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Segregated Hybrid Poly(methyl methacrylate)/Graphene/Magnetite Nanocomposites for Electromagnetic Interference Shielding Farbod Sharif, Mohammad Arjmand, Aref Abbasi Moud, Uttandaraman Sundararaj,* and Edward P. L. Roberts* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 S Supporting Information *

ABSTRACT: Nanocomposites of poly(methyl methacrylate)/reduced graphene oxide (PMMA/rGO) without and with decorated magnetite nanoparticles with a segregated structure were prepared using emulsifier-free emulsion polymerization. Various characterization techniques were employed to validate the presence of the nanofillers and the formation of the segregated structure within the nanocomposites. The percolation threshold of the nanocomposites was found to be 0.3 vol %, while a maximum electrical conductivity of 91.2 S·m−1 and electromagnetic interference shielding effectiveness (EMI SE) of 63.2 dB (2.9 mm thickness) were achieved for the PMMA/rGO nanocomposites at a loading of 2.6 vol % rGO. It was also observed that decorating rGO with magnetite nanoparticles (hybrid nanocomposites) led to a tremendous increase in EMI SE. For instance, 1.1 vol % PMMA/rGO nanocomposites indicated an EMI SE of 20.7 dB, while adding 0.5 vol % magnetite nanoparticles enhanced EMI SE to 29.3 dB. The excellent electrical properties obtained for these nanocomposites were ascribed to both superiorities of the segregated conductive structure and magnetic properties of the magnetite nanoparticles. KEYWORDS: electromagnetic interference shielding, segregated structure, magnetite nanoparticle, reduced graphene oxide, hybrid nanocomposite

1. INTRODUCTION Nowadays, electronics has set foot in very high-tech applications, such as flexible electronic circuits and transparent electrical conductors, and found a broad spectrum of employments in various scientific fields.1,2 Electromagnetic (EM) waves from electronic devices have the potential to harm people’s health or disturb the operation of other electronic devices.3 In this regard, the proliferating market of electronics has driven the demand for new, efficient electromagnetic interference (EMI) shields. The performance of such shields is typically characterized using the EMI shielding effectiveness (SE) to evaluate the amount that the EM waves are diminished by the shield. The EMI SE in decibels (dB) is defined as follows EMI SE = 20· log10(E I /E T) = 20·log10(HI/HT)

where PI is the incident power, and PT is the transmitted power. A SE value of at least 20 dB is typically required for a shield to be commercially applicable in electronic devices.4,5 In recent years, application of nanocomposites of conductive fillers and polymers (CPNs) for EMI shielding application has attracted interest owing to their attractive characteristics, including variable electrical conductivity, low density, low cost, corrosion resistance, and processability.6−8 Among the range of nanofillers used in CPNs,9−14 graphene has drawn a great deal of interest owing to its large specific surface area, layered structure, high conductivity, and flexibility.15,16 Liang et al. recorded the first study on using a graphenebased nanocomposite as an EMI shield, recording a SE of 21 dB for a 15 wt % loading of graphene in an epoxy matrix.15 Since then, many studies have been devoted to CPNs holding graphene or reduced graphene oxide (rGO) for EMI shielding applications.17,18 Nevertheless, it has been observed that satisfactory EMI shielding always accompanies high loading of graphene, increased cost, and deteriorated mechanical properties of nanocomposites. In order to obtain a conductive CPN,

(1)

where EI and ET are the root-mean-square of incident and transmitted electric fields, respectively, and HI and HT are the root-mean-square of incident and transmitted magnetic fields, respectively. Considering that the power is proportional to the square of amplitude of an electric field or a magnetic field leads to EMI SE = 10·log10(PI/PT) © 2017 American Chemical Society

