Highly Ordered 3D Graphene-Based Polymer Composite Materials

Article pubs.acs.org/Macromolecules

Highly Ordered 3D Graphene-Based Polymer Composite Materials Fabricated by “Particle-Constructing” Method and Their Outstanding Conductivity Liang Yang, Zhaoqun Wang,* Yucheng Ji, Jianing Wang, and Gi Xue* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology (Nanjing University), Ministry of Education, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093 P. R. China S Supporting Information *

ABSTRACT: The fabrication of graphene-based polymer composite materials is of interest and significance from an academic and an application viewpoint. The widely used method to obtain such composites was liquid-phase blend of graphene nanosheets (GNSs) and polymer solutions followed by casting or heat pressing. Until now, the challenge of dispersing the GNSs uniformly in the polymer matrix to form controllable and regular structure still remains. Here, we developed a unique “particleconstructing” method for fabricating highly ordered 3D graphene-based polymer composite materials, throughout which the GNSs formed intact, uniform and welldefined network structure. The strategy contains two steps: wrapping polymer microspheres with GNSs and mold-compressing them at room temperature, followed by an appropriate heat treatment. The composite materials exhibited outstanding electrical properties involving extremely low percolation threshold and much higher conductivity. The method can be easily extended to fabricate highly ordered GNS aerogels and more GNS-based composite materials. The results represent an important step toward developing GNS-based composite materials with high performance.

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INTRODUCTION Graphene has attracted tremendous attention and research interest for its fascinating physical properties such as giant electron mobility, high thermal conductivity, excellent mechanical flexibility and strength, and large specific surface area,1−4 which holds great promise for potential applications in many technological fields such as batteries,5 sensors,6 supercapacitors,7−9 energy storage devices,10 nanocomposites, and so on.11−14 How to apply the graphene nanosheet (GNS) in an effective way is still a scientific challenge, even if a low-cost, large-scale production of GNS is achieved.15 One possible route would be the preparation of GNS-based polymer composite materials,16 and naturally, the key for fully realizing the value of graphene is its homogeneous distribution in the resultant composites. Until now, the widely used method to obtain such composites was common liquid-phase blend of GNSs and polymer solutions (or polymer latices in a few researches), followed by a direct cast or a two-step process of drying and then heat pressing.11,16−21 Altogether, dispersing GNSs uniformly in the polymer solutions and continuing in the uniform state at the molding stage are still the most important and challenging parts. To avoid the reaggregation or restacking of GNSs during the subsequent period, some particular treatments, such as rapid precipitation in a nonsolvent11,19 or freeze-drying,20 were also used to rapidly fix the previously established uniform structure. Stankovich et al.11 mixed polystyrene (PS) with GNSs in N,N-dimethylformamide and added the solution dropwise into a larger volume of methanol © 2014 American Chemical Society

with vigorous stirring, thus accomplishing the rapid coagulation of the polymer composites. Brinson and co-workers19 used similar rapid precipitation to remain good dispersion of GNSs in poly(methyl methacrylate). Loose et al.20 mixed water dispersion of pretreated GNSs with PS latex, froze the mixture in liquid nitrogen and freeze-dried it overnight to obtain composite power. However, all these additional processes are complicated, energy-consuming and environmentally unfriendly. What’s more, these methods could not afford much more effective control over the uniformity of GNSs dispersed in polymer matrix. In a word, it is impossible to attain highly ordered 3D composite structure in strict sense by these conventional methods. Herein we developed a simple and environment-friendly technique for fabricating unique GNS-based polymer composite materials, throughout which GNSs formed highly ordered 3D network structure with uniform well-defined shape and controllable network density. In the proposed strategy, the intact GNS network was constructed with preformed graphenewrapped polymer microspheres as building blocks via conventional compression molding. Therefore, it can be called as “particle-constructing” method, which is quite distinct from the widely used random liquid-phase blend. The method can be further used to prepare highly ordered graphene aerogel Received: November 16, 2013 Revised: February 10, 2014 Published: February 28, 2014 1749

