Composites Part A 121 (2019) 411–417
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High conductive and mechanical robust carbon nanotubes/waterborne polyurethane composite ﬁlms for eﬃcient electromagnetic interference shielding ⁎
Hui Lia, Du Yuanb, Pengcheng Lia, , Chaobin Heb,c,
Key Laboratory for Green Chemical Process of Ministry of Education, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, 117574 Singapore, Singapore c Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 117602 Singapore, Singapore b
A R T I C LE I N FO
A B S T R A C T
Keywords: CNT/WPU composites Filtration process High conductive Mechanical property Electromagnetic interference
Highly conductive multi-walled nanotubes (MWCNT)/waterborne polyurethane (WPU) composites were prepared using facile latex technology without surfactant, in which the solid composites were achieved by ﬁltration process to prevent the re-aggregation of MWCNT during drying process. MWCNT was distributed along the interfaces of polyurethane particles, constructing segregated structure without insulating surfactant. Therefore, high conductivity of 362.6 ± 23.1 S m−1 was obtained at CNT content of 10.6 wt%, resulting in excellent electromagnetic interference (EMI) shielding eﬀectiveness (SE) of 24.7 dB at thin ﬁlm thickness of 0.4 mm in Xbands. Considering lightweight and thin thickness, outstanding speciﬁc shielding eﬀectiveness (SSE) of 537 dB cm2 g−1 was achieved, higher than most values of CNT composites ever reported. Furthermore, the composites exhibited desirable mechanical properties with high elongation at break of 62%, demonstrating promising applications for commercial EMI shielding, especially for ﬂexible and stretchable devices.
1. Introduction With the increasing application of electrical devices, wireless communication, and automation, electromagnetic radiation has become a serious problem. Every electronic device could generate electromagnetic waves which may cause malfunctioning of the devices and electromagnetic pollution on the surrounding environment and human body [1,2]. As a result, electromagnetic interference (EMI) shielding materials are greatly required for reliable applications of electronic devices. Conventional metals were the ﬁrst choice for industry until the development of electrically conducting polymer composites (CPCs). Compared with traditional metals which are suﬀering from high mass density, poor ﬂexibility, and corrosion, CPCs have become a promising candidate for EMI shielding, due to the advantages of ﬂexibility, lightweight, excellent mechanical property, and tunable electrical properties [3–5]. In general, EMI shielding eﬃciency (SE) value of 20 dB is required to satisfy commercial application which implies 99% of incident microwave radiation can be dissipated . Since EMI SE is strongly dependent on the electronic conductivity of the materials, thus
the materials which possess high conductivity are attracting for EMI shielding applications [1,7,8]. In order to satisfy the practical applications, the composites with high content of conductive ﬁller were usually required to achieve high conductivity. However, this would result in high cost, inferior mechanical property, and processing diﬃculties [6,9]. Therefore, achieving high conductivity composite at relative low content of conductive ﬁller is a promising strategy for practical applications. Carbon nanotubes (CNT), with the unique structure, high aspect ratio, good mechanical strength, and high electric conductivity, are considered as one of the most promising candidates as the conductive ﬁller for CPCs [10–12]. The dispersion of conductive ﬁller in the polymer matrix has signiﬁcant eﬀect on the conductive behaviour of the composites. With strong Van Der Waals force, CNT tend to aggregate as bundles, making it diﬃcult to be uniformly dispersed in polymer matrix to form network. The poor dispersion in the polymer matrix would increase the contact resistance and on the other hand deteriorate the mechanical performance of the composites. In recent years, great eﬀorts have been made to improve the dispersion of
Corresponding authors at: Key Laboratory for Green Chemical Process of Ministry of Education, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China (P. Li). Department of Materials Science & Engineering, National University of Singapore, 9 Engineering Drive 1, 117574 Singapore, Singapore (C. He). E-mail addresses: [email protected]
(P. Li), [email protected]
(C. He). https://doi.org/10.1016/j.compositesa.2019.04.003 Received 8 January 2019; Received in revised form 27 March 2019; Accepted 5 April 2019 Available online 05 April 2019 1359-835X/ © 2019 Published by Elsevier Ltd.
