Poly(vinyl alcohol

Apr 26, 2017 - Our nacre-mimetic rGO/PVA composite films were even more remarkable in terms of its fracture strain and toughness (Figure 5b and Table ...
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Superstretchable Nacre-Mimetic Graphene/ Poly(vinyl alcohol) Composite Film Based on Interfacial Architectural Engineering Nifang Zhao,† Miao Yang,† Qian Zhao,† Weiwei Gao,‡ Tao Xie,† and Hao Bai*,† †

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Through designing hierarchical structures, particularly optimizing the chemical and architectural interactions at its inorganic/organic interface, nacre has achieved an excellent combination of contradictory mechanical properties such as strength and toughness, which is highly demanded yet difficult to achieve by most synthetic materials. Most techniques applied to develop nacre-mimetic composites have been focused on mimicking the “brick-and-mortar” structure, but the interfacial architectural features, especially the asperities and mineral bridges of “bricks”, have been rarely concerned, which are of equal importance for enhancing mechanical properties of nacre. Here, we used a modified bidirectional freezing method followed by uniaxial pressing and chemical reduction to assemble a nacre-mimetic graphene/poly(vinyl alcohol) composite film, with both asperities and bridges introduced in addition to the lamellar layers to mimic the interfacial architectural interactions found in nacre. As such, we have developed a composite film that is not only strong (up to ∼150.9 MPa), but also tough (up to ∼8.50 MJ/m3), and highly stretchable (up to ∼10.44%), difficult to obtain by other methods. This was all achieved by only interfacial architectural engineering within the traditional “brick-and-mortar” structure, without introducing a third component or employing chemical cross-linker as in some other nacre-mimetic systems. More importantly, we believe that the design principles and processing strategies reported here can also be applied to other material systems to develop strong and stretchable materials. KEYWORDS: biomimetic, nacre, bidirectional freezing, superstretchable, interfacial engineering infiltration,17 spraying,18 extrusion,19 and freeze-casting20−23 have been applied to develop nacre-mimetic composites. Most of these techniques have been focused on mimicking “brickand-mortar” layered structure of nacre. However, the interfacial architectural features, especially the asperities and mineral bridges of “bricks”, have been rarely concerned, which are of equal importance for enhancing mechanical properties of nacre. Recently, dip-coating,24 freeze-casting,25 and magnetic assembly26 were utilized to prepare nacre-mimetic composites with nanoasperities or mineral bridges at the interfaces of inorganic bricks, obtaining improved mechanical properties. Despite of these progresses, there is still insufficient manipulation at the interfacial architectures in synthetic nacre-mimetic composites, especially in scalable manufacturing. Freeze-casting,27 also known as ice-templating, have been considered as a powerful technique to make nacre-mimetic

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any natural materials have obtained outstanding mechanical properties by building sophisticated hierarchical architectures ranging from nano/microscopic to macroscopic scales. As a prime example, nacre has achieved an excellent combination of strength and toughness due to its hierarchical structures, usually serving as a source of inspiration or even “golden standard” for the development of high-performance composites.1 Nacre arranges 95 vol % aragonite platelets and 5 vol % organic biopolymers into “brick and mortar” layered structure,2 which helps to endure large inelastic deformations and dissipate more energy during fracture. Moreover, mineral bridges and nanoasperities are observed between adjacent “bricks”.3,4 Breaking of mineral bridges and inelastic shearing resisted by nanoasperities also play a critical role in nacre’s toughening mechanisms.5 Nacre-mimetic composites based on different inorganic building blocks have been fabricated during the past decade, such as aluminum oxide,6 clay,7−9 carbon nanotubes,10,11 and graphene.12−14 Various techniques, such as layer-by-layer assembly,15 slip/tape casting and hot pressing,16 vacuum © 2017 American Chemical Society

Received: February 16, 2017 Accepted: April 26, 2017 Published: April 26, 2017 4777

