Facile and Green Method To Structure Ultralow ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2019, 7, 9904−9915

Facile and Green Method To Structure Ultralow-Threshold and Lightweight Polystyrene/MWCNT Composites with Segregated Conductive Networks for Efficient Electromagnetic Interference Shielding Jia Chen, Xia Liao,* Wei Xiao, Jianming Yang, Qiuyue Jiang, and Guangxian Li

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College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, Sichuan China S Supporting Information *

ABSTRACT: Porous conductive polymer composites (CPCs) with lower density and high performance are more favorable for practical electromagnetic interference (EMI) shielding application. In this work, a judicious combination of high-speed mechanical mixing and supercritical carbon dioxide (scCO2) foaming was applied to prepare lightweight polystyrene (PS)/multiwall carbon nanotube (MWCNT) composites with segregated conductive networks. The composite foam presented an ultralow percolation threshold of 0.07 vol % and exhibited excellent electrical conductivity of 8.05 S/m and EMI shielding effectiveness of 23.2 dB, exceeding the requirement of EMI shielding materials in commercial application, with density of 0.47 g/cm3 at such a low thickness of 1.8 mm and 1.88 vol % MWCNT loading. In addition, the EMI shielding mechanism was further clarified and the results showed that absorption was the primary EMI shielding mechanism for such conductive porous composites in the frequency range of 8.2−12.4 GHz. Meanwhile, the effect of foaming on the EMI shielding mechanism was also investigated. Results indicated that the electromagnetic absorption was increased from 90.5% to 95.9% at the same MWCNT content of 7 wt %. Consequently, this material with high performance and its facile, versatile, green fabrication method provide a novel idea for preparing lightweight EMI shielding materials. KEYWORDS: Segregated structure, Lightweight, Supercritical carbon dioxide, Conductive polymer composites, Electromagnetic interference shielding

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INTRODUCTION Along with the sustained advance in technology, applications of electronic and electrical equipment in military, communication and civil fields are growing swiftly. Despite so much convenience such equipment brings, at the same time they emit electromagnetic waves with different frequencies and ineluctably leads to serious contamination, which is awfully detrimental to human health and normal operation of electronic devices.1−3 The EMI shielding materials could surmount mentioned barriers by reflecting or absorbing electromagnetic waves and draw great attention in both academic and industrial domains.4,5 Compared to conventional metal-based materials, conductive polymer composites (CPCs) have become particularly popular and have exerted enormous potential in EMI shielding application owing to their advantages of low density, easy processing, corrosion resistance, low cost and tunable mechanical and functional property.6−8 In order to construct satisfactory interconnected conductive networks and meet the demand of EMI shielding materials in commercial application (≥20 dB), a great number of conductive fillers are necessary but this in turn contributes to © 2019 American Chemical Society

higher density, poorer processability, and mechanical property.9−11 Currently, manufacturing lightweight CPCs with high performance remains a huge challenge and is urgently required for practical application. Nowadays, many researchers are committing themselves to improving the electrical conductivity and EMI shielding performance of CPCs by elaborating structural design. The formation of a segregated structure with major conductive fillers selectively distributing at the interface of polymer particles rather than dispersing uniformly in polymer matrix is one of the effective methods to increase electrical conductivity and EMI shielding effectiveness.11−13 Jia et al. reported the effects of diverse conductive networks in carbon nanotube/polyethylene (CNT/PE) composites on electrical conductivity and EMI shielding performance.14 Segregated structure (s-CNT/PE), partially segregated structure (p-CNT/PE), and randomly Received: February 1, 2019 Revised: April 24, 2019 Published: May 13, 2019 9904

DOI: 10.1021/acssuschemeng.9b00678 ACS Sustainable Chem. Eng. 2019, 7, 9904−9915

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of the fabrication procedure of the porous PS/MWCNT composites with segregated structure.

reducing percolation threshold and improving electrical conductivity, specific EMI SE and the absorption of electromagnetic interference shielding via scCO2 foaming process due to the redistribution and interconnection of conductive fillers caused by cell growth.31 Zhang et al. prepared microcellular epoxy/multiwall carbon nanotube (EP/MWCNT) composite foams by a batch foaming process with scCO2 and investigated the electromagnetic interference shielding performances of composite foams. The results showed that the absorption was increased from 59.5% for solid composites to 79.5% for composite foams at 3 wt % MWCNT content.32 Other CPC foams consisting of different polymer matrixes and conductive fillers prepared by scCO2 foaming have also been intensively investigated and similar results that the introduction of air into solid composites could no doubt increase the absorption of electromagnetic interference shielding were achieved.29 In our previous study,33 scCO2 foaming was applied to enhance the electrical conductivity of PS/graphene composites. It was found that the electrical conductivity of composite foams could be controlled by adjusting CO2 pressure and temperature and significantly increased under proper conditions. Nevertheless, the resultant composite foam still failed to exhibit superior electrical conductivity (10−2 S/m at 1.8 vol % graphene loading). Consequently, the bubbles-induced “segregated structure” with conductive fillers concentrating in cell walls or struts to produce an electron transport path is usually distributed irregularly and is not perfect; therefore, we usually have no proof method but to conjecture the microstructure by the variation of electrical conductivity. To sum up, it is reasonably envisaged that CPCs with porous segregated structure are a fairly promising option for the design of lightweight EMI shielding materials with higher absorption. In the current study, we first put forward a facile, environmentally friendly and versatile method which combines highspeed mechanical mixing and scCO2 foaming to prepare lightweight, density-tunable, and highly conductive polymer composite foams with efficient EMI shielding effectiveness. The whole preparation process completely avoids the employment of organic solvents and could be applicable to most thermoplastic polymers, escaping the dilemma that increasing the EMI shielding effectiveness detrimentally compromises the environmental friendliness. In order to compare with our previous work and highlight the advantage of composite foam with segregated conductive network, PS as a commonly used polymer matrix and

