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A Green Plastic Constructed from Cellulose and Functionalized Graphene with High Thermal Conductivity Na Song,*,† Xingshuang Hou,† Li Chen, Siqi Cui, Liyi Shi, and Peng Ding* Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China S Supporting Information *

ABSTRACT: It is urgent to fabricate a class of green plastics to substitute synthetic plastics with increasing awareness of sustainable development of an ecological environment and economy. In this work, a novel green plastic constructed from cellulose and functionalized graphene has been explored. The mechanical properties and thermal stability of the resultant cellulose/ functionalized graphene composite plastics (CGPs) equal or even exceed those of synthetic plastics. Moreover, the in-plane thermal conductivity of CGPs can reach 9.0 W·m−1·K−1 with only 6 wt % functionalized graphene loading. These superior properties are attributed to the strong hydrogenbonding interaction between cellulose and functionalized graphene, the uniform dispersion of functionalized graphene, and the alignment structure of CGPs. Given the promising synergistic performances and ecofriendly features of CGPs, we envisage that CGPs as novel green plastics could play important roles in thermal management devices. KEYWORDS: green plastics, cellulose, graphene, thermal conductivity, NH2/PEG/NH2

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INTRODUCTION Nowadays, environmental plastics are becoming highly valued materials in automotive and electronics industries.1,2 Besides, it is meaningful to fabricate a novel kind of green plastic with superior heat dissipation performance owing to the rapid rising demand in effective thermal management systems.3,4 Moreover, most substrates (such as polyamide, epoxy, polypropylene, and so on)5−11 of thermally conductive plastics are processed from synthetic polymers that are rooted in petrochemicals, which eventually bring about concerns in terms of both economic and environmental sustainability.2 Thus, using renewable and environmental energy to fabricate green plastics with excellent thermal transfer performance has great application potential. One of the most economical and promising substrates is cellulose, which is the most abundant natural polymer on the earth.12,13 Because of their good strength, toughness, and transparency, cellulose compounds have a variety of application areas,14 for instance, electronics, supercapacitors, biomaterials, and aerospace.15−20 Nevertheless, abundant hydroxyl groups on cellulose tend to form large proportions of intra- and intermolecular hydrogen bonds, which go against the dissolution or melting of cellulose.12,21 Fortunately, Zhang et al.22−25 successfully dissolved cellulose in an aqueous alkali hydroxide and urea solution at low temperature (−12 °C). Moreover, on the basis of the hydrogen bonding in cellulose and mobility of cellulose molecular chains in a hydrogel state, a new cellulose bioplastic (CBP) was constructed by a simple hot-pressing technique.12,21,26 CBP shows superphysical properties including high tensile strength, great thermal stability, biodegradability, and renewability,21 in contrast to © 2017 American Chemical Society

common plastics. Therefore, together with the unique graphene, a two-dimensional material with excellent thermal conductivity (TC; 2000−5300 W·m−1·K−1),27−30 with cellulose as the matrix, dramatically increases our interest in constructing novel green cellulose/graphene composite plastics (CGPs) with superior properties for thermal management devices. To the best of our knowledge, thermally conductive nanocomposite papers or films prepared from nanofibrillated cellulose by a filtration technique have been reported, and these composites have achieved high TC,16,31,32 whereas the applications of these papers or films are still confined to some areas such as lightemitting-diode (LED) lampshades, the shell of an electron device, and so on. In this paper, novel CGPs with superior heat dissipation performance have a potential advantage in domains. Moreover, the fabrication of thermally conductive composite plastics starting from hybrid hydrogels has never been explored yet. Because of the high van der Waals attraction between graphene and their large surface area, hydrophilic bis(3aminopropyl)-terminated poly(ethylene glycol) (NH2/PEG/ NH2) was used to covalently functionalize graphene oxide (GO),33,34 and then the functionalized GO was reduced by hydrazine hydrate and denoted as GP. Covalent functionalization is one of the most effective ways of improving fillers’ dispersion by introducing additional functional groups.35−37 For example, Brinson et al.7 demonstrated that functionalized Received: March 1, 2017 Accepted: May 3, 2017 Published: May 3, 2017 17914