DOI: 10.1021/acsnano.7b01089 ACS Nano 2017, 11, 4777−4784

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Figure 1. Fabrication of highly stretchable nacre-mimetic rGO/PVA composite film. (a) Bidirectional freezing method. By introducing a low thermal conductive PDMS wedge between the GO/PVA aqueous suspension and the cooling stage, dual temperature gradients, both vertical (ΔTV) and horizontal (ΔTH), were generated simultaneously during freezing, which played as a template to assemble GO and PVA into a long-range aligned lamellar monolith. (b) Optical image showed an as-prepared monolith (around 1.2 × 1.2 × 3 cm) after freeze-drying. (c) Optical image showed a flexible rGO/PVA film after hot-pressing and chemical reduction. (d) SEM image and schematic illustration in (b) from the cross-section parallel to the cooling stage showed lamellar structure of the as-prepared monolith with both dendrites and bridges. (e) SEM image of the cross-section of a rGO/PVA film showed that lamellar layers were successfully densely packed. Schematic illustrations in (c) showed the detailed lamellar structure of the rGO/PVA film with curled dendrites and bridges, which mimicked (f) nacre’s “brick-and-mortar” structure, especially the interlayer bridges (the inset).

can also be applied to other material systems to develop strong and stretchable composites.

composites with the ability to effectively control over multiple scale architectures. Recently, a bidirectional freezing technique was developed to assemble small building blocks (ceramic particles, platelets, and/or polymer) into large-sized ordered lamellar structure (centimeter-scale), which can be subsequently pressed and infiltrated to fabricate nacre-mimetic composites with “brick-and-mortar” architecture and outstanding mechanical properties.28,29 In this method, the chemical and physical properties of the aqueous suspension greatly influenced the nucleation and growth of ice crystals, resulted in scaffolds with different architectures. More interestingly, when modifying the viscosity, the number of dendrites and bridges in the scaffold could be simultaneously adjusted in addition to the common lamellar layers. Here, we show as a proof of concept that a graphene oxide and poly(vinyl alcohol) (GO/PVA) scaffold with long-range aligned lamellar structure and designed interlayer dendrites and bridges can be fabricated by a modified bidirectional freezing technique. Graphene oxide sheet was chosen here because of its easy processability and outstanding mechanical properties.30,31 Owing to its water solubility and rich functional groups on the surface, it has become a favorable candidate for fabricating nacre-mimetic materials with excellent mechanical properties. Subsequent hot-pressing and chemical reduction with hydroiodic acid (HI) resulted in a reduced GO/PVA (rGO/PVA) composite film with unusual interfacial architectural features mimicking the asperities and bridges of nacre in addition to its typical “brick-and-mortar” structure. With such subtle engineering at interfacial architectures, these flexible nacre-mimetic composite films are not only strong (up to ∼150.9 MPa), but also tough (up to ∼8.50 MJ/m3) and highly stretchable (up to ∼10.44%), showing great potential in soft electronics such as electronic skin and sensors.32 More importantly, we believe that the design principles and processing strategies reported here

RESULTS AND DISCUSSION The fabrication route for the nacre-mimetic rGO/PVA composite film is illustrated in Figure 1. First, a GO/PVA porous scaffold was prepared by a modified bidirectional freezing method. An aqueous suspension consisting of GO and PVA with a suitable viscosity was prepared and then frozen at a given cooling rate from the cold copper plate (Figure 1a). A low thermal conductive polydimethylsiloxane (PDMS) wedge was introduced between the aqueous suspension and the cooling stage where it simultaneously generated both the horizontal (ΔTH) and vertical (ΔTV) temperature gradients when cooling. With ice crystals grew preferentially along the vertical and horizontal temperature gradients, GO and PVA were both expelled from the freezing suspension into the space between the ice crystals to replicate the lamellar ice pattern (Figure 1a). After freeze-drying at −60 °C for more than 48 h, a brown-colored GO/PVA scaffold (around 1.2 × 1.2 × 3 cm) was obtained (Figure 1b). From the SEM images of the crosssection parallel to the cooling stage, a long-range aligned lamellar structure with both dendrites and bridges was obtained (Figure 1d). The freeze-dried GO/PVA monolith was then hotpressed perpendicular to lamellar layers and cured under pressure at 95 °C for 30 min to generate a GO/PVA film. After chemical reduction in hydroiodic acid solution, the rGO/PVA film became flexible (Figure 1c). The dark color indicated effective reduction of GO. The X-ray diffraction (XRD) results showed that the interlayer distance of the rGO/PVA film increased when increasing PVA content (Figure S1 and Table S1), indicating that PVA was successfully embedded in between the GO sheets. The rGO/PVA film was then cut into strips (length: ∼30 mm; width: ∼3−4 mm) for tensile test. Lamellar 4778

