Strong and Highly Conductive Graphene Composite Film Based on

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Strong and Highly Conductive Graphene Composite Film Based on the Nanocellulose-Assisted Dispersion of Expanded Graphite and Incorporation of Poly (ethylene oxide) Weixing Yang, Yichen Gong, Xuefan Zhao, Tianyu Liu, Yiyin Zhang, Feng Chen, and Qiang Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05850 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Sustainable Chemistry & Engineering

Strong and Highly Conductive Graphene Composite Film Based on the Nanocellulose-Assisted Dispersion of Expanded Graphite and Incorporation of Poly (ethylene oxide) Weixing Yang a 1, Yichen Gong a 1, Xuefan Zhao a, Tianyu Liu a, Yiyin Zhang a, Feng Chen a * and Qiang Fu a * a

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China. 1 These

authors contributed equally to this work.

Corresponding author E-mail: [email protected] (Q. Fu), Tel. /Fax: +86- 28-8546 1795. E-mail: [email protected] (F. Chen), Tel. /Fax: +86-28-85460690. ABSTRACT: Graphene films promise great application potential in modern electronic devices due to their superior electrical and thermal conductivities. However, the green manufacturing of graphene films is still faced with challenges. Besides, the graphene films prepared by oxidation and exfoliation method are expensive and exhibit poor mechanical properties. In this work, the highly conductive graphene-based film with reinforced mechanical strength is fabricated by employing cellulose nanofiber (CNF) to help expandable graphite (EG) exfoliating directly in aqueous solution and poly(ethylene oxide) (PEO) to construct a nacre-like structure. Herein, we succeeded in addressing the issue of graphene films’ unsatisfactory cost and mechanical properties by using very cheap EG as raw material and taking advantage of the synergistic performance of two-dimensional EG nanoplatelet, one-dimensional CNF and flexible PEO. When the mass ratio of EG, CNF and PEO reaches 95:5:3, the graphene-based film displays a relatively high tensile strength (about 63.3 MPa), which shows 587% increase over that of EG film (9.2 MPa) and is much higher than those of the reported graphene films prepared through physical exfoliation to our knowledge. Moreover, it shows extraordinary electrical

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conductivity (1226 S cm-1), thermal conductivity (302.3 W m-1 K-1) and electromagnetic interference shielding effectiveness (44 dB with a thickness of 12 μm). In summary, the manufacturing route of the EG/CNF/PEO composite film is efficient, economical and promising for commercial applications in the contemporary electronic industry. KEYWORDS:

Expandable

graphite,

Nanofibrillated

cellulose,

Electrical

conductivity, Thermal conductivity.

 INTRODUCTION Graphene films are drawing wide interests in electronics and automotive industries owing to their marvelous electrical and thermal conductivities, which allow the films to effectually shield harmful electromagnetic waves and transmit heat generated from electronic equipment.1-7 According to the pertinent literatures reported up to now, chemical/thermal reduction of graphene oxide (GO) and physical exfoliation of graphite are the most common methods to prepare graphene films.1,8-13 Especially, the former method has attracted the most attention owing to the cost-effective mass production of GO. However, a great deal of wastewater containing strong acids and oxidizing substances produced during the oxidation step of GO preparation process would create enormous pressure on circumstances. More importantly, the irreversible damage of the structure and properties of graphene can be hardly restored by chemical or thermal reduction.7,14 The chemical reduction of GO usually requires the use of toxic chemicals and gives rise to relatively low conductive properties,8,15-17 meanwhile the thermal reduction requires extremely high temperature which leads to excessive cost and energy consumption.2,10 Moreover, long-time exposure at high temperatures may take a toll on the well-stacked structure, and consequently doing harm to the mechanical performance of prepared graphene films.18-20 As for the latter route, although the physical exfoliation of graphite could perfectly solve the above-mentioned problems and obtain graphene sheets with less defects, these graphene films possess low thermal, electrical and mechanical performances due to


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the fluffy porous structure generated from the poor overlap between graphene nanoplatelets.12,14,21-24 Besides, the physical exfoliation generally requires the special solvents/stabilizer and brings about the low-yield and low-concentration graphene dispersion, which hardly satisfy the large-scale applications.25-31 Furthermore, high temperature and pressure are generally required for better electrical and thermal properties, which also increase cost and difficulty in processing.7,32-34 Considering these problems in the reported approaches to graphene-based films, it’s urgent to develop a facile, cost-effective, and environment-friendly approach for the preparation of the graphene-based films with excellent comprehensive performances. Hence, to achieve the above goal, it is of crucial importance to prepare homogenous dispersion with high concentration and yield of graphene nanoplatelets via cost-effective and environment-friendly route. There have been related literatures reporting that cellulose nanofiber (CNF) could serve as an auxiliary dispersant for functional carbonaceous nanofillers in water owing to the hydrophobic interactions between CNF and nanofillers, electrostatic repulsion and fluctuations of counterions on the surfaces of the CNF.35-39 More importantly, CNF with high aspect ratio is beneficial to the improvement of interaction between nanofillers, sequentially enhancing the mechanical properties of these fabricated composites.40,41 In our previous study, the CNF has been applied to assist the dispersion of commercial graphene nanoplatelets (GNPs), and thus obtaining composite films with relatively high comprehensive performances.42 Nevertheless, the exorbitant price and limited properties (relatively low C/O) of GNPs raw material greatly hinder the commercial applications of these fabricated composite films in practice. Therefore, our goal is to fabricate graphene-based films with excellent comprehensive performances through more economical, practical and effective method, and thereby broadening their application ranges. In this contribution, we employed cost-effective expanded graphite (EG) as raw material, which could be fully utilized without complicated processes such as purification, and then CNF was added to assist EG exfoliate and stably disperse in water under the action of short-time ultrasound. It have been convinced that, when the



