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Hierarchical Layered Heterogeneous Graphene-Poly(N-Isopropylacrylamide)Clay Hydrogels with Superior Modulus, Strength and Toughness Chao Teng, Jinliang Qiao, Jianfeng Wang, Lei Jiang, and Ying Zhu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05120 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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Hierarchical Layered Heterogeneous Graphene-Poly(N-Isopropylacrylamide)-Clay Hydrogels with Superior Modulus, Strength and Toughness Chao Teng,1 Jinliang Qiao,2Jianfeng Wang,1,* Lei Jiang,1 Ying Zhu1,* 1

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key

Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang

University, Beijing 100191, China. 2

Sinopec Beijing Research Institute of Chemical Industry, Beijing 100013, China. Correspondence and requests

for

materials

should

be

addressed

to

Y.Z.

(email:

[email protected])

or

to

J.W.

(email:

[email protected])

ABSTRACT: Biological composites are renowned for their elaborate heterogeneous architectures at multiple scales, which lead to unique combination of modulus, strength and toughness. Inspired by biological composites, mimicking the heterogeneous structural design principles of biological composites is a powerful strategy to construct high-performance structural composites. Here we creatively transfer some heterogeneous principles of biological composites to the structural design of nanocomposite hydrogels. Unique heterogeneous conductive graphene-PNIPAM-clay hydrogels are prepared through combination of inhomogeneous water removal processes, in situ free-radical polymerization and chemical reduction of graphene oxide. The nanocomposite hydrogels exhibit hierarchical layered heterogeneous architectures with alternate stacking of dense laminated layers and loose porous layers. Under tensile load, the stiff dense laminated layers serve as sacrificial layers that fracture at a relatively low strain, while the stretchable loose porous layers 1

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serve as energy dissipation layers by large extension afterwards. Such local inhomogeneous deformation of the two heterogeneous layers enables the nanocomposite hydrogels to integrate superior modulus, strength and toughness (9.69 MPa, 0.97 MPa, 5.60 MJ/m3, respectively). The study might provide meaningful enlightenments for rational structural design of future high-performance nanocomposite hydrogels.

KEYWORDS: nanocomposite hydrogels, clay, graphene, mechnical properties, heterogeneous structure

2


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Improving the mechanical properties of polymer hydrogels has always been an important topic in the field of gel science, since their mechanical weakness and brittleness seriously limit the scope for their application.1 2 Up to now, different kinds of tough hydrogels have been developed, including double-network hydrogels,3, 4 nanocomposite hydrogels,5, 6 sliding-ring hydrogels,7, 8 macromolecular microsphere composite hydrogels,9-11 tetra-arm poly(ethylene glycol) hydrogels,12, 13

polyampholytes hydrogels,14 microgel-reinforced hydrogels,15 nanostructured hydrogels16 and

others.17-19 Among them, nanocomposite hydrogels are attracting much attention because of their improved mechanical properties20-24 and introduced additional functionalities, such as transparency, conductivity,25 self-healing,26, 27 controllable swelling ratio,28 property anisotropy29-31 and others.32 In these nanocomposite hydrogels, randomly dispersed inorganic nanoplatelets act as effective multifunctional cross-linkers to homogenize polymer network. The homogeneous cross-linked network avoids stress concentration on single polymer chain and thus imparts large strain deformation ability to hydrogels. The large strain deformation of these nanocomposite hydrogels, however, often comes at the expense of stiffness. Although great progress has been made in toughness, in only very rare cases are nanocomposite hydrogels with integration of high modulus, strength and toughness obtained.33 Differing from synthetic composites with homogeneous structure, biological composites develop well-designed heterogeneous structure at multiple length scales to simultaneously achieve remarkable modulus, strength and toughness.34 At molecular level, biopolymers integrate strong covalent bond with weak hydrogen bond35, 36 or ionic bond37-39 or coordinate bond40 for graded breakage. The biopolymers are combined with hard building blocks to form well-ordered nanoscale structure, such as layered aragonite platelet/protein nanostructure in nacre41 and layered 3