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Fig. 1. (a) Size distribution of WPU particles; SEM images of MWCNT/WPU composites with CNT content of 0.75 wt% (b) and 1.6 wt% (c), inset is the cross-section image of the composite ﬁlm; (d) Schematic illustrations of latex technology for the fabrication of segregated MWCNT/WPU composites. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
conductive ﬁllers and conductivity of the composites , such as melt mixing, surface chemical modiﬁcation, and surfactant method [14–17]. However, relatively large amount of conductive ﬁller is required to achieve high conductivity as most of CNT do not take part in the formation of conductive network, which may sacriﬁce the mechanical properties of the composites [3,18]. Selective localization of CNT in one preferred phase of biphasic blends provides an alternate way to improve both the electrical property and the mechanical property of conducting composites [19–21]. However, this approach requires precise control on the biphasic mixing conditions, such as mixing time and biphasic ratios. Recently, constructing segregated structure has become a promising strategy for forming well-established conductive network with minimal ﬁller loading [6,19,22]. In this morphology, conductive ﬁllers distribute along the boundary of the polymer domains to form conductive pathways, which eﬃciently increases the utilization ratio of the conductive ﬁller to form conducting network and increase conductivity [23–25]. For instance, Zeng et al. fabricated segregated MWCNT/high density polyethylene (HDPE) composites by alcohol-assisted dispersion and hot-pressing process, which exhibited low percolation thresholds of 0.15 vol% . Segregated MWCNT/poly(vinylidene ﬂuoride) (PVDF) composites were prepared by mechanical mixing and hot compaction, which possessed high conductivity and SE of 6 S m−1 and 30.9 dB respectively, with 7 wt% ﬁller content . The signiﬁcant results promote the improvement of CPCs. However, most of the fabrication processes are complicated, which need melt processing or hot compaction [27–29]. In contrast, facile and environment friendly latex technology has attracted extensive attention to synthesize segregated CPCs [30–32]. The emulsions create excluded volume and essentially push conductive ﬁller lie on the interfaces of polymer particles, resulting in segregated conductive network with high conductivity. Grunlan et al. incorporated SWCNT into poly(vinyl acetate) (PVAc) latex with gum Arabic(GA) as stabilizer to enhance the dispersion of conductive ﬁller. With CNT content of 4 wt%, the composites exhibited high electrical conductivity of ∼32 S m−1 . Loos et al. prepared conductive MWCNT/PS ﬁlms by latex technology with
sodium dodecyl sulfate(SDS) as stabilizer, which exhibited low percolation threshold about 1.5 wt% . However, the insulating surfactants would deteriorate the electrical property of the composites . Moreover, most of the ﬁlms were obtained by freeze-drying and dropcasting process [31,33], which complicate the fabrication processing. In this work, we developed a facile latex technology to fabricate high conductive MWCNT/waterborne polyurethane (WPU) ﬁlms by ﬁltration process without surfactant. WPU emulsions were used as polymer matrix, due to the advantages of environmental friendly and excellent elastic property . As CNT are relatively hydrophobic, stabilizer becomes crucial to achieve stable suspension and prevent reaggregation of CNT for the typical drying process of drop-casting ﬁlms and freeze drying. In order to preserve the segregated network without stabilizer, we propose a facile strategy of ﬁltration process to achieve solid composites. Although various CNT/polymer composites have been studied for EMI shielding, there is few work to fabricate ﬁlms directly by ﬁltration process, especially for CNT/WPU composites. Technically, this process is superior to most of the reported latex technologies which require complicated freeze-drying and compaction processing [26,31]. Moreover, segregated network without insulating surfactant was beneﬁcial for the electrical property of composites. Finally, high conductive MWCNT/WPU composites were achieved by this facile latex process. Besides, the elastic property of WPU enables the composites to possess superior ﬂexible and stretchable feature, making it promising composites for application in commercial EMI shielding. These results demonstrate the feasibility of this approach to achieve high conductive CPCs. With facile process, high conductivity, and desirable mechanical property, this material demonstrated potential applications especially for stretchable devices and wearable electronics.
