Flexible, Ultrathin, and High-Efficiency Electromagnetic Shielding

Research Article www.acsami.org

Flexible, Ultrathin, and High-Efficiency Electromagnetic Shielding Properties of Poly(Vinylidene Fluoride)/Carbon Composite Films Biao Zhao, Chongxiang Zhao, Ruosong Li, S. Mahdi Hamidinejad, and Chul B. Park* Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada S Supporting Information *

ABSTRACT: In this study, we fabricated conductive poly(vinylidene fluoride) (PVDF)/carbon composites simply by dispersing multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets into a PVDF solution. The electrical conductivity and the electromagnetic interference (EMI) shielding of the PVDF/carbon composites were increased by increasing the conductive carbon filler amounts. Moreover, we also found that the EMI shielding properties of the PVDF/ CNT/graphene composites were higher than those of PVDF/ CNT and PVDF/graphene composites. The mean EMI shielding values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composite films with a thickness of 0.1 mm were 22.41, 18.70, and 27.58 dB, respectively. An analysis of the shielding mechanism showed that the main contribution to the EMI shielding came from the absorption mechanism, and that the EMI shielding could be tuned by controlling the films’ thickness. The total shielding of the PVDF/CNT/graphene films increased from 21.90 to 36.46 dB as the thickness was increased from 0.06 mm to 0.25 mm. In particular, the PVDF/carbon composite films, with a thickness of 0.1 mm, achieved the highest specific shielding values of 1 310 dB cm2/g for the PVDF/5 wt %-CNT composite and 1 557 dB cm2/g for the PVDF/CNT/graphene composite, respectively. This was due to the ultrathin thickness. Our study provides the groundwork for an effective way to design flexible, ultrathin conductive polymer composite film for application in miniaturized electronic devices. KEYWORDS: conductive polymer, poly(vinylidene fluoride)/carbon composite film, electric conductivity, electromagnetic interference shielding, conduction loss devices, will need to be integrated with very thin and flexible EMI shielding films.17 The CPCs combine intrinsic polymer properties with a tunable electrical conductivity that results from their adjustable filler morphology (that is, their conductive network) within the polymer matrix.18−20 Carbon-based materials, such as carbon nanotubes (CNTs) and graphene, have been used as effective conductive fillers to assemble EMI shielding composite structures. This has been due to their controllable aspect ratio, lightweight, excellent electrical conductivity, and flexibility.21−24 Their EMI SE has the capacity to dissipate electromagnetic energy, and this is generally expressed in decibels (dB).25 For applications that need high-efficiency shielding materials, the specific SE (dB cm2/g), which is defined as SE divided by the mass density and thickness, is also a crucial criterion.26 Sundararaj and coworkers27 studied the EMI SE of polypropylene/MWCNT composite plates with different CNT concentrations at various

1. INTRODUCTION In recent decades, the large-scale development and use of electronic devices and instruments have made life easier for humans. However, these advantages have come with a price. Our dependency on electronic gadgets and equipment has generated a pollutant, namely, unwanted electromagnetic energy. Our electronic and/or electrical devices produce electrical and magnetic fields that interrupt the operation of other nearby devices. This phenomenon is called electromagnetic interference (EMI).1−7 High-performance EMI shielding (or screening) materials are necessary to resolve the issues of electromagnetic wave attenuation in both civil and military applications.8−12 Traditional metal-based shielding materials have several drawbacks. These include high density, poor flexibility, undesirable corrosion susceptibility, and the limited tuning of shielding effectiveness (SE). By contrast, the positive features of conductive polymer composites (CPCs) have attracted large numbers of researchers. These attractive qualities include their mass density, shaping capability, chemical stability, and design flexibility.13−16 In addition, future soft and flexible electronic devices, such as foldable phones and wearable © 2017 American Chemical Society

Received: April 7, 2017 Accepted: May 30, 2017 Published: May 30, 2017 20873

DOI: 10.1021/acsami.7b04935 ACS Appl. Mater. Interfaces 2017, 9, 20873−20884

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ACS Applied Materials & Interfaces

Figure 1. Diagram of the film casting + compression molding process for the formation of a PVDF/CNT composite film.

In our study, we synthesized the PVDF/carbon composite films using a simple solution-mixture method. The resultant conductive PVDF films had excellent EMI shielding properties. High specific SE values of 1 310 dB cm2/g and 1 557 dB cm2/g were observed in the PVDF/5 wt %-CNT and PVDF/CNT/ graphene films with a thin thickness of 0.1 mm, respectively. Compared with other polymer/carbon composites, these PVDF/carbon composite films are competitive in terms of their low carbon material concentration and their ultrathin characteristic.