Received: November 1, 2016 Accepted: April 4, 2017 Published: April 4, 2017

(2) 14171

DOI: 10.1021/acsami.6b13986 ACS Appl. Mater. Interfaces 2017, 9, 14171−14179

Research Article

ACS Applied Materials & Interfaces graphene sheets link together through plane-to-plane or edgeto-edge contacts. However, the two-dimensionality of graphene sheets makes their interlacing difficult within the nanocomposites compared to one-dimensional fillers like carbon nanotubes, thereby hampering the formation of conductive network.19−22 Therefore, preparing graphene/polymer nanocomposites with an acceptable EMI shielding at low filler loadings has still remained as a challenge. It is well-known that formation of a well-established conductive network is of prime significance to achieve improved EMI shielding. That is, CPNs with an enhanced conductive network present higher EMI shielding.9,10 A segregated microstructure is one of the proposed techniques to improve the dispersion state of nanofillers and thus to enhance the level of conductive network formation in CPNs.23−25 In this kind of structure, nanofillers tend to be distributed at the surface of polymer microspheres, where they contact each other, resembling a cell-like configuration. Most of the research studies on segregated CPNs have targeted electrical conductivity as the final property; however, there are a few studies employing CPNs with a segregated structure toward EMI shielding applications. Gelves et al. developed a segregated structure for copper nanowire/ polystyrene nanocomposites and were successful in obtaining an EMI SE of 26 and 42 dB at 10 and 13 wt %, respectively.25 In another study, using a thermal reduction technique, a novel reduced graphene oxide/polyethylene nanocomposite was developed by Yan et al.26 The developed nanocomposite had a segregated architecture, and an EMI SE of 28.3−32.4 dB was obtained at a graphene loading of 0.66 vol %. In all these studies, the authors claimed that their results are superior to the literature, where they attributed that to the formation of segregated conductive network. Besides constituting a well-formed conductive network, incorporation of magnetic nanofiller into CPNs can also improve their EMI shielding performance. According to Poynting’s theorem,27 the directional energy flux density of an EM wave is defined as the cross product of the vector associated with an electric field and the vector associated with a magnetic field. Hence, efficient shielding of the EM wave requires attenuation of both the electric field and the magnetic field. Carbonaceous fillers are nonmagnetic and contribute to EM wave absorption mostly because of their conductive nature; accordingly, decorating their structure with magnetic nanoparticles can improve their efficiency for EMI shielding much further. Therefore, in recent years, many studies have been devoted to hybrid systems of conductive fillers and magnetic fillers to achieve enhanced EMI shielding.28,29 For example, Che et al.30 reported that encapsulation of iron in carbon nanoshells increased the microwave absorption, mainly ascribed to the magnetic interactions. By the same rationale, researchers have utilized graphene in combination with iron oxide to further improve EMI shielding efficacy. For instance, Xu et al.31 investigated the EMI shielding of bowl-like iron oxide hollow spheres/rGO nanocomposites, revealing a maximum EM absorption of 24 dB at 30 wt % as-synthesized nanofillers. Sun et al.9 developed a simple solvothermal method for synthesizing laminated magnetic rGO (decoration of Fe3O4 nanoparticles on the surface of rGO). The results demonstrated that there was a considerable improvement in the EMI shielding performance of magnetic graphene when compared with pure graphene.9

In this study, we introduce a segregated hybrid poly(methyl methacrylate)/reduced graphene oxide/Fe3O4) (PMMA/rGO/ magnetite) nanocomposite to benefit from both superiorities of the segregated structure of rGO and magnetic properties of magnetite. Our study is unique compared to the literature as we employed both a segregated structure and a hybrid filler system to enhance EMI shielding. The developed segregated nanocomposites exhibited a remarkable EMI shielding performance, as shown in Table 2, endorsing their high competence.

