reduced graphene oxide composite foams for electromagnetic interference shielding with high absorption characteristic

reduced graphene oxide composite foams for electromagnetic interference shielding with high absorption characteristic

Composites Part A 123 (2019) 310–319 Contents lists available at ScienceDirect Composites Part A journal homepage: www.elsevier.com/locate/composite...

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Composites Part A 123 (2019) 310–319

Contents lists available at ScienceDirect

Composites Part A journal homepage: www.elsevier.com/locate/compositesa

Flexible thermoplastic polyurethane/reduced graphene oxide composite foams for electromagnetic interference shielding with high absorption characteristic

T



Qiuyue Jianga, Xia Liaoa, , Junsong Lia, Jia Chena, Gui Wanga, Jian Yib, Qi Yanga, Guangxian Lia a b

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, China Deyang Tianci Plastic Co. LTD, Deyang 610065, Sichuan, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermoplastic polyurethane/graphene foam Segregated structure Electromagnetic interference shielding Supercritical carbon dioxide foaming

Lightweight and flexible thermoplastic polyurethane/reduced graphene oxide (TPU/RGO) composite foams for electromagnetic interference shielding (EMI SE) were fabricated through a supercritical CO2 foaming method. The hydrogen-bond interaction formed through in-situ reduction by L-ascorbic acid in samples contributed to good interface adhesion and good flexibility of foamed samples. Multistage cellular were formed after scCO2 foaming, in this way, the density of samples was effectively reduced and the size of cells depends on the content of RGO. And the constructing of segregated structure in TPU/RGO composites contributed significantly to the electrical conductivity of samples. Shielding effectiveness of 21.8 dB was achieved with only 3.17 vol% RGO loading owing to the multistage cellular structure with good conductive network. In addition, the introduction of cellular structure enhanced the electromagnetic shielding absorption characteristic of the samples. This result demonstrated a promising method to fabricate flexible and lightweight composite foams for absorption-dominated electromagnetic interference shielding.

1. Introduction Nowadays, with the increasing use of various electronic devices, electromagnetic interference pollution becomes a serious problem because it would not only bring about a great deal of harms to electronic products but also cause adverse effects on human health [1–4]. Therefore, considerable attention has been paid to the fabrication of promising EMI shielding materials. Comparing with metal-based materials, conductive polymer composites (CPCs) with conductive nanofillers are more attractive for EMI shielding applications because of their lightweight, low cost and corrosion-resistant with a good conductivity [5,6]. As is reported, the electromagnetic interference shielding effectiveness (EMI SE) value of conductive polymer composite (CPC) should be at least 20 dB when it is commercially applicable, that means a volume electrical conductivity of at least 1 S/m is required to achieve this [1,4]. In order to realize such high conductivity, the concentration of nano-fillers in composites should be reached above the electrical percolation threshold (φc) to construct continuously conductive networks [7-9]. Among carbon nano-fillers, graphene and carbon nanotube (CNT) have been most extensively researched because of their excellent



electrical conductivity [10]. Even so, the relatively high φc and high nano-fillers loading are still required in these polymer/nano-filler composites. For example, Liang et al. reported an EMI SE of 21 dB for a graphene/epoxy composite with a graphene loading of 15 wt% [11]. Ling et al. introduced graphene to a polyetherimide composite, exhibiting an EMI SE of 20 dB with graphene loading about 10 wt% [12]. Yan et al. fabricated graphene/polystyrene composite with an improved EMI SE at a particularly high loading of 30 wt% [13]. Other polymer composites such as poly (methyl methacrylate) [14], polypropylene [15] and water-borne polyurethane [16,17] with graphene or CNTs were also reported in literatures, yet satisfactory electrical conductivity and EMI SE always requires amounts of nano-fillers, which result in high material density, high costs and poor mechanical properties. What’s more, the EMI reflection shielding of these composites was too high. The formation of cellular structure can reduced the density and enhanced the EMI absorption shielding of polymer/nano-filler composites [18–24]. As is reported, the introduction of cellular structures would induce the orientation of nano-filler around cells, thus forming more effective conductive networks throughout the sample, which consequently enhanced the electrical conductivity [25,26] and the EMI