distributed structure (r-CNT/PE) were all fabricated for comparison and the results showed that the electrical conductivity of s-CNT/PE composite exhibited 2 orders of magnitude higher than that of p-CNT/PE and r-CNT/PE composites at the same CNT content. In addition, s-CNT/PE composite at 5 wt % CNT loading realized an excellent EMI SE as high as 46.4 dB, which was 20% and 46% higher than that for p-CNT/PE and r-CNT/PE composites, respectively. Zhao et al. first put forward that high-speed mechanical mixing could be an available, simple, low-cost, and less time-consuming approach to embedding CNTs on polymer particle surfaces.15 Gu et al. has proved that this coating was ascribed to electrostatic adsorption and investigated the influence of the electrostatic voltage on the final conductivity of composites with segregated structure.13 Although such segregated architecture with conductive fillerwrapped polymer particles plays a role in absorbing and transferring electromagnetic waves to heat, thus leading to absorption-dominated shielding mechanism, further increasing absorption and reducing the density of composites are very limited. Porous CPCs could simultaneously achieve both higher electromagnetic absorption and lower density. The lightweight materials tally with sustainable development on account of materials and energy savings and are especially favorable for practical EMI shielding application in aircraft and spacecraft fields.16−19 Yang et al. first made use of chemical foaming to prepare CNT/PS composite foam and the resultant foam with density of 0.56 g/cm3 exhibited an effective EMI shielding effectiveness of 19 dB at 7 wt % CNT loading.20 Yan et al. utilized salt leaching to fabricate porous graphene/polystyrene composites, and specific shielding effectiveness of the lightweight material was as high as 64.4 dB cm3/g at 30 wt % graphene loading with density of 0.45 g/cm3.9 Li et al. produced polyimide (PI)/graphene composite foams via phase separation, and the average EMI SE reached about 17−21 dB with 16 wt % graphene.21 Apart from the mentioned processing methods, physical foaming, especially scCO2 foaming, is an effective and environmentally friendly way to manufacture composite foams.22−30 Compared to traditional chemical and other physical blowing agents (such as halohydrocarbon or alkane with low boiling point), scCO2 has the advantages of nontoxicity, inexpensive, eco-friendly, and high foaming efficiency that is more in line with social sustainable development. Numerous studies have reported the possibility of 9905


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ACS Sustainable Chemistry & Engineering Table 1. Foaming Conditions and Density Values of PS/MWCNT Foams with Various MWCNT Contents foaming conditions MWCNT content in unfoamed samples (wt %)

MWCNT content in unfoamed samples (initial vol %)

0

0

1

0.6

2

1.21

3

1.82

5

3.06

7

4.32

sample name

soaking temperature (°C)

soaking pressure (MPa)

soaking time (h)

MWCNT content in foams (vol %)

foam density (g/cm3)

C0T80 C0T90 C1T80 C1T90 C2T80 C2T90 C3T80 C3T90 C5T80 C5T90 C7T80 C7T90

80 90 80 90 80 90 80 90 80 90 80 90

20

2

20

2

20

2

20

2

20

2

20

2

0 0 0.23 0.18 0.48 0.35 0.74 0.55 1.29 0.97 2.4 1.88

0.40 0.31 0.40 0.31 0.42 0.31 0.43 0.32 0.45 0.34 0.60 0.47

MWCNT as the promising conductive fillers were selected as an example of this facile and green method. The morphology, electrical conductivity, and EMI shielding performance of the solid and porous composites are investigated contrastively. In addition, their respective shielding mechanism has also been explored.

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environmentally friendly scCO2 foaming accords with the green design of lightweight CPCs. For convenience, foamed samples with different weight fractions of MWCNT and distinct foaming conditions were identified. The samples were named CXTY, where “X” refers to the weight content of MWCNT (0, 1, 2, 3, 5, 7) and “Y” stands for the saturation temperature (80, 90). For example, CXT80 refers to composite foam prepared at 80 °C and C1T90 refers to composite foam containing 1 wt % MWCNT loading prepared at 90 °C. Table 1 lists the foaming conditions, weight content and corresponding volume content of MWCNT, sample name, and foam density. Characterization. Optical microscopy observation was employed to clarify the distribution of MWCNT in the PS matrix and the morphological structure. The 10 μm thick specimens were cut by a microtome and performed using an Olympus BX53 M microscope equipped with a digital camera. For scanning electron microscopy (SEM) observation, the samples were frozen in liquid nitrogen for 20 min and then quickly fractured. Prior to observation, the fractured surfaces were sputter coated with gold and observed using a field emission SEM (Quanta 250, FEI, America) with the accelerating voltage of 20 kV. Cell diameter was calculated by analyzing the resulting SEM photographs via Image-Pro Plus 6.0 software. The densities of composite foams were measured via water displacement method according to ASTM D792−00. For the determination of electrical conductivity, PS/MWCNT composites with the volume conductivity above 10−6 S/m were conducted using a four-probe measurement system (RTS-9, Guangzhou Four Probe Technology Co., Ltd., Guangzhou), and the volume conductivity below 10−6 S/m were measured using a highresistance meter (ZC-90F, Shanghai Taiou Electronics Co., Ltd., Shanghai). EMI shielding characteristics of all PS/MWCNT composites were conducted using an Agilent N5247A vector network analyzer. The sample with 1.8 mm in thickness was placed in the specimen holder. The scattering parameters (S11 and S21) in the frequency range of 8.2−12.4 GHz were recorded to calculate total EMI SE (SEtotal), microwave reflection (SER), and microwave absorption (SEA), using the following equations

EXPERIMENTAL SECTION

Materials. The MWCNT (Nanocyl 7000) fabricated by catalytic carbon vapor deposition (CCVD) process with carbon purity of 90% was purchased from Nanocyl S.A., Belgium. The density, mean diameter, and mean length are 1.75 g/cm3, 9.5 nm, and 1.5 μm, respectively. The commercial PS pellets (GP5250) with density of 1.05 g/cm3 and melt flow index of 7.0 g/10 min were obtained from Formosa Chemicals & Fiber Corporation, Ningbo. CO2 with purity of 99.99% was supplied by Qiaoyuan Gas Co., Ltd., Chengdu. The pellets of PS were pulverized cryogenically and sifted out by sieves with different mesh apertures to acquire particles with the average size of ∼100 μm, as shown in Figure S1. To decrease volatile content and moisture level, the MWCNT and PS powders were dried in vacuum ovens at 60 and 80 °C, respectively, for 24 h before use. Fabrication of Solid PS/MWCNT Composites with Segregated Structure. A facile and green method of electrostatic adsorption was chosen to prepare PS/MWCNT composites. Static electricity was generated in the process of high-speed mechanical friction. First, 20 g of dried PS powders were placed in a high speed mixer and stirred for 2 min with rotation speed of 26 000 rpm. Then, the desired amount of MWCNT was added into the high speed mixer and restirred for 1 min with rotation speed of 26 000 rpm to produce MWCNT-coated PS powders. The intermittent and short operation could make polymer particles avoid high temperature which was caused by high-speed mechanical friction. Next, the obtained mixtures were compression molded into sheets of 1.8 mm in thickness at 160 °C, 10 MPa for 10 min, whereupon the PS/MWCNT composites with segregated structure were achieved. The fabricating procedure of solid PS/MWCNT composites with segregated structure was time saving, maneuverable, and organic solvent-free. Preparation of Porous PS/MWCNT Composite Foams with Segregated Conductive Networks. Figure 1 depicts a schematic for the fabrication procedure of porous PS/MWCNT composite foams with segregated structure. The above-fabricated solid samples with segregated structure were employed for batch foaming, using scCO2 as blowing agent. All specimens were soaked at constant pressure of 20 MPa for 2 h. Foam density was controlled by saturation temperature. At the end of the saturation process, the pressure was rapidly released at an average rate of about 5 MPa/s. The stabilized foamed structure was obtained via cooling down the autoclave to ambient temperature. The employment of solvent-free high speed mechanical mixing and