thicknesses. The EMI SE value was 34.8 dB with 16 wt % CNT amounts at a 1.0 mm thickness, and this reached a specific SE of 356.9 dB cm2/g. Das et al.28 used the coagulation method to prepare poly(methyl methacrylate)/ single-walled-CNT. The EMI SE and the specific SE of the poly(methyl methacrylate)/ single-walled-CNT composite were about 30 and 356.9 dB cm2/g, respectively, at a 4.5 mm thickness with 20 wt % singlewalled CNT. The EMI shielding of epoxy/CNT sponge nanocomposites was prepared by an infiltration method using a 3-D CNT sponge as the 3-D reinforcement and conducting framework.29 The specific SE, which was around 169.2 dB cm2/ g in the X-band, was obtained for the epoxy nanocomposite with 0.66 wt % of the CNT sponge at a 2.0 mm thickness. Zhang and co-workers30 investigated the EMI shielding of CNT/waterborne polyurethane (WPU) composites. It was found that the EMI SE values could be tuned by controlling the thickness and the CNT content. The specific SE value of 778.5 dB cm2/g can be obtained with a high CNT content (61.5 wt %) at a 0.32 mm thickness. Shen et al.31 fabricated polyurethane/graphene foams through the solution dip-coating method. When the foam thickness was 60 mm, the specific SE value could be reached at 332 dB cm2/g with a graphene concentration of 10 wt %. Li et al.32 reported on the EMI SE of polystyrene/functionalized-graphene nanocomposite foams, which were prepared using supercritical carbon dioxide. The EMI SE reached 18 dB when 10 wt % graphene was added with a 2.8 mm thickness. Yan et al.33 used high-pressure compression molding and salt-leaching to prepare porous polystyrene/graphene composites. The specific SE of 260 dB cm2/g was reported when 30 wt % graphene was introduced into the polystyrene matrix. In the noted conductive polymer composites, the EMI shielding capabilities could be enhanced by introducing conductive carbon fillers. Also, the EMI SE values would be effectively tuned by adjusting the carbon materials’ content and the samples’ thickness. Typically, a large EMI SE value is obtained with a higher thickness. This would not be suitable for the flexible and wearable electronic devices, which require much thinner EMI shielding films. Much research has been done on the conductive thin films34−39 used in light transmittance and EMI shielding. For example, Yoon and his co-workers34,35 developed outstanding silver nanowire and copper nanofiber flexible transparent conducting films, even though both of these materials oxidize easily. Shen et al. studied the EMI shielding properties of thin graphene and thermoplastic polyurethane/graphene composite films.36,37 When the graphene content in each of these films was 12 wt %, the EMI SE was around 10 dB. Cao et al. prepared poly(vinyl alcohol)/ graphene sandwich films and investigated their EMI shielding properties.38 When the film’s thickness was 1 mm, the poly(vinyl alcohol)/60 vol %-graphene’s EMI SE was only 14 dB, and this limited their applications.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. PVDF was acquired from Solvay. The MWCNTs (NC7000) were purchased from Nanocyl SA. Graphene nanoplatelets were supplied by Group Nanoxplore, Inc. N,NDimethylformamide (DMF) was provided by Caledon Laboratories Ltd. 2.2. Preparation of PVDF/Carbon Composite Films. We prepared PVDF/carbon composite films using solvent casting followed by compression-molding. In Figure 1, the CNT materials are used as an example. First, the MWCNTs were uniformly dispersed in the DMF solution through the ultrasonication process. Then, the PVDF particles were dissolved by magnetic stirring in the DMF mixture. Finally, the flexible PVDF/CNT film was obtained through the evaporation and compression-molding processes. The CNT fillers can also easily be replaced by graphene nanoplatelets and a mixture of the CNT and graphene. To investigate how the filler content affected the electric conductivity and electromagnetic shielding properties, we prepared a series of PVDF/CNT films (0.05 wt % CNT, 0.1 wt % CNT, 0.2 wt % CNT, 0.5 wt % CNT, 1 wt % CNT, 2 wt % CNT, 5 wt % CNT, or 8 wt % CNT), PVDF/graphene films (0.2 wt % graphene, 0.5 wt % graphene, 1 wt % graphene, 2 wt % graphene, 5 wt % graphene, 10 wt % graphene, or 15 wt % graphene), and a PVDF/ CNT/graphene film (5 wt % CNT + 10 wt % graphene). 2.3. Characterization. We used a field-emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) and transmission electron microscopy (TEM, JEOL JEM-2100) to observe the morphologies of the CNTs and the graphene nanoplatelets as well as their PVDF composite films. The crystal structures and samples’ phases were investigated by powder X-ray diffractometer (XRD, Rigaku Ultima IV, Cu Kα radiation, λ = 0.15418 nm). The structural features of the pristine MWCNTs, the graphene nanoplatelets, and the PVDF composite films were identified by a high-resolution Raman spectrometer (the excitation source of the laser was 532 nm; Lab RAM HR Evolution; Horiba Scientific). Room-temperature electrical conductivities of the polymer composites were measured using an Alpha-N analyzer from Novocontrol Technologies GmbH & Co. KG in a frequency range of 0.1 Hz to 100 000 Hz at 1 V initial potential. For the purpose of comparison, the direct-current conductivity, σdc, was considered at a frequency of 0.1 Hz. At least five sample replications were performed in each case, and the average values were recorded. The polymer composite films with sizes of ∼10.6 mm × 4.3 mm × 0.1 mm were measured according to their EMI SE characterization in a frequency range of 18−26.5 GHz (K-band) using the waveguide method via the vector network analyzer (Agilent N5234A). The S20874


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Figure 2. (a) dc electrical conductivity versus the CNT content for the PVDF/CNT composites and (b) ac electrical conductivity of PVDF/CNT composites vesus the frequency at various CNT contents. The inset in part a is the log σ vs log (φ − φc) plot, which is based on the power law relation for various PVDF/CNT composites.