Correspondence author. E-mail address: [email protected] (X. Liao).

https://doi.org/10.1016/j.compositesa.2019.05.017 Received 11 February 2019; Received in revised form 14 May 2019; Accepted 16 May 2019 Available online 17 May 2019 1359-835X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic showing the fabrication process of TPU/RGO composite foams. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

mechanical properties, which could provide the composites with good flexibility. Then the in-situ reduction method was used to improve the poor interface between the fillers and matrix, and L-ascorbic acid (LAA) was used as the reducing agent. In order to obtain TPU/RGO composites with low content of fillers as well as high absorption EMI shielding performance, we constructed segregated structure in the TPU/ RGO composite followed by introducing cellular to it through scCO2 foaming technology. Therefore, in this research work, GO sheets were successfully bonded to the surface of TPU particles in aqueous solution through the hydrogen bonding, followed by in-situ reduction by a mild reducing agent L-ascorbic acid (L-AA), improving the adhesion between TPU matrix and RGO. Subsequently the TPU/RGO composite with segregated conductive networks was foamed through scCO2 foaming method, in order to prepare flexible and lightweight foams for electromagnetic interference shielding with high absorption characteristic. The process of in-situ reduction mechanism of GO, the influence of RGO content on cellular structure and the EMI shielding of TPU/RGO composite foams, and the EMI shielding mechanism of foams had been studied comprehensively.

absorption shielding of samples because of the improved paths for electromagnetic wave in cellular structure [27–31]. Tran et al. [32] reported conductive microcellular foams of poly (methyl methacrylate)/carbon nanotubes (PMMA/CNT) nanocomposites, and established structure/properties relationships for further optimizing EMI shielding performances of the materials. Monnereau et al. [33] prepared polycarbonate/carbon nanotube (PC/CNT) nanocomposites foams with a gradient morphology that could be used as EMI absorbers and the experimental results indicated the gradient foams were advantageous for EMI shielding compared to the homogeneous foams. Hence, using the foaming technology to fabricate cellular polymer/graphene nanocomposites is an extremely valuable attempt for EMI shielding applications. Recently, structural design of polymer/nano-filler composites is reported as an effective ways to reduce the electrical percolation threshold and improve electrical conductivity. For example, some researchers constructed graphene foam framework and then compounded it with polymer [4,34,35], in this way, the 3D network not only resolved the agglomeration problem of fillers, but also provided efficient bunch of channels for electrical transport [36]. However, this kind of method was mainly used for thermosetting polymers [37,38]. A more general approach was to fabricate the segregated structure in conductive polymer nanocomposites [39]. In such kind of structure, electrical nano-fillers were selectively distributed at the interfaces of polymer particles, resulting in more efficient conductive pathways and better conductive networks. Yoonessi et al. [40] compared electrical conductivity of the segregated and homogeneous graphene/polycarbonate composites, as a result, the composites with segregated structure possessed conductivity higher by 220% than that of the homogeneous samples while the content of graphene was 4 wt%, in addition, the percolation threshold of the former composite was one third of that for the latter. Yan et al. [41] used in-situ thermal reduction method to prepare UHMWPE/graphene with a segregated structure, achieving the EMI shielding of 28.3–32.4 dB at a relatively low content of graphene. Even though the electrical conductivity and EMI shielding of composites with segregated structure were improved, the interface adhesion between polymer matrix and nano-fillers were not strong enough, leading to poor flexibility of polymer/nano-fillers composites [42]. To solve above problems, thermoplastic polyurethane (TPU) was used as the polymer matrix due to its flexibility, stretchability and good