2. Experimental 2.1. Materials MWCNT were purchased from Nanocyl (NC-3100) with the average 412
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particles was obtained by dynamic light scattering (DLS) measurements (BI-200SM-3 DLS system) with a He-Ne laser operating at wavelength of 632.8 nm. The conductivity of the composite was measured by a fourprobe method with a Keithley 2400 source/meter. Tensile tests were performed using an Instron Universal Tester 5569 at a crosshead speed of 10 mm min−1. Rectangle shape tensile specimens were cut with gauge length of 10 mm and width of 3 mm. At least 5 specimens were measured for tensile test. Young’s modulus was calculated by ﬁtting linear relationship between stress and strain, and the slope of the stressstrain curve at low strain was calculated to be Young’s modulus. EMI shielding measurements were conducted using Agilent/HP 8510C Vector Network Analyzer (VNA) over the frequency range of 8.2–12.4 GHz (X-band). 3. Results and discussion Waterborne polyurethane (WPU), with excellent mechanical property and environmental friendly, has attracted extensive attention for practical applications as polymer matrix [3,35]. WPU emulsions consisted of polymer particles prior to ﬁlm formation which were around 85 nm and 250 nm (Fig. 1a). Since MWCNT is diﬃcult to penetrate into the particles, they are excluded to lie on the interfaces of particles as evidenced by the scanning electron microscope (SEM) images. Fig. 1b showed that CNT was dispersed well in the composites without any large aggregates. Meanwhile, as WPU domains (marked with yellow lines) create excluded volume, CNT were pushed to enrich around the polymer domains and form connected network rather than uniformly distributed within the composites, thus constructing segregated structure. These results indicate the feasibility of the ﬁltration process for the construction of segregated structure. In the typical slow drying process of drop-casting, CNT prone to re-aggregated during the solution evaporating. Therefore, insulating stabilizer was crucial to achieve stable suspension and disperse CNT along the interfaces of emulsion particles. In this work, ﬁltration process could remove water solution in a few minutes and preserve the segregated structure without large CNT aggregates. With increasing the content of MWCNT, the bright domains were increased and MWCNT were interconnected to enhance charge transport within the composites, which indicates the outstanding electrical properties of the composites. Furthermore, as shown in the crosssectional SEM images, the obtained ﬁlm is dense without porosity, beneﬁcial for the desirable mechanical performance and electrical property. Electrical conductivity of the composites as a function of MWCNT volume fractions was investigated and shown in Fig. 2. Considering that the conductivity of WPU was in the order of 10-14 S m−1, the conductivity of the composites displayed a dramatic increase with increasing MWCNT loadings of 0.53 wt%, indicating a typical percolation transition behaviour [26,36,37]. Further increasing the content of MWCNT lead to a slightly increase of the conductivity of the composites. The experimental data could be ﬁtted using the following equation to estimate the percolation threshold concentration (φ c) ,
Fig. 2. (a) Electrical conductivity of MWCNT/WPU composites with diﬀerent CNT loadings. The straight line in the inset is a ﬁt to the data according to statistical percolation theory; (b) Comparison of the conductivity of the CNT composites. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
diameter of 9.5 nm, average length of 1.5 μm, and purity > 95 wt%. Waterborne polyurethane (WPU) was obtained from Taiwan PU Corporation (WPU-372D) with concentration of 50 wt%. The WPU suspension consists PU particles of both 85 nm and 250 nm as shown in Fig. 1. 2.2. Preparation of MWCNT/WPU ﬁlms Firstly, MWCNT were dispersed in H2O with the concentration of 2.5 mg g−1 by probe ultrasonic on SCIENTZ ultrasonic homogenizer JY92-IIN (650 W) with ultrasonic power of 30% for 10 min. Then CNT dispersion was blended with WPU suspension, and the blended dispersion was vacuum ﬁltrated using Advantec mixed cellulose ester ﬁlter membrane with pore size of 0.20 μm for about 3 min. After that, the composites were cured in the oven at 70 °C for 2 h and the ﬁlms were obtained. The weight ratios of MWCNT were estimated by the initial weight of MWCNT and ﬁnal weight of the composites, and the ﬁlms with various MWCNT weight ratios of 0.53–21.1 wt% were obtained. The ﬁlm thickness was varied from 0.05 mm to 0.4 mm by tuning the amount of mixture.