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Figure 2. Mechanical properties of nacre-mimetic rGO/PVA composite films with different GO/PVA ratio. Tensile tests were performed to evaluate the (a) tensile strength, (b) fracture strain, (c) toughness, and (d) Young’s modulus of nacre-mimetic rGO/PVA composite films with different GO content. It showed that the best performance was achieved at GO/PVA ratio of 7:5, i.e., ∼58 wt % of GO in the film. Red points in (a−d) were measured from the films prepared by common vacuum filtration method with a diluted suspension of the same GO/PVA ratio (7:5). The reduction and postprocessing procedures of the films were the same as in the bidirectional freezing technique. The as-prepared nacre-mimetic film showed both higher strength, fracture strain and toughness, but lower Young’s modulus.

structure was clearly observed in the film cross-section, showing that the lamellar layers were successfully densely packed (Figure 1e). We proposed two schematic illustrations (Figure 1b and c) to indicate changes caused by hot-pressing. As shown in Figure 1b, the bridges were straight and the dendrites were just slightly curled, and packed in between the layers. After hotpressing, the dendrites and bridges were curled without changing position (Figure 1c). The structure of the rGO/ PVA composite film with both curled dendrites and bridges is consistent with that of nacre, which has a “brick-and-mortar” structure with asperities on the surface of aragonite brick and mineral bridges between adjacent bricks (Figure 1f). Tensile tests were performed to evaluate the tensile strength, fracture strain, toughness, and Young’s modulus of the nacremimetic rGO/PVA composite films with different GO content or GO/PVA ratio (Figure 2 and Table S2). As GO content was raised from 55 wt % to 67 wt %, the average tensile strength first increased from 119.3 ± 1.9 MPa to 147.6 ± 5.6 MPa, and then decreased to 114.4 ± 9.8 MPa (Figure 2a), while the average fracture strain first increased from 5.84 ± 0.26% to 8.19 ± 0.79%, and then decreased to 6.18 ± 0.26% (Figure 2b). Consequently, the average toughness first increased from 3.87 ± 0.20 MJ/m3 to 6.75 ± 0.52 MJ/m3, and then decreased to 3.83 ± 0.43 MJ/m3 (Figure 2c). Besides, the average Young’s modulus first decreased from 2.84 ± 0.13 GPa to 2.18 ± 0.20 GPa, and then increased to 2.37 ± 0.10 MJ/m3 (Figure 2d). When the GO content was relatively low (around 44 wt %), the

average tensile strength, fracture strain, toughness, and Young’s modulus was 104.0 ± 7.3 MPa, 9.19 ± 0.69%, 6.01 ± 0.40 MJ/ m3 and 2.19 ± 0.07 GPa, respectively. In this condition, the PVA content was higher than GO, which resulted in a higher fracture strain and relatively lower tensile strength. When the GO content was high (around 71 wt %), the average tensile strength, fracture strain, toughness, and Young’s modulus was 69.5 ± 7.7 MPa, 5.46 ± 0.17%, 2.06 ± 0.30 MJ/m3 and 1.61 ± 0.22 GPa, respectively. Although the GO content was much higher than the PVA content, the mechanical properties were ordinary, which was because the GO/PVA suspension was too viscous to obtain long-range aligned lamellar structure. The GO content for optimal mechanical properties was thus found at around 58 wt %, namely the optimal GO/PVA ratio was 7:5, in which condition the tensile strength, fracture strain, toughness, and Young’s modulus reached 150.9 MPa, 8.84%, 7.15 MJ/m3 and 2.84 GPa, respectively. For comparison, we fabricated a rGO/PVA composite film by common vacuum filtration with a diluted suspension of the same GO/PVA ratio (7:5). Note that the reduction and postprocessing procedures of the films were the same as in the bidirectional freezing technique. Tensile test showed that the tensile strength, fracture strain, and toughness were 118.4 ± 2.7 MPa, 6.79 ± 0.04%, 4.98 ± 0.15 MJ/m3, much lower than that of our nacre-mimetic composite film with the same GO/PVA ratio (Figure 2a−c). Besides, the Young’s modulus was 3.05 ± 0.03 GPa, slightly higher than that of our nacre-mimetic composite film (Figure 2d). Typical stress− 4779