σ = σ0(φ‐φc)−t

parameters (S11 and S21) of each sample were recorded and were then used to calculate the EMI SE. The parameter S11 symbolizes the tested reflection coefficient data, and S21 stands for the transmission data. Because of the negative values of the measured S11 and S21, the values of the |S11| and |S21| were identified as the attenuations of reflection and transmission in the incident electromagnetic waves, respectively. The power coefficients of reflection (R), transmission (T), and absorption (A) were evaluated based on the following equations:25,27,30,36,40

R = |S11|2

(1)

T = |S21|2

(2)

A=1−R−T

(3)

In this formula, σ is the composite electrical conductivity, σ0 is a scaling factor, φ is the volume fraction of the filler, φc is the electrical percolation threshold volume fraction, and t is a critical exponent dependent on the electrical network mechanism. The inset in Figure 2a shows the best fit result using the power law equation, and φc is calculated to be 0.31 wt %. The fitted value of t is 2.96, which is much higher than the universal critical exponents (1.1−2.0) derived from the classical conduction model.42 The high t-value of around 3 demonstrated that the conductivity resulted from the contact between individual CNTs. Figure 2b depicts the alternating current (ac) electrical conductivity of various PVDF/CNT composites as a function of the frequency. It is clear that the electrical conductivity sharply increased with the CNT content below 0.5 wt % and then gradually increased above 0.5 wt %. For example, we found that the conductivity values of pure PVDF, 0.05 and 0.5 wt % CNT-reinforced PVDF were 6.39 × 10−14 S/ cm (see Figure S1 of the Supporting Information), 2.93 × 10−13 S/cm and 1.75 × 10−6 S/cm at 0.1 Hz, respectively. When the CNT amount was further increased to 5 wt %, the electrical conductivity value of the PVDF/CNT composite was 2.53 × 10−3 S/cm at 0.1 Hz. A further increase in the CNT content did not seem to dramatically improve the conductivity value. This was due to the formation of 3-D conductive networks of CNTs in the PVDF matrix with a high content of 5 wt % CNT. Thus, the electrical conductivity of the PVDF/CNT composite was only slightly increased. The XRD patterns of the as-synthesized PVDF/CNT composites with various CNT contents are shown in Figure S2a of the Supporting Information. In the XRD patterns, the raw MWCNTs showed a peak at 26.6°, which was attributed to the (002) plane of the interplanar graphite. With regard to the PVDF/CNT composites, besides the characteristic CNT peak, the other peaks were attributed to the PVDF crystal polymer. This indicated that the polymer composite films were composed of PVDF and CNTs. To confirm the existence of the carbon material, we used Raman spectroscopy, which is a proven and powerful tool for the analysis of carbon and its derivatives.43 Figure S2b of the Supporting Information shows the representative Raman spectra of pristine CNTs and difference as-fabricated PVDF/CNT composites. The Raman spectra of carbon were characterized by two main features: the peak at 1 574 cm−1, which is described as the G-band, and that at 1 340 cm−1, which is described as the D-band. The D-band can result from the structural imperfections in the carbon basal

Here, the absorption coefficient is given in terms of the power of the incident wave. After the first reflection, the relative intensity of the effective incident wave inside the material is (1 − R). Therefore, the coefficient of effective absorbance (Aeff) can be defined as follows:

Aeff = (1 − R − T )/(1 − R )

(4)

The total EMI SE (SET) is also composed of three parts, including reflection (SER), absorption (SEA), and multiple reflections (SEM). Above 15 dB, the SET can be simplified as follows:25,40 SE T = SE R + SEA + SEM ≈ SE R + SEA

(5)

Because of its reflectance and effective absorbance, the EMI SE can be described as follows:30,36,40

SE R = − log10(1 − R )

(6)

⎛ T ⎞ ⎟ SEA = − log10(1 − Aeff ) = − log10⎜ ⎝1 − R ⎠

(7)

(8)

3. RESULTS AND DISCUSSION 3.1. Electrical and EMI Shielding Properties of PVDF/ CNT Composite Films. The EMI shielding performance of composite materials is closely associated with their intrinsic electrical properties. To explore how the introduction of MWCNTs would affect the electrical conductivity, we investigated its activity in the PVDF/CNT composites, and Figure 2 shows the results. It is generally known that when the conductive nanofiller content reaches the electrical percolation threshold, a conductive path forms in the composite matrix due to the conductive filler’s network formation. Figure 2a shows the dependence of the PVDF/CNT composite’s electrical conductivity (σDC) on the CNTs’ loading activity. To determine the electrical percolation threshold for the PVDF/ CNT composites, a power law relation, which was derived from the percolation theory, was used as follows:41 20875


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Figure 3. (a) FE-SEM images of pristine CNTs; (b, c) FE-SEM images of the plane surface and cross section of the PVDF/5 wt %-CNT composite; and (d) TEM image of the PVDF/5 wt %-CNT composite.