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hydroxyapatite platelet/collagen nanostructure in bone.42 The nanoscale structure is further arranged into multiple levels of self-similar macroscale structure in a highly inhomogeneous way, such as fractal bone.43, 44 Quite often, the concentration and spatial orientation of reinforcing elements at different regions are changed locally for adjusting the site-specific mechanical properties,45 such as tooth with highly mineralized collagen fibers oriented out-of-plane in the outer layer and less mineralized collagen fibers oriented in-plane in the inner layer.46 Another universal heterogeneous structural design from nature is that ordered dense motif coexists with random porous one, such as bone osteons and dentin tubules. From a mechanical point of view, the well-designed heterogeneous elements at multiple length scales respond cooperatively to external mechanical stress, and thus enable biological composites to achieve outstanding modulus, strength and flaw tolerance.47, 48 Mimicking the heterogeneous structures of biological composites has been proved to be a powerful strategy towards high-performance bioinspired structural composites.49-55 We hope that applying the heterogeneous structural design principles lying in biological materials to synthetic nanocomposite hydrogels may be a feasible route to develop new hydrogels with integration of high modulus, strength and toughness. In

the

present

work,

we

create

unique

heterogeneous

graphene

oxide-

poly(N-isopropylacrylamide)-clay (GO-PNIPAM-clay) hydrogels by combining inhomogeneous water removal processes with in situ free-radical polymerization. The nanocomposite hydrogels have hierarchical layered heterogeneous architectures with alternate stacking of dense laminated layer and loose porous layer. Cooperative interaction of the two heterogeneous layers makes the hydrogels to integrate superior modulus, strength and toughness that are much higher than those of homogeneous nanocomposite hydrogels. In addition, GO-PNIPAM-clay hydrogels are reduced to 4


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form reduced graphene oxide-PNIPAM-clay (RGO-PNIPAM-clay) hydrogels that display good electrical conductivity. We believe that it could offer an innovative insight into the design and preparation of robust, conductive nanocomposite hydrogels.

RESULTS AND DISCUSSION The preparation process of GO-PNIPAM-clay hydrogel is shown in Figure 1a. In a first step, homogeneous aqueous solution of GO, NIPAM monomer, clay and 2, 2’-diethoxyacetophenone photoinitiator was condensed into a layer of paste on filter membrane through vacuum-assisted filtration. The condensed paste contains GO, clay, water, part entrapped NIPAM and photoinitiator. The employed GO has a diameter of about several hundred nanometers and an average thickness of about 0.66 nm (Supporting Information, Figure S1), while the diameter and thickness of clay are about several ten nanometers and 1 nm, respectively. Then, nitrogen gas flow was applied to exclude oxygen and moderately evaporate the water on the upper surface of the paste. Subsequently, UV light was immediately applied to induce in situ radical polymerization of NIPAM monomer. After polymerization, the deep yellow GO-PNIPAM-clay hydrogel is obtained, which is highly stable without dissolution when kept in water for a long time, indicating that NIPAM is successfully polymerized and forms a cross-linked network structure. We altered filtration and evaporation times to prepare three GO-PNIPAM-clay hydrogels with solid content of 18

wt%,

21

wt%

and

26

wt%,

which

are

designated

as

GO-PNIPAM-clay-18,

GO-PNIPAM-clay-21 and GO-PNIPAM-clay-26, respectively. Here, the clay plays three roles: multifunctional cross-linker, reinforcement and isolator, which prevents GO sheets from stacking during dehydration and subsequent reduction. If clay is absent, water and NIPAM cannot be stably trapped within the paste during filtration, which leads to dry GO film. The addition of GO has two 5