2. MATERIALS AND METHODS 2.1. Nanofillers and Nanocomposites Preparation. Preparation of GO. GO was prepared using natural graphite with the modified Hummers method. A full description of the method can be found in our previous study.32 Preparation of Positively Charged PMMA Latex. PMMA was selected as the polymer matrix because of its growing demand in the electronics industry, optical features, weathering resistance, high transparency, and chemical resistance. PMMA latex with a positive charge was prepared via an emulsifier-free emulsion polymerization method. A full description of the method can be found in the study by Pham et al.33 Preparation of the PMMA/rGO Nanocomposite. Thirty milliliters of GO dispersion with different concentrations was slowly added to 50 g of 10 wt % PMMA aqueous solution under vigorous stirring to prepare PMMA/rGO nanocomposites with a wide spectrum of loadings, i.e. 0, 0.1, 0.2, 0.3, 0.6, 1.1, 1.6, and 2.6 vol %. The mass ratio was converted to volume ratio using 1.18 and 2.20 g cm−3 as the density of PMMA and rGO, respectively.33 The dispersion of the PMMA/GO nanocomposite was reduced in the presence of 3 mL of hydrazine (Alfa Aesar, 98%) at 80 °C for 3 h. The such-prepared dispersion of the PMMA/rGO nanocomposite was filtered, washed with DI water, and then dried in a vacuum oven at 80 °C for 24 h. The obtained material was ground by pestle and mortar. Preparation of the PMMA/rGO/Magnetite Nanocomposite. Similar to the PMMA/rGO nanocomposite, 30 mL of GO dispersion was mixed with PMMA latex. A known mass of FeCl3 (Alfa Aesar, 98%) and FeCl2 (Ward’s Science, 97%), with a mole ratio of 2:1, dissolved in 20 mL of DI water was slowly mixed with the as-prepared dispersion. Five mL of DI water containing 0.5 mL of NH3 (EMD Millipore, ACS grade) was mixed with the as-prepared dispersion and stirred for 2 h. The dispersion was reduced in the presence of hydrazine at 80 °C for 3 h to obtain the PMMA/rGO/magnetite nanocomposite. The nanocomposite was filtered and washed with DI water several times and then dried in a vacuum oven at 80 °C for 24 h. Dried material was ground using pestle and mortar. The hybrid nanocomposites were developed for the PMMA/rGO nanocomposites with 0.3 and 1.1 vol % rGO loadings with 0.2 and 0.5 vol % decorated magnetite nanoparticles (NPs) on the surface of rGO sheets. Figure 1 depicts a schematic elucidating the preparation procedure of PMMA/ rGO/magnetite nanocomposites.

Figure 1. Schematic depicting preparation procedure of the PMMA/ rGO/magnetite nanocomposite. 14172


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Figure 2. (a) Raman spectra of PMMA/rGO and PMMA/GO nanocomposites, (b) XRD patterns of rGO/magnetite and PMMA/rGO/magnetite nanocomposites, and (c) TGA plots of rGO and GO. Nanocomposites Molding. Nanocomposites were molded into rectangular cavities, 22.9 × 10.2 × 2.9 mm3, with a Carver compression molder (Carver Inc., Wabash, IN). The molding temperature, pressure, and time were 220 °C, 38 MPa, and 10 min, respectively. 2.2. Nanofillers and Nanocomposites Characterization. Raman Spectroscopy, X-ray Diffraction, and Thermogravimetric Analysis. The Raman spectra of nanofillers were obtained using a WITec alpha 300 R Confocal Raman microscope (WITec GmbH, Germany). Measurement conditions can be found in our previous paper.32 The X-ray diffraction (XRD) analysis on the nanofillers was carried out via a Rigaku ULTIMA III X-ray diffractometer. The thermal stability of GO and rGO was examined with a Thermogravimetric Analyzer (TA Instruments, Model: Q500). Scanning and Transmission Electron Microscopy. High-resolution scanning and transmission electron microscopies (HRSEM and HRTEM) were utilized to study the morphology of PMMA latex and the dispersion state of the nanofillers within the PMMA matrix. SEM was performed using a Philips XL30 setup. HRTEM was obtained on a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) at 200 kV acceleration voltage with a standard single-tilt holder. Electrical Conductivity and EMI Shielding. Electrical conductivity of the nanocomposites was measured using two different conductivity meters. A Loresta GP resistivity meter (MCP-T610 Model, Mitsubishi Chemical Co., Japan) was used for conductive samples, and a Keithley 6517A electrometer connected with a Keithley 8009 test fixture (Keithley Instruments, USA) was employed for insulative samples. Shielding properties of the nanocomposites were measured via an E5071C network analyzer (ENA series 300 kHz−20 GHz). The scattering parameters obtained from the shielding setup were used to obtain the permittivity and permeability of the nanocomposites using the Reflection/Transmission Mu and Epsilon Nicolson-Ross model. More information about TEM and SEM imaging, TGA, and electrical conductivity and EMI shielding measurements can be found in our previous studies.6,14

