skeleton shows an interesting fracturing behavior during the stretching process. Initially, it bears most of the applied stress until rupture takes place locally at multiple positions at a relatively small strain (Figure 2d). Rather than fracturing globally, the stress is transferred to the soft rubber matrix; upon further stretching, other regions


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of the chitosan skeleton deform and sequentially rupture to dissipate more energy (Figure 2e, 2f). Therefore, the fracture process of chitosan skeleton continues many times at different localized positions during the stretching process until the fracturing of the material, making it very dissipative from small to large strains and therefore enhance the toughness. Moreover, the energy dissipation efficiency increases with the chitosan loading (Figure 2g), as a higher chitosan content leads to a more intact skeleton (Figure S5, Supporting Information). Self-healing behavior of DTST-SM It has been reported that Schiff-base linkage is dynamic and reversible at elevated temperature.34,35 To verify the reversibility of the Schiff-base linkage in the chitosan skeleton, tensile-recovery tests are performed by stretching a sample of DTST-SM5 to a strain of 300% and then allowing it to recover by releasing the stress. The first cycle shows an obvious hysteresis loop (Figure 2h), due to the rupture and deformation of the rigid chitosan skeleton, in which the dynamic imine bonds preferentially break as they have lower bond energy.36 In the unloading process, the rubber network recovers quickly as a result of its dominant elastic contraction, while the ruptured/deformed rigid skeleton cannot return to its original state in such a short time because of the lack of elasticity, thus leading a notable residual strain. Conforming to the Mullins effect in most soft materials,37 a much smaller hysteresis loop is found in the second cycle following immediately the first cycle, as the broken skeleton does not have enough time to be reconstructed, and a fractured chitosan skeleton cannot effectively dissipate energy, similar to what is found in NR/CS dispersed samples. As the waiting time prolongs, the hysteresis loop and the normalized residual strain recover extremely slowly, and the loading curve cannot recover to the original one at room temperature even after a relatively long wait time (Figure 2h; Figure S6, Supporting Information). The slow recovery process may be attributed to the competition between the elasticity of covalent rubber network and the plasticity


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of the chitosan skeleton: the chitosan skeleton is locally ruptured and deformed, which restricts the recovery of the rubber network to its equilibrium state. Nevertheless, heat-treatment can diminish the restriction and accelerate the self-healing process of the skeleton. Compared with the recovery at room temperature for 1 hour, heat treatment at 90 ℃ for 1 hour obviously increases the hysteresis loop and reduces the residual strain (Figure 2i). Prolonging the heattreatment time to 10 hours, the loading-unloading curve almost overlaps with that of the first cycle, suggesting the complete healing of the chitosan skeleton. Such a phenomenon can be attributed to the fact that high temperature promotes the reformation of broken imine bonds so as to heal internal damages of the fractured inverse opal-mimetic chitosan skeleton (Figure 2j). The samples with other chitosan loadings also exhibit similar self-repair behavior (Fig. S6, Supporting Information). Defect-tolerance of DIST-SM The ability to sacrifice the chitosan skeleton is critical for enhancing the resistance to crack propagation. As a proof of concept, a sample of DTST-SM3 (approximately 25 mm in width, 1.4 mm in thickness) with an initial edge notch of 6 mm in width is subjected to tensile test. Upon stretching, the notch widens gradually and the crack propagation is extremely slow until disappear when reaching the top and bottom edge of the DTST-SM sample, suggesting the extraordinary blunting at the crack tip thanks to the presence of rigid skeleton. (Figure 3a; Movie S1, Supporting Information). The color of the unstretched sample is dark brown due to the crosslinking of chitosan (Figure S2, Supporting Information). Interestingly, it changes from dark brown to faint yellow in front of the crack tip at small strains ascribing to the internal fracture of the brittle skeleton. Upon further stretching, such a change in color gradually propagates from