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roles: 1) it acts as conductive filler after reduction, 2) it structures the hydrogels and facilitate the formation of micron-sized pores, which will be mentioned below. To fully isolate RGO, the initial mass ratio of clay to GO is controlled to be 30 : 1.25 The composition in the resulting GO-PNIPAM-clay gels was determined by thermogravimetric analysis, as shown in Figure S2 in Supporting Information. The concentration of inorganic nanoplatelets (GO plus clay) in GO-PNIPAM-clay hydrogels is about 10.0-14.5 wt%. GO-PNIPAM-clay-18 hydrogel exhibits trilayered heterogeneous architecture, in which two skin layers (referred to as layer 1) have well-defined layered nanostructure, and a middle layer has porous

microstructure (Figure

1b-b2

and

Supporting

Information,

Figure S3). The

energy-dispersive X-ray (EDX) element mapping verifies that both skin layers and middle layer contain clay, GO and PNIPAM (Supporting Information, Figure S4). X-ray diffraction (XRD) curves display that the interlayer spacing of clay (1.38 nm) and GO (0.89 nm) is increased to 3.12 nm, confirming the intercalation of PNIPAM into clay and GO (Supporting Information, Figure S5). According to the results mentioned above, the structural model of the composite hydrogel is illustrated, as shown in Figure 1b3. Such structure can be deemed to embrace two levels of hierarchy; that is, GO and clay are enveloped by PNIPAM to form laminated layer 1 (first order), and the two laminated layer 1 are glued together by a porous layer to form trilayered heterogeneous architecture (second order). The formation mechanism of the trilayered heterogeneous architecture is related to inhomogeneous water removal processes. It has been widely demonstrated that colloidal inorganic nanoplatelets can self-assemble into nacre-like stiff layered films through complete water removal due to the excluded volume effects.56, 57 Herein, water is removed partly through two commonly 6


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used methods: vacuum-assisted filtration and evaporation. As water is drained out through filtration, GO and clay gradually condenses on the surface of filtration membrane58 and form one laminated layer 1. When nitrogen gas flow was applied, evaporation induces GO and clay to co-assemble at gas-water interface into the other laminated layer 1. Differently, due to relatively high water content in middle layer, GO and clay co-assembles into the well-defined microporous structure. For comparison, we used the same method to prepare clay-PNIPAM hydrogel without addition of GO, which exhibits a homogeneous layered structure without micron-sized pores (Supporting Information, Figure S6). It indicates that GO plays an important role in the formation of middle porous layer. In essence, GO are amphiphilic nanosheets with hydrophilic oxygen-containing regions and hydrophobic aromatic regions. The hydrophobic aromatic regions are prone to held together by hydrophobic interaction. However, parallel stacking of GO is suppressed to a great extent because of spatial isolation by clay as well as hydration repulsions between hydrophilic regions of GO.59 As a result, GO and clay co-assembles into well-defined microporous structure in the middle layer with relatively high water content. Subsequent UV light irradiation induced in situ polymerization of NIPAM monomer and formation of cross-linked polymer network, thus fixing the preformed trilayered heterogeneous architecture. Increasing the degree of water removal has a profound effect on the structure of GO-PNIPAM-clay hydrogels. GO-PNIPAM-clay-21 hydrogel evolves into multilayered heterogeneous architecture (Figure 1c), in which the thickness of layer 1 increases compared to GO-PNIPAM-clay-18 hydrogel. Interestingly, some new laminated layers (referred to as layer 2) appear between the two surface layer 1 (Figure 1c1), which is ascribed to that the decrease water content induces partial collapse of the porous structure. The interlayers between laminated layers 7