mass ratio of EG to CNF is 95:5 and 90:10 respectively, the ultrasonically treated EG/CNF form satisfactory dispersions, and the mixtures can be directly applied for the fabrication of graphene-based films. Furthermore, we added water-soluble poly(ethylene oxide) (PEO) into the EG/CNF system to act as adhesive for the purpose of the construction of a nacre-like structure in the composite films. By means of the synergistic enhancement effect of CNF, PEO and EG nanoplatelets, the strong and highly conductive graphene-based films have been successfully achieved. Especially, when the mass ratio of EG, CNF and PEO is 95:5:3, the graphene-based film possesses optimal mechanical properties (63.3 MPa) as well as extraordinary electrical conductivity (1226 S cm-1), thermal conductivity (302.3 W m-1 K-1) and remarkable electromagnetic interference shielding performance.

 EXPERIMENTAL SECTION Materials. Microfibrillated cellulose (MFC, 25 wt %) was acquired from Daicel Chemical Industries; TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, 98 wt %) was obtained from Sigma-Aldrich; Sodium Hydroxide (NaOH), Sodium Hypochlorite (NaClO) and Sodium Bromide (NaBr) were purchased from Chengdu Kelong Chemical Reagent Factory; Intercalated Expanded Graphite was obtained from Qingdao Jinrilai Graphite Co., Ltd.; Poly(ethylene oxide) (PEO) was purchased from Sigma-Aldrich. Composite films preparation. Figure 1 shows the preparation schematic of the composite films. The experiments in this work utilized TEMPO-oxidized CNF, which was prepared according to the reported method.43,44 And the detailed preparation process of CNF showed in Supporting Information. A certain amount of EG/CNF aqueous dispersion (0.63 wt %) and water were mixed at room temperature to obtain a EG/CNF dispersion, in which the total solid content of EG and CNF was fixed at 5 mg g-1. After sonicated for 0.5 h (the ultrasonic mode was pulsed with 5s on and 1s off, and the power was 150 W), a certain amount of PEO aqueous solution (mass fraction of 1 wt%) was added to the EG/CNF mixture. The mixture was stirred at room temperature for 1 h until the formation of a homogeneous EG/CNF/PEO


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mixture. The mixture was then filtered for an initial EG/CNF/PEO film product, which was dried at room temperature in the air for 24 h. After drying, it was treated

Figure 1. Preparation schematic of the composite films.

with cold pressing (150 MPa, 10 min) at room temperature, followed by heated pressing (75 °C, 20 MPa, 10 min) to finally acquire an EG/CNF/PEO composite film. For convenience, we label the final EG/CNF/PEO film as GxFyOz, where x, y, and z represent the mass ratios of EG, CNF, and PEO in the composite film. For example, G95F5O3 represents that the mass ratio of EG, CNF and PEO is 95:5:3 in the EG/CNF/PEO film. In particular, when the PEO content is 0, the EG/CNF film is labeled as GxFy, where x and y means the mass ratios of EG and CNF in the EG/CNF film. Characterization. The microstructure and morphology of EG were characterized by scanning electron microscopy (SEM, USA Fei) and transmission electron microscopy (TEM, FEI Tecnai 20). The X-ray Photoelectron Spectroscopy (XPS) was conducted on an X-ray photoelectron spectrometer (Axis Ultra DLD, KRATOS, UK). Thermogravimetric analysis under nitrogen atmosphere was measured by TGA 500 (TA company). UV-1800PC Spectroscopy (UV/Vis spectrophotometer) was used to