Figure 4. (a) Plots of EMI SE versus frequency for the PVDF composites filled with various CNT content with a 0.1 mm specimen; (b) SET, SEA, and SER values of the PVDF composite with 5 wt % CNT at a 0.1 mm film thickness; (c) EMI SE values of the PVDF composite with 5 wt % CNT at various film thicknesses; (d) SET, SEA, and SER with a frequency of 20.0 GHz at various film thicknesses.

plane or edge site. The G-band represents the graphite’s peak, and this corresponds to the sp2 carbon bond stretching in the E2g mode.43,44 Generally, the value of the intensity ratio from the D-band to the G-band (ID/IG) is used to evaluate the degree of disorder and defects in the carbon materials.45 Figure S1b shows the degree of disorder and how the PVDF/CNT composites’ defects increased with various CNT amounts. This helped to improve the EMI SE values, as shown below. Figure S3 of the Supporting Information displays the XRD patterns and Raman curves of the as-synthesized PVDF/graphene composites with various graphene contents. Compared with

PVDF/CNT composites, it can be found that PVDF/graphene composites present similar XRD results and Raman change trends. To disclose the distribution of CNTs in the PVDF/CNT composite film, the SEM images of the raw CNTs and the PVDF/5 wt %-CNT composites were obtained, and Figure 3 shows the results. In Figure 3a, the diameter of the CNTs is about 20−50 nm, and the CNTs tend to form bundles and entanglements because of the van der Waals forces of attraction. Figure 3b−d shows the microstructure images of the plane surface and the cross section of the PVDF/5 wt 20876


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Figure 5. (a) dc electrical conductivity versus the graphene content for the PVDF/graphene composites and (b) ac electrical conductivity of PVDF/ graphene composites versus the frequency at various graphene-nanoplatelet contents.

Figure 6. (a) FE-SEM images of pristine graphene nanoplatelets, (b, c) FE-SEM images of plane surface and cross section of the PVDF/10 wt %-graphene composite, and (d) TEM image of the PVDF/10 wt %-graphene composite.

tion. As the amount rose to 8 wt %, the mean SE of the composite film was found to be 25.02 dB. We analyzed the EMI shielding mechanism of the PVDF/ CNT films. The average values of the SET, the SEA, and the SER, calculated from the measured scattering parameters, were 22.41, 14.50, and 7.91 dB, respectively, for the PVDF/5 wt %-CNT composite with a 0.1 mm film thickness (see Figure 4b). Thus, the contribution of absorption to the total EMI SE (65%) was much larger than that of the reflection (35%), and this indicated the presence of an absorption-dominated shielding mechanism. It is well-known that the EMI SE depends not only on the intrinsic electrical conductivity, the aspect ratio, and the fillers’ dispersion quality but also on the thickness of the polymer specimens.48 The SE value of the PVDF/5 wt %-CNT composite film was increased with an increased thickness (Figure 4c), because of the increased conductive-filler content interacting with the electromagnetic fields.25,49 The PVDF/5 wt %-CNT film showed mean SE values ranging from 21.58 to 35.37 dB at thicknesses ranging from 0.08 mm to 0.4 mm. To investigate the contributions of the SEA and the SER to the SET, a comparison of the SET, the SEA, and the SER at 20 GHz as a

%-CNT composite. The random and uniform dispersion of the CNT fillers in the PVDF matrix is visible. As seen in Figure 2a, the percolation threshold for the formation of the initial conductive CNT network was around 0.3 wt %. In the PVDF/5 wt %-CNT composite, more physical connections of CNT or CNT bundles (Figure 3b,c) can be found, and such connections are responsible for the high electrical conductivity and EMI shielding values. Furthermore, the distribution and connection of CNTs in the PVDF matrix were also confirmed by TEM (Figure 3d). The amount of the incident EM waves’ dissipation is described by evaluating the total SE (SET). In general, when the values of SET reach 10 and 20 dB, the material is capable of blocking 90% and 99%, respectively, of the incident EM waves.46,47 Figure 4a displays the SET values of the PVDF/ CNT composite films with a 0.1 mm thickness as a function of the frequency (18−26.5 GHz). For the 0.05 wt % CNT, the average EMI SE was only 0.24 dB. However, the EMI SE values of the PVDF/CNT films were increased dramatically as the filler loading was increased. For example, the composite film containing 5 wt % of CNTs had an average of 22.41 dB, which exceeded the target level of 20 dB required for commercializa20877


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Figure 7. (a) EMI SE values of various PVDF/graphene composite films with a sample thickness of 0.1 mm; (b) SET, SEA and SER values of PVDF/ 15 wt %-graphene film with a thickness of 0.1 mm; (c) EMI SE values of PVDF/15 wt %-graphene film with various thicknesses; (d) SET, SEA, and SER values of PVDF/15 wt %-graphene film with various thicknesses at a frequency of 20 GHz.