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Figure 3. Comparison of structures and mechanical properties of nacre-mimetic rGO/PVA composite films prepared with suspensions that had different GO concentration, but the same GO/PVA ratio (7:5). (a−d) SEM images showed the cross sections parallel to the cooling stage of the freeze-dried samples before hot-pressing, with a fixed GO/PVA ratio (7:5) but increasing GO concentration in the suspension: 3, 5, 7, 9 mg/mL, respectively. All the samples showed lamellar layers, but with different amounts of asperities and bridges. (e−h) Magnified SEM images of (a−d) showed the differences between four kinds of samples. The number of asperities and bridges increased with the increase of GO concentration. As GO concentration increased from 3 to 9 mg/mL, (i) the average tensile strength first increased from 98.2 ± 1.7 MPa to 147.6 ± 5.6 MPa, and then decreased to 129.3 ± 12.2 MPa, the average Young’s modulus first increased from 2.54 ± 0.09 GPa to 3.05 ± 0.26 GPa, and then decreased to 2.02 ± 0.12 GPa, (j) the average fracture strain increased from 5.39 ± 0.18% to 9.86 ± 0.52%, and (k) the average toughness increased from 3.03 ± 0.09 MJ/m3 to 7.30 ± 1.08 MJ/m3. Note that when further increased the GO concentration the suspension became too viscous to obtain long-range aligned lamellar structure.

strain curves for the films prepared by two methods were illustrated in Figure S2. These results demonstrate that the introduction of the interlayer dendrites and bridges can effectively improve the mechanical properties, especially the stretchability of the nacre-mimetic composite film. It was reported that the viscosity of the aqueous suspension would affect the growth of ice crystals and thus alter the number of dendrites and bridges between adjacent lamellar layers.33 Here, to further investigate the role of dendrites and bridges, we fabricated four kinds of nacre-mimetic composite films from aqueous suspensions with different viscosity using the modified bidirectional freezing method, i.e., fixed GO/PVA ratio (GO:PVA = 7:5) but different GO concentration in suspension (3, 5, 7, 9 mg/mL, respectively) (Figure 3 and Table S2). SEM images of the freeze-dried scaffolds before hotpressing with fixed GO/PVA ratio but different GO concentration in suspension were taken from the cross sections parallel to the cooling stage, Figure 3a−h. All samples showed similar lamellar structure but with different amounts of asperities and bridges. When the GO concentration in suspension was 3 mg/mL, the viscosity of the suspension was low, and ice crystals were easier to grow along the horizontal and vertical temperature gradients with less restriction from particles, resulted in little dendrites and no bridges (Figure 3a,

e). Further increasing the GO concentration raised the suspension viscosity, which restricted the growth of ice crystals to generate more dendrites and bridges (Figure 3b−d, f−h). As the GO concentration was raised from 3 mg/mL to 9 mg/mL, the average tensile strength first increased from 98.2 ± 1.7 MPa to 147.6 ± 5.6 MPa, and then decreased to 129.3 ± 12.2 MPa, the average Young’s modulus first increased from 2.54 ± 0.09 GPa to 3.05 ± 0.26 GPa, and then decreased to 2.02 ± 0.12 GPa (Figure 3i), the average fracture strain increased from 5.39 ± 0.18% to 9.86 ± 0.52% (Figure 3j), and thus the average toughness increased from 3.03 ± 0.09 MJ/m3 to 7.30 ± 1.08 MJ/m3 (Figure 3k). The ultimate tensile strength, Young’s modulus, fracture strain, and toughness reached 150.9 MPa, 3.34 GPa, 10.44%, and 8.50 MJ/m3, respectively. These results further demonstrate that the introduction of the interlayer dendrites and bridges can effectively improve the mechanical properties of the nacre-mimetic composite film. Further increase of GO concentration made the suspension too viscous for the growth of lamellar ice crystals. Therefore, nonlamellar structure was obtained. To further understand the facture mechanism of the nacremimetic rGO/PVA composite film, we compared the fracture surfaces of the films prepared by both vacuum filtration and bidirectional freezing. Fracture models were proposed accord4780