nanocomposites, crucial to enhance the conductivity and consequently EMI shielding. In the Raman spectrum of a carbonaceous material, the relative motion of sp2 carbon atoms is illustrated by the G-band, and the disorders in the graphitic structure are represented by the D-band.34 The degree of disorder and average size of the sp2 domains are related to the D-band to G-band intensity ratio. Comparison of the Raman spectra of PMMA/GO and PMMA/rGO nanocomposites, shown in Figure 2a, indicates an increase in the D-band to Gband intensity ratio after reduction of GO in the presence of hydrazine. This suggests formation of new graphitic domains, leading to an increase in the sp2 cluster number and reduction of the average size of the sp2 domains. Furthermore, following the reduction of GO, there are changes in D-band and G-band locations from 1355 and 1591 cm−1 to 1348 and 1586 cm−1, respectively; this implies the formation of rGO within the nanocomposite, which is consistent with the literature.35 Figure 2b shows the XRD patterns of rGO/magnetite and PMMA/rGO/magnetite nanocomposites. In the XRD pattern, the formation and presence of magnetite nanoparticles, beneficial for EMI shielding, can be validated. Discernible peaks at about 30°, 35°, 43°, 53°, 57°, and 62° are indexed to (220), (311), (400), (422), (511), and (440) planes of magnetite, respectively, corresponding to the cubic inverse spinel structure of magnetite (JCPDS Card No. 19-0629).36,37 Hence, we can claim that magnetite nanoparticles were successfully synthesized and incorporated within the nanocomposites. Thermogravimetric analysis (TGA) of GO and rGO can provide useful information about the presence of oxygen functional groups and thermal stability of the synthesized nanofillers.38 Since GO is an oxidized form of graphene synthesized by the mild sonication of graphite oxide, its surface is decorated with a large amount of oxygen functional groups.39 Therefore, these sites are highly prone to decomposition at

3. RESULTS AND DISCUSSION 3.1. Raman Spectroscopy, XRD, and Thermogravimetric Analysis. Raman spectroscopy is a powerful tool to confirm the conversion of GO to rGO in the generated 14173


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sheets do not span the entire nanocomposite, leading to an imperfect segregated structure and conductive network. Nevertheless, by increasing the concentration of rGO within the nanocomposite, a perfect, uniform, continuous segregated structure was formed, resulting in formation of well-established conductive electron pathways. It is also interesting to note that PMMA particles kept their original structure within the nanocomposite, having a particle size around 250 nm. As depicted in the TEM image of the PMMA/rGO/ magnetite nanocomposite in Figure 4c, due to the electrostatic interaction between the positively charged iron ions (Fe2+ and Fe3+) and the negatively charged rGO sheets, magnetite NPs tend to grow on the surface of GO sheets. Despite the in situ growth of iron oxide nanoparticles onto PMMA/rGO assembly, the gradual buildup of iron ions on the surface of graphene keeps PMMA/rGO segregated structure intact. During the reduction of PMMA/GO nanocomposites, the oxygen functional groups of GO are removed, and the conjugated structure is restored. This accompanies an increase in the hydrophobicity of the graphene sheets and formation of π−π stacking sites,44,45 resulting in strong bindings between the overlapped rGO sheets.46 Therefore, it can be said that the conductive interconnected structure of graphene is constituted owing to the combined effect of both hydrophobicity and π−π interactions.47,48 This interconnectivity of graphene sheets in such-prepared nanocomposites signifies its superiority to other methods such as solution mixing49 or melt mixing.50 3.3. Electrical Conductivity of PMMA/rGO Nanocomposites. In CPNs, electrical conductivity increases in a nonlinear trend beyond the percolation threshold. Evolution of conductive paths at and above the percolation threshold improves electron transference inside the nanocomposite. In highly conductive CPNs, conduction happens through direct contact between nanofillers coupled with tunneling and hopping mechanisms.51,52 The percolation curve of the PMMA/rGO nanocomposite is plotted in Figure 5. According to the percolation theory, electrical conductivity after the percolation threshold follows a power law equation:53

elevated temperatures in the presence of air. Upon thermolysis of GO its main mass loss occurs at around 200 °C, probably due to eliminating the unstable oxygen functional groups on the basal planes and the edges of the GO.40,41 Reduction of GO in the presence of hydrazine leads to elimination of unstable oxygen functional groups and enhances the stability of rGO. Hence, rGO did not show any significant mass loss in the investigated temperature spectrum, indicating successful reduction and restoration of the conjugated structure of rGO. 3.2. Electron Microscopy of Segregated Nanocomposites. Inherent conductivity of the nanofillers and their dispersion within the polymer matrix are among the most important factors impacting the EMI shielding performance of the nanocomposites.42,43 Accordingly, we employed different microscopy techniques (SEM and TEM) to investigate the dispersion of rGO and rGO/magnetite within the PMMA matrix. As disclosed by the SEM image shown in Figure 3a, spherical PMMA particles were uniformly synthesized with a particle size