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Figure 3. The deformation and failure behaviors of notched DTST-SM. (a) Snapshots of the crack propagation in the DTST-SM sample at elevated tensile strains. (b) The gray scale distribution of the notched DTST-SM in the tensile direction, obtained by analyzing the snapshots using a commercial software ImageJ. The abscissa represents a particular position between the bottom and top edge of the sample and the initial position of the crack is approximately 1550. (c) Intensity-colorized images of propagating cracks on notched DTST-SM samples, obtained by digital image correlation (DIC) using a commercial software MATLAB. The inserted diagram is a schematic of a sharp crack grows within DTST-SM: the crack (dark


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line) propagates in a unique pattern with multiple crack deflections owing to the crack tortuosity yielded by the skeleton. (d) SEM image of the multiple crack deflections in front of the notch of the DTST-SM. The SEM image is obtained after the tensile fracture of DTST-SM3. (e) Stressstrain response for the uncracked and precracked DTST-SM3 samples with different sizes of initial notches. The normalized strength (σc/σc0) (f) and toughness (UTc/UTc0) (g) of the notched DTST-SM3 samples with different notch depths, in comparison with the NR/CS3 samples as well as other hydrogel and composite materials compiled from our previous publication.39 Reproduced with permission from ref 39. Copyright 2019 Elsevier. crack tip to other regions and finally the whole sample turns into faint yellow (Figure 3a). Through the gray scale analysis of the notched sample along the tensile direction (Figure 3b), it can be seen that at the very beginning of the tensile, the gray scale distribution is quite narrow and concentrated near the crack. With increasing strain, the gray scale distribution broadens rapidly and eventually becomes uniform throughout the sample. Such a phenomenon indicates that the internal fracture of the brittle skeleton initiates at the crack tip and then spreads throughout the whole sample. Accompanying this process, the stress is dispersed in the same way, thereby avoiding stress concentration. To better clarify the stress distribution in the elastomer upon stretching, intensity-colorized images is obtained via digital image correlation (DIC) to reflect the strain field in the notched DTST-SM (Figure 3c). Under small strains, the strain is extremely localized in front of the crack tip caused by the stress concentration in this area. Nevertheless, as the strain increases, the strain concentration around the notch is dramatically delocalized and extends over a large region throughout the whole sample, suggesting great stress dispersion benefited from the internal fracture of the brittle skeleton. Moreover, the incorporation of rigid skeleton arises significant stiffness variation in soft matrix,


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which significantly hinder crack propagation and increase toughness.38 Cracks mainly travel along the interface of rigid skeleton and soft elastomer instead of through the matrix directly, thus significantly increasing crack propagation path and tortuosity. Besides, the deformation and fracture of the skeleton near the crack will inevitably cause the crack to deflect, differentiate into multiple cracks and even disappear, as schematically depicted in Figure 3c. Indeed, microscopic observation of the fracture sections clearly demonstrates that the crack does not propagate along its initial direction; instead, it deflects into multiple small cracks that extend in different paths, until one of the cracks evolves into a large one and leads to the fracture of the bulk material (Figure 3d). As a result, the DTST-SM is highly resistant to crack propagation. To quantitatively characterize the crack-tolerant property of DTST-SM, the width of the edge notch (L) is changed systematically in relative to the central width of each dumbbell shaped sample (L0), thus the ratio of L/L0 varies from 0 to 0.8. Thereafter, tensile tests are conducted under a controlled strain rate of 100 mm min-1 to obtain the stress-strain curves for both unnotched and notched samples. Clearly, the mechanical response of DTST-SM is not significantly affected when the L/L0 value is less than 0.2 (Figure 3e). Moreover, even when the samples have quite large notches, they do not break catastrophically. These phenomena indicate that DTST-SM is remarkably defect-tolerant. By contrast, the dispersed counterpart and other soft materials, such as conventional rubber composites and hydrogels are extremely sensitive to defects.39 In these materials, drastic decrease in their tensile strength and fracture strain happens as long as there is a small notch on the sample (Figure S7, Supporting Information). To illustrate this difference, we plot the normalized stress (σc/σc0, where σc and σc0 are the tensile strength of notched sample and unnotched sample, respectively) and normalized fracture energy (UTc/UTc0,