2. Experimental 2.1. Materials Thermoplastic polyurethane powders (1185A) with a density of 1.12 g/cm3 and hardness of 85 (Shore A) were purchased by Tesulang Chemical raw Materials Co., Ltd. (Dongguan, China). The expanded graphite (200 μm) was obtained from Ruisheng Graphite Co., Ltd. (Qingdao, China). CO2 (> 99.9% purity) was supplied by Qiaoyuan Industry Co., Ltd. (Chengdu, China) and it was used as a physical blowing agent. Other reagents such as KMnO4, NaNO3, H2SO4, HCl, H2O2 and ascorbic acid (L-AA) were all analytical grade and provided by Kelong Chemical Co., Ltd. (Chengdu, China). 2.2. Fabrication of TPU/RGO composites Graphene Oxide (GO) was prepared by the modified Hummers method [43,44], as reported in our previous work. The fabrication of TPU/RGO composites with a segregated structure was shown in Fig. 1. The TPU particles coated with RGO were prepared by solution 311

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was used to measure the density of TPU/RGO composites and TPU/ RGO composite foams. Firstly, the weight of sample was measured in the air, and then the weight of sample in the water was measured by submerging it in the water, which was equal to the weight of samples in air minus buoyancy. The density of the sample in air and water had been known, thus the density of sample could be calculated as follows:

impregnation method. First of all, A certain amount of GO were dispersed in deionized water by stirring and ultrasonic. And 5 g TPU powders were then added to GO solution under vigorous stirring for another 1 h. After that, a desire quantity of L-AA were added to the mixed solution, followed by water-bath heating at 90 °C for 3 h. In this progress, the GO was in situ reduced to RGO by L-AA. Then vacuum filtration and freeze drying were used to remove residual solvent. And then the TPU/RGO composite powders were obtained. The TPU/RGO composite particles were compression molded into sheets by subjecting to 10 MPa at 120 °C for 10 min. And then the TPU/ RGO composites with various RGO were obtained, TPU particles deformed into multi-facets, and RGO were sandwiched into the contact interfaces and joints of TPU particles. The thickness of the samples was 1.8 mm. What’s more, the content of RGO on TPU particles was measured by weighting the weight of samples before and after solution impregnation, and the difference is the content of RGO coating on TPU. By the way, the content of RGO in composite depended on the concentration of GO solutions.

ρ=

Wa ρ Wa − Wi i

(1)

where Wa represented the weight of samples in the air, Wi represented the weight of samples in the water, and ρi represented the density of water. The electricity of TPU/RGO composites and TPU/RGO composite foams were measured by a four-point probe instrument and a high insulation resistance meter. EMI SE of TPU/RGO composites and TPU/ RGO composite foams were measured by the Agilent N5230 vector network analyzer in the frequency of 8.2–12.4 GHz (X-band) [46]. The scattering parameters (S11 and S21) of samples were gathered to calculate the power coefficients absorption (A), reflection (R) and transmission (T) and EMI SE depending on the following equations:

2.3. Fabrication of TPU/RGO composite foams The supercritical CO2 foaming method was employed as our previous work [25,45]. Firstly, TPU/RGO composite sheets were put in a high pressure vessel and the vessel was flushed with low pressure CO2 for about 5 min. After that, samples were exposed to scCO2 under 14 MPa and 80 °C for 2 h. During this progress, samples were saturated with enough CO2. Then the vessel was depressurized as quick as possible and simultaneously cooled to room temperature. Finally, these foamed samples were taken out of the high pressure vessel.

R = |S11 |2

(2)

T = |S21 |2

(3)

A=1−R−T

(4)

SEtotal = SEA + SER + SEM = −10 log T

(5)

SEA = −10 log (T/ (1−R))

(6)

SER = −10 log (1−R)

(7)

where the EMI SE (SEtotal) is the summation of the reflection shielding (SER), the absorption shielding (SEA) and the multiple reflections shielding (SEM), the SEM can be neglected while the SEtotal > 10 dB [47].