σ = σ0 (φ − φc )t where σ is the conductivity of the composite, σ0 is a constant related to the intrinsic conductivity of MWCNT, φ is the volume fraction of MWCNT, φc is the volume fraction of conductive ﬁller at percolation threshold, and t is the critical exponent reﬂecting the system dimensionality of the composites. The best ﬁtted value for the percolation threshold was 0.277 vol% (0.53 wt%), lower than most values of MWCNT/polymer composites (0.5 vol% for MWCNT/TPU composites prepared by solution cast , 1.0 wt% for MWCNT/TPU composites prepared through melt blending and compression moulded , 2.0 wt % for MWCNT/HDPE composites prepared by solution precipitation ), and even lower than SWCNT/polymer composites (4 wt% for SWCNT/HDPE by melt processing , 1.8 vol% for SWCNT/PMMA composites prepared using coagulation and compression processing
2.3. Characterizations The morphology of the synthesized MWCNT/WPU composite was investigated under Hitachi SU3500 scanning electron microscope (SEM) at an accelerating voltage of 15 kV. The size distribution of WPU 413
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Fig. 3. EMI curves of MWCNT/WPU composites with various MWCNT contents(a), 5.3 wt% MWCNT/WPU composites with various thickness(c), 10.6 wt% MWCNT/ WPU composites with various thickness (e); corresponding shielding eﬃciency of the composites with various MWCNT contents(b), 5.3 wt% MWCNT/WPU composites with various thickness (d), 10.6 wt% MWCNT/WPU composites with various thickness (f). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
10 min before ﬁltration to enhance the dispersion of CNT. It exhibited much lower conductivity of 2.2 ± 0.67 S m−1 and 77.2 ± 19.1 S m−1 with CNT mass content of 5.3 wt% and 10.6 wt%, respectively. These results indicate the unique segregated structure of CPCs and feasibility of latex technology with ﬁltration process to construct high conductive CPCs, which demonstrates potential application for EMI shielding. To demonstrate the potential of the MWCNT/WPU composites for EMI shielding, the EMI shielding eﬃciency (SE) which characterizes the ability of an EMI shielding material to attenuate electromagnetic radiation were investigated over the frequency range of 8.2–12.4 GHz. EMI shielding mechanism mainly consists of reﬂection, absorption, and multiple internal reﬂections, which are largely dependent on mobile
). Besides, the critical exponent was estimated to be 1.74, which means two-dimensional conductive network was constructed . Furthermore, the composites displayed high conductivity of 123.8 ± 18.2 S m−1 and 362.6 ± 23.1 S m−1 with CNT mass content of 5.3 wt% and 10.6 wt%, respectively, higher than most of the reported values of the composites with comparable conductive ﬁller content (20 S m−1 for 15 wt% SWCNT/epoxy [45,46], 12.4 S m−1 for 10 wt% MWCNT/TPU , 45.6 S m−1 for 10 wt% MWCNT/TPU ). The low percolation threshold and high electric conductive could be attributed to an existence of segregated structure. For comparison, MWCNT/WPU composite with CNT better dispersed within the matrix was prepared by ultrasonic treatment of the MWCNT/WPU mixture for 414
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Table 1 Comparison of various CNT composites for EMI shielding. Materials
SSE/dB cm2 g−1
MWCNT/PS MWCNT/PS MWCNT/EMA MWCNT/PC MWCNT/PVDF MWCNT/ABS A-MWCNT/TPU MWCNT/TPU SWCNT/epoxy SWCNT/epoxy SWCNT/PMMA
10 5 10 10 7 10 10 10 15 15 20
2 2 2 2 2 1.1 1.5 2 2 1.5 4.5
∼1.05 ∼1 ∼1 1.1 ∼1.8 1.05 ∼1.3 ∼1.3 1.3 ∼2 ∼1.2
10 7.1 0.001 ∼100 6 100 12.4 45.6 20 20 2
50 17.2 20 27 33.2 40.7 29 22 25 20 30
238 86 ∼100 123 ∼92 ∼352 ∼149 ∼85 96.2 ∼67 ∼56
          
absorption of the electromagnetic wave is expressed as the following equations [35,49],
SE(R) = 39.5 + 10log(σ/(2πfμ))