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Figure 4. Mechanical properties of DTST-SM. (a) Representative stress-strain curves of DTSTSM with various CS contents, the tensile rate is 100 mm min−1. (b) Tensile strength and strain at break of DTST-SM with increasing CS content. (c) Modulus and fracture toughness of DTSTSM with various CS contents. (d) Ashby diagram displaying fracture toughness for as-fabricated DTST-SM with other natural elastomers, synthetic structural materials and composites as a function of their strength (CFRP:Carbon Fiber Reinforced Plastic; GFRP: Glass Fiber Reinforced Plastic).40,41 Reprinted in part with permission from ref. 40 and 41. Copyright 2014 Springer-Verlag (40) and 2004 Taylor & Francis (41). where UTc and UTc0 are the fracture energy obtained by integrating the stress-strain curves for notched sample and unnotched sample, respectively) as a function of L/L0. Interestingly, both σc/σc0 and UTc/UTc0 of the notched samples reduce linearly with L/L0 for DTST-SM, which indicates that it is extraordinarily damage-tolerant; while the NR/CS dispersed samples and other


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conventional hydrogels and rubber composites show sharp decreases in σc/σc0 and UTc/UTc0 as long as a small notch is introduced (Figure 3f, 3g). Mechanical properties of DTST-SM To explore the influence of the chitosan skeleton on the mechanical strength and toughness of DTST-SM, uniaxial tensile tests are carried on unnotched dumbbell shaped samples and the mechanical properties are summarized in Table S2 (Supporting Information). Obviously, the incorporation of chitosan skeleton pronouncedly increases the strength and modulus of NR, as shown by the typical stress-strain curves in Figure 4a. With increasing chitosan loading, the elastic modulus increases monotonically, while the tensile strength increases first and then decreases. In particular, when the chitosan loading is 4 phr, the DTST-SM4 shows an extraordinary 20-fold increase in elastic modulus (from 0.9 MPa of NR to 18.0 MPa) and a 2.2fold increase in tensile strength (from 15.3 MPa of NR to 33.4 MPa) (Figure 4b, 4c). Moreover, the fracture toughness (Gc) of DTST-SM, which is calculated using the Greensmith method (see Materials and Methods for details),33 shows astonishing improvement, reaching a remarkably high value of 122 KJ m-2 as compared with other natural and artificial elastomers (Figure 4c, 4d). Notably, the DTST-SM also shows much higher modulus and fracture toughness than NR/CS dispersed samples (Figure S8, Supporting Information). Collectively, the interconnected skeleton can not only effectively bear load, but also act as sacrificial skeleton that preferentially rupture to dissipate energy, avoiding the stress concentration and resisting crack propagation. Ultimately, the strength, modulus and toughness of DTST-SM are simultaneously enhanced. Since the hydroxyl and amino groups of chitosan can form strong interactions with water molecules, the chitosan skeleton is highly hydrophilic. The combination of such hydrophilic skeleton with the hydrophobic matrix may lead to unexpected humidity-responsive behavior of DTST-SM. To testify the humidity-responsive behavior, we fabricate a thin film with a thickness


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Figure 5. Soft robots and smart garment based on DTST-SM. (a) Adaptive actuation movement of a DTST-SM10 film (55 mm × 12 mm × 36 µm) in response to local humidity change. The fixed thin film of DTST-SM10 can bend to the opposite side when a finger comes close to it and recover to its original state after the finger is removed. (b) Time course of the walking motion of the walking device (Length = 32 mm; Width = 10 mm; Thickness = 120 µm), which is cut and shaped into a worm-like strip with several body segments in stable ambient condition. (c) Fabrication of the triangular patterned mini-windows-switch array using scissor. (d) The membrane (thickness = 97 µm) with mini-windows array able to curl in an outward direction is applied on a commercial climbing shoe to develop a personal humidity and heat management system. The time-dependent internal humidity of the shoe (e) and temperature on the surface of foot (f). of 36 μm, which is then fixed between two pieces of glass slide. Due to gravity and internal stress generated during the film casting process, the film initially bends to the right side under