2.4. Characterization X-ray diffraction patterns (XRD) were employed to characterize the structure of RGO using a Philips X’Pert pro MPD diffractometer with Cu Kα radiation (λ = 0.154 nm) with the generator voltage of 40 KV and a current of 40 mA. The testing range was 5–60° and the sweeping speed was 5°/min. Fourier transfer infrared (FTIR) spectroscopy analysis was used to characteristic oxygen-containing groups of samples using a Thermo Nicolet Nexus 6700 spectrometer, and the testing wavenumber was from 400 to 4000 cm−1. A scanning electron microscope (SEM) (FEI Company, America) was employed to observe the microstructures of solid and foamed TPU/RGO composite samples with accelerating voltage of 20 kV. The optical microscope (OM) (Olympus bx53m) was used to observe the distribution of RGO in TPU/RGO composites. In order to improve the quality of observation, 10 μm thick specimens were prepared by a rotary slicer, and then observed with optical microscope equipped with a CCD camera. A specific gravity measurement kit (Shimadzu Corporation, Japan)

3. Results and discussion 3.1. The progress of in-situ reduction of GO The shape and size of GO sheets were shown in Fig. 2(a), most of the as-prepared GO sheets were single layer and the average diameter of GO sheets was ∼4 μm. Solution mixing method was used to prepare TPU/RGO composite with segregated structure. During this progress, TPU particles were coated with lots of GO sheets at first, after in-situ reduced by L-AA, most of the oxygen-containing groups on GO were removed, then the TPU coated RGO particles were obtained. And compression molding method was used to prepare TPU/RGO samples with segregated structure. The shape of the segregated structure of TPU/RGO had a great influence on the electricity and the EMI SE of

Fig. 2. (a) The SEM image of GO; (b) The XRD spectra of graphite, GO and RGO; (c) The FTIR spectra of GO, Graphene, TPU and TPU/RGO composite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 312

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L-AA was shown in Fig. 3. There were lots of oxygen-containing groups on GO sheets, such as hydroxyl groups, carboxyl groups, carbonyl groups and epoxy groups. Thus, the driving forces of coating may be hydrogen bonding between the urethane groups and carboxyl groups in TPU chains and oxygenated groups in GO sheets [48]. In order to make the composites conductive, it was very necessary to reduce GO coated on TPU. And the reason we chose L-AA was that the reduction effect was comparable to that of hydrazine hydrate and sodium borohydride, and itself and oxidized products were all environmentally friendly [49]. And the reduction progress was shown as follows. There were two active hydroxyl groups in the five-membered ring double bond of L-AA, which was easily for protons to dissociate out. On the one hand, an open-loop reaction was reacted for epoxy groups of GO, The hydroxyl group is formed at one end and connected with the L-AA oxygen anion at the other end. Then another hydroxyl group on the five-element ring double bond of L-AA attacked the hydroxyl group formed by the openloop reaction through the back and removed the water molecule, forming a intermediate. After that, the intermediate might recover C]C conjugate by removing small molecules. On the other hand, the hydroxyl group was replaced by the oxygen anions of L-AA and simultaneously be attacked on the reverse side, which was then reduced by the elimination reaction. And dehydroascorbic acid is readily decomposed into glucuronic acid and other oxidative products. However, there were a small amount of oxygen-containing groups remained in RGO. That is to say, the hydrogen bonding still existed between the TPU chain and RGO. And the RGO with little oxygen-containing groups was going to deliver good electrical conductivity for TPU/RGO composites.