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are still porous structure, but the pore size is smaller than that of GO-PNIPAM-clay-18 (Figure 1c2). The corresponding structural model is illustrated in Figure 1c3. With further removal of water, GO-PNIPAM-clay-26 hydrogel evolves into complex multilayered heterogeneous architecture (Figure 1d). The thickness of layer 2 obviously increases, compared with that in GO-PNIPAM-clay-21 (Figure 1d1). In addition, the thickness of pore wall in porous layers also increases so that some minor sub-layers (referred to as layer 3) are generated (Figure 1d2). The corresponding structural model is simplified, as shown in Figure 1d3. The GO-PNIPAM-clay hydrogels were converted to RGO-PNIPAM-clay hydrogels through chemical reduction. Due to partial restoration of conjugated structure and increase of hydrophobility, RGO tends to cause the fusion of pore walls in porous layers. Herein, reduction by hydrazine in an aqueous ammonia solution is adopted to alleviate the fusion trend. Residual carboxyl groups at edges help to increase the hydration and repulsion between RGO,60 thus prevent the pores from excessive fusion during the reduction process. As a result, reduced nanocomposite hydrogels basically maintain original hierarchical layered heterogeneous architectures. RGO-PNIPAM-clay-18 hydrogel still exhibits trilayered heterogeneous architecture, except the thickness of pore wall in middle porous layer increases (Figure 1e-e3 and Supporting Information, Figure S7). RGO-PNIPAM-clay-21 hydrogel shows complex multilayered heterogeneous architecture, which is more like GO-PNIPAM-clay-26 hydrogel (Figure 1f-f3). The primary layers (layer 1 and layer 2) contain many narrow gaps which reflect the incomplete fusion of original pores (Supporting Information, Figure S8). In addition, the porous layers contain forked laminated sub-layers (layer 3). For RGO-PNIPAM-clay-26 hydrogel, its structure is the same as that of RGO-PNIPAM-clay-21 hydrogel, except the thickness of layer 1, layer 2 and layer 8


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3 increases. The mechanical properties of GO-PNIPAM-clay hydrogels were studied by tensile test, as shown in Figure 2a. The whole stretching process can be divided into three typical parts: 1) an initial elastic deformation with high modulus, where the stress increases sharply and linearly with strain; 2) progressive transition from elastic to inelastic deformation (rounding of the curves, yielding); 3) large plastic elongation with strain hardening up to failure. The curve profile differs markedly from common J-shaped stress-strain curve of homogeneous nanocomposite hydrogels only with porous structure,25 indicating that the hierarchical layered heterogeneous structures determine their unique tensile behaviour. The modulus, yield stress, ultimate strength, ultimate strain and toughness were extracted from the stress-strain curves, as shown in Table S1 in Supporting Information. It can be found that the ultimate strain is as high as 600-800%, almost independent of the degree of water removal. Importantly, the modulus, yield stress and ultimate strength increase sharply with structure evolution from the trilayered to the multilayered, and then to the complex multilayered, indicating that the formation of laminated layers (layer 1, layer 2 and layer3) is the primary cause of such obvious reinforcement. In addition, the toughness that is calculated from the area under stress-strain curves increases monotonically with structural evolution. The GO-PNIPAM-clay-26 hydrogel exhibits highest mechanical properties with modulus of 9.69 MPa, tensile strength of 0.97 MPa and toughness of 5.6 MJ/m3. The comprehensive mechanical properties are much higher than those of reported homogeneous nanocomposite hydrogels with relative low nanoplatelet concentration (Figure 2c and Supporting Information, Table S2).20-25 RGO-PNIPAM-clay hydrogels exhibit the same stress-strain curve profile as GO-PNIPAM-clay 9


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hydrogels (Figure 2b). Moreover, the large ultimate strain is basically maintained. The difference is that strength and modulus are slightly decreased after reduction. The decrease of mechanical properties is probably due to the reduced interfacial interaction between PNIPAM and RGO with removal of oxygen-containing groups on basal plane (Supporting Information, Figure S9). Again, modulus, yield stress and ultimate strength of RGO-PNIPAM-clay hydrogels increase with structure evolution, except RGO-PNIPAM-clay-26 hydrogel in which the slope of strain hardening region is degraded. The degradation is probably due to excessively high volume ratio of laminated layer to porous layer. When the solid content is higher than 26%, the hydrogels obviously become brittle. The RGO-PNIPAM-clay-21 hydrogel exhibits best comprehensive mechanical properties with modulus of 2.52 MPa, tensile strength of 0.69 MPa and toughness of 3.5 MJ/m3, which are, respectively, about two orders of magnitude, 5.5 times and one order of magnitude higher than those of reported homogeneous RGO-clay-PNIPAM hydrogels with low nanoplatelet concentration (Figure 2c and Supporting Information, Table S2).25 The comparison firmly confirms that the hierarchical layered heterogeneous structure enables