Figure 3. SEM micrographs of (a) PMMA latex and (b) the PMMA/ rGO nanocomposite.

of about 250 nm. These particles were employed for the nanocomposite preparation using electrostatic interactions between positively charged PMMA particles and negatively charged GO. Figure 3b exhibits that rGO sheets were effectively self-assembled on PMMA particles. The rGO sheets were evenly distributed between PMMA particles, suggesting formation of a highly conductive interconnected network of rGO through the PMMA matrix. This led to superior exploitation of the potential surface area of graphene sheets in the polymer nanocomposites, a necessity to achieve high EMI shielding performance. TEM images shown in Figure 4 further endorse the formation of a segregated structure within the generated

σ = σ0(v − vc)t

(3)

Figure 5. Electrical conductivity of PMMA/rGO nanocomposites as a function of rGO content. Figure 4. TEM image of (a) PMMA/rGO (0.3 vol %), (b) PMMA/ rGO (1.1 vol %), and (c) PMMA/rGO/magnetite (1.1/0.5 vol %) nanocomposites.

Through this equation, electrical conductivity of the nanocomposite (σ) is related to scaling factor (σ0), nanofiller volume fraction (v), electrical percolation threshold volume fraction (vc), and critical exponent (t). The percolation threshold and critical exponent obtained were 0.3 vol % and 1.91, respectively, through linear regression fitting of Log (σ) against Log (v−vc) by maximizing the correlation coefficient R2.

nanocomposites. Comparing the TEM images of 0.3 vol % and 1.1 vol % PMMA/rGO nanocomposites elaborates vividly the relationship between rGO concentration and developed conductive network. As shown in the TEM images, when the concentration of rGO in the nanocomposite is low, the rGO 14174


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Table 1. Comparison of the Percolation Threshold and Maximum Electrical Conductivity of Current Work with the Literature

a

polymer matrix

nanofiller

percolation threshold (vol %)

nanofiller loading (vol %)

σmax (S·m−1)

ref

PMMA PSa PS PS PETb PMMA

rGO graphene graphene graphene graphene rGO

0.16 0.10 0.15 0.10 0.47 0.30

2.7 2.5 4.8 4.19 3 2.6

64 1 1083 13.84 2.1 91.2

Pham et al.33 Stankovic et al.54 Wu et al.55 Liu et al.56 Zhang et al.57 this work

Polystyrene. bPolyethylene terephthalate.

thickness of l, shielding by absorption in SI units is defined as the following:

The developed nanocomposites also showed a maximum electrical conductivity of 91.2 S·m−1 at 2.6 vol % rGO loading. Such a low percolation threshold obtained in this study is believed to be due to the very high aspect ratio of the rGO sheets and their exfoliation and assembly in the segregated microstructure. Moreover, high maximum conductivity is attributable to π−π stacking sites, creating strong bindings between the overlapped rGO sheets. Comparing the percolation threshold of this study with the literature (Table 1) confirms the efficacy of the processing method employed in this study. 3.4. EMI Shielding of PMMA/rGO Nanocomposites. Intrinsic impedance, the ratio of the electric field to the magnetic field in a propagating wave, is defined as58

η=

jωμ σ + jωβ

l

SEA = 20·log(e δ ) = 131l fμσ r r

The equation above elucidates that a material with high conductivity and permeability can absorb an EM wave efficiently. In addition, there is a linear relationship between shielding by absorption and thickness of a shield. As a matter of fact, the higher the thickness of a conductive shield, the greater is the amount of interacting free charge carriers and/or electric/ magnetic dipoles. Figure 6 depicts the average EMI SE of the nanocomposite with respect to graphene loading over the X-band regime. It can