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test the mixed liquid sample at a wavelength of 200-800 nm. The measurement of fourier transform infrared spectromotry (FT-IR) was performed on Nicolet 6700 under reflection mode at room temperature. Differential scanning calorimetry (DSC, Q20, USA TA) was used to measure the crystal melting behavior of the sample. The test was performed under a nitrogen atmosphere and the temperature was raised at a rate of 10 °C min−1. The mechanical properties were tested on Instron 5576. The sensor applied in the test was 1 kN. The test temperature, humidity, and tensile rate were 25 °C, 60%, and 20 mm min-1, respectively. The sample size was consistent, with 30 mm and 3 mm length and width, and at least five sets of data were measured for each ratio. Dynamic mechanical analyzer (DMA Q800, American TA) was adopted to test the mechanical properties of the sample at a speed of 3 N min-1, and the test temperature was 25 °C and 75 °C respectively. The measurement of the conductivity of the films was operated on the Keithley 6487 instrument (USA) at room temperature. The EMI SE measurements were carried out by the APC-7 connected with Agilent N5230. The diameter for the samples is 13 mm and the EMI SE testing frequency was in X-band. Specifically, the measured S11 and S12 scattering parameters were applied to calculate the coefficients of transmission (T), absorption (A), and reflection (R). The total EMI SE (SETotal) can be also obtained, which was the summation of the absorption (SEA), reflection (SER) and multiple internal reflections (SEM). The Supporting Information showed the specific measurement process of SETotal, SER, SEA and SEM. The thermal conductivities of the film, including the in-plane thermal conductivity (K) and the cross-plane thermal conductivity (K⊥), are calculated by the following formula: K⊥=α⊥×Cp×ρ K=α×Cp×ρ

(1) (2)

Among them, in-plane thermal diffusion coefficient (α) and cross-plane thermal diffusivity (α ⊥ ) are measured by a laser thermal conductivity meter (LFA 447 Nanoflash). The density (ρ) of the sample is calculated from the sample size and weight, and the specific heat capacity (Cp) was measured by DSC (1/700, Mettler-Toledo, Switzerland) at a rate of 10 °C min-1.


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 RESULTS AND DISCUSSION To begin with, a variety of characterization methods have been conducted for EG to demonstrate the comprehensive characteristics of EG and make full use of the EG raw material. As illustrated in Figure S1, the EG utilized in this work is a black powdered material. Based on SEM characterization (Figure S1a), we can clearly observe that the EG has a worm-like morphology and its size has reached the millimeter level. And in the further magnified SEM image, the EG exhibits an uneven surface with a loose layered structure. Then, in order to investigate the thermal stability of EG, TGA was used to characterize its weight loss at 25-600 °C (Figure S1b) and the results presented the remarkable thermal stability of EG. Concretely, even if the temperature climbed up to 600 °C, almost no weight loss occurred, indicating that the EG had not undergone any degradation reaction and existed as a very stable conductive carbon material. The oxygenated functional groups of EG were next characterized by FT-IR and XPS tests. It can be apparently seen that there is almost no infrared absorption peak of oxygenated functional groups (Figure S1c), and it’s difficult to find such peaks from XPS test results as well, as the C/O atomic ratio reaches 58.1. And the C1s spectrum also confirms that there is almost no oxygenated functional group on the GNs (Figure S1d). These above results illuminate that the EG is a thermally stable carbon material with ideal conductivity, which hardly contains oxygenated functional groups and preserving its complete structure consequently. Reported studies have revealed that the graphene films prepared by the physical exfoliation graphite method have inferior mechanical properties due to poor alignments and interactions between graphene nanoplatelets.7,33 In our system, in order to maintain high electrical and thermal conductivities, it’s necessary that the content of expanded graphite is as high as possible. But unfortunately, high content of EG will inevitably lead to poor mechanical properties of the fabricated film. To deal with this problem, a large number of published researches have revealed that two-dimensional layered nanomaterials, one-dimensional fibrous nanomaterials,



together with flexible macromolecules could construct a bio-inspired nacre-like structure with splendid mechanical property.45-47 Based on these encouraging results, in this study, CNF, as a low-cost, inexhaustible and green 1D fibrous nanomaterial derived from cellulose,48,49 is taken as an auxiliary exfoliating and dispersing agent for EG in water to further transform EG from millimeter loose layered structure to nanoscale graphite nanoplatelet. As shown in Figure S2, the CNF manifests remarkable dispersion and stability in the water. And the CNF suspension retains high transparency at the visible wavelength from 390 nm to 780 nm even if its concentration reaches 0.5%, thanks to electrostatic repulsion of the carboxyl groups on the surface of CNF. The average length and diameter of the obtained CNF are 0.45 μm and 5 nm, respectively (Figure S3). In the meantime, we employ poly (ethylene oxide) with high molecular weight (PEO, with a molecular weight of 1 × 106 g mol-1) as the flexible macromolecules in view of its ideal thermoplasticity, crystallinity, and ability to form hydrogen bonds with CNF in this current work.50,51 Besides, it is a kind of flexible macromolecule with biodegradability, hydrophilicity and biocompatibility. Specifically, its hydrophilicity offers the possibility to employ water as a dispersion medium system, which would result in less environment problems. In addition, the thermoplasticity and crystallinity of PEO would facilitate subsequent processing and promote the mechanical properties of the prepared films. Figure S4 indicates that the pure PEO looks white and its melting point is 68.8°C (Figure S4a). The TGA result illustrates that PEO can keep heat stability within 300°C (Figure S4b). Above all, these results show promise of the preparation of strong and highly conductive graphene composite film through a green route. As shown in Figure 2a, both PEO and CNF can completely dissolve in water to form a transparent solution, while obvious agglomeration and sedimentation occurred in the EG dispersion without CNF. It may be attributed to the fact that CNF can be uniformly dispersed in water and maintain stability for a long time owing to electrostatic repulsion of the negatively charged carboxyl groups (introduced during the TEMPO oxidation process) on the CNF surface. And EG can be assisted to be stably dispersed in water with only 5 wt % CNF. The results of the UV absorption