SE(A) = 8.7t πfμσ while f is the frequency of the electromagnetic wave, μ is the magnetic permeability, σ is the conductivity, and t is the sample thickness. It is apparent that both shielding by reﬂection and absorption represent a positive correlation with its electrical conductivity. And SE(A) is also dependent on the thickness of materials. The EMI curves for the composites with varying MWCNT contents and thickness over the frequency range of 8.2–12.4 GHz were shown in Fig. 3. With MWCNT contents of 3.2 wt%, SE of the composites was only 2.5 dB at 10 GHz. In general, CNT attenuate electromagnetic (EM) radiations into thermal and internal electrical energies to dissipate EM waves, contributing to higher SE(A) with increasing CNT contents . While SE(R) relies on the interactions between incoming EM waves and mobile charge carriers on the surface of the materials, which related with the conductivity of the materials . Therefore, with increasing CNT contents, more CNT resulted in large amount of charge carriers to interact with incoming waves and dissipate EM energies, leading to improved EMI shielding performance. Consequently, SE(R) and SE(A) exhibited a signiﬁcant increase and SE of 8.0 dB and 12.8 dB were obtained for MWCNT/WPU composites with MWCNT content of 5.3 wt% and 10.6 wt%, respectively. Furthermore, as the MWCNT content increased to 21.1 wt%, SE of 24.5 dB was achieved with thin ﬁlm thickness of only 0.1 mm (as shown in Fig. 3b). The EMI SE for the MWCNT/WPU composites with MWCNT content of 5.3 wt% and 10.6 wt% with various thicknesses was investigated and shown in Fig. 3(c)–(f). SE was greatly increased with increasing the thickness of the composites from 0.1 mm to 0.4 mm. Samples with higher thickness provided more conductive ﬁllers to attenuate electromagnetic waves, resulting in enhanced EMI shielding performance. With the ﬁlm thickness of 0.4 mm, SE values of 21.1 dB and 24.7 dB were obtained for 5.3 wt% and 10.6 wt% MWCNT/WPU respectively, which was satisfy for practical applications. For comparison, most of reported materials for EMI SE applications required higher thickness about 1–2 mm or higher content of conductive ﬁllers (as summarized in Table 1), which inevitably increase the weight of the electronic devices and deteriorate the ﬂexibility feature of the composites . In order to evaluate the shielding eﬃciency of materials more reality, speciﬁc shielding eﬀectiveness (SSE = SE/ρt) is derived to compare the eﬀectiveness of shielding materials taking into account of ﬁlm thickness (t) and density (ρ). As shown in Fig. 4, the MWCNT/WPU composites with CNT content of 10.6 wt% exhibited high SSE of 537 dB cm2 g−1, which is higher than most value of the reported CNT/polymer ﬁlms (20–400 dB cm2 g−1, Table 1). This outstanding EMI shielding performance is attributed to optimum segregated network structure of the
Fig. 4. Comparison of SSE of the CNT composites ever reported as a function of thickness. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
Fig. 5. Stress-strain curves of MWCNT/WPU ﬁlms with various MWCNT content of 0, 1.1 wt%, 5.3 wt%, and 10.6 wt%. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)
charge carrier, electric or magnetic dipoles, and interfaces in the shielding materials, respectively . Total EMI SE is usually the sum of reﬂection (SE(R)) and absorption (SE(A)), as the multiple reﬂections (SE(M)) of electromagnetic radiation can be neglected when SE is greater than 10 dB . For conductive materials, the reﬂection and 415
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composites with interconnected conductive network, which leading to high conductivity at relative low conductive ﬁller content. Moreover, the excellent segregated structure and electrical property demonstrate the feasibility of the latex technology to fabricate high conductive CPCs. With the facile process, low cost, and environmental friendly, this approach could well be applied to other conductive composites. The mechanical property of WPU and MWCNT/WPU composites was shown in Fig. 5. With addition of MWCNT, the Young’s modulus increased from 9.3 MPa of pristine WPU to 110 MPa of MWCNT/WPU with CNT content of 10.6 wt%, attributing to the stiﬀness of conductive ﬁllers and good interfacial interactions between the CNT and polymer [57,58]. Meanwhile, the elongation at break decreased with the increasing of CNT loading. At CNT content of 10.6 wt%, the composite exhibited elongation of 62%. The superior ﬂexibility and stretchability demonstrate its potential application on commercial industries, such as sensors, actuators, and wearable electronics which have been intensively studied to realize smart practical applications.