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ambient condition. However, when a finger approaches the film, it spontaneously stands up gradually and eventually changes from right bending to left bending in a few seconds (Figure 5a and Movie S2, Supporting Information). This phenomenon clearly demonstrates that even a slight and local increase in humidity close to the DTST-SM film readily induces fast change in the configuration of the sample. Furthermore, after removing the finger, the film recovers back to its initial position within few seconds, indicating that the bending is completely reversible. However, when the approached finger is in a rubber glove, the film does not bend, indicating that the bending process is not driven by temperature. The mechanism of this bending process is that the chitosan in the film can absorb water molecules at the surface facing the moisture, and moreover the water molecules can quickly transport in the seamlessly interconnected skeleton to generate a water gradient. This causes the film to expand vertically and horizontally on the side facing the moisture, generating a net folding force on the film and finally enabling the film to bend.42 It must be pointed out that NR/CS with dispersed chitosan does not show obvious humidity-responsive property (Figure S9, Supporting Information), indicating that the inverse opal-mimetic structure is critical for such property. The humidity-responsive property of DTST-SM enables us to design and fabricate smart soft robots. As a proof of concept, we fabricate a walking device, showing unidirectional worm-like motion on a piece of paper stimulated by humidity gradients (Fig. 5b; Movie S3, Supporting Information). Moisture is provided by a humidifier and applied to the walking device from left upside by a rubber tube. When the water vapor is on, the strip bends into an arch as the left upside of the walking device absorbs the moisture and swell, causing the left part to bend inward immediately and serve as a stationary point. After removing the moisture, the left edge gradually unbends and stretches, allowing the whole film to move forward. The walking device can


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achieve a unidirectional crawling driven by local humidity stimulation, and the walking velocity is 40 mm min-1 approximately. This humidity-responsive actuator mimics the crawling of worms, which may find many applications such as soft robots or other smart devices. It is worth mentioning that the water absorption of DTST-SM increases with the increase of chitosan content, but mechanical properties of DTST-SM is not seriously influenced after water absorption (Figure S10, Figure S11, Supporting Information) . Integrating excellent mechanical performance and pronounced moisture sensitivity, DTST-SM might be applied in smart garments under certain extreme conditions that require both acute response and sufficient strength as well as toughness. For instance, during outdoor climbing and long-distance running or marching, shoes are exceedingly vital, as they directly affect the comfort and health of feet. Aiming at intelligent control of personal humidity and temperature in shoes, the DTST-SM is partially cut and patterned to obtain a film with multiple triangular humidity switches (Figure 5c), which can open up with increased humidity and shut down when the humidity decreases (Movie S4, Supporting Information). To evaluate the efficiency of the intelligent humidity management system, the DTST-SM film is integrated into the man-made openings of climbing shoes, as shown in Figure 5d. For comparison, neat NR is cut and patterned in the same way. Then, a test that involves a man running for 30 minutes is conducted and the results are displayed in Figure 5e. It is clear that the relative humidity (RH) in the shoe with the patterned DTST-SM film reaches equilibrium more quickly at RH≈70%, which is relatively comfortable and is about 20% lower than the case with patterned NR film. Moreover, the temperature within the shoe with the patterned DTST-SM film is lower, of which the temperature difference is about 1.4 ℃ (Figure 5f). Therefore, the DTST-SM enables effective


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control of the humidity and temperature of human body, which holds great prospect of being applied in smart garments.

CONCLUSIONS Inspired by the periodic structural design of soft and rigid ingredients in living organisms, we develop a facile latex-assembly method to fabricate a class of soft material with a rigid inverse opal-mimetic skeleton and a soft continuous rubber matrix. The soft material is ultra tough, strong and defect-tolerant. Such multiple mechanical properties, which are exclusive in most synthetic materials, are simultaneously improved by the synergistic toughening and reinforcing mechanisms originating from the load-bearing and internal fracture of the stiff skeleton, the stress redistribution and extensibility of the soft matrix and the stiffness variation between the hard and rigid components. As a result, the soft material exhibits toughness as high as 122 KJ m2

which is even higher than that of the current state-of-the-art titanium alloys, and strength up to

33.4 MPa which is higher than most rubber nanocomposites. Additionally, the skeleton of the soft material shows self-healing ability, as it is composed of dynamically crosslinked chitosan. Moreover, the skeleton is hydrophilic and can easily generate water gradient, which enables the humidity-responsive property of the soft material. Taking advantages of these properties, soft robots with a few simple motions are realized and a smart garment is exploited for personal humidity and heat management.