samples. Thus it was important to research the processing parameters. In order to research the process of the in-situ reduction of GO, the XRD and FTIR analysis were employed in this work. The XRD spectra of Graphite, GO and RGO was shown in Fig. 2(b). The Bragg equation was used to calculate the XRD parameters. The specific diffraction peak of the GO appeared at 2θ = 9.7°, which means the layer spacing for RGO was 0.91 nm. That was to say, the GO synthesis by modified Hummers method had lots of oxygen-containing groups. After in-situ reduction by L-AA, the diffraction peak of (0 0 2) lattice plane of samples appeared at 2θ = 23.8°, thus the layer spacing of RGO was 0.37 nm, which was larger than the layer spacing of graphite (0.33 nm). As a result, most of the oxygen-containing groups in GO were reduced by L-AA, and the residual oxygen-containing groups of RGO contributed to the interfacial adhesion between the TPU and RGO. The FTIR spectra of GO, TPU and TPU/RGO composite were shown in Fig. 2(c). There were four characteristic absorption peaks of GO at wavelength 3427 cm−1, 1724 cm−1, 1625 cm−1 and 1046 cm−1, which respectively correspond to deformation vibration peaks of eOH, C]O vibration peaks, vibration peaks of benzene ring skeleton (C]C) and stretching vibration of CeO. For RGO reduced by L-AA, intensity of these characteristic absorption peaks decreased a lot, indicating that most of oxygen-containing groups in GO were successfully removed. For pure TPU, the characteristic absorption peaks at 3345 cm−1, 1600 cm−1 and 817 cm−1 referred to stretching vibration peaks of NeH, and peaks at 1735 cm−1 referred to C]O vibration peaks. For TPU/RGO composite, peaks at 3445 cm−1 and 1730 cm−1 proved that RGO were in-situ reduced by L-AA. The in-situ reduction mechanism of GO coated on TPU particles by

Fig. 3. Schematic illustration of the in-situ reduction of GO with L-AA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 313

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Fig. 4. SEM images of pure TPU powders and RGO-coated TPU powders. (a)–(g) respectively refer to the RGO content of 0, 0.5, 1.5, 2.5, 3.5 and 4.5 wt%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.2. The microstructure of and the flexible properties of TPU/RGO composites and TPU/RGO composite foams

which provide insight into the structure formation of the samples. It was easy to distinguish the distribution of RGO in the TPU/RGO composite because of the different light transmittance between RGO and TPU using the optical microscopy. That was to say, the RGO scarce region appeared to be bright and the RGO enriched region appeared to be dark. It can be observed that RGO were selectively distributed at the interfaces of TPU multi-facets, which formed interconnected networks throughout the composites. To be sure was that the segregated structure of TPU/RGO composites was successfully prepared. And TPU multifacets that RGO failed to penetrate into were regarded as “excluded volume”, increasing the effective RGO concentration and the possibility to form more conducting pathways. While the volume content of RGO increased, the dark region among TPU particles increased and the interconnected networks became more complete. And the dense RGO region between adjacent TPU particles formed highly conductive network, which enhanced the electrical conductivity.

SEM images of pure TPU powders and RGO-coated TPU powders with 0, 0.5, 1.5, 2.5, 3.5 and 4.5 wt% RGO were shown in Fig. 4. The particle size of TPU was about 100 μm, and the surface of TPU particles was relatively smooth. In addition, the particles had irregular shapes due to the preparation method of freezing grinding. The TPU particles were coated with RGO sheets through solution infiltration and in-situ reduction by L-AA. As presented in Fig. 4(b)–(f), some wrinkled loadings appeared on the surface of TPU particles, which indicated that RGO sheets were successfully coated on TPU particles. And the amount of wrinkled structure increased as the volume content of RGO increase, indicating that more RGO sheets were bonded with TPU chains. Fig. 5 show the optical microscope images of the film cut from the TPU/RGO composite containing 0.5, 1.5, 2.5, 3.5 and 4.5 wt% RGO,

Fig. 5. Optical microscope images of composite sections of TPU/RGO. (a)–(e) respectively refer to the RGO content of 0.5, 1.5, 2.5, 3.5, 4.5 wt%. 314