the incorporation of

high-concentration inorganic nanoplatelets into nanocomposite hydrogels to integrate high modulus, strength and toughness. The superior mechanical properties of GO-PNIPAM-clay and RGO-PNIPAM-clay hydrogels are ascribed to the unique hierarchical layered heterogeneous architecture, high nanoplatelet concentration and efficient interfacial adhesion. At first-order interface within the laminated layers (layer 1 or layer 2), clay platelets with rich hydroxyl groups acts as multifunctional cross-linkers to interact with amide side groups of multiple PNIPAM chains through hydrogen bond.61, 62 In addition, GO and RGO contain more or less oxygen-containing groups which can also form a 10


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certain amount of hydrogen bond with PNIPAM chains.25 The multipoint hydrogen bond interaction avoids local stress concentration on single PNIPAM chains. At second-order interface between laminated layers (layer 1 or layer 2) and porous layers, efficient stress transfer is expected because both layers contain the same PNIPAM matrix. More importantly, at microscopic level, the laminated layer and porous layer respond cooperatively to external tensile stress, and further enhance the mechanical properties of the hydrogels. To shed light on the cooperative mechanism, the deformation process of RGO-PNIPAM-clay-21 hydrogel under tensile stress is explored (Figure 3). During yielding where the slope of stress-strain curve sharply degrades, cracks are initiated in laminated skin layer (Figure 3a, b, f). It indicates that the high modulus of the hydrogel stems from the laminated skin layers which contain high-concentration inorganic platelets orientated in-plane. The generated cracks are bridged and stabilized by ductile porous interlayer, which enables the laminated skin layers to generate more cracks over a large region of the hydrogel. The continuous microcracking within laminated layer, in turn, decreases the stress concentration of porous interlayer. After microcracking, the hydrogel still elongates more than 600%, indicating that the hydrogel has excellent crack tolerance. In the elongation process, clay platelets and RGO sheets in porous layer are substantially oriented to tensile stress direction (Figure 3c, g). That is, the porous layer with relatively low concentration and orientation of inorganic platelets reserve deformation space to enhance the toughness of hydrogel. After fracture, the cross section of hydrogel shows homogeneous layered structure with disappearance of original pores (Figure 3d, h). Such inhomogeneous deformation process is in stark contrast to homogeneous deformation of common porous nanocomposite hydrogels which often generate large strains, but low modulus. In essence, 11


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the deformation mechanism of the heterogeneous architecture has some common features with those of natural composites such as bone, tooth and mollusc shells, which realize inhomogeneous strain distribution through hierarchical structures and local change in concentration and orientation of building blocks.48 Apart from excellent mechanical properties, RGO-PNIPAM-clay hydrogels show good electrical conductivity. Their conductivity is about 2.6-3.7×10-3 S/cm, comparable with pure graphene hydrogel (4.9×10-3 S/cm) which was synthesized by hydrothermal method.63 Furthermore, the conductivity is constant when immersed into water, because inorganic nanoplatelets substantially suppress the motion of PNIPAM chain and the hydrogels are non-swellable.28 The electrical conductivity of the porous layer is probably lower than the laminated layers, due to relatively high water content. This means that the electrical conductivity level of the hydrogels is limited by the porous layer. Thanks to the well-defined and interconnected porous structure, RGO form effective conductive network in porous layer through partial π-π stacking, which contributes to the good electrical conductivity of RGO-PNIPAM-clay hydrogels.64 CONCLUSION In summary, we successfully demonstrate that the heterogeneous structural principles of biological composites are applied to the structural design of nanocomposite hydrogels. Unique heterogeneous GO-PNIPAM-clay hydrogels were constructed by combination of two inhomogeneous water removal process (vacuum-assisted filtration and evaporation) and in situ free-radical polymerization. The heterogeneous hydrogels have hierarchical layered architectures with alternate stacking of dense laminated layer and loose porous layer. With the increase of water 12