EXPERIMENTAL SECTION Materials. NR latex (60 wt% solid content) is kindly provided by Chinese Academy of Tropical Sciences. Chitosan (CS) with a deacetylation degree not less than 85% is purchased from


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Shanghai Titan Scientific Co., Ltd. Glutaraldehyde (GA, 25% in water) is purchased from Adamas Reagent Co., Ltd. Potassium hydroxide (KOH), Acetic acid (36%, analytical purity), Fatty alcohol polyoxyethylene ether (Peregal O) are obtained from Sichuan Xilong chemical Co., Ltd. Potassium Laurate (30%) is supplied by Shanghai Ziyi Reagent Co., Ltd. Curing reagents including sulfur, zinc oxide (ZnO), Zinc diethyldithiocarbamate (ZDC) and are provided by Sichuan Haida Rubber Group Co., Ltd. Preparation of acidified prevulcanized natural rubber latex. Natural rubber latex is compounded as per the formulation given in Table S1. The NR latex is mixed with the curing ingredients along with continuous stirring by a homogenizer at 70 ℃ for 1 hour. Then, the prevulcanized latex is cooled to room temperature and kept overnight for maturation. The obtained latex is filtered, adequately stabilized with Peregal O and then acidified with 36 wt % acetic acid to a pH of 4. Preparation of DTST-SM samples. The DTST-SM with inverse opal-mimetic skeleton is prepared by a facile latex assembly method. Chitosan is dissolved in 1% (v/v) acetic acid to get a 2 wt% solution. The obtained solution is added into the acidified prevulcanized latex under vigorous stirring for 12 hours followed by sonicating for 30 min so as to get stable homogeneous blends with various chitosan concentrations ranging from 0 to 10 phr, respectively. After standing in room temperature for several hours to remove bubbles, the blends are cast onto flat petri dishes and dried in air to obtain the films with a thickness around 1 mm. Then, the dried films are further treated with GA solution to crosslink CS and dried again. Finally, postvulcanization is conducted on all samples at 130 ℃ for 30 minutes. Preparation of NR/CS dispersed samples. NR/CS samples with randomly dispersed chitosan is prepared as a control. Typically, NR latex is compounded as per the formulation given in Table


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S1 and acidified followed by blending with chitosan solution under vigorous stirring for 12 hours followed by sonicating for 30 min so as to get stable homogeneous blends with various chitosan concentrations ranging from 0 to 10 phr, respectively. After film formation, glutaraldehyde treating and drying, films are then subjected to twin-roll mixing followed by hot-pressing to obtain the randomly dispersed chitosan in NR matrix. Characterization. Fourier transform infrared (FTIR) spectra are recorded at room temperature using Thermo Scientific Nicolet iS50 FTIR, the scan range is from 4000 cm-1 to 650 cm-1 with a resolution of 4 cm-1. The dispersion and distribution state of chitosan within the NR matrix is examined by a scanning electron microscope (SEM, JEOL JSM-5900LV). Atomic force microscopy (AFM) is performed on a SPI4000 AFM instrument of Seiko using silicon tips (NSG10) with a spring constant of 3 N m-1 at a resonance frequency of 228.9 kHz. All measurements are carried out in tapping (AC) mode. Mechanical properties are measured on a universal testing machine (Instron 5966) at room temperature with an extension rate of 100 mm min-1. At least three specimens have been tested, and the average data are adopted. The loadingunloading cycles are performed by a universal testing machine (Instron 5966) with extension rate of 100 mm min-1. In tensile-recovery tests, the sample is stretched to a strain of 300% and then relaxed at room temperature for a certain waiting time (0, 600, 1800, 3600 s) prior to the subsequent loading process, heat-treatment of samples is carried out in the environment box of the universal testing machine. Energy dissipation efficiency is quantified by the ratio of the integrated area in the hysteresis loop to that under the loading curve. For every cycle, the energy dissipation efficiency (η) is calculated from Equation (1):