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and TPU/RGO composite foams with 0, 0.5, 1.5, 2.5, 3.5, 4.5 wt% RGO content of RGO was investigated by SEM, as shown in Fig. 7. For sample of pure TPU, the diameter of cell was inhomogeneous because of the interface between the contiguous TPU particles. And there were some huge cellular because of the air bubble wrapped in samples during the compression molding steps. And the TPU/RGO composite foams exhibited smaller cell diameter and higher density. Thanks to the hydrogen bond between TPU and RGO, the interface between TPU particles were be instead by wrinkled RGO sheets, and air bubbles wrapped less in samples while more RGO were added, thus little huge cell formed in TPU/RGO composite foams. Multistage cell structure with regular region formed in samples with 0, 0.5, 1.5, 2.5, 3.5, 4.5 wt% RGO. And the dividing line between adjacent regions was clearer as RGO loading increased, the diving lines were shown as these red lines in Fig. 7. In other words, the distribution of RGO in TPU/RGO composites greatly affected the cellular structure of samples. RGO was acted as heterogeneous nucleation points during the process of scCO2 foaming, resulting in lots of cell with small dimension along the interface of TPU. Heterogeneous nucleation points increased as the content of RGO increased, resulting in broaden boundary of big cell and small cell. However, the network of RGO limited the diffusion of CO2 while the content of RGO was high, then the dimension of cell in the region of TPU became smaller. The cell diameter in regions without RGO were about 15–30 μm while the cell diameters in regions with enriched RGO enriched were smaller than 10 μm. In general, several big cells were surrounded by lots of small cell. In this way, RGO moved to cell walls of these small cells, which further increased the effective RGO concentration and formed more conducting pathways. Fig. 8(a) showed the optical image of TPU/RGO composite foam. It was obviously to see the hierarchical pores in the foaming samples. And the inside of sample was divided into similar regions by different size of cells. Several big cells were encircled by some small cells. And Fig. 8(b) showed the SEM image of the cell wall of TPU/RGO composite foam. For larger cells in the center of each region, the cell wall was smooth and flat, and for smaller cells on the edge of each region, the cell wall was rough and had plenty of wrinkled structures. Therefore, it proved that the segregated structure remained in samples after foaming. And the density of TPU/RGO composites and TPU/RGO composite foams were shown in Fig. 9. After scCO2 foaming, the density of foamed samples were much lower than solid samples, which was beneficial to

Fig. 6. SEM images of TPU/RGO composites. (a) and (b) respectively refer to the RGO content of 0.5 and 4.5 wt%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For further showed the interface structure between RGO and TPU particles in TPU/RGO composites, the high resolution SEM images of samples with low and high content of RGO were shown as Fig. 6(a) and (b). The result showed that the TPU/RGO composite possessed typical segregated structure, RGO sheets were squeezed around the interface of TPU particles and formed continuous 3D networks. The magnified images interpretations proved that RGO and TPU were combined tightly, which result from the hydrogen-bond interaction among TPU particles and residual oxygen-containing functional groups of GO during the process of in-situ reduction. Additionally, the RGO region became widen as the RGO content increased, which was consistent with the results in Fig. 5. In order to prepared samples with lighter weight and higher electrical conductivity, the mild and environmentally friendly scCO2 foaming technology was used. And the cellular morphology of pure TPU

Fig. 7. SEM images of TPU/RGO foams with different content of RGO, (a)–(f) respectively refer to the RGO content of 0, 0.5, 1.5, 2.5, 3.5, 4.5 wt%. The foaming condition is 80 °C–14 MPa–2 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 315

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Fig. 8. (a) Optical image of TPU/RGO composite foam; (b) SEM images of the cell wall of TPU/RGO composite foam. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. The density of TPU/RGO composites and TPU/RGO composite foams loaded with different content of RGO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. The electrical conductivity of TPU/RGO composites and TPU/RGO composite foams loaded with different content of RGO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. (a)–(c) Photos of the TPU/RGO composite and the TPU/RGO composite foam before and after bending. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 316

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Fig. 12. EMI SE values of (a1) the TPU/RGO composites and (b1) the TPU/RGO composite foams as a function of frequency; Average A, R and T values of (a2) the TPU/RGO composites and (b2) the TPU/RGO composite foams. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Average EMI SE of TPU/RGO composites in this work and other conductive polymer/graphene composites in literatures in the X-band frequency range. Polymer

RGO content

Thickness (mm)

EMI SE (dB)

Structure

Ref.