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spectrum test in Figure 2b also further confirm the above results. Compared with no

Figure 2. (a) The digital images of the PEO, CNF, EG, EG90/CNF10, and EG95/CNF5 dispersions. (b) UV-vis absorption spectra of the PEO, CNF, EG, EG90/CNF10, and EG95/CNF5 dispersions. TEM images of EG (c), EG90/CNF10 (d), EG95/CNF5 (e) dispersions.

incident light absorption of the pure EG dispersion, the EG90/CNF10 and EG95/CNF5 dispersions absorb light at wavelength from 200 to 800 nm, especially represent a large absorption peak at a wavelength of 280 nm, which indicates that EG is uniformly dispersed in the aqueous solution and therefore could absorb incident light thanks to the hydrophobic interaction and dipolarization between CNF and EG. And then, to explain the interaction between CNF and EG in aqueous solution through the preparation method in our work, the thicknesses of EG in the pure EG, EG90/CNF10 and EG95/CNF5 dispersions were examined by TEM, separately (Figure 2c-e). Surprisingly, the thicknesses of EG in EG90/CNF10 and EG95/CNF5 dispersions became significantly thinner than that of pure EG (Figure 2c), illuminating that EG turned into graphene nanoplatelets with the exfoliation effect of CNF. As well, with the increasing content of CNF, the graphite nanoplatelets



continuously decrease in thickness, which proves that CNF not only has a dispersing effect on EG, but also act as the auxiliary of its exfoliation. Notably, it can be observed that CNF is adsorbed on the surface of graphite nanoplatelets (GNs) in EG90/CNF10 and EG95/CNF5 dispersions, which endows the successful construction of bio-inspired nacre-like structure with great potential access to splendid mechanical property. And the SEM images in Figure S5 also greatly manifest the dispersion and exfoliation effect of CNF on EG in water. Figure S5(a, a1) reveal that the graphite sheets are agglomerated together in the pure EG dispersion, which is harmful to the alignment and interaction between the graphite sheets. With the dispersion and exfoliation effect of CNF (Figure S5b, c, and b1-c1), the EG in EG90/CNF10 and EG95/CNF5 dispersions were exfoliated into nanoplatelets and the agglomeration was significantly reduced in pace with the increasing CNF content. Moreover, as exhibited in Figure S6, both the SEM and TEM images of 95EG/5CNF and 90EG/10CNF dispersions indicated that CNF were absorbed on the surface of EG nanoplatelets. All the above results prove that the hydrophobic interaction and dipolarization between CNF and EG allow the CNF to be adsorbed onto the EG surface, and then the electrostatic repulsion and steric hindrance of CNF can assist the dispersion and exfoliation of EG in water.36,38,40 Consequently, the EG/CNF and EG/CNF/PEO composite films were successfully fabricated by vacuum-assisted filtration based on the prepared homogeneous and stable EG/CNF dispersion. However, the composite films produced by filtration display loose structure and poor mechanical properties. To solve this problem, the cold and heated pressing were utilized as post-treatment process to further augment the mechanical and conductive properties of the composite films. First of all, cold pressing was applied to eliminate pores within the films and align the EG nanopaltelets in the plane to form a compact layered structure. And the another step of heat pressing (75°C, 20 MPa, 10 min) above the melting point of PEO could force the PEO to melt and crystallize to firmly reinforce the interaction between PEO, EG and CNF. For comparison, pure EG film and EG/CNF composite films were treated with the same process.