into contact with each other in the confined volume, although the number of contact points may not be large. Consequently, this would produce a charge transport through the contact points. The charge transport can induce conduction loss when the composite is irradiated by electromagnetic waves. The TEM image (Figure 6d) also validates the distribution and connection of graphene nanopatelets in the PVDF matrix. The attenuation or loss of the incoming microwave radiation, while it traveled through the PVDF/graphene composite films, was assessed across a frequency region ranging 18 to 26.5 GHz. Figure 7a shows the EMI SE values of the PVDF/graphene composite films with various graphene amounts and a 0.1 mm film thickness. As in the case of the electric conductivity, the PVDF/graphene composites having a low graphene content (≤2 wt %) resulted in very low SET values, almost below 1.0 dB because of the poor contact. For the PVDF/10 wt %-graphene and PVDF/15 wt %-graphene films, the SET values were ∼18.70 dB and ∼22.58 dB, respectively. These SE values were very close to the desired value for shield materials (≥20 dB) in commercial applications.52 Figure 7b shows the variations of the EMI SE of the PVDF/15 wt %-graphene film with a 0.1 mm thickness as a function of the frequency. We found that the average SEA and SER values were 13.82 dB and was 8.76 dB, respectively, which suggests that absorption was higher than reflection. Figure 7c shows the EMI SE values of a PVDF/15 wt %-graphene film with various thicknesses. The SET values of the PVDF/graphene composite increased with the film’s thickness because of the increased conductive-filler content interacting with the electromagnetic fields53,54 as in the case of PVDF/ CNT composite films. A comparison of the SET, SEA, and SER values of the PVDF/15 wt %-graphene composite films with various thicknesses at a frequency of 20 GHz is shown in Figure 7d. Clearly the SEA was increased by the sample thicknesses while the SER remained almost constant, as in the case of the PVDF/CNT case. The increase in the EMI SE was primarily

function of the sample thickness was analyzed and is shown in Figure 4d. It is clear that both the SEA and SER were increased by the samples’ thicknesses. However, the incremental tendency of the SEA was stronger than that of the SET, which suggests that the absorption mechanism dominated the shielding mechanism. The reflection mechanism of EMI shielding depends on the conductivity of the shielding material, which is correlated with the interaction between mobile charge carriers (electrons or holes) of the material and the electromagnetic field.50,51 It is well-known that refection loss is a function of the ratio of σr/μr, where σr is the electrical conductivity of the shielding material and μr is the relative magnetic permeability. The relative magnetic permeability is supposed to be as 1.0 due to the absence of the magnetic constituents,51 and therefore, the refection loss is just related to the electrical conductivity. 3.2. Electrical and EMI Shielding Properties of PVDF/ Graphene Composite Films. The dc and ac electrical conductivities of the PVDF/graphene composites at room temperature are shown in Figure 5. The electrical conductivity of the PVDF/graphene composite containing 0.2 wt % graphene nanoplatelets was 1.25 × 10−12 S/cm at 0.1 Hz. The conductivity values of 10 and 15 wt % graphene-reinforced PVDF were 2.22 × 10−3 S/cm and 6.56 × 10−3 S/cm, respectively. With a lower reinforcement of graphene (≤2 wt %), the interparticle distance of the graphene was higher in the PVDF matrix. However, a higher graphene content of ≥5 wt % in the PVDF matrix can enhance the composites’ conductivity. Figure 6a shows the SEM image of graphene nanoplatelets. A curled morphology consisting of a thin, rippled, and wrinkled paper-like structure can be observed. To show the graphene distribution in the PVDF matrix, the surface and cross section of the PVDF/10 wt %-graphene composite were characterized using SEM. Figure 6b,c does not show the contact points of the graphene nanoplatelets clearly. However, it seems that a large number of graphene nanoplatelets were forced to be compacted 20878


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Figure 8. (a) Comparison of ac electrical conductivity of the PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composites and (b) TEM image of the PVDF/CNT/graphene composite.

Figure 9. (a) EMI SET values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and the PVDF/CNT/graphene composite films at a 0.1 mm thickness; (b) SET, SEA, and SER values of the PVDF/CNT/graphene composite films at a 0.1 mm thickness; (c) EMI SET values of the PVDF/ CNT/graphene composites with various film thicknesses; (d) SET, SEA, and SER values of PVDF/CNT/graphene films with various thicknesses at a frequency of 20 GHz.

the mean EMI SET values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composite films were 22.41, 18.70, and 27.58 dB, respectively. This indicated that the PVDF/CNT/graphene composite film possessed better EMI blocking abilities than the PVDF/5 wt %-CNT and PVDF/10 wt %-graphene films. For the sake of exploration, we investigated the shielding mechanism of the PVDF/CNT/ graphene composite film and the SET, SEA, and SER values of the PVDF/CNT/graphene composite film at a 0.1 mm thickness, as shown in in Figure 9b. Interestingly, the SER was higher than the SEA below 18.6 GHz. Meanwhile, the SEA offered a higher contribution to the EMI screening properties in a frequency of 18.6−26.5 GHz. The special EMI shielding mechanism of the PVDF/CNT/graphene composite film was attributed to the interconnected 3-D CNT-graphene heterostructure.17 To investigate how the film thickness affected the EMI’s SE, several PVDF/CNT/graphene films with different thicknesses were measured in a frequency range of 18−26.5 GHz. Generally, the EMI SE of the screening materials increased with the film thickness.56 The SET of the PVDF/CNT/ graphene films increased as the thickness increased from 0.06