DOI: 10.1021/acsami.7b02675 ACS Appl. Mater. Interfaces 2017, 9, 17914−17922

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the construction of CGPs. −12 °C before use. The solution was used to dissolve cellulose at 0 °C in an ice bath by stirring and formed a 5 wt % transparent cellulose solution (Figure S1a) within 2 h in terms of the previous work. Subsequently, a certain amount of GP [for details, see the Supporting Information (SI-2)] dispersed in deionized (DI) water was sonicated in a water bath for 2 h and then added into a cellulose solution to obtain a CGP mixture solution (Figure S1b). The mixture solution underwent vacuum filtration for 30 min at 0 °C in an ice bath to remove air bubbles. The obtained mixture solution was spread on poly(tetrafluoroethylene) molds and subjected to physical regeneration with ethanol and then thorough washing with DI water to construct CGP physical hydrogels with 3.0 mm thickness. In contrast, a pure cellulose hydrogel was also prepared to test. Fabrication of CGPs. The resultant CGP and cellulose hydrogels were sandwiched between stainless steel plates and then hot pressed at 70 °C. After 48 h, CGPs and CBP sheets with 0.2 mm thickness were obtained. Characterization. An Avatar 370 Fourier transform infrared (FTIR) spectrometer using potassium bromide pellets was used to record the FTIR spectra. A D/MAX2200/PC X-ray diffractometer with Cu Ka radiation (λ = 0.154 nm) was used to collect the X-ray diffraction (XRD) patterns. A T. A. Instruments Q500 thermogravimetric analyzer was used to conduct thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C·min−1. All samples were dried at 50 °C for 24 h under vacuum before TGA. Emission scanning electron microscopy (SEM; JSM-6700F) was used to observe the fracture morphology of the samples. Transmission electron microscopy (TEM; 200CX 178) was used to acquire the TEM images. A Netzsch LFA 447 Nanoflash was used to test the thermal diffusivity (α) and specific heat (Cp) of the CBP and CGPs at 25 °C. Each thermal diffusivity test was repeated six times, and values with large errors were ignored. A density balance (JA3003J, SOPTOP, China) was used to measure the density (ρ). TC was obtained by the formula TC = αCpρ. An Agilent 4339B high-resistance meter was introduced to measure the volume resistivity, and the applied voltage and current were 100 V and 10 mA, respectively. Dynamic mechanical thermal analysis (DMTA) was performed on the samples using a dynamic thermomechanic analyzer (Q800 from T. A. Instruments) in tensile mode at a heating rate of 5 °C·min−1 in the range from 25 to 240 °C with a frequency of 1 Hz and an amplitude of 10. Rectangular strips of 3 × 20 mm were cut from the plastics and tested. The stress− strain curves were performed at a temperature of 21.7 °C and a humidity of 43.4% on a universal tensile tester (Instron 5569A), and

graphene sheets exhibit better dispersion and provide better interface interaction with poly(methyl methacrylate) compared to expanded graphite. Thus, it is expected that the amino group at the end of NH2/PEG/NH2 can form hydrogen bonding with hydroxyl on cellulose, giving rise to homogeneous dispersion of graphene in a cellulose solution. Besides, it has been demonstrated that functionalization is an effective way of decreasing the interface thermal resistance and improving the interface thermal transport.37 Considering these factors, GP nanosheets (GPs) with amino groups were introduced to promote the TC of the composites. In the present work, we constructed a novel green plastic based on cellulose and GPs through the transition of an aggregated structure from hybrid hydrogels by hot pressing (Figure 1). We find that not only are the cellulose molecular chains apt to be oriented along the in-plane direction but also GPs exhibit well alignment in a cellulose substrate. It is favorable to form effective heat conduction paths, which assist to improve the in-plane TC. Therefore, TC can achieve 9.0 W· m−1·K−1 with only 6 wt % GP loading. Moreover, the mechanical properties and thermal stability increase preferably by incorporating of GPs into the cellulose matrix. We believe that the green plastics fabricated from cellulose and GPs with these superior comprehensive properties could exhibit high values in connectors, semiconductors, and other high-performance thermal management devices.