R = |S11|2 ,

T = |S21|2

(1) (2)

A=1−R−T SE R = − 10 log(1 − R ),

SEA = − 10 log

SEtotal = SE R + SEA + SEM

T 1−R

(3) (4)

where SEM is the microwave multiple internal reflections and could be negligible when SEtotal ≥ 10 dB.14 9906


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Figure 2. SEM images of (a,b) neat PS powders; (c,d) 7 wt % MWCNT coated PS powders.

Figure 3. Cross-sectional optical microscopy images of the PS/MWCNT composites containing 3 wt % MWCNT. (a) The unfoamed sample. (b−d) C3T80: (b) optical reflection image; (c) optical transmittance image; (d) the magnified region of (c). (e−g) C3T90: (e) optical reflection image; (f) optical transmittance image; (g) the magnified region of (f).

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The compressive strength was measured using a universal testing

RESULTS AND DISCUSSION

SEM observation was first carried out to demonstrate the validity of MWCNT with agglomerated microstructure adsorbed on PS particles (Figure 2c,d). The dispersion of

machine (Instron 4302, U.S.A.) at room temperature according to ASTM standard D1621 with crosshead speed of 0.5 mm/min on samples with 16 mm in diameter and 12 cm in height. 9907


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Figure 4. Scanning electron micrographs and cell size distribution of the porous PS/MWCNT composites with different foaming conditions and MWCNT contents.

shows cross-sectional optical microscopy images of the PS/ MWCNT composites containing 3 wt % MWCNT. A typical segregated structure with clear boundaries between PS particles is formed in both solid and porous PS/MWCNT composites, indicating hot pressing and scCO2 foaming do not obviously destroy the MWCNT conductive networks. However, if we further increase the temperature, the reduction in viscosity of PS

MWCNTs in aggregated forms was due to the specific surface area difference between PS particles and MWCNTs, and conducive to constructing dense electrical networks. In order to further examine the special structure still retain after hot pressing and scCO2 foaming, optical microscopy was employed to observe the conductive networks. Both the transmission and reflection light mode were used for composite foams. Figure 3 9908


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Figure 5. (a) Electrical conductivity versus MWCNT loading for the PS/MWCNT composites. (b) Power law fitting of the electrical conductivity data from (a).

conductivity surpassing the target value (1 S/m) for target EMI SE of 20 dB in commercial application is 1.19, 1.37, 1.85 S/m at ultralow MWCNT volume content of 0.6 vol % MWCNT loading for unfoamed composites, 0.74 vol % MWCNT loading for CXT80, and 0.97 vol % MWCNT loading for CXT90. For unfoamed PS/MWCNT composites, a sharp increase of nearly 9 orders of magnitude in electrical conductivity is observed from 0 to 0.2 vol % MWCNT content, presenting a typical percolation behavior. Similar results also exist in composite foams. The power law equation σ = σ0(φ − φc)t is applied to further evaluate the relationship between electrical conductivity and MWCNT content, wherein σ is the electrical conductivity of composites, σ0 is a constant in connection with the intrinsic conductivity of MWCNT, φ is the volume content of MWCNT, φc is the percolation threshold, and t is the critical exponent related to filler dispersion and dimensionality. The best fitting results of composite and composite foams using the power law equation are presented in Figure 5b. The optimum fitted t values are 2.41, 2.36 and 2.66, respectively, for unfoamed composites, CXT80, and CXT90, indicating the construction of three-dimensional MWCNT conductive networks in PS/ MWCNT composites.29 The fitted φc is 0.09 vol %, 0.08 vol %, and 0.07 vol %, respectively for unfoamed composites, CXT80, and CXT90, which are very low MWCNT contents to endow the composites with semiconductor. Their percolation thresholds are very close, but foamed samples with larger expansion ratio exhibit lower value. Many researchers have reported that the foamed nanocomposites assuming lower percolation thresholds and higher electrical conductivities were due to volume exclusion, making conductive fillers concentrate in the cell wall or struts, and thus were conducive to better interconnection of fillers.31 However, the mechanism is not suitable for segregated structure system. For desired segregated structure, the conductive fillers completely distribute at the interface of polymer particles, interconnecting well with each other without penetrating into polymer particles. Foaming cannot shorten filler-to-filler distance, without improving the electrical conductivity of composites. For our PS/MWCNT system, it is actually not formed with desired segregated conductive networks because PS is an outstanding thermoplastic resin with good processing fluidity,33,37 unlike those high viscosity polymers such as ultrahigh molecular weight polyethylene (UHMWPE) and cross-linked polymers. In other words, the process of compression molding, especially the temperature, could make for the activation of PS molecular chains and viscosity reduction in PS melt, inevitably resulting in part MWCNT diffusing into PS matrix. Having thought of the problem before, the compression molding temperature of 160