the result of the increased SEA. The stacking architecture of the 2-D graphene nanoplatelets with strong absorption keeps the electromagnetic wave going through the surface until it is adequately absorbed within the film.55 3.3. Electrical and EMI Shielding Properties of PVDF/ CNT/Graphene Composite Film. Figure 8a shows the ac electrical conductivities of the PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene (containing 5 wt % CNT and 10 wt % graphene) composite films. It should be noted that the electrical conductivity of the PVDF/CNT/ graphene composite was much higher than the addition of the electrical conductivity of the PVDF/5 wt %-CNT composite and the electrical conductivity of PVDF/10 wt %-graphene composite. This indicated that more conductive networks had formed in the ternary PVDF/CNT/graphene composite in a synergistic way as reported by others,17 which was favorable to the EMI SE. From Figure 8b, we note that both CNTs and graphene nanoplatelets were distributed in the PVDF matrix. To demonstrate their superior EMI screening properties, we studied the EMI SET values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composite films at a 0.1 mm thickness. Figure 9a shows the results. We found that 20879


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g. Moreover, the specific EMI SE of the PVDF/CNT/graphene film was greater than that of the PVDF/CNT and PVDF/ graphene films. We attribute this outcome to the synergy that occurred between the conductive CNTs and the graphene nanoplatelets. Table 1 shows some EMI shielding performances of carbon-based polymer composites. The PVDF composite films were much thinner than the other carbon-based polymer composites that obtained 20−25 dB SE values; thus, they achieved a much higher specific SE (1 310 dB cm2/g for PVDF/5 wt %-CNT, 1 430 dB cm2/g for PVDF/8 wt %-CNT, and 1 557 dB cm2/g for PVDF/CNT/graphene, respectively). In terms of the relatively low CNT loading and the ultrathin thickness, we fabricated the PVDF composite films simply by mixing the carbon materials with a polymer matrix in a PVDF solution. This method offers outstanding advantages when compared with other carbon- based polymer composites. Figure 11 shows the schematic EMI shielding mechanisms for (a) PVDF/CNT, (b) PVDF/graphene, and (c) PVDF/ CNT/graphene composite films. On the basis of the above analysis, we surmised that the EMI screening capabilities of the PVDF composite films had resulted from the reflection and absorption mechanisms. The reflection is correlated to the impedance mismatch between the air and the absorber. The presence of surface charges or mobile charge carriers (electrons or holes) is assumed to be the most important factor with respect to the reflection mechanism.66,67 In these PVDF composite films, the contribution of EMI shielding resulted from absorption rather than from reflection. Therefore, we focused on the absorption mechanism. Absorption depends on the shield’s thickness, which always originates from the conduction loss (i.e., the ohmic loss) and the polarization loss.52 The conduction loss is associated with attenuation of energy through the current flow via the conduction, hopping, and tunneling mechanisms. The polarization loss represents the energy consumed in overcoming the momentum needed to reorient the dipoles in the alternated EM wave. The polarization relates to the materials’ functional groups, defects, and interfaces. In the PVDF composite films, due to the interfaces which existed between the carbon materials and the PVDF, the interfacial polarization loss induced by the EM waves contributed to the absorption. Generally, electrical transport in polymer composites can take place either through direct “contacts” between the conductive fillers or by the “hopping” of electrons between sufficiently close conductive particles. In the case of “contacts,” the conductive fillers physically connect with one another to form a conductive network, and the electrons can transport energy just like any other inherently conductive material. However, in the “hopping” situation, the electrons can literally jump between the two sufficiently close conductive fillers which are separated by the polymer. In the PVDF/CNT composite film (Figure 11a), the high conductivity resulted from the contacting and hopping conductive mechanisms, which produce conduction loss in alternating electromagnetic fields. In the PVDF/ graphene composite film (Figure 11b), due to the graphene nanoplatelets’ 2-D structure, the multiple reflections would promote the microwaves absorption.68 In the PVDF/CNT/ graphene (Figure 11c), the multiple reflections and greater interfacial polarizations contributed to the enhanced the microwave absorption properties. Furthermore, the greater number of charges coupled with their uniform state of dispersion generated a conductive network, which consequently

mm to 0.25 mm, as shown in Figure 9c. The average SET value of a PVDF/CNT/graphene film with a 0.25 mm thickness increased to 36.46 dB, which means it blocked 99.97% of the electromagnetic wave. Figure 9d shows the SET, SEA, and SER values of the PVDF/CNT/graphene films with their various thicknesses at a frequency of 20 GHz. The SET and SEA values increased when the samples’ thicknesses were increased. However, the SER first increased and then decreased when the samples’ thicknesses were increased. There were two main reasons for this: (i) The number of free electrons available to interact with the EM radiation was increased by the added CNT and graphene-nanoplatelet contents; (ii) most of the incident radiation’s power had been absorbed within the structure; that is, within the interfacial regions of each layer. This was caused by multiple absorptions and by scattering phenomena in the 3-D interconnected network structure located between the CNTs and the graphene nanoplatelets.57 Thus, we concluded that the increased total EMI SE of the PVDF/CNT/graphene film was mainly based on the increased absorption rather than on the reflections of the electromagnetic waves. Generally, lightweight and a reduced thickness should be considered when designing and evaluating shielding materials. Therefore, to more comprehensively compare the materials’ performances, we applied the concept of a specific SE value. This means that we divided the SET by the product’s density and thickness, and integrated this with both its density and thickness.31 This concept corresponds to the shielding materials’ practical efficiency, which is critical in the fabrication of lightweight and smart EMI shields. Figure 10 shows the specific EMI SE values of the PVDF/CNT, PVDF/graphene, and PVDF/CNT/graphene composite films at a thickness of 0.1 mm. We found that all of the PVDF/CNT, PVDF/ graphene, and PVDF/CNT/graphene composite films had high specific EMI SE values, which were greater than 1000 dB cm2/