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Actually, the construction of nacre-like structure of EG/CNF/PEO is to obtain greatly enhanced mechanical property. Hence, the characterization and discussion of mechanical properties of these fabricated films are deemed to be the most critical part of our research and the mechanical properties of the EG, G100O3, EG/CNF and EG/CNF/PEO composite films have been characterized (Figure 3). Figure 3a and Figure 3b represent the tensile stress-strain curves of pure EG films, G100O3 and GxFyOz composite films with various composition ratios. In Figure 3a, it can be clearly seen that the mechanical properties of pure EG films are extremely poor, with

Figure 3. (a,b) Stress–strain curves of the EG film, G100O3, GxFy and GxFyOz composite films. The



comparison of tensile strength (b), Young’s modulus (c), Toughness (e) and elongation at break strain (d) of these films.

low tensile strength, Young's modulus and elongation at break remaining 9.2 MPa, 2.5 GPa and 0.43%, respectively. Surprisingly, when 5 wt% CNF is incorporated, there is a significant elevation of the tensile strength (37.8 MPa), Young's modulus (9 GPa) and elongation at break (0.87%) of the G95F5 film, illuminating the enhancement effect of CNF on the EG films. Moreover, with a small amount of PEO added, the mechanical property of the G100O3 only shows a slight increase when compared to that of pure EG film. Specifically, its tensile strength, Young's modulus, and elongation at break is 10.7 MPa, 2.8 GPa, and 0.48%, respectively. As for G95F5Oz composite films, the tensile strength and Young's modulus of the EG/CNF/PEO films increase at first and then decrease with rising PEO content, while the elongation at break keep increasing consistently. Especially, when the mass ratio of EG:CNF:PEO reached 95:5:3, the tensile strength and Young's modulus of the G95F5O3 obtain the optimum values, as high as 63.3 MPa and 8.8 GPa, and the elongation at break also raised up to 1.05%. Compared with the EG film’s tensile strength (9.2 MPa), Young's modulus (2.5 GPa) and elongation at break (0.43%), the corresponding values of G95F5O3 increase by 587%, 252% and 144%, respectively. Note that the G95F5O3 possessed the greatest mechanical property compared with the previously reported graphene films prepared through physical exfoliation to our knowledge.42 Furthermore, the tensile test results of G90F10 and G90F10Ox shown in Figure 3b indicated that a further increase of CNF content could effectively enhance the mechanical properties. More precisely, compared to pure EG and G95F5 films, the tensile strength, Young's modulus and elongation at break of G90F10 were vigorously climbed to 50.7 MPa, 7.1 GPa and 0.97%. It also demonstrated that, only with the simultaneous incorporation of CNF and PEO, the mechanical properties of the EG/CNF/PEO composite films could be improved fully. Besides, the mechanical properties of G90F10Ox composite films exhibited the same trend as G95F5Ox composite films in the wake of the increasing PEO content. Both the tensile strength


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and Young’s modulus climbed up firstly and then declined with the increasing PEO content, and acquired the highest values when the weight ratio of PEO to the total mass of EG nanoplatelets and CNF was 3:100. The reason may be that, when the PEO content was below 3 wt%, the incorporation of PEO could effectively fill up the gaps between CNF and EG nanoplatelets and enhance the interaction between CNF and EG nanoplatelets, thus improving the tensile strength and Young’s modulus of these composite films. However, when the PEO content was higher than 3 wt%, excess PEO which obstructed between CNF and EG nanoplatelets possessed weak PEO chain interaction and resulted in lowered tensile strength and Young’s modulus. And the reason why the elongation at break of the composite films decreased with the increasing PEO content may be the strong interaction between CNF and EG nanoplatelets along with the relative slip of flexible PEO chain. This work further sorted out the tensile strength, Young's modulus, elongation at break, and fracture toughness of EG, G100O3, EG/CNF, and EG/CNF/PEO composite films (Figure 3c-f). As a consequence, ascribed to the exfoliation and dispersion effect of CNF on EG together with the synergetic enhancement effect of CNF and PEO on EG nanoplatelets, these fabricated EG/CNF/PEO composite films in this work possessed exceedingly excellent mechanical properties, which were superior to the most of graphene films reported as so far.

Figure 4. The digital photos and the fracture surfaces SEM images of the EG film (a), G95F5O3 (b) and



G90F10O3 (c) composite films, in which the G95F5O3 and G90F10O3 composite films exhibiting excellent flexibility.

And then, to make clear the internal structure of these fabricated films and confirm the formation of the nacre-like structure, the flexibility test and SEM characterization of the pure EG film, G95F5O3, and G90F10O3 films have been carried out. As we can see in Figure 4, contrary to the brittleness and poor flexibility of pure EG, G95F5O3 and G90F10O3 possessed much better flexibility. Meanwhile, their cross-sectional SEM photos indicated that all of them possessed ordered layered structure inside, but large graphite platelets still exist inside the pure EG film because of incompletely exfoliation. With regard to the G95F5O3 and G90F10O3 films, the nanoplatelets of EG are uniformly arranged to form compact layered structure. Moreover, compared those of G90F10O3 with G95F5O3 films, it can be concluded that the EG nanoplatelets inside the former film display the smallest thickness, which proves the exfoliation effect of CNF on EG more intuitively. In order to further explore the enhancement mechanism of CNF and PEO on the composite films, the cross-sectional and surface morphologies of the EG/CNF and EG/CNF/PEO composite films after tensile testing have also been studied by SEM. As shown in Figure 5, The fracture surface morphologies of G95F5, G95F5O3, G90F10, and G90F10O3 show that the tensile profiles of these films are not a flat section. And the fibrous CNF absorb in the surface of EG nanoplatelets and fill the interspace among layered structure of EG nanoplatelets. As for the fracture of these samples, the fibrous CNF pull out at the fracture to cover the entire cross section, manifesting that the incorporation of CNF could greatly improve the strength, modulus, and toughness of these EG/CNF/PEO composite films. Moreover, it occurs that thin EG nanoplatelets are pulled out of the cross section, which owes to two reasons: (i) on one hand, the assisted exfoliation and dispersion effect of CNF produce thin EG nanoplatalets and their stable dispersion, which endow the tight stack and strong interaction between EG nanoplatelets a greater chance; (ii) on the other hand, the mechanical pressing treatment further make the layered structure of EG nanoplatelets