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Figure 4. Proposed fracture mechanism of superstretchable nacre-mimetic rGO/PVA composite film. (a) Schematic illustration showed the structure of the superstretchable nacre-mimetic film with lamellar layers interconnected with both asperities (in yellow) and bridges (in pink). (b) During stretching, the dendrites were stretched with high friction between adjacent lamellar layers. (c) Until they were fully elongated, the lamellar layers began to crack. However, with the bridges connecting the adjacent lamellar layers, the crack could not propagate all the way through the film. All these structural features involved extrinsic toughening phenomena contributing to the outstanding stretchability of the film. (a−c) The proposed mechanism indicated that the bridges functioned as the mineral bridges of nacre, while the dendrites functioned as asperities between adjacent bricks. SEM images showed the fracture surfaces of typical rGO/PVA composite films prepared by (d−f) the bidirectional freezing technique (60 to 120 μm thick) and (g−i) common vacuum filtration method (50 to 70 μm thick). (f, i) The magnified SEM images showed that the graphene sheets played as bricks during stretching and were finally “pulled-out”. (f) Superstretchable nacremimetic film had a rougher fracture surface than (i) the common film, indicating the “zigzag” crack path and contribution of bridges to the film toughness.

Figure 5. Comparison of mechanical properties among nacre-mimetic rGO/PVA composite film, nacre and other composite films based on GO or rGO sheets. (a) Comparison of tensile strength and toughness, showing that the nacre-mimetic rGO/PVA composite films have similar strength with nacre but are much tougher than nacre and other composite films based on GO or rGO sheets. (b) Comparison of fracture strain and toughness, showing that the nacre-mimetic rGO/PVA composite films are both superstretchable (up to 10.44% strain), and tough (up to 8.50 MJ/m3) resulted from interfacial architectural engineering.

dendrites introduced additional friction energy dissipation between adjacent lamellae and toughened the nacre-mimetic film while stretching. When stretching continued, the dendrites and bridges were finally pulled out. As the bridges cracked, the nacre-mimetic film finally fractured (Figure 4c). The proposed mechanism indicated that the bridges functioned as the mineral

ingly (Figure 4a−c). Lamellar structure with curled dendrites and bridges before stretching were shown in Figure 4a. When start stretching, the lamellae consisting of rGO and PVA cracked at first. Owing to the bridges connecting the adjacent crack lamellae, the composite film did not rupture promptly and could be further stretched (Figure 4b). Meanwhile, the 4781

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Figure 6. Electrical properties of nacre-mimetic rGO/PVA composite films. (a) The nacre-mimetic composite film retained over 80% of its original tensile strength after 100 stretching cycles with 5% strain. (b) As the GO content increased from 44 wt % to 71 wt %, the electrical conductivity increased from 16.5 ± 1.0 S/m to 249.2 ± 11.4 S/m. (c) The electrical conductivity variation was measured during stretching cycles. The resistance showed no significant increase after 100 cycles. (d) The electrical conductivity of superstretchable composite film responded rapidly upon the finger bending−unbending cycles at an applied voltage of 0.01 V.

remarkable in terms of its fracture strain and toughness (Figure 5b and Table S3), as it could be stretched to as high as around 10.44%, making it very promising for soft electronics. It is noteworthy that this was all achieved by only interfacial architectural engineering within the traditional “brick-andmortar” structure, without introducing a third component or employing chemical cross-linker as in some other nacremimetic systems. In addition, we further explored the potential applications of our superstretchable nacre-mimetic composite films in soft electronics. Cycling stretch with 5% strain was performed on a nacre-mimetic composite film (9 mg/mL GO in suspension; GO/PVA ratio of 7:5). The composite film retained over 80% of its original tensile strength after 100 stretching cycles with 5% strain (Figure 6a), demonstrating its robustness and antifatigue property. The static electrical conductivity of films with different GO content was measured at an applied voltage of 0.01 V. As the GO content increased from 44 wt % to 71 wt %, the electrical conductivity increased from 16.5 ± 1.0 S/m to 249.2 ± 11.4 S/m (Figure 6b and Table S4). The conductivity of our film is comparable to the previously reported films with similar amount of GO, which could be further improved by increasing GO content. We further explored the change of the film electrical conductivity during stretching with an applied voltage of 0.01 V. The resistance showed no significant increase after 100 cycles (Figure 6c), which is crucial for flexible electronic devices. The same film was also attached to a finger to test its fast response to strain change, by recording its