Figure 10. (a) Specific EMI SE values of PVDF/CNT, PVDF/ graphene, and PVDF/CNT/graphene composite films at a 0.1 mm thickness and (b) average specific EMI SE values of PVDF/CNT, PVDF/graphene, and PVDF/CNT/graphene films with a thickness of 0.1 mm. 20880


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ACS Applied Materials & Interfaces Table 1. EMI Shielding Performance of Carbon-Based Polymer Composites materials

carbon contents

thickness (mm)

EMI SE (dB)

specific SE value (dB cm2/g)

PMMA/graphene PS/graphene WPU/graphene PEI/graphene cellulose/CNT PP/CNT PMMA/CNT WPU/CNT epoxy/CNT PE/CNT ABS/CNT PANI/CNT PI/graphene foam PS/graphene foam PUG/graphene foam phenolic/CNT WPU/CNT PVDF/CNT PVDF/CNT PVDF/graphene PVDF/graphene PVDF/CNT/graphene

4.23 vol % 10 wt % 5 vol % 10 wt % 33 wt % 16 wt % 20 wt % 76.2 wt % 0.66 wt % 10 wt % 10 wt % 10 wt % 16 wt % 30 wt % 10 wt % 60.6 wt % 61.5 wt % 5 wt % 8 wt % 10 wt % 15 wt % 5 wt % CNT + 10 wt % graphene

3.4 2.8 2.0 2.3 0.64 1.0 4.5 2.3 2.0 3.0 1.1 2 0.8 2.5 60 0.14 0.32 0.1 0.1 0.1 0.1 0.1

30 18 32 20 30 34.8 30 35 33 35 40 35 21 29.3 57.7 32.4 35 22.41 25.02 18.70 22.58 27.58

74 62 112 68 312.7 356.9 49.0 2 143.0 169.2 117.0 317.9 194.4 937 260 332 1 257.6 778.5 1 310 1 430 1 067 1 265 1 557

ref 58 32 59 26 60 27 28 25 29 61 62 63 64 33 31 65 30 this this this this this

work work work work work

shielding properties. For the PVDF/5 wt %-CNT, PVDF/8 wt %-CNT, and PVDF/5 wt %-CNT/10 wt %-graphene composite films with a 0.1 mm thickness, the EMI values were 22.41, 25.02, and 27.58 dB, respectively. The EMI shielding properties of the PVDF composite films were related to the films’ thicknesses, and the major contribution to the EMI shielding came from the absorption mechanism. The PVDF/5 wt %-CNT film had mean SE values that ranged from 21.58 to 35.37 dB at thicknesses ranging from 0.08 mm to 0.4 mm. The average SET value of the PVDF/CNT/graphene film with a 0.25 mm thickness reached 36.46 dB. The PVDF composite films, in particular, at a 0.1 mm thickness achieved the highest specific SE values (1 310 dB cm2/g for PVDF/5 wt %-CNT, 1 430 dB cm2/g for PVDF/8 wt %-CNT, and 1 57 dB cm2/g for PVDF/CNT/graphene, respectively) in terms of their relatively low CNT loading and ultrathin thickness. These PVDF/carbon composite films can be used as flexible, ultrathin, and high-efficiency electromagnetic screening 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.7b04935.

Figure 11. Schematic EMI shielding mechanisms for (a) PVDF/CNT, (b) PVDF/graphene, and (c) PVDF/CNT/graphene composite films.

dissipated more electrical energy and thus resulted in a higher ohmic loss.

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4. CONCLUSION In summary, the PVDF/carbon (CNT, graphene, or CNT/ graphene) composites were prepared by the distribution of conductive carbon fillers in the PVDF solution. The electrical conductivity and the EMI shielding property of the PVDF/ CNT and PVDF/graphene composites were increased by the increased CNT and graphene concentrations. Compared with the PVDF/CNT and PVDF/graphene composite films, the PVDF/CNT/graphene composite film possessed superior EMI

Plot of ac electrical conductivity as a function of frequency, XRD curves, and Raman profiles (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-416-978-3053. Fax: +1-416-978-0947. ORCID