could be adverse to very well interconnected electrical networks on account of cell growth. Interestingly enough, we can distinctly see bubbles in the magnified optical transmittance image (Figure 3d,g). This will help us have luminous comprehension of the structure in composites and sort out the relationship between structure and property. SEM photos of the porous PS/MWCNT composites with different foaming conditions and MWCNT contents are shown in Figure 4 to further verify the structure of foams. The morphology of absolutely closed spherical or polygonal cells was observed in all prepared foams and the cell size of CXT90 (e.g., average cell diameter of C0T90 = 16.8 μm) is a little larger than that of CXT80 (e.g., average cell diameter of C0T80 = 14.8 μm) but not too obvious because of small temperature difference (80 and 90 °C). At higher temperature, the viscosity of the polymer reduces, conductive to cell growth.32 It is apparent that the cell size gradually decreases and the cell size distribution tends to be narrower with the increase of MWCNT content. As is known to all, MWCNT plays a significant role of heterogeneous nucleating agents in polymer matrix, leading to the increase in cell density as a result of reduction in nucleation activation energy. In addition, MWCNT with its solidity and nondeformability hampers cell growth, contributing to the decrease in cell size, as shown in the interfaces between PS particles marked in Figure 4.34−37 The pure polymer foams have a fine uniform microcellular structure while present with bimodal cell structure with the addition of MWCNT. Large cells occur in the PS core and small cells appear near the boundaries of PS particles and MWCNT. However, the efficiency of heterogeneous nucleating is gradually decreased with the increase of MWCNT content because of severe agglomeration between nanofillers.32,38 In addition, the dense interconnected MWCNT layers could act as barriers which inhabit the growth of cells, resulting in the reduction of cell size with the increase of MWCNT content. We can clearly observe the morphology of porous PS/MWCNT foam is segregated structure. At lower MWCNT content, the interface is distinct with evidence of bimodal cell morphology. At higher MWCNT content, the dense MWCNT layers have the incapability of foaming and could be directly observed and the morphology of which is much like the immiscible polymer foaming system.39 Figure 5 illustrates the electrical conductivity of PS/MWCNT composites as a function of MWCNT volume fraction. It is obvious that regardless of foaming or not, the electrical conductivity always increases with MWCNT content. The continuity and denseness of MWCNT conductive pathways are constantly improved with a greater amount of MWCNT introduced into the composites. The remarkable electrical 9909


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Table 2. Electrical Conductivity and Average EMI SE in the X-Band Frequency Range of the PS/MWCNT Composites and Several Carbon-Based CPCs Reported in Literaturea composite

filler content (wt%)

conductivity (S/m)

percolation threshold

PS/MWCNT PS/MWCNT PS/MWCNT PS/graphene epoxy/CNT WPU/graphene PMMA/graphene PS/MWCNT foam PS/MWCNT foam PS/CNT foam PS/graphene foam PS/graphene foam PMMA/graphene foam PVDF/graphene foam PEI/graphene@Fe3O4 foam

7 20 19.1 7 15 7.7 7 7 7 7 30 30 5 7 10

61.9 10 90.3 45.5 4 5.1 10 11 8.05 N/A 1.25 0.22 3.11 ∼1 N/A

0.09 vol % N/A N/A 0.09 vol % 0.32 wt % N/A 0.62 vol % 0.08 vol % 0.07 vol % N/A N/A N/A 0.6 vol % 0.5 wt % N/A

density (g/cm3)

thickness (mm)

EMI SE (dB)

0.60 0.47 0.57 0.45 0.27 0.79 N/A 0.43

1.8 2 0.2−0.3 2.5 2.0 2.0 3.4 1.8 1.8 N/A 2.5 2.5 2.4 N/A 2.5

42 30 18 45.1 20−30 35 30 26.3 23.2 19 29 17 19 28 17.8

specific SE divided by thickness (dB·cm2/g)

ref.

243.5 274.2 N/A 257.8 251.9 100.2 N/A 165.6

this work 40 41 11 42 43 44 this work this work 20 9 9 45 46 47

a

The weight content is used for comparison.

Figure 6. EMI SE as a function of frequency in X-band range for the PS/MWCNT composites: (a) the unfoamed samples; (b) CXT80; (c) CXT90.

°C we selected in this work is an appropriate choice with sufficient PS−PS binding force and higher viscosity. Consequently, the segregated structure we prepared is virtually a “partially segregated structure” with MWCNT scattering in a narrow region between PS particles, as shown in Figure S2. For high MWCNT content, the “partially segregated structure” has slight influence on the structure and electrical conductivity of composites because the MWCNT diffused into PS particles is negligible compared to MWCNT at the interface. Foaming with certain expansion ratio could enlarge the distance between fillers, resulting in the decrease in electrical conductivity. For

example, the electrical conductivity is 15.91, 10.97, and 8.05 S/ m, respectively for unfoamed PS/MWCNT composites containing 1.82 vol % MWCNT, CXT80 containing 2.4 vol % MWCNT, and CXT90 containing 1.88 vol % MWCNT. Nevertheless, for low MWCNT content, the “partially segregated structure” could contribute to the loss of filler interconnections. After foaming, the MWCNT diffused into PS particles could trigger heterogeneous nucleation, giving rise to the orientation of MWCNT around the cells and a decrease in distance between fillers. This is the reason why the foamed samples exhibit lower percolation threshold compared to 9910


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Figure 7. Comparison of total EMI SE (SEtotal), microwave absorption (SEA),, and microwave reflection (SER) at the frequency of 12.4 GHz for the PS/MWCNT composites with various MWCNT loadings: (a) the unfoamed samples, (b) CXT80, and (c) CXT90.

which is in accordance with the regularity of electrical conductivity. The more continuous and denser conductive layers at higher MWCNT loadings could contribute to strong interaction with incoming electromagnetic waves, accordingly leading to the high EMI SE. What needs illustration is that MWCNT volume fraction is used for comparing the total EMI SE of solid or porous composites, and foaming also has little influence on the measured value. For example, the average EMI SE is 32.1 dB at 3.06 vol % MWCNT loading for unfoamed PS/ MWCNT composites, 26.3 dB at 2.4 vol % MWCNT loading for CXT80, and 23.2 dB at 1.88 vol % MWCNT loading for CXT90. The conclusion is consistent with the above morphology and electrical conductivity characterization that foaming has little impact on segregated conductive work. However, if MWCNT weight fraction is used for comparison (same color of curves in Figure 6a−c represents for identical MWCNT weight fraction), the total EMI SE dramatically decrease. This appears to show the adverse effect of foaming. Nevertheless, there is a remarkable fact that the EMI SE of a shield is considerably dependent on its thickness. Yan et al. prepared PS/graphene composites containing 7 wt % graphene with different thickness for EMI SE measurement.11 The average EMI SE of composites with the thickness of 2 mm was 30 dB, and the value increased to 45 dB with the thickness of 2.5 mm. Thus it can be seen that the thickness has significant influence on EMI SE. Taking composite foam, for example, the thickness of foamed sample used for EMI SE measurement is about 1.8 mm, whereas the actual compacted thickness is probably only 0.8 mm. Composite foams with 1.8 mm in thickness possess a much smaller amount of conductive fillers that interact with incoming electromagnetic wave compared to solid composites with 1.8 mm in thickness at the same MWCNT weight content. Consequently, the EMI SE of