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removal, the structures of composite hydrogels evolve from trilayered to multilayered, then to complex multilayered heterogeneous architecture. After chemical reduction of GO, the obtained RGO-PNIPAM-clay hydrogels basically maintain the original hierarchical layered heterogeneous architecture. As a result, both the heterogeneous GO-PNIPAM-clay and RGO-PNIPAM-clay hydrogels exhibit excellent mechanical strength, modulus and toughness which increase with the structural evolution and are far superior to traditional homogenous nanocomposite hydrogels. The impressive comprehensive mechanical properties of the heterogeneous nanocomposite hydrogels are mainly attributed to cooperative deformation of stiff laminated layers and stretchable porous layers, essentially similar to some biological composites. Besides, the RGO-PNIPAM-clay hydrogels show good electrical conductivity, comparable to pure RGO hydrogel prepared by hydrothermal method. The robust, conductive hydrogels may be potentially applied in the design of soft biomimetic machines.1 We believe that implement of heterogeneous structural principles of biological materials in synthetic hydrogels along the direction will bring about fascinating nanocomposite hydrogels with extraordinary mechanical properties and additional functionalities. METHODS Materials: GO was obtained from XianFeng NANO Co., Ltd. NIPAM (TCI, >98% purity), 2,

2’-diethoxyacetophenone (TCI, > 95% purity), hydrazine hydrate (J&K, 64% hydrazine), ammonium hydroxide

solution (Sigma-Aldrich, 28% NH3 in H2O) and clay (Laponite XLG, [Mg5.34Li0.66Si8O20(OH)4]Na0.66, MW = 762.24, platelet size = 25-30 nm in diameter and 1 nm in thickness, Rockwood Ltd., UK) were used as received.

Preparation of GO-PNIPAM-Clay and RGO-PNIPAM-Clay hydrogels: GO (3 mg) was dispersed in

deionized water (15 ml) by ultrasonic bath. Clay (90 mg) was added into GO dispersion and stirred for 30 min.

NIPAM monomer (0.4 g) was added and stirred until dissolution. Then, photo-initiator 2, 2’-diethoxyacetophenone

13


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(40 µl) was added and mixed for 30 min. The mixture was condensed under vacuum-assisted filtration on filter

membrane (50 nm pore size) into a layer of paste. The condensed paste along with filter membrane was transferred

to a glass vessel and further condensed through nitrogen gas flow. The solid content was controlled through

altering filtration and nitrogen gas flow times. After UV light radiation (λ = 365 nm, 45 min) initiate the

polymerization of NIPAM, GO-PNIPAM-clay hydrogels were produced and stored in deionized water. For

reduction, GO-PNIPAM-clay hydrogels were immersed into hydrazine hydrate/ ammonium hydroxide aqueous

solution (H2O : N2H4 : NH3 = 100 : 0.03 : 1) for three days. The resultant RGO-PNIPAM-clay hydrogels were washed in deionized water to remove excessive hydrazine and ammonia.