Chul B. Park: 0000-0002-1702-1268 Notes

The authors declare no competing financial interest. 20881


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(16) Ameli, A.; Kazemi, Y.; Wang, S.; Park, C. B.; Pötschke, P. Process-Microstructure-Electrical Conductivity Relationships in Injection-Molded Polypropylene/Carbon Nanotube Nanocomposite Foams. Composites, Part A 2017, 96, 28−36. (17) Lee, S.-H.; Kang, D.; Oh, I.-K. Multilayered Graphene-Carbon Nanotube-Iron Oxide Three-Dimensional Heterostructure for Flexible Electromagnetic Interference Shielding Film. Carbon 2017, 111, 248− 257. (18) Zhang, K.; Yu, H.-O.; Shi, Y.-D.; Chen, Y.-F.; Zeng, J.-B.; Guo, J.; Wang, B.; Guo, Z.; Wang, M. Morphological Regulation Improved Electrical Conductivity and Electromagnetic Interference Shielding in Poly(L-lactide)/Poly(ε-caprolactone)/Carbon Nanotube Nanocomposites via Constructing Stereocomplex Crystallites. J. Mater. Chem. C 2017, 5 (11), 2807−2817. (19) Ameli, A.; Nofar, M.; Park, C. B.; Pötschke, P.; Rizvi, G. Polypropylene/Carbon Nanotube Nano/Microcellular Structures with High Dielectric Permittivity, Low Dielectric Loss, and Low Percolation Threshold. Carbon 2014, 71, 206−217. (20) Ameli, A.; Wang, S.; Kazemi, Y.; Park, C. B.; Pötschke, P. A Facile Method to Increase the Charge Storage Capability of Polymer Nanocomposites. Nano Energy 2015, 15, 54−65. (21) Li, N.; Huang, Y.; Du, F.; He, X.; Lin, X.; Gao, H.; Ma, Y.; Li, F.; Chen, Y.; Eklund, P. C. Electromagnetic Interference (EMI) Shielding of Single-Walled Carbon Nanotube Epoxy Composites. Nano Lett. 2006, 6 (6), 1141−1145. (22) Yousefi, N.; Sun, X.; Lin, X.; Shen, X.; Jia, J.; Zhang, B.; Tang, B.; Chan, M.; Kim, J.-K. Highly Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for HighPerformance Electromagnetic Interference Shielding. Adv. Mater. 2014, 26 (31), 5480−5487. (23) Deng, H.; Skipa, T.; Bilotti, E.; Zhang, R.; Lellinger, D.; Mezzo, L.; Fu, Q.; Alig, I.; Peijs, T. Preparation of High-Performance Conductive Polymer Fibers through Morphological Control of Networks Formed by Nanofillers. Adv. Funct. Mater. 2010, 20 (9), 1424−1432. (24) Cao, M.-S.; Wang, X.-X.; Cao, W.-Q.; Yuan, J. Ultrathin Graphene: Electrical Properties and Highly Efficient Electromagnetic Interference Shielding. J. Mater. Chem. C 2015, 3 (26), 6589−6599. (25) Zeng, Z.; Jin, H.; Chen, M.; Li, W.; Zhou, L.; Zhang, Z. Lightweight and Anisotropic Porous MWCNT/WPU Composites for Ultrahigh Performance Electromagnetic Interference Shielding. Adv. Funct. Mater. 2016, 26 (2), 303−310. (26) Ling, J.; Zhai, W.; Feng, W.; Shen, B.; Zhang, J.; Zheng, W. g. Facile Preparation of Lightweight Microcellular Polyetherimide/ Graphene Composite Foams for Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2013, 5 (7), 2677−2684. (27) Al-Saleh, M. H.; Sundararaj, U. Electromagnetic Interference Shielding Mechanisms of CNT/Polymer Composites. Carbon 2009, 47 (7), 1738−1746. (28) Das, N. C.; Liu, Y.; Yang, K.; Peng, W.; Maiti, S.; Wang, H. Single-Walled Carbon Nanotube/Poly(methyl methacrylate) Composites for Electromagnetic Interference Shielding. Polym. Eng. Sci. 2009, 49 (8), 1627−1634. (29) Chen, Y.; Zhang, H.-B.; Yang, Y.; Wang, M.; Cao, A.; Yu, Z.-Z. High-Performance Epoxy Nanocomposites Reinforced with ThreeDimensional Carbon Nanotube Sponge for Electromagnetic Interference Shielding. Adv. Funct. Mater. 2016, 26 (3), 447−455. (30) Zeng, Z.; Chen, M.; Jin, H.; Li, W.; Xue, X.; Zhou, L.; Pei, Y.; Zhang, H.; Zhang, Z. Thin and Flexible Multi-walled Carbon Nanotube/Waterborne Polyurethane Composites with High-performance Electromagnetic Interference Shielding. Carbon 2016, 96, 768− 777. (31) Shen, B.; Li, Y.; Zhai, W.; Zheng, W. Compressible GrapheneCoated Polymer Foams with Ultralow Density for Adjustable Electromagnetic Interference (EMI) Shielding. ACS Appl. Mater. Interfaces 2016, 8 (12), 8050−8057. (32) Li, C.; Yang, G.; Deng, H.; Wang, K.; Zhang, Q.; Chen, F.; Fu, Q. The Preparation and Properties of Polystyrene/Functionalized

ACKNOWLEDGMENTS The authors gratefully acknowledge Solvay’s donation of PVDF, NanoXplore’s donation of graphene nanoplatelets, and the financial support of the Consortium for Cellular and Microcellular Plastics (CCMCP).

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