unfoamed composites. In addition, the obtained percolation threshold is very low in comparison with those reported values for polymer/carbon-based filler composites with randomly distributed structure and most with segregated structure in literature, indicating the high efficiency of MWCNT in forming electrically conductive path, as shown in Table 2. In our previous work,33 the method of taking advantages of scCO2 foaming to improve the electrical conductivity of composites only realized the conductivity of 10−2 S/m, nearly 3 orders of magnitude lower than this work at similar nanofiller content. It is noteworthy that hardly any attention has paid to conductive polymer foams with segregated structure. The composite foams we prepared almost have no effect on electrical conductivity in contrast to that of solid composites, but the density reduces a lot (as shown in Table 1). In order to acquire comparable results, all samples despite foaming or not were worn to circular sheets with 1.8 mm in thickness for EMI SE measurement. Figure 6 illustrates the EMI SE of solid and porous PS/MWCNT composites over the frequency range of 8.2−12.4 GHz (X-band). It is obvious that regardless of the MWCNT content, the total EMI SE is almost independent of the frequency. As shown in Figure 6a, the average EMI SE of unfoamed PS/MWCNT composites achieves as high as 42 dB at only 4.32 vol % MWCNT loading, exceeding, or approaching previously reported polymer/carbonbased composites, at thinner sample thickness or lower conductive filler loadings, as listed in Table 2. It has been well established that the EMI SE of CPCs depends heavily on its conductivity.11 Therefore, the superior EMI SE in this work should also be attributed to excellent electrical conductivity, demonstrating efficient conductive networks. Moreover, the total EMI SE increases with MWCNT content, 9911


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Figure 8. (a) Schematic representation of microwave transfer across the unfoamed PS/MWCNT composites; (b) schematic representation of microwave transfer across the PS/MWCNT foams.

previously reported works.50,8 For porous PS/MWCNT foams (Figure 7b,c), the SER presents much the same value (below 1.5 dB) with the increase of MWCNT content. The average SEtotal, SEA, and SER are 25.42, 23.92, 1.50 dB for C7T80, whereas the corresponding values are 23.69, 22.72, 0.98 dB for C7T90, respectively, suggesting that the electromagnetic wave is mainly attenuated by the electromagnetic absorption (94.1% for C7T80 and 95.9% for C7T90) in porous PS/MWCNT composite foams instead of being reflected back from the shields’ surface and absorption is the dominating shielding mechanism in both solid and porous composites. The absorption-dominated mechanism was attributed to ohmic loss and polarization loss. The ohmic loss derived from interconnected MWCNT conductive networks that could dissipate more electromagnetic energy and the polarization loss which relates to defects, interfaces, and functional groups of shields was enhanced by polymer−gas and polymer−MWCNT interfaces.51 In light of the transmission line theory, modulating the impedance of the shield close to air is undoubtedly an effective way to reduce reflection. Before foaming, a larger amount of MWCNT content means the increase in the impedance of the sample, contributing to higher SER (as shown in Figure 7a). After foaming, the impedance of sample becomes closer to air with the introduction of air. So it is reasonable to speculate and achieve the results that reflection is reduced with the increase of air volume percent.32 It can be observed that C7T80 and C7T90 with the same MWCNT weight fraction of 7 wt %, while C7T90 exhibited a bit higher absorption than C7T80. This was due to a slight difference in expansion ratio (reflected on the difference in density, as shown in Table 1) between C7T80 and C7T90 with the same sample dimension, resulting in different air volume percent. This character of high absorption is much more preferred in electronic fields because the absorption dominant EMI shielding feature could hinder reflection from disturbing normal operation of electronic devices.52 Figure 8 schematically depicts the microwave propagation across the solid and porous PS/MWCNT composites. As shown Figure 8a, in solid composites, once electromagnetic wave enters into the composites consisting of the interconnected CNT−

composite foams decreases unavoidably. In addition, many researchers proposed that the specific EMI SE (EMI SE divided by density) was more appropriate for comparing the EMI SE performance between solid and porous materials in view of the efficiency of material consumption.20,48 The specific EMI SE for C7T80 and C7T90 are approximately 43.8 and 49.4 dB·cm3/g, respectively. However, the specific EMI SE for solid composites containing 7 wt % MWCNT loading is only 38.9 dB·cm3/g. These results indicate that foaming process could prominently improve the specific EMI SE and a larger expansion ratio reveals a higher specific EMI SE. Taking the effect of thickness into consideration, we calculate specific SE divided by thickness to compare with others’ work in this study, as listed in Table 2. It is well-known that the total EMI SE is the combination of absorption and reflection due to ignoration of multiple reflection.49 The reflection originates from impedance mismatch between shields and air, and the absorption is derived from ohmic loss, polarization loss, and magnetic loss. Owing to the nonmagnetic properties of PS and MWCNT, the SEA was only attributed to ohmic loss and polarization loss in this work. In order to investigate the EMI shielding mechanism in PS/ MWCNT composites, and in view of the frequencyindependence of EMI SE (Figure 6), the respective total EMI SE (SEtotal), microwave absorption (SEA), and microwave reflection (SER) as a function of MWCNT content at a frequency of 12.4 GHz are separately calculated according to the measured scattering parameters, and the results are presented in Figure 7. As shown in Figure 7a, for unfoamed PS/MWCNT composites, as the MWCNT content increases, both the SEtotal and SEA substantially improve while the SER just slightly increases. For example, the SEtotal, SEA, and SER of the unfoamed PS/MWCNT composite containing 4.32 vol % MWCNT are 43.3, 39.2, and 4.1 dB, respectively, indicating that the contribution of absorption (90.5%) to the total EMI SE is much larger than that of reflection (9.5%), assuming the great advantages of segregated structure with abundant interfaces to reflect, scatter, and absorb incoming electromagnetic wave. Similar electromagnetic absorption superiority of solid CPCs with segregated conductive networks was also found in 9912


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Figure 9. Compressive stress−strain curves of porous PS/MWCNT composites: (a) CXT80; (b) CXT90.