TPU TPU TPU TPU PS WPU Epoxy PMMA PS PMMA PEI PVDF

6.5 wt% 6.5 wt% 12 wt% 20 wt% 10 wt% 7.7 wt% 15 wt% 8 wt% 30 wt% 5 wt% 10 wt% 7 wt%

1.8 1.8 N/A 0.05 2.8 2.0 2.0 3.4 2.5 2.4 2.3 N/A

24.7 21.8 28.3 20 18 18 or 32 21.0 30 29 19 13 28

solid foam solid solid solid solid solid solid foam foam foam foam

This work This work [17] [50] [51] [16] [11] [14] [13] [52] [12] [53]

N/A: not available.

achieve the lightweight of TPU/RGO composites. With the RGO loading increased, the density of foamed samples increased because of the thicker segregated layers limited the growth of cellular. Fig. 10 showed the optical photos of the TPU/RGO composite and the TPU/RGO composite foam before and after bending. Both solid and foamed sample could be bent and twisted freely by force, and the flatness of the sample after bending was nearly the same as the initial sample, which proved that TPU/RGO composite foam could be restored to its original state after releasing bending force. It was revealed that the sample possessed good flexibility.

Fig. 13. EMI SE of the 3.17 vol% RGO loaded composite (1.8 mm) before and after repeated bending to the radius of 2.0 mm for 1000 times. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.3. The electrical conductivity and EMI SE of TPU/RGO composites and TPU/RGO composite foams While the EMI shielding performance of polymer-based composites depended on the electric properties. The electrical conductivity of the solid and foamed TPU/RGO nanocomposites were investigated in this work. The relationship between the electrical conductivity and RGO loadings was shown in Fig. 11. It was observed that both the electrical 317

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to other samples at similar RGO loading. Therefore, it was indicated that the method we used in this paper was practicable for preparing polymer composites with EMI shielding properties. In order to fully evaluate the ability of samples to dissipate EM waves, we obtained the absorption coefficient (A), reflection coefficient (R) and transmission coefficient (T) of the solid and foamed TPU/RGO composite by calculating S-parameters, which were used respectively to express the ability of samples to absorb, reflect and transmit electromagnetic waves, as showed in Fig. 12(a2) and (b2). It was obtained that while the content of RGO was too low, EM waves would mainly transmission out of samples because of the imperfect conductive works. While the content of RGO increased, the T value decreased and the A values and R values increased. However, although the EMI absorption shielding was in the majority because of the segregated structure of TPU/RGO composite, the reflection characteristic of samples was relatively high. Thanks to the introduction of cellular structure, the A value of the TPU/RGO composite foams enhanced resulted from the orientated RGO around cell walls in TPU/RGO further after foaming. And the R value of foamed samples decreased a lot. To discuss the change of properties of TPU/RGO composite foams after bending, the electrical conductivity and the EMI shielding performance of TPU/RGO composite foams with 3.17 vol% RGO before and after repeated bending to the radius of 2.0 mm for 1000 times have been measured. The electrical conductivity of TPU/RGO composite foams was 2.77 S/m before bending, which was decreased to 2.61 × 10−3 S/m after bending. As shown in Fig. 13, the EMI shielding of TPU/RGO composite foam showed limited decreases over the frequency range, m of EMI shielding properties was retained after the repeated bending test. Fig. 14 showed the EMI shielding mechanism of the TPU/RGO composites and the TPU/RGO composite foams over the X-band frequency range. When the incident EM waves transfer to foam surface, this cell structure can significantly reduce the impedance mismatch between nanocomposite foam and air. As a result, EM waves can easily enter the foam interior with a small amount of reflection on the surface. The multistage cellular structure of TPU/RGO composite foams would promote the incident microwaves to be absorbed and transferred to heat through multiple reflecting and scattering on the interfaces between the RGO and TPU matrix. In a word, introducing the cellular structure in TPU/RGO composites would enhance their absorption property for EMI shielding, which would be beneficial to improve the absorption of electromagnetic waves of TPU/RGO samples.