(4)

where η is intrinsic impedance, σ is electrical conductivity, ω is angular frequency, and μ and ε are permeability and permittivity, respectively. The permittivity and permeability of free space are equal to 8.85 × 10−12 F·m−1 and 4π × 10−7 H· m−1, respectively. Given the low conductivity of free space, its intrinsic impedance is equal to 377 Ω. When an incident EM wave infiltrates into a conductive shield, the intrinsic impedance decreases remarkably; thus, the electric field transforms partially to the magnetic field. The level of transformation is proportionate to the level of impedance mismatch between two media. Hence, in order to provide a desirable shielding, both electric and magnetic fields must be attenuated efficiently.59 Shielding of an EM wave occurs chiefly by reflection and absorption mechanisms. In a conductive monolithic material, shielding by reflection in SI units is defined as58,59 SE R = 168 − 10· log

(6)

Figure 6. Shielding by reflection, shielding by absorption, and overall shielding of the PMMA/rGO nanocomposite as a function of rGO content. The thickness of the samples was 2.9 mm.

be observed that shielding by absorption made by far the major contribution to EMI shielding, particularly at high nanofiller contents. For instance, at 0.3 vol %, where the nanocomposite was at the percolation threshold, shieldings by reflection and absorption were 1.8 and 6.5 dB, respectively (overall EMI SE equal to 8.3 dB). At 2.6 vol %, where the nanocomposite was well above the percolation threshold, shieldings by reflection and absorption were 6.1 and 57.1 dB, respectively (overall SE equal to 63.2 dB). That is, at 2.6 vol %, shielding by absorption was over 9 times more than shielding by reflection. Shielding by absorption in nonmagnetic CPNs mainly arises from ohmic loss (dissipation of energy by free charge carriers moving in phase with the applied EM wave) and polarization loss (dissipation of energy by electric dipoles reorienting out of phase with respect to the applied EM wave).42,58 The energy dissipation by ohmic loss and polarization loss is represented by imaginary permittivity and real permittivity, respectively. It is also well-known that an intricate network of conductive filler inside the CPNs results in higher EMI shielding due to elongated mean free paths for free charges carriers to move through the conductive network in each half of an electromagnetic wave cycle.27 Both high energy dissipation and enhanced conductive network formation are signatures of highly conductive CPNs. The permittivity (complex value) was

μr f σr

(5)

where SER is shielding by reflection, μr is the permeability of the conductive shield relative to that of free space, f is frequency, and σr is the conductivity of the shield relative to the conductivity of copper. According to the equation above, the reflection loss is proportional to σr , i.e. the materials with higher μr

conductivity present higher reflection loss, while the magnetic materials deteriorate the reflection loss. Absorption attenuates an EM wave through interaction with free charge carriers and/or electric/magnetic dipoles.58 The amplitude of an EM wave infiltrating through a conductive z material is attenuated by the factor e(− δ ), where z is the distance inside the conductive shield, and δ is the skin depth of the conductive shield. Therefore, for a conductive shield with a 14175


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indicated an EMI SE of 20.75 dB, while adding 0.5 vol % magnetite NPs to this nanocomposite led to an EMI SE of 29.3 dB. Table 2 presents a comparison of EMI shielding results obtained in the current study with the literature, denoting high performance of the developed CPNs. We believe that high electrical conductivity and geometrical configuration of graphene sheets in the segregated structure coupled with magnetic loss coming from magnetite NPs caused high EMI shielding in the hybrid nanocomposites. Adding magnetite NPs to the PMMA/rGO nanocomposites did not change the permittivity significantly (see the Supporting Information). This can be attributed to the inferior surface area and electrical conductivity of magnetite NPs compared to rGO sheets. Nonetheless, magnetite NPs increased the permeability of PMMA/rGO nanocomposites, thereby enhancing magnetic absorption. Furthermore, it has been reported that the cell-like configuration formed with dense, highly conductive rGO layers could entrap incoming EM waves.26,28,64 The entrapped EM waves continue to bounce off the rGO cell walls until they are fully absorbed and quenched (see Figure 8). This is due to high conductivity of rGO sheets and high permeability of magnetite NPs, where the nanofillers can attenuate the EM wave in each cell via multiple-reflection.