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CONCLUSION In summary, porous PS/MWCNT composites with segregated conductive networks were obtained using high-speed mechanical mixing and scCO2 foaming. Ascribing to the segregated structure with interconnected CNT−CNT networks selectively distributing at the interface of polymer particles, the foam exhibited an extremely low percolation threshold of 0.07 vol %. At only 1.88 vol % MWCNT loading and slim thickness of 1.8 mm, the PS/MWCNT composite foam with density of 0.47 g/ cm3 presented electrical conductivity of 8.05 S/m and EMI SE of 23.2 dB, already meeting the requirement of EMI shielding materials in commercial application (≥20 dB). The specific EMI SE increased from 38.6 dB·cm3/g (solid PS/MWCNT composites containing 7 wt % MWCNT) to 49.4 dB·cm3/g (C7T90), fully revealing the advantages of the foaming process on improving the EMI shielding performance. Moreover, the EMI shielding mechanism of solid and porous PS/MWCNT composites were investigated systematically. The respective SE parameters (SEtotal, SEA, and SER) with different MWCNT contents at a specific frequency were discussed. The results indicated that absorption was the dominant EMI shielding mechanism for composites regardless of foaming or not in the frequency range of 8.2−12.4 GHz (X-band). The contribution of absorption to the total EMI SE increased from 90.5% (solid PS/MWCNT composites containing 7 wt % MWCNT) to 95.9% (C7T90). This study presents a facile and green but versatile method to structure porous composite foam with segregated conductive networks and provides a novel idea for preparing lightweight EMI shielding materials, which is suitable for most thermoplastic polymers/diverse conductive fillers system.

CNT networks, it is multiply reflected and scattered by the conductive MWCNT layer. It is difficult for electromagnetic waves to escape from the polygonal conductive networks before they are dissipated in the form of heat. For porous composites, the mode of electromagnetic wave transference does not change. Actually, foams prepared from polymer/randomly distributed conductive filler composites also appear segregated structure with conductive filler aggregating in cell walls or struts. Consequently, these two kinds of foams prepared from different solid structure show the same mechanism like solid PS/ MWCNT composites with segregated conductive networks. Be that as it may, porous foams prepared from solid composites with segregated structure possess higher shielding efficiency and the structure of foams could be controlled more easily compared with the other one. In this work, PS with good processing fluidity was conducive to enhancing the binding force between PS particles and improving the mechanical properties, as shown in Figure 9. PS foams and PS/MWCNT foams assume similar compressive behavior, that is, an initial linear elastic region followed by a yielding region. The incorporation of MWCNTs into neat PS foams largely improves the compressive strength under both foaming conditions. For example, the compressive stress of C7T80 and C7T90 increased by 48% and 41%, respectively, compared to C0T80 and C0T90 at 60% compressive strain. Furthermore, the specific compressive stress (compressive stress divided by density) of the composite foam achieves ∼66 MPa/ (g/cm3) for C7T90 and ∼90 MPa/(g/cm3) for C7T80, which is higher than other reported PS composites foams in the open literatures.53−55 Compared to the conventional preparation process of porous conductive foams applied in literature, the technique reported here presents certain superiorities. First, the procedure combining high-speed mechanical mixing and scCO2 foaming is time-saving, organic solvent-free, and environmentally friendly, which overcomes the defect of solution mixing and chemical foaming. Meanwhile, the weakness of melt blending such as the degradation of polymer and energy intensive can also be avoided. Second, measures to uniformly disperse nanofillers in polymer matrix involve the modification or functionalization of fillers, which in turn impairs the electrical conductivity of composites. Third, the porous conductive foam with segregated conductive networks exhibits better conductive paths and higher efficiency in EMI shielding application compared to that prepared from solid composites with uniformly distributed conductive fillers.

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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/acssuschemeng.9b00678.

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Additional figures and references (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 28 8540 8361. ORCID

Xia Liao: 0000-0002-4093-0507 9913


Research Article

ACS Sustainable Chemistry & Engineering Notes

polyethylene composites with devisable carbon nanotube networks. J. Mater. Chem. C 2015, 3, 9369−9378. (15) Liu, Q.; Tu, J.; Wang, X.; Yu, W.; Zheng, W.; Zhao, Z. Electrical conductivity of carbon nanotube/poly(vinylidene fluoride) composites prepared by high-speed mechanical mixing. Carbon 2012, 50, 339−341. (16) Tran, M. P.; Thomassin, J. M.; Alexandre, M.; Jerome, C.; Huynen, I.; Detrembleur, C. Nanocomposite Foams of Polypropylene and Carbon Nanotubes: Preparation, Characterization, and Evaluation of their Performance as EMI Absorbers. Macromol. Chem. Phys. 2015, 216, 1302−1312. (17) Min, Z.; Yang, H.; Chen, F.; Kuang, T. Scale-up production of lightweight high-strength polystyrene/carbonaceous filler composite foams with high-performance electromagnetic interference shielding. Mater. Lett. 2018, 230, 157−160. (18) Ma, X.; Shen, B.; Zhang, L.; Liu, Y.; Zhai, W.; Zheng, W. Porous superhydrophobic polymer/carbon composites for lightweight and selfcleaning EMI shielding application. Compos. Sci. Technol. 2018, 158, 86−93. (19) Wang, G.; Zhao, G.; Dong, G.; Mu, Y.; Park, C. B. Lightweight and strong microcellular injection molded PP/talc nanocomposite. Compos. Sci. Technol. 2018, 168, 38−46. (20) Yang, Y. L.; Gupta, M. C.; et al. Novel carbon nanotubepolystyrene foam composites for electromagnetic interference shielding. Nano Lett. 2005, 5, 2131−2134. (21) Li, Y.; Pei, X. L.; Shen, B.; Zhai, W. T.; Zhang, L. H.; Zheng, W. G. Polyimide/graphene composite foam sheets with ultrahigh thermostability for electromagnetic interference shielding. RSC Adv. 2015, 5, 24342−24351. (22) Tang, W.; Bai, J.; Liao, X.; Xiao, W.; Luo, Y.; Yang, Q.; Li, G. Carbon nanotube-reinforced silicone rubber nanocomposites and the foaming behavior in supercritical carbon dioxide. J. Supercrit. Fluids 2018, 141, 78−87. (23) Bai, J.; Liao, X.; Huang, E.; Luo, Y.; Yang, Q.; Li, G. Control of the cell structure of microcellular silicone rubber/nanographite foam for enhanced mechanical performance. Mater. Des. 2017, 133, 288− 298. (24) Wang, Y. W.; Liao, X.; Li, S. J.; Luo, Y.; Yang, Q.; Li, G. X. Poly(methyl methacrylate) nanocomposites based on graphene oxide: a comparative investigation of the effects of surface chemistry on properties and foaming behavior. Polym. Int. 2016, 65, 1195−1203. (25) Wang, Y.; Liao, X.; Luo, Y.; Yang, Q.; Li, G. Influence of Surfacefunctionalized Graphene Oxide on the Cell Morphology of Poly(methyl methacrylate) Composite. J. Mater. Sci. Technol. 2015, 31, 463−466. (26) Huang, E.; Liao, X.; Zhao, C.; Park, C. B.; Yang, Q.; Li, G. Effect of Unexpected CO2’s Phase Transition on the High-Pressure Differential Scanning Calorimetry Performance of Various Polymers. ACS Sustainable Chem. Eng. 2016, 4, 1810−1818. (27) Kuang, T.; Chen, F.; Chang, L.; Zhao, Y.; Fu, D.; Gong, X.; Peng, X. Facile preparation of open-cellular porous poly (L-lactic acid) scaffold by supercritical carbon dioxide foaming for potential tissue engineering applications. Chem. Eng. J. 2017, 307, 1017−1025. (28) Kuang, T.; Ju, J.; Yang, Z.; Geng, L.; Peng, X. A facile approach towards fabrication of lightweight biodegradable poly (butylene succinate)/carbon fiber composite foams with high electrical conductivity and strength. Compos. Sci. Technol. 2018, 159, 171−179. (29) Kuang, T.; Chang, L.; Chen, F.; Sheng, Y.; Fu, D.; Peng, X. Facile preparation of lightweight high-strength biodegradable polymer/multiwalled carbon nanotubes nanocomposite foams for electromagnetic interference shielding. Carbon 2016, 105, 305−313. (30) Li, J.; Zhang, G.; Zhang, H.; Fan, X.; Zhou, L.; Shang, Z.; Shi, X. Electrical conductivity and electromagnetic interference shielding of epoxy nanocomposite foams containing functionalized multi-wall carbon nanotubes. Appl. Surf. Sci. 2018, 428, 7−16. (31) 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.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project is sponsored by the National Natural Science Foundation of China (Nos. 51773138, 51373103, and 51721091) and the Science and Technology Department of Sichuan Province, China (No. 2015HH0026).