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EXPERIMENTAL SECTION

Materials. Cellulose samples (form: fibers), NH2/PEG/NH2 (Mw ∼ 1950 g·mol−1), and N-hydroxysuccinimide were purchased from Sigma-Aldrich Co. (St. Louis, MO). 1-Ethyl-3-[3-(dimethylamino)propyl]carbodiimide in hydrochloric acid was provided by Shanghai GL Biochem Ltd. (Shanghai, China). Graphite powders, ethanol, sodium hydroxide (NaOH), urea, sulfuric acid, and hydrazine hydrate (50%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other reagents and solvents were of analytical grade and were used without further purification. Fabrication of CGP Hydrogels. A 7 wt % NaOH/urea aqueous solution was obtained after bath sonication in water and precooled to 17915


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Figure 2. (a) Photographs of RGO and GP dispersion. Inset: Schematic illustration of GPs. (b) XRD patterns of GO, RGO, and GPs. (c) FTIR spectra of CBP and CGPs. (d) Schematic of the intermolecular hydrogen bonding between cellulose and GPs. the samples were cut into 10 × 50 mm strips. An IR imaging spectrometer was used to characterize the thermal transfer performance of CBP and CGPs. The diameter of the samples was 25.0 mm. The CBP or CGPs were stuck on LED chips with a thin layer of heat conductive silicone. The LED chip with 3 W power acts as a heat source.

To investigate the functional group changes and the interfacial interaction between the cellulose substrate and GPs, FTIR spectra of pure CBP and CGPs were measured in Figure 2c. The peak at 3447 cm−1 is attributed to the characteristic stretching vibration of the hydroxyl groups in pure CBP, which is sensitive to hydrogen bonding. The characteristic peaks shift to lower wavenumber after the incorporation of GPs, indicating the formation of hydrogen bonding between cellulose and GPs.5,43,44 Figure 2d shows the illustration of how the amino on GPs formed hydrogen bonding with the hydroxyl group on the cellulose molecular chains. The amino groups correspond to peaks at 2889 and 1109 cm−1 in the FTIR spectrum of NH2/PEG/NH2 (Figure S3), and the hydroxyl group corresponds to 3447 cm−1 in the spectrum of CBP. In addition, the XRD patterns of CBP and CGPs in Figure S5 also suggest that a strong interaction exists between cellulose and GPs. For comparison, we fabricated CGPs and RGO−CBP with 6 wt % filler content, respectively. In Figure S6, the surface of the CGPs is uniform and smooth, while that for RGO−CBP is detached and coarse, which also suggests the aggregation of RGO nanosheets. The phenomenon also proves that covalent functionalization with NH2/PEG/ NH2 is contributed to the homogeneous dispersion of GPs. Consequently, we believe that hydrogen bonding between GPs and the cellulose substrate and the well dispersion of GPs are all responsible for achieving homogeneous nanocomposites with desired performances. Structure and Morphology of CGPs. One of the important criteria for plastic is its transition of the aggregated structure.21,26 The photograph of cellulose hydrogel with 3 mm thickness in Figure 3a1 is semitransparent. The SEM image in Figure 3b1 shows a freeze-dried cellulose hydrogel with a network structure, which demonstrates that the hydrogel consists of interconnected cellulose fibrils.45 Subsequently, water was rapidly removed from the cellulose hydrogel, resulting in plastic deformation through the transition of an aggregated structure from hybrid hydrogels after hot pressing. In Figure 3a2, the obtained CBP with 0.2 mm thickness is transparent and presents a relatively high transmittance (75% at