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more compact and force the flexible PEO molecules and the one-dimensional CNF with high aspect ratio to be tightly stacked into the surface and interspace between the two-dimensional EG nanoplatelets, further strengthening the interaction between EG

Figure 5. The cross-sectional morphologies of G95F5, G95F5O3, G90F10, and G90F10O3 composite films after the tensile strength test.

nanoplatelets. This exactly accounts why the mechanical properties of G95F5, G95F5O3, G90F10, and G90F10O3 composite films are several times higher than that of pure EG film. The proposed synergistic enhancement mechanism is presented in Figure S7.



Moreover, it is a remarkable fact that the cold and heat mechanical post-treatments developed and utilized in our work are fatal and meaningful for the preparation of composite films under such high graphene contents. The mechanical post-treatment is

Figure 6. The comparison of the diameter, weight, and thickness between the G95F5 and G95F5O3 composite films.

capable of effectively eliminating inner pores of the graphene-based films and align the graphene nanoplatelets along the planar direction more orderly, thereby obtaining a more compact layered structure and stronger interaction between the graphite nanoplatelet, finally obtaining outstanding mechanical and electrical conductive properties of these EG/CNF/PEO composite films. As concluded in these above tensile results, the EG/CNF/PEO composite films with high EG content have been tremendously reinforced by means of the corporation of one-dimensional CNF and flexible PEO, and the enhancement effect of CNF on these composite films has been evidenced. Interestingly, the Figure 3a and 3b illustrate that, when compared to EG/CNF composite films with the same CNF content, the EG/CNF/PEO composite films possess superior mechanical properties, and the G95F5O3 film is taken as a typical sample, which has much superior mechanical property than that of G95F5 film. Therefore, the interpretation of the role of PEO in the greatly improved mechanical


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properties of EG/CNF/PEO composite films is of great concern. For this purpose, the weight and thickness of G95F5 and G95F5O3 with the same diameter have been measured and the results are listed in Figure 6. Surprisingly, at the same diameter, the heavier G95F5O3 film possesses smaller thickness, when compared with G95F5 film. The reason may be that PEO molecules can bond the CNF and EG nanoplatelets much closer and augment the interaction between CNF and EG, thus giving rise to the more compact layered structure.

Figure 7. (a) The DSC melting profiles of the G95F5 and G95F5O3 composite films. (b) Stress–strain curves of the G95F5 and G95F5O3 composite films at 25 °C and 75 °C measured by DMA.

Furthermore, the G95F5 and G95F5O3 composite films were selected as samples to deeply clarify the enhancement mechanism of PEO on these composite films. The DSC profile of G95F5O3 composite film in Figure 7a exhibits that the melting peak of PEO appears at 67°C, approximately. It not merely confirms the existence of PEO in G95F5O3, but also manifests that, during the preparation process of these composite films, the crystallizable PEO molecules distributed inside the films homogeneously through hot pressing and then crystallized via the following cold pressing, rather than existed as single molecular chains. Besides, the tensile tests of G95F5 and G95F5O3 composite films have been performed at 25°C and 75°C (higher than the PEO melting point) and the results are shown in Figure 7b. Obviously, the tensile properties of the G95F5 film at 25°C and 75°C are similar, but in regard to the G95F5O3, it appears a divergent result. Specifically, the tensile strength and Young's modulus of G95F5O3



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measured at 25°C were higher than those of the test results at 75°C, while elongation at break at 25°C turned out lower, because PEO could not

provide strong bonding

force to enhance these films any more owing to the melting of PEO at 75°C. It also indirectly demonstrated the significance of PEO for EG/CNF/PEO films. In conclusion, the great improvement of the mechanical properties of these composite films can be completely summed up as follows: firstly, the millimeter-sized EG turned into 2D EG nanoplatelets and the layered structure come to being because of the exfoliation effect of CNF; and then the CNF adsorbed on the surface of EG nanoplatelets and filled up the gaps among the EG nanoplatelets, thereby enhancing interaction between EG nanoplatelets and obtaining more compact layered structure; finally, the flexible and crystallizable PEO molecules constituted bonds between EG and CNF and augment the interaction between CNF and EG, consequently resulting in much denser layered structure and greatly improved mechanical properties. In other words, 2D EG nanoplatelets, 1D fibrous CNF and flexible PEO molecules successfully constructed the bio-inspired nacre-like structure and provided these EG/CNF/PEO composite films with excellent mechanical performances.