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REFERENCES

(1) Chen, Z. P.; Xu, C.; Ma, C. Q.; Ren, W. C.; Cheng, H. M. Lightweight and Flexible Graphene Foam Composites for HighPerformance Electromagnetic Interference Shielding. Adv. Mater. 2013, 25, 1296−1300. (2) Ren, F.; Song, D.; Li, Z.; Jia, L.; Zhao, Y.; Yan, D.; Ren, P. Synergistic effect of graphene nanosheets and carbonyl iron−nickel alloy hybrid filler on electromagnetic interference shielding and thermal conductivity of cyanate ester composites. J. Mater. Chem. C 2018, 6, 1476−1486. (3) Manna, K.; Srivastava, S. K. Fe3O4@Carbon@Polyaniline Trilaminar Core−Shell Composites as Superior Microwave Absorber in Shielding of Electromagnetic Pollution. ACS Sustainable Chem. Eng. 2017, 5, 10710−10721. (4) Cheng, H.; Wei, S.; Ji, Y.; Zhai, J.; Zhang, X.; Chen, J.; Shen, C. Synergetic effect of Fe3O4 nanoparticles and carbon on flexible poly (vinylidence fluoride) based films with higher heat dissipation to improve electromagnetic shielding. Composites, Part A 2019, 121, 139− 148. (5) Arief, I.; Biswas, S.; Bose, S. Tuning the Shape Anisotropy and Electromagnetic Screening Ability of Ultrahigh Magnetic Polymer and Surfactant-Capped FeCo Nanorods and Nanocubes in Soft Conducting Composites. ACS Appl. Mater. Interfaces 2016, 8, 26285−26297. (6) Thomassin, J.-M.; Jérôme, C.; Pardoen, T.; Bailly, C.; Huynen, I.; Detrembleur, C. Polymer/carbon based composites as electromagnetic interference (EMI) shielding materials. Mater. Sci. Eng., R 2013, 74, 211−232. (7) Wang, P.; Chong, H.; Zhang, J.; Lu, H. Constructing 3D Graphene Networks in Polymer Composites for Significantly Improved Electrical and Mechanical Properties. ACS Appl. Mater. Interfaces 2017, 9, 22006−22017. (8) Cui, C.-H.; Yan, D.-X.; Pang, H.; Xu, X.; Jia, L.-C.; Li, Z.-M. Formation of a Segregated Electrically Conductive Network Structure in a Low-Melt-Viscosity Polymer for Highly Efficient Electromagnetic Interference Shielding. ACS Sustainable Chem. Eng. 2016, 4, 4137− 4145. (9) Yan, D. X.; Ren, P. G.; Pang, H.; Fu, Q.; Yang, M. B.; Li, Z. M. Efficient electromagnetic interference shielding of lightweight graphene/polystyrene composite. J. Mater. Chem. 2012, 22, 18772− 18774. (10) Liang, J.; Wang, Y.; Huang, Y.; Ma, Y.; Liu, Z.; Cai, J.; Zhang, C.; Gao, H.; Chen, Y. Electromagnetic interference shielding of graphene/ epoxy composites. Carbon 2009, 47, 922−925. (11) Yan, D. X.; Pang, H.; Li, B.; Vajtai, R.; Xu, L.; Ren, P. G.; Wang, J. H.; Li, Z. M. Structured Reduced Graphene Oxide/Polymer Composites for Ultra-Efficient Electromagnetic Interference Shielding. Adv. Funct. Mater. 2015, 25, 559−566. (12) Li, T.; Ma, L. F.; Bao, R. Y.; Qi, G. Q.; Yang, W.; Xie, B. H.; Yang, M. B. A new approach to construct segregated structures in thermoplastic polyolefin elastomers towards improved conductive and mechanical properties. J. Mater. Chem. A 2015, 3, 5482−5490. (13) Wang, B. J.; Li, H. Y.; Li, L. Z.; Chen, P.; Wang, Z. B.; Gu, Q. Electrostatic adsorption method for preparing electrically conducting ultrahigh molecular weight polyethylene/graphene nanosheets composites with a segregated network. Compos. Sci. Technol. 2013, 89, 180− 185. (14) Jia, L. C.; Yan, D. X.; Cui, C. H.; Jiang, X.; Ji, X.; Li, Z. M. Electrically conductive and electromagnetic interference shielding of 9914