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Scheme 1. Schematic Procedure for the Fabrication of Highly Ordered 3D GNS-Based Polymer Composite Materials and GNS Aerogel Materials

Figure 1. SEM images PS/GNSs composite particles prepared at GONS/PS weight ratio of 0.05 wt % (a), 0.1 wt % (b), 0.15 wt % (c), and 0.3 wt % (d). Preparation of Polystyrene Microspheres and Graphene Oxide Nanosheets. Polystyrene (PS) microspheres were prepared by dispersion polymerization25 and graphene oxide nanosheets (GONSs) were synthesized via oxidation of graphite (Hummers method), followed by exfoliation of graphite oxide by ultrasonic treatment.24 Preparation of PS/GNSs Composite Particles. As a typical synthetic process, PS emulsion (10 g, 10 wt %), deionized water (15 mL) and suspension of graphene oxide nanosheets (4 mL, 0.5 wt %) were added into a 50 mL round flask. Then the suspension was reduced with HI solution (2 mL, 45 wt %) at 100 °C for 24 h with magnetic stirring. The resultant PS/GNSs composite particles were purified by repeated centrifugation and redispersion cycles, washing with ethanol until the centrifugate became colorless. Finally, the composite particles were dried at 60 °C in vacuum. Fabrication of the GNS-Based PS Composite Materials. The PS/GNSs composite particles powders were pressed into disks in an oilostatic press with 3000 or 8500 MPa, and then the samples were heated at 130 °C for 2 h. The procedures of preparing the P(S-EA)/ GNSs composite particles and GNS-based P(S-EA) composite

materials and tune the composition and structure of highly ordered GNS-based polymer composite materials. The conductivity of the resulting GNS-based composites as electrical percolation systems was fully investigated as well.

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EXPERIMENTAL SECTION

Materials. Styrene (St, AR) and ethyl acrylate (EA, AR) were purchased from Shanghai Chemical Reagent Co. and were purified by distillation under reduced pressure. Azodiisobutyronitrile (AIBN) of chemical reagent grade (Shanghai Chemical Reagent Co.) was purified by recrystallization in 95% ethanol. Polyvinylpyrrolidone with an average molar mass of 58 kg/mol (PVP K-30) was purchased from Acros Organics. Graphite with an average size of 30 μm and a purity of >95% was obtained from Shanghai Chemical Reagent Co. KMnO4, Na2SO3, hydriodic acid, concentrated sulfuric acid, hydrogen peroxide, hydrochloric acid, absolute ethanol, 95% ethanol, and isopropanol were purchased from Nanjing Chemical Reagent Co. and used as received. Deionized water (18.2 MΩ·cm) was prepared in a Sartorius Arium 611 system and used throughout the experiment (unless otherwise specified). 1750

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materials are also similar. The GNS/PS weight ratio of the PS/GNSs composite particles and GNS-based polymer composite materials mentioned below is the initial feed weight ratio of GNS/PS as preparing the composite particles. Preparation of Highly Ordered Graphene Aerogel Materials and Graphene/MnO2 Composite Aerogel Materials. Removing of PS were carried out by calcining the resultant GNS-based PS composites with a GNS loading of 8 wt % at 550 °C for 1 h in nitrogen atmosphere or soaking them in THF overnight. The graphene/MnO2 composite aerogel materials were fabricated by dipping the graphene aerogel materials in a solution of 0.1 M KMnO4/0.1 M Na2SO3 at a neutral pH at room temperature for 1 h. Characterization. Scanning electron microscope (SEM) images were taken on an S-4800 instrument (Hitachi Co., Japan). Raman spectra were collected on an Aramis confocal microscope Raman spectrometer (Horiba Johib Yvon, Edison, NJ). A 633 nm He−Ne laser served as the excitation light source and was kept below 0.5 mW to prevent thermal damage of the samples. The spatial resolution of the beam spot was around 1 μm2, attained using a 100× objective microscope lenses. The electric conductivity of the samples was measured by a four probe tester (SB100A/2, Shanghai Qianfeng Electronic Instrument Co., Ltd.). Nitrogen adsorption−desorption isotherms at 77 K were measured by using an ASAP 2020 analyzer. The pore size distribution (PSD) was calculated from the desorption and adsorption branches, respectively, using the Barrett−Joyner− Halenda (BJH) method.