found to increase significantly with rGO content, whereas permeability stayed constant (see the Supporting Information). Thus, an outstanding EMI shielding performance observed at high rGO loadings can be ascribed to both the rGO content (free charge carriers and electric dipoles) and the conductive network (segregated structure and formation of π−π stacking sites between overlapped rGO sheets). Furthermore, it was observed that at the percolation threshold shielding by absorption was high. Referring to the TEM image depicted in Figure 4a, it can be perceived that at low rGO contents localized networks were formed within the nanocomposite; however, they did not span well across the entire nanocomposite. We believe that these localized networks brought about high shielding by absorption around the percolation threshold. 3.5. EMI Shielding of Hybrid PMMA/rGO/Magnetite Nanocomposites. As mentioned before, absorption loss in conductive monolithic materials is proportional to electrical conductivity times the permeability;59−61 hence, the presence of both conductive and magnetic fillers results in enhanced attenuation of an EM wave. In fact, the presence of magnetic entities increases total shielding of CPNs through interactions of the magnetic field with natural resonance, exchange resonance, and eddy currents.62,63 Therefore, in this study, magnetite NPs were decorated on rGO sheets, as verified by XRD and TEM. The hybrid nanocomposites were developed by decorating 0.2 and 0.5 vol % magnetite NPs on the surface of rGO sheets for the PMMA/rGO nanocomposites with 0.3 and 1.1 vol % rGO loadings (Figure 7). It was observed that

Figure 7. Shielding by reflection, absorption, and total shielding of hybrid PMMA/rGO/magnetite nanocomposites. The thickness of the samples was 2.9 mm.

Figure 8. Synergy of rGO segregated structure and magnetite NPs to attenuate the incident EM wave.

adding magnetite NPs to PMMA/rGO nanocomposites improved EMI SE via increasing shielding by absorption, whereas shielding by reflection remained almost constant. For instance, the PMMA/rGO (1.1 vol %) nanocomposite

4. CONCLUSIONS Nanocomposites of poly(methyl methacrylate)/reduced graphene oxide (PMMA/rGO) without and with decorated magnetite nanoparticles were developed via the self-assembly

Table 2. Comparison of EMI SE of Nanocomposites in This Work with the Literature

a

polymer matrix

nanofiller

total filler concn

thickness (mm)

EMI SE (dB)

ref

PDMSa epoxy PEIb PUc PMMA PSd PEI PS PMMA PMMA

graphene graphene graphene graphene graphene graphene graphene:Fe3O4 graphene graphene graphene:magnetite

0.8 wt % 8.8 vol % 10 wt % 5.0 vol % 1.8 vol % 30 wt % 5 wt % 3.47 vol % 2.6 vol % (1.1:0.5 vol %)

1.0 1.0 2.3 2.0 2.4 2.5 2.5 2.5 2.9 2.9

20 21 19.7 32 19 29.3 6.5−9.2 45.1 63.2 29

65 14 66 17 67 68 28 69 this work

Polydimethylsiloxane. bPoly(ether imide). cPolyurethane. dPolystyrene. 14176


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technique, followed by compression molding. The developed nanocomposites showed a segregated 3D architecture, kept intact even following the incorporation of magnetite nanoparticles. The segregated structure helped the nanocomposites gain a very low percolation threshold and high electrical conductivity and electromagnetic interference (EMI) shielding. The EMI shielding performance was immensely improved by decorating the surface of rGO sheets with magnetite nanoparticles (hybrid nanocomposites). The outstanding EMI shielding performance of the hybrid nanocomposites was ascribed to a well-established conductive network (segregated structure and formation of π−π stacking sites between overlapped rGO sheets), magnetic attenuation of magnetite nanoparticles, and cell-like configuration formed with dense highly conductive rGO layers surrounding.

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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.6b13986. Data on the permeability and permittivity of nanocomposite materials in the X-band frequency range, including data for the PMMA/rGO nanocomposites at a range of rGO loadings and for the hybrid PMMA/rGO/ magnetite nanocomposites with a range of rGO and magnetite loadings (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +1 403 220-4466. E-mail: [email protected] (E.P.L.R.). *Phone: +1 403 210-6549. E-mail: [email protected] (U.S.). ORCID

Mohammad Arjmand: 0000-0002-8812-5638 Uttandaraman Sundararaj: 0000-0003-4124-3917 Edward P. L. Roberts: 0000-0003-2634-0647 Author Contributions

F.S. and M.A. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI) is highly appreciated. We are grateful to Dr. Ahmed Abouelmagd for carrying out the latex synthesis and to Ms. Samaneh Dordani Haghighi for designing and drawing the schematics.

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REFERENCES

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