Figure 8. The electrical conductivity (σ) of the EG film, EG/CNF and EG/CNF/PEO composite films.


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As mentioned above, EG nanoplatelets exfoliated via the exfoliation and auxiliary dispersion of CNF possess less defects and larger lamella size, when compared to those graphene nanoplatelets constructed through chemical/thermal reduction of GO. Hence, it is reasonable to believe that these EG/CNF/PEO composite films have outstanding electrical performance in spite of the slight incorporation of insulating CNF and PEO. The electrical conductivities of pure EG film, EG/CNF and EG/CNF /PEO composite films have been measured subsequently and the results are presented in Figure 8. Obviously, the pure EG film displays the highest electrical conductivity (σ) of 1870 S cm-1. However, EG/CNF composite films possess inferior electrical properties than pure EG film, and the electrical conductivity of G95F5 film (1481 S cm-1) is higher than that of G90F10 film (1240 S cm-1), representing that the increment of CNF content would reduce the electrical property. Similarly, when it comes to the effect of PEO on the electrical property of these composite films, it can be concluded that the electrical conductivities gradually decreased with the increasing PEO content. Specifically, the electrical conductivity of G95F5O1, G95F5O3, G95F5O5, and G95F5O10 are 1377 S cm-1, 1226 S cm-1, 1123 S cm-1, and 910 S cm-1, respectively, and the conductivities of G90F10Ox films exhibit the same trend with the increasing PEO loading. In general, although the electrical conductivities of these EG/CNF/PEO composite films represent a little decrease when compared with that of EG film, taken the relatively poor electrical conductivity of graphene films reported in other works into account, the EG/CNF/PEO composite films still generally retain outstanding electrical properties. Moreover, the electrical conductive mechanism of these composite films has been further discussed as follows. Firstly, to elucidate why the pure EG film displays the highest electrical conductivity compared with other EG/CNF and EG/CNF/PEO composite films, it should be noted that the insulated CNF and PEO impede the electron conduction between the EG nanoplatelets. On the contrary, there is no insulator such as CNF and PEO in the pure EG film, enabling electrons to move without hindrance, so the pure EG film possesses the optimal electrical property.42 In other words, in the wake of the increasingly higher CNF and PEO content, the



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insulating layers which deter the electron conduction between the EG nanoplatelets become much stronger, consequently leading to the gradual decrease of the electrical conductivity of these EG/CNF/PEO composite films. But satisfyingly, the electrical properties of these prepared composite films with relatively low content of CNF and PEO maintain a high level and possess excellent comprehensive performances.

Figure 9. (a) EMI SE for the G95F5O3 composite films with various thicknesses. (b) The SETotal,

SER

and SEA for the G95F5O3 composite films with various thicknesses at the frequency of 10 GHz. (c) The power coefficients of reflection (R), transmission (T), and absorption (A) of G95F5O3 composite films as a function of thickness at the frequency of 10 GHz, which are similar at other frequencies.

Taking their optimal electrical properties into account, the G95F5O3 composite films were chosen to be characterized their electromagnetic interference shielding performances. Therefore, the G95F5O3 films with different thicknesses in the X-band range have been carried out and the results are listed in Figure 9. As for all of these samples with various thicknesses, their electromagnetic interference shielding effectiveness (EMI SE) in the X-band always displays a stable value with respect to


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frequency. And as the thickness of the G95F5O3 film elevating, EMI SE also increases accordingly. In particular, when the thickness of the G95F5O3 film is nearly 12 μm, the EMI SE turns out as high as 44 dB, which already meets the requirements of commercial applications,52 and as expected, the EMI SE reaches its maximum (71 dB) with 73 μm in thickness. In brief, the G95F5O3 composite film can be endowed with the required electromagnetic interference shielding performance by controlling its thickness. In order to make clear the electromagnetic interference shielding mechanism of these films, the SETotal, SEA and SER of G95F5O3 are presented in Figure 9b. The results illuminate that both SETotal and SEA increase with film thickness, while the SER appears the opposite trend. Simultaneously, Figure 9c summarizes the variation of the electromagnetic wave absorption (A), reflection (R), and transmission (T) power coefficients as a function of thickness. It can be clearly observed that the R attains much higher than the A for these film with various thickness, which clarifies that the reflection of microwaves dominates the EMI shielding mechanism. The major of microwaves were reflected on the surfaces of G95F5O3 before being absorbed by reason of impedance mismatches between the air and films due to the extraordinary electrical conductivity. Moreover, it is noteworthy that the R and T of the G95F5O3 decrease while A ascends with increasing thickness, which results from that the increment of thickness gives rise to the rising EG content, inevitably improving the interaction between EG and electromagnetic microwave.53,54 In summary, G95F5O3 is endowed with tremendous electromagnetic interference shielding performance.