their spatial conformation under control, they almost perfectly wrapped around the PS microspheres by a simple blend. Two kinds of interaction between the PS and GNS have been used to explain the surprising formation of the PS/GNSs composite particles in our previously reported work.25 Hydrophobic interaction was proposed as a main force that drives the both to approach each other based on colloid thermodynamics. In addition, π electron interaction begins to take effect as the GNSs approach the surface of microspheres to a fairly close distance. It is interesting that the PS/GNSs composite particles had well-controlled coverage as expected. If lower weight ratios of GONS/PS such as 0.05 to 0.15 wt % were used, the PS microspheres (3.4 μm) were incompletely wrapped with GNSs. As shown in Figure 1a−c, some shape-irregular pits irregularly distribute on the spherical surface, which just provide a visual evidence for the coating of PS particles with the ultrathin GNSs. These faults, i.e., less than perfect domains, become smaller and less along with an increase of GONSs, finally achieving a complete wrapping of PS microspheres with GNSs as shown in Figure 1d. The difference in GNS coverage was further confirmed by the evidence of Raman spectra as shown in Figure 2. In curve a for the PS microspheres, there are some

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RESULTS AND DISCUSSION On the basis of the aforementioned conception, as shown in Scheme 1, the strategy of fabricating the highly ordered 3D GNS-based polymer composites contains mainly two steps: polymer microspheres are first wrapped with flexible GNSs by our proposed thermodynamic driving heterocoagulation method.22,23 Then, the GNS-wrapped particles are used as building blocks to construct the 3D GNS network by a general compression molding at room temperature. Finally, the mold compressed composite was heat treated at an appropriate temperature. If the composite particles are prepared with partial covering of GNSs onto the PS microsphere and then mold pressed, not only GNSs but also the polymer phase could form continuous structure in the composites, in which case we can obtain a bicontinuous polymer−GNS network. On the other hand, it should be easy to manufacture highly ordered 3D graphene aerogel materials by removing the polymer phase from the resultant GNS-based polymer composites. Furthermore, a series of GNS-based aerogel composite can be prepared via in situ generating of metal or metal oxide nanoparticles on the inner surface of the GNS aerogels. Very common polystyrene (PS) microspheres with monodispersed diameter were used in our system, which were prepared by dispersion polymerization, going without any surface modifications and functionalizations. Graphene oxide nanosheets (GONSs) synthesized according to the Hummers method24 have compatible size to the PS microspheres, with thickness less than 2 nm as shown in Figure S1 (Supporting Information). We blended GONS suspension with PS emulsion (number-average diameter: 3.4 μm), simultaneously reduced them to GNSs with HI at 100 °C for 24h, and finally obtained the GNS-wrapped particles (PS/GNSs composite particles) after routinely washing and drying. In SEM image of Figure 1, we can observe many fine wrinkles irregularly distributed on the surface of microspheres, which can be evidently assigned to the flexible and ultrathin nanosheets. It suggests that the GNSs were wrapped closely around the PS microspheres. Though the graphene nanosheets are ultrathin, flexible and difficult to keep

Figure 2. Raman spectra of PS microspheres (a), PS/GNSs composite particles prepared with different weight ratios of GO/PS: 0.1 wt % (b), 0.2 wt % (c), 0.4 wt % (d), 1.0 wt % (e), 2.0 wt % (f), and GO (g).