bridges of nacre to connect open cracks, while the dendrites functioned as asperities between adjacent bricks to enhance frictional energy dissipation. Furthermore, the fracture surface morphology of typical rGO/PVA composite film prepared by our bidirectional freezing technique and common vacuum filtration methods were compared. The fracture surface of our nacre-mimetic film (60 to 120 μm thick, Figure 4d−f) was rough and showed a “zigzag” crack path. While for the film prepared by vacuum filtration method (50 to 70 μm thick, Figure 4g−i), its fracture surface was much smoother, indicating a straighter crack propagation during fracture. All these observations supported our proposed model: the introduction of the interlayer dendrites and bridges significantly influenced the fracture mechanism and improved the stretchability of the nacre-mimetic composite film. In literature, different kinds of interfacial interactions have been utilized to enhance the mechanical properties of the nacre-mimetic composite films based on GO or rGO sheets, such as π−π interaction,34,35 hydrogen bonding,36 ionic bonding,37 and covalent bonding.38 Here, our nacre-mimetic rGO/PVA composite films with interlayer dendrites and bridges were compared with nacre39 and other synthetic composite films in terms of their strength and toughness (Figure 5a and Table S3).36−38,40−50 As indicated, our film showed similar strength with nacre but much higher toughness. Although some of the nacre-mimetic films reached relatively higher tensile strength, their toughness values were modest. Our nacre-mimetic rGO/PVA composite films were even more 4782

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mm and a width of 3−4 mm. At least five samples were tested for each experimental condition to obtain statistically reliable values. X-ray diffraction (XRD) was collected from the Panalytical X’Pert Pro Diffractometer. The electrical conductivity was measured with a Keithley 2611B source meter.

CONCLUSION In conclusion, we have fabricated a nacre-mimetic rGO/PVA composite film with interfacial architectural engineering between the adjacent lamellae using a modified bidirectional freezing method. By carefully controlling the viscosity and chemical component in the suspension, the ultimate tensile strength, ultimate fracture strain, and ultimate toughness of the resulted film reached 150.9 MPa, 10.44% and 8.50 MJ/m3, respectively, combining high strength with high stretchability. More importantly, this was achieved by only interfacial architectural engineering within the traditional “brick-andmortar” structure, without introducing a third component or employing chemical cross-linker as in some other nacremimetic systems. We further studied the fracture mechanism and concluded that the bridges functioned as the mineral bridges of nacre to connect open cracks, while the dendrites functioned as asperities between adjacent bricks to enhance frictional energy dissipation. With interfacial architectural design, our research provides feasible strategy and insights to adjust the mechanical response of composite films, which have wide applications in smart materials such as soft electronics.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01089. XRD spectra of nacre-mimetic rGO/PVA composite films with different GO/PVA ratio and prepared by different methods; Typical stress−strain curves for the film prepared by the bidirectional freezing technique and common vacuum filtration method; Tables of the dspacing and mechanical properties of the nacre-mimetic rGO/PVA composite films with different GO content and prepared by different methods; Table of the mechanical properties of the nacre-mimetic rGO/PVA composite films with nacre and other composite films based on GO or rGO sheets; Table of the electrical conductivity of the nacre-mimetic rGO/PVA composite films with different GO content (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