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4. Conclusions High conductive MWCNT/WPU composite was prepared by facile latex technology without surfactant. The interconnected network of MWCNT at the interfaces of PU matrix domains improved the conductivity of the composites, resulting in low percolation threshold of 0.277 vol%, lower than most values of MWCNT composites ever reported. Meanwhile, high conductivity of 362.6 ± 23.1 S m−1 was achieved with CNT mass content of 10.6 wt%, leading to excellent SE of 24.7 dB with thin ﬁlm thickness of 0.4 mm. Furthermore, the composites exhibited desirable mechanical property for practical applications, especially for the ﬂexible and stretchable conductive devices. These results demonstrate the feasibility of this approach to construct high conductive CPCs and pave the way for constructing mechanical robust conductive composites for eﬃcient EMI shielding. Acknowledgments This work was supported by A-star research grant [R-284-000-135305], the National Natural Science Foundation of China [Grant No. 51803156], and Hubei Provincial Natural Science Foundation of China [2018CFB102]. References  Shahzad F, Alhabeb M, Hatter CB, Anasori B, Hong SM, Koo CM, et al. Electromagnetic interference shielding with 2D transition metal carbides (MXenes). Science 2016;353(6304):1137–40.  Chung DDL. Electromagnetic interference shielding eﬀectiveness of carbon materials. Carbon 2001;39(2):279–85.  Zeng Z, Chen M, Jin H, Li W, Xue X, Zhou L, et al. Thin and ﬂexible multi-walled carbon nanotube/waterborne polyurethane composites with high-performance electromagnetic interference shielding. Carbon 2016;96:768–77.  Du F-P, Yang W, Zhang F, Tang C-Y, Liu S-P, Yin L, et al. Enhancing the heat transfer eﬃciency in graphene-epoxy nanocomposites using a magnesium oxide-graphene hybrid structure. ACS Appl Mater Interf 2015;7(26):14397–403.  Zeng F, Feng G, Nguyen ST, Hai MD. Advanced multifunctional graphene aerogel – poly (methyl methacrylate) composites: experiments and modeling. Carbon 2015;81(1):396–404.  Yan DX, Pang H, Li B, Vajtai R, Xu L, Ren PG, et al. Structured reduced graphene oxide/polymer composites for ultra-eﬃcient electromagnetic interference shielding. Adv Funct Mater 2015;25(4):559–66.  Shen B, Zhai W, Zheng W. Ultrathin ﬂexible graphene ﬁlm: an excellent thermal conducting material with eﬃcient EMI shielding. Adv Funct Mater 2014;24(28):4542–8.  Zeng Z, Chen M, Pei Y, Shahabadi Seyed SI, Che B, Wang P, et al. Ultra-light and ﬂexible polyurethane/silver nanowire nanocomposites with unidirectional pores for highly eﬀective electromagnetic shielding. ACS Appl Mater Interf 2017;9(37):32211–9.  Kuang T, Chang L, Chen F, Sheng Y, Fu D, Peng X. Facile preparation of lightweight high-strength biodegradable polymer/multi-walled carbon nanotubes nanocomposite foams for electromagnetic interference shielding. Carbon 2016;105:305–13.  Du FP, Ye EZ, Yang W, Shen TH, Tang CY, Xie XL, et al. Electroactive shape memory polymer based on optimized multi-walled carbon nanotubes/polyvinyl alcohol
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