very strong signals at about 620, 795, 1000, 1030, and 1200 cm−1, whereas the G peak and D peak of GNSs at about 1600 and 1330 cm−1 are displayed obviously in curves b-f for the PS/ GNSs composite particles prepared using different GONS amounts. More significantly, the characteristic signals of PS weaken steadily in accordance with an increased weight ratio of GONS/PS. This phenomenon can be regarded as a powerful evidence for the gradient changes in coverage of PS/GNSs composite particles since the Raman signal belonging to PS core could be obscured or absorbed by the GNS layer. In addition, an increased D/G intensity ratio of composite particles compared to that of GONS (curve g) indicates the successful reduction of GONS. Figure 3 illustrates clearly a gradual process of constructing the GNS-based composites with the PS/GNSs composite particles. Figure 3b shows cross sectional image of the highly ordered GNS-based PS composite material fabricated by pressing the PS/GNSs composite particles shown in Figure 3a under 3000 MPa at room temperature. A highly ordered 1751

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Figure 3. SEM images of PS/GNSs composite particles (a), cross-section of GNS-based PS composite material obtained by mold pressing the PS/ GNSs composite particles at 3000 MPs (b), subsequently heat-treating at 130 °C for 2 h (c), and a high magnification of a local image (d).

melt processing at temperature typically greater than 200 °C to enable extrusion or molding into desired forms. This process consumes energy and can disorder the structure. It was recently reported that the application of stress can also make a glass flow: soft glasses, including polymers, yield when subjected to sufficiently large stress.28 The equivalence of two routes (heating or stress) to flow is a basic tenet of jamming, a conceptual means of unifying the glassy behavior with that of granular materials. Under quasi-static compression, GNS wrapped PS microspheres yielded and deformed to form tightly packed composites. It is an enduring puzzle why a boxful of ball bearings, even when compacted by many taps, never packs denser than ∼0.64 (See Figure S2a). This is much less dense than Kepler’s stacking as the optimal packing of balls (packing density ∼0.74) (Figure S2b).29 Annealing at temperature above glass transition for sufficient long time can disclose the packing voids and reach packing maximum (density factor of 1, Figure S2c). We applied a sufficient compressive force to PS particles, they began to deform through movement of the polymer chain. Ediger et al. reported that segmental mobility increases by up to a factor of 1000 during uniaxial tensile creep. 30 The distribution of relaxation times narrows substantially, indicating a more homogeneous ensemble of local environments. Under compression, chain mobility is strongly accelerated and displays a low temperature flow (unjamming transition). As a result, the density of the deformed plastic PS microspheres by room-temperature molding can reach nearly 0.99 (Figure 3b). This tightly packing makes the ordered structure stable in the subsequent thermal treatment process.

array of hexagonal blocks comes into view, which is well consistent with the anticipatory plastic deformation of sizemonodispersed PS/GNSs composite particles under quasistatic compression.26,27 Figure 3c depicts the same sample after heated at 130 °C for 2 h. We observed distinct change in the cross sectional structure, in which the GNSs connect with each other and their fracture lines form an interconnected network. In other words, we successfully constructed an extremely intact, uniform and well-defined 3D graphene network throughout the composite material via the simple and conventional pressing process. It is noteworthy that these heat-treated samples always left smooth and flat fracture as broken off by hand. In a very real sense, it is the strict cross section of GNS and PS region, whereas the untreated samples were broken always at the interface among the original composite particles as shown in Figure 3b. With a much higher magnification (Figure 3d), we can clearly observe the fracture morphology of graphene. From these phenomena, it would be reasonable to believe that the PS chains and graphene sheets have closely and firmly knitted each other. Altogether, to the best of our knowledge, such a highly ordered 3D GNS-based composite material embedded with an intact, uniform and well-defined graphene network had not been reported before. And what’s more, few organic solvents were required all along in the preparation process and hence, it is assuredly an environment-friendly technique. We stress the importance of successfully fabricating the PS/ GNSs composite particles as necessary groundwork for constructing the highly ordered GNS 3D network. However, the room-temperature compressive molding that leaded to the regular deformation of the microspheres is also a significant and key step. Traditionally, the manufacturing of plastics involves 1752

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Figure 4. Cross-sectional SEM images of PS/GNSs composite materials prepared by “particle-constructing” method with different graphene weight loading of 0.1 (a), 0.2 (b), 0.6 (c), and 1.0 wt % (d).