Fig. 14. Schematic of EM waves transfer across (a) the TPU/RGO composites and (b) the TPU/RGO composite foams. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

conductivity of the solid and foamed TPU/RGO composites exhibited apparent increase as the content of RGO increased. While the content of RGO was low, the conductivity increased rapidly with RGO loading increased, and when the content of RGO was high, the conductivity of samples was increased slowly. What’s more, the electrical conductivity of foamed samples was lower than that of solid samples because of the volume exclusion effect. When the RGO loading was 0.27 vol%, the electrical conductivity of solid TPU/RGO composites was 3.21 × 10−4 S/m, and foamed TPU/RGO composites was 6.68 × 10−6 S/m, While RGO loading increased to 3.71 vol%, the value of the electrical conductivity of solid sample was 2.77 S/m, while that of the foamed samples was 2.53 × 10−1 S/m. Although the foaming process would decrease the electrical conductivity of samples, constructing the segregated structure in TPU/RGO composites played an important role in forming conductive network. The electrical conductivity of TPU/RGO composite foams was high than which prepared by traditional melt mixing method or solution mixing method, which was in advantage of the improving of EMI shielding performance of samples. Fig. 12(a1) and (b1) respectively showed the EMI SE of the TPU/ RGO composites and the TPU/RGO composite foams in the X-band (8.2–12.4 GHz). It was observed that EMI shielding performance exhibited frequency dependence across the measured frequency range, and EMI shielding of samples was increased with the increase of the content of RGO. For solid samples, the EMI shielding of TPU/RGO composites with 1.37 vol% RGO was 10.3 dB, indicating that approximately 90% of the electromagnetic radiation was blocked by the sample. While the content of RGO rose to 3.71 vol%, the EMI shielding increased to 24.7 dB, which was mainly attributed to the completed of segregated conductive networks. After scCO2 foaming, the EMI shielding performance of samples also increased with the RGO loading increased. Although the EMI SE of TPU/RGO composites decreased after foaming, it could be seen that the specific EMI SE (EMI SE divided by density) of TPU/RGO composite foam was higher than that of solid samples, for example, the specific EMI SE of TPU/RGO composite with 2.5 wt% was 10.4 dB cm3/g, while the specific EMI SE of the same sample after scCO2 foaming was 16.6 dB cm3/g. Thus the introduction of cellular structure in composites could help to improve the EMI SE performance. Table 1 compared the EMI shielding performance of sample in this work and that of solid or foamed polymer composites with RGO in other study. It seemed that the TPU/RGO composite and TPU/RGO composite presented comparable EMI shielding performance

4. Conclusion TPU/RGO composite foams were manufactured successfully through constructing a segregated structure combing with the scCO2 foaming method. The results demonstrated that the interface of TPU and RGO bonds well through the hydrogen bonding, and the segregated structure of solid TPU/RGO composites resulted in multistage pore structure foamed in samples after scCO2 foaming, which played positive roles to enhance the flexibility and realize the lightweight of foamed TPU/RGO composites. With the increase of RGO content, the electrical conductivity and EMI shielding of samples increased owing to the improvement of conductive network. For solid samples, the electrical conductivity of 2.77 S/m and the EMI SE of 24.7 dB were achieved with only 3.71 vol% RGO loading. After scCO2 foaming, the special cellular structure in TPU/RGO composite foams provided more paths for electromagnetic waves, leading to the EMI shielding with lower reflection characteristic and higher absorption characteristic at relatively low RGO content. The TPU/RGO composite foams could achieve 21.8 dB in the X band with a power coefficient of absorption (A) of 0.66. This result highlighted a potential method of constructing a segregated structure combing with a scCO2 foaming method in developing flexible and lightweight electromagnetic shielding materials with high absorption characteristic. 318

Composites Part A 123 (2019) 310–319

Q. Jiang, et al.

Acknowledgements [26]

This work was supported by the National Natural Science Foundation of China (Nos. 51773138 and 51721091) and the Science and Technology Department of Sichuan Province, China (No. 2019YFG0246).

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