METHODS

ORCID

Materials. Graphene oxide sheets (diameter: 0.5−5 μm; thickness: 0.8−1.2 nm, 99%) were purchased from Nanjing XFNANO Materials Tech Co., Ltd., China. Poly(vinyl alcohol) (PVA, Mw = 205000, 99%) and hydroiodic acid (HI, 55.0−58.0%) were purchased from Aladdin Chemistry Co., Ltd., China. Polydimethylsiloxane (PDMS, Sylgard 184) were purchased from Dow corning, USA. The copper foil was purchased from a local store. Preparation of PDMS Wedge. Square Teflon tubes (12 × 12 × 30 mm) were sealed with a copper plate on one end and tilted to an angle of around 15°. PDMS precursor solution was then poured carefully into the mold to completely cover copper plate. After curing at 60 °C for more than 4 h, PDMS wedges were obtained. Preparation of GO/PVA Suspension. Aqueous solution of PVA (50 mg/mL) was formed by dissolving PVA in deionized water at 95 °C. Various amount of GO and PVA solution (50 mg/mL) were mixed in deionized water to form suspensions with different concentrations. Before freezing, all the suspensions were sonicated for 30 min by noise isolating chamber (JY 92-IIN, Ningbo Scientz Biotechnology Co., Ltd., China) at 30% power, and vacuumed to remove air bubbles. Fabrication of rGO/PVA Films. The GO/PVA suspension was poured into a square Teflon tube with PDMS wedge, and then frozen by cryogenic ethyl alcohol bath. After being entirely frozen, the sample was tapped out of the mold and freeze-dried for more than 48 h at −60 °C with Freeze-dryer under 0.05 mbar pressure (Labconco 8811, Kansas City, USA). The freeze-dried sample was pressed at 95 °C for 30 min to form the GO/PVA film. In comparison, the same sonicated suspension was also assembled into GO/PVA films by vacuum filtration, followed by air drying and peeling off the filter paper. All the GO/PVA films were reduced in hydroiodic acid (HI) solution. After washing, drying and hot-pressing at 95 °C for 15 min, dense rGO/ PVA films were obtained. Characterization. Scanning electron microscopy (SEM) images were gathered by Hitachi S-3700 at an acceleration voltage of 10 kV. Mechanical properties were tested in the tensile mode by an electronic universal testing machine (UTM2102, ShenZhen Suns Technology Stock Co., Ltd., China) with gauge length of 10 mm, and loading rate of 0.5 mm/min. All the samples were cut into strips with a length of 30

Tao Xie: 0000-0003-0222-9717 Hao Bai: 0000-0002-3348-6129 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51603182 and 21674098) and the “1000 Youth Talents Plan” of China. REFERENCES (1) Wang, J.; Cheng, Q.; Tang, Z. Layered Nanocomposites Inspired by the Structure and Mechanical Properties of Nacre. Chem. Soc. Rev. 2012, 41, 1111−1129. (2) Mayer, G. Rigid Biological Systems as Models for Synthetic Composites. Science 2005, 310, 1144−1147. (3) Wang, R. Z.; Suo, Z.; Evans, A. G.; Yao, N.; Aksay, I. A. Deformation Mechanisms in Nacre. J. Mater. Res. 2001, 16, 2485− 2493. (4) Evans, A. G.; Suo, Z.; Wang, R. Z.; Aksay, I. A.; He, M. Y.; Hutchinson, J. W. Model for the Robust Mechanical Behavior of Nacre. J. Mater. Res. 2001, 16, 2475−2484. (5) Wegst, U. G.; Bai, H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Bioinspired Structural Materials. Nat. Mater. 2015, 14, 23−36. (6) Bonderer, L. J.; Studart, A. R.; Gauckler, L. J. Bioinspired Design and Assembly of Platelet Reinforced Polymer Films. Science 2008, 319, 1069−1073. (7) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured Artificial Nacre. Nat. Mater. 2003, 2, 413−418. (8) Guo, T.; Heng, L.; Wang, M.; Wang, J.; Jiang, L. Robust Underwater Oil-Repellent Material Inspired by Columnar Nacre. Adv. Mater. 2016, 28, 8505−8510. (9) Wang, J.; Lin, L.; Cheng, Q.; Jiang, L. A Strong Bio-Inspired Layered PNIPAM-Clay Nanocomposite Hydrogel. Angew. Chem., Int. Ed. 2012, 51, 4676−4680. 4783

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ACS Nano

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DOI: 10.1021/acsnano.7b01089 ACS Nano 2017, 11, 4777−4784