Research Article

ACS Sustainable Chemistry & Engineering (32) Li, J.; Zhang, G.; Ma, Z.; Fan, X.; Fan, X.; Qin, J.; Shi, X. Morphologies and electromagnetic interference shielding performances of microcellular epoxy/multi-wall carbon nanotube nanocomposite foams. Compos. Sci. Technol. 2016, 129, 70−78. (33) Xiao, W.; Liao, X.; Jiang, Q.; Zhang, Y.; Chen, J.; Yang, Q.; Li, G. Strategy to Enhance Conductivity of Polystyrene/Graphene Composite Foams via Supercritical Carbon Dioxide Foaming Process. J. Supercrit. Fluids 2018, 142, 52−63. (34) Yeh, S. K.; Huang, C. H.; Su, C. C.; Cheng, K. C.; Chuang, T. H.; Guo, W. J.; Wang, S. F. Effect of Dispersion Method and Process Variables on the Properties of Supercritical CO2 Foamed Polystyrene/ Graphite Nanocomposite Foam. Polym. Eng. Sci. 2013, 53, 2061−2072. (35) Shen, J.; Zeng, C.; Lee, L. J. Synthesis of polystyrene−carbon nanofibers nanocomposite foams. Polymer 2005, 46, 5218−5224. (36) Zhai, W.; Yu, J.; Wu, L.; Ma, W.; He, J. Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams. Polymer 2006, 47, 7580−7589. (37) Xiao, W.; Liao, X.; Li, S.; Xiong, J.; Yang, Q.; Li, G. The distinctive nucleation of polystyrene composites with differently shaped carbon-based nanoparticles as nucleating agent in the supercritical CO2 foaming process. Polym. Int. 2018, 67, 1488−1501. (38) Chen, L. M.; Rende, D.; Schadler, L. S.; Ozisik, R. Polymer nanocomposite foams. J. Mater. Chem. A 2013, 1, 3837−3850. (39) Liao, X.; Zhang, H.; Wang, Y.; Wu, L.; Li, G. Unique interfacial and confined porous morphology of PLA/PS blends in supercritical carbon dioxide. RSC Adv. 2014, 4, 45109−45117. (40) Arjmand, M.; Apperley, T.; Okoniewski, M.; Sundararaj, U. Comparative study of electromagnetic interference shielding properties of injection molded versus compression molded multi-walled carbon nanotube/polystyrene composites. Carbon 2012, 50, 5126−5134. (41) Mathur, R. B.; Pande, S.; Singh, B. P.; Dhami, T. L. Electrical and mechanical properties of multi-walled carbon nanotubes reinforced PMMA and PS composites. Polym. Compos. 2008, 29, 717−727. (42) Huang, Y.; Li, N.; Ma, Y. F.; Feng, D.; Li, F. F.; He, X. B.; Lin, X.; Gao, H. J.; Chen, Y. S. The influence of single-walled carbon nanotube structure on the electromagnetic interference shielding efficiency of its epoxy composites. Carbon 2007, 45, 1614−1621. (43) Hsiao, S. T.; Ma, C. C. M.; Tien, H. W.; Liao, W. H.; Wang, Y. S.; Li, S. M.; Huang, Y. C. Using a non-covalent modification to prepare a high electromagnetic interference shielding performance graphene nanosheet/water-borne polyurethane composite. Carbon 2013, 60, 57−66. (44) Zhang, H. B.; Zheng, W. G.; Yan, Q.; Jiang, Z. G.; Yu, Z. Z. The effect of surface chemistry of graphene on rheological and electrical properties of polymethylmethacrylate composites. Carbon 2012, 50, 5117−5125. (45) Zhang, H. B.; Yan, Q.; Zheng, W. G.; He, Z. X.; Yu, Z. Z. Tough Graphene-Polymer Microcellular Foams for Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2011, 3, 918−924. (46) Eswaraiah, V.; Sankaranarayanan, V.; Ramaprabhu, S. Functionalized Graphene-PVDF Foam Composites for EMI Shielding. Macromol. Mater. Eng. 2011, 296, 894−898. (47) Shen, B.; Zhai, W. T.; Tao, M. M.; Ling, J. Q.; Zheng, W. G. Lightweight, Multifunctional Polyetherimide/Graphene@Fe3O4 Composite Foams for Shielding of Electromagnetic Pollution. ACS Appl. Mater. Interfaces 2013, 5, 11383−11391. (48) Shui, X. P.; Chung, D. D. L. Nickel filament polymer-matrix composites with low surface impedance and high electromagnetic interference shielding effectiveness. Mater. Res. Soc. Symp. Proc. 1995, 390, 135−139. (49) Bhattacharjee, Y.; Chatterjee, D.; Bose, S. Core−Multishell Heterostructure with Excellent Heat Dissipation for Electromagnetic Interference Shielding. ACS Appl. Mater. Interfaces 2018, 10, 30762− 30773. (50) Yan, D.-X.; Pang, H.; Li, B.; Vajtai, R.; Xu, L.; Ren, P.-G.; Wang, J.-H.; Li, Z.-M. Structured Reduced Graphene Oxide/Polymer Composites for Ultra-Efficient Electromagnetic Interference Shielding. Adv. Funct. Mater. 2015, 25, 559−566.

(51) Zhan, Y.; Wang, J.; Zhang, K.; Li, Y.; Meng, Y.; Yan, N.; Wei, W.; Peng, F.; Xia, H. Fabrication of a flexible electromagnetic interference shielding Fe3O4@reduced graphene oxide/natural rubber composite with segregated network. Chem. Eng. J. 2018, 344, 184−193. (52) Thomassin, J. M.; Pagnoulle, C.; Bednarz, L.; Huynen, I.; Jerome, R.; Detrembleur, C. Foams of polycaprolactone/MWNT nanocomposites for efficient EMI reduction. J. Mater. Chem. 2008, 18, 792−796. (53) Shen, J.; Han, X.; Lee, L. J. Nanoscaled Reinforcement of Polystyrene Foams using Carbon Nanofibers. J. Cell. Plast. 2006, 42, 105−126. (54) Okolieocha, C.; Köppl, T.; Kerling, S.; Tölle, F. J.; Fathi, A.; Mülhaupt, R.; Altstädt, V. Influence of graphene on the cell morphology and mechanical properties of extruded polystyrene foam. J. Cell. Plast. 2015, 51, 413−426. (55) Arora, K. A.; Lesser, A. J.; McCarthy, T. J. Compressive behavior of microcellular polystyrene foams processed in supercritical carbon dioxide. Polym. Eng. Sci. 1998, 38, 2055−2062.

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