Figure 5. (a) Electrical conductivity of the highly ordered GNS-based PS composites (heat treated) as a function of volume fraction of graphene. (b) comparison of the electrical conductivity of the highly ordered GNSs-based PS composites before (○) and after (∇) heat treated.

Figure 4 shows cross sectional images of a series of GNSbased PS composite materials that were fabricated using the PS/GNSs composite particles with different coverage of GNSs. The intact and clear network structure can be observed on the cross section as the PS microspheres (number-average diameter: 2.2 μm) had a complete wrapping with GNSs. Besides, we also can fabricate GNS-based PS composite materials with a bicontinuous structure as purposefully using the partially wrapped PS microspheres. As shown in Figure 4a, in addition to the GNS network, the PS chains form the other continuous phase in the composite materials and it should be due to interparticle diffusion through the uncoated parts on the microspheres. In this way the composites should get excellent

mechanical property because of their bicontinuous structure. Figure S3 shows photographs of some of the samples in different forms. The pressing blocks get darker with an increased GNS content. But after heat treated, all of them have metallike appearances, which present a similar and uniform hue of blackness. The highly ordered 3D graphene network structure undoubtedly can make a great contribution to the properties of the composite materials. Figure 5a shows the conductivity variation of the GNS-based PS composite materials as a function of GNS volume fraction. The percolation in the composites occurs when the graphene concentration, φ, is near 0.057 vol % obtained from the fitting according to model of a 1753

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Figure 6. Cross-sectional SEM images of highly ordered graphene aerogel materials prepared by calcining the resultant GNS-based PS composites with a GNS loading of 8 wt % at 550 °C for 1h in nitrogen atmosphere (a, b, c) and soaking them in THF overnight (d, e), and the graphene/MnO2 aerogel composite materials (f) prepared via in situ generating of MnO2 nanoparticles on the internal surface of the graphene aerogel material. The inset in part a is the appearance photograph of the highly ordered graphene aerogel materials.

typical electrical percolation system (σ = σf[(φ − φc)(1 − φc)]t). This percolation threshold (φc) is the lowest so far and only about a half of the reported value.11 Moreover, the conductivity of our composites with a GNS loading of 0.096 vol % reaches as high as 1.57 S/m, which is at least 3 orders of magnitude higher than reported. As shown in Figure 5b, the electrical conductivities of all these composites had a little improvement after heat treated at 130 °C. On the basis of a slight shrink of sample pieces, we suggest that the PS segment motion may contribute to eliminating the gaps among the deformed particles though they stacked tidily and compactly, thus reducing their contact resistance. Obviously, the outstanding conductivity is absolutely of great benefit from the unique 3D network structure of graphene in the composites, since it exhibits the conductivity as an electrical percolation system in the optimal and the most effective way. In

other words, the conductive filler forms an intact connected path through the insulating matrix with its content as few as possible, and hence, it can be regarded as the most optimal electrical percolation material. Besides, the rather high goodness of fit to the model, as shown in the inset of Figure 5, should reflect the structure characteristics of the composites. It was further confirmed by a theoretical estimation for the extent of GNS wrapping the PS microspheres. According to eq 1 in the Supporting Information, the lowest amount of GNS to reach a complete wrapping of a PS microsphere (2.2 μm) is about 0.19 wt % to the PS (see Supporting Information for details), which was supported by the experiment result. As shown in Figure S4, we can observe an almost perfect GNSwrapped particle except only a few bare pits on the particle surface. In other words, it is also the lowest amount of GNS to form a complete GNS network in polymer matrix. A more 1754

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Figure 7. (a) SEM image of P(S-EA)/GNSs composite particles. (b) Cross sectional SEM image of GNS-based P(S-EA) composite material with bicontinuous structure.