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RESULTS AND DISCUSSION Covalent Functionalization and Hydrogen Bonding. Graphene as a nanofiller has an obvious tendency to agglomerate in polymer substrates,38,39 and it can exhibit outstanding properties, with its favorable dispersibility in the polymeric matrix as reported.40,41 Herein, covalent modification was carried out for a high level of graphene dispersion. For a typical preparation (see the SI-2), GO was first functionalized with NH2/PEG/NH2 and then reduced by hydrazine. The obtained GP suspension (Figure 2a) dispersed homogeneously after sitting in DI water for almost 3 months, while agglomeration is obviously observed in a reduced GO (RGO) suspension, which suggests that GPs are dispersed more homogeneously than RGO. This favorable dispersion is attributed to the bigger interlayer spaces between GPs as one amino group cross-links in the graphene sheets.34 A schematic illustration of the functionalized state of GPs in the inset of Figure 2a clearly illustrates this phenomenon, and the result is verified by XRD spectra in Figure 2b. For GO, the featured diffraction peak (001) at 2θ = 9.5° suggests an interlayer spacing of 0.93 nm. After reduction, the peak at 2θ = 9.5° disappears and dramatically shifts to higher 24.2° (an interlayer spacing is 0.37 nm), whereas with NH2/PEG/NH2 crossing in, the peak shifts from 24.2° to approximately 22.7°, and the interlayer spacing of the GPs increases to 0.39 nm, which implies that the GPs are successfully functionalized. In addition, the peak at 11.3° appears, representing a better regularity and crystallinity of NH2/PEG/NH2, which may explain the formation of crystals.34,42 Besides, the FTIR spectra and TEM images in Figures S3 and S4 also indicate that GPs are successfully synthesized. 17916


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to the 0.5 wt % CGPs. With magnified SEM images (Figure 4b,d), the GPs (at the arrowhead) are well-distributed in the cellulose substrate without congregation. Meanwhile, the GPs are also well aligned, with their sheet plane parallel to the surface of the CGPs (see the details in Figure S8).46 GPs are thickly wrapped with the adsorbed cellulose substrate, which indicates strong interfacial adhesion between two surfaces due to functionalization.6 Such a uniform dispersion and the alignment of GPs, as well as a strong interface, are beneficial to improving the TC and mechanical performance of the CGPs. TC. It is necessary to construct a class of green composite plastics with a superior heat dissipation performance to substitute synthetic plastics. A laser flash system was used to evaluate the TC of the CBP and CGPs at 25 °C. The mechanism and methodology of testing were introduced in detail (see SI-8.1).34,43,44 The pure CBP has an in-plane TC of 2.59 W·m−1·K−1 in Figure 5a, which is superior to other polymer matrixes.16,47−50 As is known, the in-plane TC values of cellulose sheets vary from 0.03 to 2.47 W·m−1·K−1. This can be attributed to the different types and sources of cellulose,16,43,51,52 as well as the different preparation methods.53 In this paper, the source of cellulose was a cotton linter. The transition of the aggregated structure of CBP from cellulose hydrogel and the alignment of cellulose microfibrils could contribute to the high TC of CBP.54 Beyond that, the TC values show an obvious increase with the loading of GP additives. As the concentration of GPs reaches 6 wt %, the TC of CGPs increases to a maximum with 9.0 W·m−1·K−1 in the plane. Evidently, the in-plane TC of the CGPs is one of the highest values ever reported for lower filler-containing composites (Table 1). It is easy to understand the enhancement of the TC of CGPs with increasing GPs. However, the reason that the obtained CGPs with a lower additive amount has a higher TC than that for other reports is primarily attributed to the alignment structure in CGPs (see Figure 4). The hot-pressing process can lead to the transition of the aggregated structure through the stretching and rearrangement of the cellulose molecular chains. In CGPs, cellulose molecular chains tend to lie along the inplane direction, and GPs are rearranged so that they are parallel to the CGP surface. Besides, it has also been reported that

Figure 3. Photogram of the cellulose hydrogel (a1) and CBP (a2). SEM images of the inner part of the cellulose hydrogel (b1) and CBP (b2).