 d   d

loading

unloading

 d

100%

(1)

loading


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where σ is stress, λ is the local elongation in the tensile direction. The skeleton structure change of DTST-SM is analyzed using a laser scanning microscope (Zeiss LSM 710, Carl Zeiss, AG, Germany) under excitation with lasers operating at 644 nm. Chitosan is labeled with cyanine dye (Cy-5). Briefly, Cy-5 is dissolved in PBS buffer solution and mixes with a certain amount of chitosan solution. After stirring for 24 h in a light-free environment, prevulcanized NR latex is added to the mixture according to the ratio of DTSTSM5 and stirs for 10 hours to form a homogeneous suspension. Several drips of the suspension are dropwise to Teflon petri dish and quickly dried in oven. After remove the residual water, the film is immersed in glutaraldehyde solution to crossliink chitosan and postvulcanized to obtain an ultrathin film with inverse opal-mimetic skeleton for the next measurement. The laser scanning confocal microscopy (LSCM) images are taken at 200× magnification with a pinhole diameter of 1 Å. After stretching, the samples are fixed on the glass slide via adhesive tape. Fracture tests are conducted using the classical single edge notch test on universal testing machine (Instron 5966) with an extension rate of 6 mm min-1. An edge notch of 1 mm in length is preset in the middle of a rectangular specimen of about 1 mm in thickness and 5 mm in width. The fracture energy (Gc) is calculated by the Greensmith method: Gc 

6WC c

(2)

where C is the depth of the notch, λc is the fracture strain of notched sample, W is the fracture energy calculated by integration of the stress-strain curve of an un-notched specimen until λc; the un-notched specimen undergoes a tension process with the same strain rate as the notched sample.


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The water absorption tests are performed by immersing the samples in distilled water at 30 ℃. Weight of the samples ares recorded at different time intervals after wiping the surface water with tissue paper. Water absorption is determined using the following equation:



Wg  W0 W0

* 100%

(3)

where η is the water absorption rate, Wg is the weight after absorbing water, W0 is the oven dry weight before water treatment. The influence of water absorption on the mechanical properties is test by place the DTST-SM samples in a home-made enclosed and wet environment (RH≈80%) for two days before measuring the mechanical properties by by a universal testing machine (Instron 5966).

Moisture-responsive behaviors are performed in the laboratory where the relative humidity and temperature are maintained at 21±1% and 22 ℃ , respectively. The thickness of thin film are measured by a thickness gauge and the humidity is supplied through a humidifier. The response of the hybrid films is recorded with a digital camera.

ASSOCIATED CONTENT Supporting Information. FTIR spectra and AFM pictures of DTST-SM and NR/CS, mechanical properties of DTST-SM before and after treated with glutaraldehyde, SEM images and preparation process of NR/CS dispersed samples, swelling tests of DTST-SM, stress-strain curves for self-healing ability of DTST-SM3, the comparison of mechanical performance and humidity-responsive abilitiy


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between NR/CS dispersed composites and DTST-SM. The water absorption test of DTST-SM and the influence of water absorption on the mechanical properties. (PDF) Excellent defect-tolerance of DTST-SM (Movie S1) The bending of DTST-SM membrane. (Movie S2) The worm-like crawling of the walking device. (Movie S3) The opening and closing of the humidity switches based on DTST-SM. (Movie S4) AUTHOR INFORMATION Corresponding Author * Jinrong Wu. Email: [email protected]. * Guangsu Huang. Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant Nos. 51673120, 51790501 and 51873110) and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2017-3-05). REFERENCES (1) Li, J.; Geng, L.; Wang, G.; Chu, H.; Wei, H., Self-Healable Gels for Use in Wearable Devices. Chemistry of Materials. 2017, 29 (21), 8932-8952.


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Table of Contents


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Ultra-Tough, Strong, and Defect-Tolerant Elastomers with Self-Healing

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