universality and flexibility of the approach for the polymer matrix. In addition, the GNS network density could be easily tuned by controlling the diameter of polymer microspheres, which is an important factor of influencing the properties of the GNS-based polymer composite materials.

interesting fact is that this weight ratio of GNS/PS (0.19 wt %), nearly corresponding to above-mentioned 0.096 vol % (i.e., 0.20 wt %), is precisely the crucial value at which the conductivity of the GNS-based composite materials rose perpendicularly above 1 S/m. The highly ordered 3D GNS-based PS composites can be further used to prepare highly ordered graphene aerogel materials via dissolving the PS with THF or calcining the GNSbased PS composites in nitrogen at 550 °C. Taking the case of the PS/GNSs composite materials with GNS/PS weight ratio of 8 wt %, the residual amount after calcining for 1 h was about 7.6 wt %. It suggests the full removal of PS, which is also in mainly accord with the result based on TGA. As shown in Figure 6, there is a honeycombed cross section that was constructed with the residual GNSs regardless of by calcination means (Figure 6, parts a−c) or adopting the extraction (Figure 6, parts d and e). The orderly arranged holes obviously derived from the PS microspheres. Nitrogen adsorption−desorption isotherms of the some aerogel samples were measured and a typical result of the graphene aerogel obtained by calcining the PS/GNS composites with GNS/PS weight ratio of 8 wt % is shown in Figure S5. The Brunauer−Emmett−Teller (BET) special surface area is 53.5 m2/g and it is not so large that we expected. The methods and experimental conditions should be less than perfect and require improving and we believe this is quite a promising and effective way to prepare highly ordered 3D graphene aerogel. Furthermore, we obtained graphene/ MnO2 aerogel composite material (see Figure 6f) via in situ generating of MnO2 nanoparticles on the internal surface of the graphene aerogel material. The graphene aerogel materials with the uniform and orderly arranged cavities should be fit to be applied in energy production and storage, electrochemical detection, water−oil separate and superhydrophobic materials etc. This “particle-constructing” method can be easily extended to fabricate more GNS-based composite materials and to tune their properties and functions in a broad range. For instance, we used poly(styrene−ethyl acrylate) (P(S-EA)) microspheres instead of the PS microspheres to fabricate P(S-EA)/GNSs composite particles as shown in Figure 7a. Then the composite particles were mold pressed and heat treated at 90 °C to obtain highly ordered GNS-based P(S-EA) composite material. As shown in Figure 7b, there is a clear and intact GNS network in the cross-section and a metallike appearance as shown in the photograph inserted in Figure 7b. This sample suggests the

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CONCLUSION In summary, our proposed “particle-constructing” method provided a simple, environment-friendly and scalable technique for fabricating highly ordered 3D GNS-based polymer composite materials, throughout which the GNSs connect with each other to form the intact, uniform and well-defined GNS network. The highly ordered GNS-based PS composite materials exhibited outstanding electrical property involving extremely low percolation threshold and much higher conductivity, which fully indicated the advantage of applying the graphene in the most effective way. This technique can be extended to fabricate highly ordered graphene aerogels and more GNS-based materials, and it is possible to tune their properties and functions in a broad range. It should be a promising route for moving the synthesis technique forward to provide the high performance GNS-based composite materials for a broad range of applications.

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ASSOCIATED CONTENT

S Supporting Information *

Conductivity of the GNS-based PS composite materials, theoretical estimation for the extent of GNS wrapping the PS microspheres, AFM image of graphene oxide nanosheets, schematic diagram for particle ball packing, digital images of PS/GNSs composite particles, SEM image of PS/GNSs composite particles, and nitrogen adsorptio/desorption isotherm and pore size distribution plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail:[email protected]. Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS This work was supported by Program for Changjiang Scholars and Innovative Research Team in University, National Basic 1755

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Research Program of China (973 Program, 2012CB821503) and the NSF of China (51133002, 21274060).

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