600 nm; Figure S7), further indicating that CBP has a fairly homogeneous structure.21 Importantly, it can obviously be seen in Figure 3b2 that cellulose microfibrils exhibit a parallel arrangement that is very distinct from the network structure of the cellulose hydrogel, indicating that the cellulose hydrogel, which consisted of cellulose molecular chains, was transferred into transparent CBP after hot pressing. The fast Fourier transform approach quantitatively proves the degree of cellulose microfibril orientation (see the SI-7 and Figure S8). Thus, we suggest that the process not only promotes the plastic deformation but also leads to a transition of the aggregated structure. To observe the morphology change of the nanocomposites filled with GPs, representative cross-sectional SEM of the 0.5 and 6 wt % CGP sheets was carried out. In Figure 4a,c, the aligned structures are obviously observed in the CGPs. Also, the GPs are more tightly stacked in the 6 wt % CGPs compared

Figure 4. (a and b) SEM images of 0.5 wt % CGPs. (c and d) SEM images of 6 wt % CGPs (the white arrows show the dispersed GPs). 17917


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Table 1. In-Plane TC of Polymer-Based Nanocomposites in Previous Literature ref

Figure 5. (a) In-plane TC values of CBP and CGPs with varied GP contents. (b) Proposed model of GPs for thermal conduction.

functionalized molecules can also facilitate the alignment and close packing of graphene nanosheets.55 Meanwhile, covalent functionalization is an effective way of improving the interface thermal transport.37 The design of amino groups on GPs reduces the phonon vibrational mismatch and creates a strong hydrogen-bonding interaction between the two surfaces, giving rise to improvement of the miscibility and affinity between GPs and the cellulose substrate, which also improves the TC of the nanocomposites.11,35,56 The proposed model of heat flow in Figure 5b vividly presents the formation of thermally conductive paths in CGPs. This demonstrates that both the covalent functionalization and orderly alignment of GPs in CGPs corporately contribute to the high TC. To further investigate the application of CGPs in thermal management devices, an IR imaging spectrometer was introduced to characterize the thermal transfer performance of CBP and 6 wt % CGPs. The CBP and CGPs were stuck on the LED chip with heat-conductive silicone (Figure 6a). Figure 6b presents the diagram of heat flow on CBP and 6 wt % CGPs, the LED chip acts as the heat source to heat plastic sheets. It is clear in Figure 6c that the temperature distributions on CBP and 6 wt % CGPs are dramatically different; the temperature increase in CGPs is faster than that in pure CBP with an increase of the heat time. Moreover, the temperature on 6 wt % CGPs is always higher than that of CBP at a certain moment. These results indicate that CGPs possess superior thermal dissipation properties, which is consistent with the high inplane TC of 6 wt % CGPs. In addition, the through-plane TC of CBP and CGPs was also measured (Figure S10). The value for 6 wt % CGPs achieves 0.73 W·m−1·K−1, which is far below that in the inplane TC. This significant difference between the in-plane and through-plane TC can be attributed to the alignment structure of composites and the anisotropy of graphene, as previously investigated.34,43,44 On the other hand, the electrical performances of CBP and CGPs were measured with a high-resistance meter. The data are presented in Figure S11. The CBP possesses a volume

matrix

a

filler

b

filler content (wt %)

in-plane TC (W· m−1·K−1)