Characterization: SEM images and EDX element mappings were obtained using a Hitachi S-4800 SEM. The

samples were freeze-dried and coated with a thin layer of gold. XRD experiments were carried out with a Rigaku

D/max-2500 instrument using Cu Kα radiation. AFM image was measured by a FASTSCANBIO Bruker atomic force microscope. Tensile test was carried out with a Shimadzu AGS-X testing machine. The tested rectangular

strips were 3 mm in width and 30-40 mm in length. The distance between the clamps was 10 mm and the load

speed was 5 mm/min. In the testing process, water mist was sprayed on sample surface in order to avoid water

evaporation. The conductivity of the RGO-PNIPAM-clay hydrogel was measured by method of four point probe.

Thermogravimetric analysis was performed on a Diamond TG/DTA thermal analyzer under air at a heating rate of

5 °C/min.

CONFLICT OF INTEREST The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the financial support from the National Natural Science Foundation of China (51403008, 51273008, 51473008), the Fundamental Research Funds for the Central Universities (YWF-14-HHXY-008), the National Basic Research Program of China (2012CB933200). 14


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Supporting Information Available: AFM images, XRD curves, thermogravimetric analysis, SEM images, Raman

spectra, FTIR spectra and table for comparison of mechanical properties. This material is available free of charge

via the Internet at http://pubs.acs.org.

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Figure 1 Preparation, structure and model of GO-PNIPAM-clay and RGO-PNIPAM-clay gels. (a) Preparation process illustration, including filtration, nitrogen gas-induced evaporation, UV light-initiated polymerization of NIPAM monomer and chemical reduction of GO to RGO. (b-b3) Structure of GO-PNIPAM-clay-18 gel, showing trilayered heterogeneous architecture with two laminated skin layers (layer 1) and a middle porous layer. (c-c3) Structure of GO-PNIPAM-clay-21 gel, showing multilayered heterogeneous architecture with alternate stacking of dense laminated layer (layer1, layer 2) and loose porous layer. (d-d3) Structure of GO-PNIPAM-clay-26 gel. Some laminated sub-layers are formed in porous layers. (e-e3) Structure of RGO-PNIPAM-clay-18 gel, similar to that of GO-PNIPAM-clay-18 gel. (f-f3) Structure of RGO-PNIPAM-clay-21 gel, somewhat similar to that of GO-PNIPAM-clay-26 gel. Gray background of layer 1 and layer 2 in structural models is drawn to highlight higher platelet concentration, compared with that in porous layer.

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Figure 2 Tensile stress-strain curves of (a) GO-PNIPAM-clay and (b) RGO-PNIPAM-clay gels. The side digital photographs show their high levels of deformation by elongation. (c) Comparison of mechanical properties of our heterogeneous nanocomposite gels with the reported homogeneous nanocomposite gels. The heterogeneous nanocomposite gels show both striking elastic modulus, 1-3 orders of magnitude higher than that of other homogeneous nanocomposite gels, and good toughness. (Abbreviations: LDH, layered double hydroxide; MMT, montmorillonite; GPO, graphene peroxide; PAM, polyacrylamide.). The red and black five-pointed stars represent GO-PNIPAM-clay and RGO-PNIPAM-clay gels, respectively. The percentage in parentheses is the mass concentration of inorganic nanoplatelets in corresponding hydrogels.

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Figure 3 (a-d) Deformation process of RGO-PNIPAM-clay-21 gel under tension. (a) Cross-sectional morphology during yielding, showing occurrence of cracks within the laminated skin layers. The black arrows point to cracks. (b) Surface morphology after yielding, showing extensive cracks within the laminated skin layers. (c) Cross-sectional morphology at 600% strain, showing substantial orientation of porous layer. (d) Fracture morphology of cross section viewed vertical to tensile direction, showing homogeneous layered structure without original pores. (e-h) Proposed synergistic mechanisms of stiff laminated layer and ductile porous layer. (e) Simplified structural schematic. (f) Stiff laminated skin layers crack over large range after yielding. (g) Ductile porous layers substantially elongate and bridge the crack during strain hardening. RGO and clay in porous layers are reoriented to form layered structure. (h) After fracture, the hydrogel becomes homogeneous layered structure. White arrows represent tensile direction.

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