Figure 10. (a) In-plane (K) and cross-plane (K ⊥ ) thermal conductivity of EG/CNF and EG/CNF/PEO films. (b) The anisotropy of the thermal conductivities (K/K ⊥ ) for the EG/CNF and EG/CNF/PEO films.

As for graphene-based films, their thermal transport capacities are also of great significance. Thus the thermal conductivities of the EG/CNF/PEO composite films are exhibited in Figure 10. As we can see from Figure 10a, the pure EG film possesses the highest in-plane thermal conductivity (K) of 398.2 W m-1 K-1 and cross-plane conductivity of 2.23 W m-1 K-1, respectively. With the increment of CNF and PEO contents, both in-plane and cross-plane conductivities of these composite film drop slightly. To be more specific, the K of G95F5, G95F5O3, G90F10, and G90F10O3 only reduce to 331.1, 302.3, 295.4, and 251.3 W m-1 K-1, respectively. The slight decrease of thermal conductivities of EG/CNF/PEO composite films might be ascribed to the formation of a certain amount of thermal obstacles between EG nanoplatelets due to the insertion of thermally insulated CNF and PEO into the layered structure.55,56 Whereas, when compared with most of reported graphene-based films, these films still exhibit superior in-plane conductivities. Moreover, as shown in Figure 10b, thanks to the high K and low K ⊥ of these films, unique thermal conductivity anisotropy is demonstrated in these EG/CNF/PEO films as well. The strong thermal conductivity anisotropy of the EG/CNF and EG/CNF/PEO films were ascribed to the highly ordered layered structure of EG nanoplatelets inside these films and the highly superior thermal conduction along the graphene plane direction. Specific speaking, the tightly stacked EG nanoplatelets inside the layered structure achieved the large in-plane overlapping area among the EG nanoplatelets, consequently reducing the interface thermal resistance during the heat conduction along this direction. Combined the low in-plane interface thermal resistance among EG nanoplatelets with the intrinsically high graphene plane thermal conduction, the high K was successfully achieved for these films. However, as for the cross-plane direction, the formation of the numerous thermal interfaces rendered the high cross-plane interface thermal


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resistance, subsequently giving rise to the extremely low K

⊥

. Furthermore, the

incorporation of insulated CNF and PEO would impede the heat conduction between the EG nanoplatelets, consequently leading to the lower K and K ⊥ of EG/CNF and EG/CNF/PEO films when compared with the EG film. In summary, the fabricated graphene-based films possessed extraordinary thermal transport properties.

 CONCLUSIONS In this work, by utilizing EG as raw material, with green natural material CNF to help EG effectively exfoliate and disperse in water, and then adding flexible PEO, a compact layered nacre-like structure is successfully constructed by mechanical cold and heated pressing as a post-treatment process, which effectively reinforce the mechanical properties of the obtained graphene-based films. The resultant G95F5O3 films reaching the optimum tensile strength (63.3 MPa) attributed to the synergistic enhancement of two-dimensional EG flakes, one-dimensional CNF and flexible PEO, and it retains extraordinary electrical conductivity (1226 S cm-1), thermal conductivity (302.3 W m-1 K-1) and EMI performance (electromagnetic interference shielding effectiveness up to 44 dB when the thickness is only 12 μm). Moreover, the preparation route of our composite films in aqueous solution is facile, cost-effective and environmental friendly.

■ ASSOCIATED CONTENT Supporting Information Additional experimental details and results including SEM images, TGA date, FT-IR spectra, TEM images, XPS data and DSC melting profiles.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (Q. Fu), Tel. /Fax: +86- 28-8546 1795. E-mail: [email protected] (F. Chen), Tel. /Fax: +86-28-85460690.



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■ ACKNOWLEDGMENT We appreciate the National Natural Science Foundation of China for the financial support (Grant No. 51721091 and 51573102).

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10.1016/j.carbon.2011.04.039 (55) Song, N.; Hou, X.; Chen, L.; Cui, S.; Shi, L.; Ding, P. A Green Plastic Constructed from Cellulose and Functionalized Graphene with High Thermal Conductivity. ACS Appl. Mater. Interfaces 2017, 9, 17914-17922. DOI: 10.1021/acsami.7b02675 (56) Song, N.; Jiao, D.; Ding, P.; Cui, S.; Tang, S.; Shi, L. Anisotropic thermally conductive flexible films based on nanofibrillated cellulose and aligned graphene nanosheets. J. Mater. Chem. C 2016, 4, 305-314. DOI: 10.1039/C5TC02194D


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The strong and highly conductive graphene-based film is fabricated by the nanocellulose-assisted dispersion of expanded graphite and incorporation of Poly (ethylene oxide).


Strong and Highly Conductive Graphene Composite Film Based on

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