47 48

epoxy CPE

RGO SWCNT

not given 50

1.32 1.6

36 50 57 8 58

PCL PI PI PVA epoxy

BNNS BN/g-TRG GWF BN graphene nanoplate

20 50 12 10 11

1.96 2.11 3.73 3.93 4.5

35 43 59 11

PDA NFC PVA PI

BN RGO BNNS BN

10 30 6 60

5.4 6.618 6.95 7

34 49 60

PA NFC PMMA

GP GNP BNNS

5 10 80

9.71 12.8 14.7

61

PI

BN

60

17.5

62 31

PVDF PVA

27.2 95

19.5 21.7

16

NFC

rLGO BN−Ag/ SICNW− Ag BNNS

5

26.2

this work

cellulose

GP

6

9.0

testing method laser flash four linear probe laser flash TC tester laser flash laser flash temperature wave analysis laser flash laser flash laser flash thermal constant analyzer laser flash laser flash thermowave analyzer temperature wave analysis laser flash laser flash steady-state method laser flash

a

In this column, CPE = conjugated polyelectrolyte, PCL = poly(caprolactone), PI = polyimide, PMMA = poly(methyl methacrylate), PVA = poly(vinyl alcohol), PDA = polydopamine, NFC = nanofibrillated cellulose, PA = polyamide, and PVDF = poly(vinylidene fluoride). bIn this column, SWCNT = single-walled carbon nanotube, BN = boron nitride, BNNS = boron nitride nanosheet, g-TRG = glycidyl methacrylate-grafted graphene, GWF = graphene woven fabric, rLGO = large-area reduced graphene oxide, SICNW = silicon carbide nanowire, and GNP = graphite nanoplatelet.

resistivity of 5.9 × 109 Ω·cm. After the incorporation of GPs, the values slightly decrease to 1.3 × 109 Ω·cm, which still fits in with the critical resistance for electrical insulation (109 Ω· cm).31 This means that the CGPs are electrically insulated with a high TC as well. Mechanical Properties and Thermal Stability. The mechanical properties of composite plastics are of great importance for applications in thermal management. As discussed above, it is expected to improve the mechanical performance by incorporating of GPs into the cellulose matrix. Figure 7a presents the stress−strain curves and Young’s modulus of CBP and 6 wt % CGPs. It is clear to see that the tensile strength and Young’s modulus of CGPs are up to 53.33 and 3625.855 MPa, respectively. Especially, the toughness of CGPs increases to 0.8 MJ·m−3, which increases by 167% compared to that of CBP, indicating that the incorporation of GPs into cellulose is important to improve the toughness. In Figure 7b, 6 wt % CGPs exhibit great flexibility. In addition, Figure S13 shows the tailoring process; note that brittle failure occurs during tailoring of the CBP sample, and there are some burrs on the cross section and some microcracks on the sample 17918


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Figure 6. (a) CBP and 6 wt % CGPs stuck on the LED chip with heat conductive silicone. (b) Diagram of heat flow on CBP and 6 wt % CGPs, with the LED chip acting as the heat source. (c) IR thermal images of CBP and 6 wt % CGPs at different heating times.

Figure 7. (a) Stress−strain curves of CBP and 6 wt % CGPs (local amplified pictures in Figure S12a). (b) Photogram of 6 wt % CGPs. (c) DMTA curves of CBP, CGPs, PP, PE, and PVDF that show the storage modulus. (d) TGA curves of CBP and CGPs.

the temperature increases. Obviously, CBP and CGPs possess superb thermal mechanical stability at 240 °C. Moreover, a pronounced improvement in E′ emerges when the GP content reaches 6 wt %. Meanwhile, the loss modulus is similar to the result of E′ (Figure S12b). On the one hand, the result is caused by the rigid and wormlike molecular chains of cellulose.25 On the other hand, the incorporation of GPs is more efficient to increase E′ under the same temperature because of the adhesion of cellulose and the strong hydrogenbonding interaction between the cellulose chains and GPs. In addition, CBP and CGPs show a high thermal degradation temperature over 300 °C in Figure 7d, which further proves excellent thermal stability. The cost of green plastics is one of the interesting problems that we are concerned with. It can be found that the mechanical and thermal properties improved with the addition of GPs.

surface. In contrast, the incorporation of GPs prevents this problem, CGPs become flexible, and the smooth cross section can be observed. The distinctive properties demonstrate that CGPs are potential materials for flexible electronics, whereas compared with the significant improvement in the in-plane TC, the strenthening effect is not notable. This is due to the alignment of GPs along the in-plane direction that can take full advantage of the in-plane TC of graphene. Maybe the introduction of a three-dimensional carbon honeycomb into the composites could promote the TC and mechanical properties simultaneously.63 The capability of withstanding high temperature for CGPs is essential for thermal management plastics. Figure 7c shows dynamic mechanical analyses of the synthetic polymers (PE, PP, and PVDF), CBP, and CGPs. Note that the storage modulus (E′) of synthetic polymers gradually decreases when 17919


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ACS Applied Materials & Interfaces ORCID

However, the cost per unit area of green plastic goes up with increasing loadings of GPs in current research. Considering the cost performance, composites with more than 6 wt % GP loading are not expected to be explored. According to our investigation and calculation, the price of pure CBP that was used in our experiment was slightly higher than that of common plastics, which is nowadays largely due to the largescale industrialization of common plastics. In fact, the raw materials’ cost of cellulose plastics will decrease greatly during practical production. Besides, CBP can be constructed under 70−100 °C, which also cut the cost compared to the higher processing temperature of conventional synthetic plastics. Moreover, CBP is biodegradable, while the common plastics are fabricated from synthetic polymers derived from petrochemicals; thus, CBP also has a lower cost in the environmental treatment (a detailed comparison can be found in SI-12). On the basis of the above analysis, cellulose plastics have more advantages than conventional synthetic plastics. Therefore, it can be imagined that cellulose plastics with excellent TC, outstanding mechanical properties, and high thermal stability will show enormous potential in advanced thermal management devices.

Na Song: 0000-0002-3343-3000 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Shanghai (Grant 17ZR1440700), the Program of Shanghai Academic/Technology Research Leader (Grant 17XD1424400), the PetroChina Innovation Foundation (Grant 2016D-5007-0508), and the National Natural Science Foundation of China (Grant 51303101). The authors thank Prof. Yanyan Lou and Prof. Weijun Yu for help with the SEM and TEM measurements. The authors are grateful to Prof. Dehong Li, Engr. Jianhai Zhi, and Associate Prof. Xuefei Wang from Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, for help in the mechanical property test.

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CONCLUSION In summary, we constructed a novel, green plastic based on cellulose and GPs through the transition of the aggregated structure from hybrid hydrogels by hot pressing. Cellulose molecular chains are apt to be oriented along the in-plane direction, and GPs exhibit well alignment in the cellulose substrate after processing. Moreover, GPs can disperse uniformly in cellulose as a result of the formation of hydrogen bonding between them. Herein, effective heat conduction paths are obtained, which assist to improve the TC along the plane. The value can achieve 9.0 W·m−1·K−1 with 6 wt % GP loading, which is one of the highest values ever reported for lower fillercontaining composites. Besides, the mechanical properties and thermal stability increased by incorporating of GPs into the cellulose matrix. What is more, 6 wt % CGPs exhibit excellent thermal mechanical properties at high temperature in contrast to common synthetic plastics. Thus, these excellent properties make green cellulose/graphene plastic very useful in highperformance thermal management devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02675. Pictures of cellulose and CGP mixture solutions, preparation of GPs, covalent functionalization and hydrogen bonding, XRD test, photographs of composite plastics, UV−vis spectroscopy of CBP, quantitative characterization of the degree of orientation, TC, electrical performance, mechanical properties and thermal stability, tailoring process of the composites, and cost of green plastics compared with synthetic plastics (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.S.). *E-mail: [email protected] (P.D.). 17920


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