Multilayer WPU conductive composites with controllable electro-magnetic gradient for absorption-dominated electromagnetic interference shielding

Journal Pre-proofs Multilayer WPU conductive composites with controllable electro-magnetic gradient for absorption-dominated electromagnetic interference shielding An Sheng, Wei Ren, Yaqi Yang, Ding-Xiang Yan, Hongji Duan, Guizhe Zhao, Yaqing Liu, Zhong-Ming Li PII: DOI: Reference:

S1359-835X(19)30441-5 https://doi.org/10.1016/j.compositesa.2019.105692 JCOMA 105692

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Composites: Part A

Received Date: Revised Date: Accepted Date:

9 October 2019 7 November 2019 9 November 2019

Please cite this article as: Sheng, A., Ren, W., Yang, Y., Yan, D-X., Duan, H., Zhao, G., Liu, Y., Li, Z-M., Multilayer WPU conductive composites with controllable electro-magnetic gradient for absorption-dominated electromagnetic interference shielding, Composites: Part A (2019), doi: https://doi.org/10.1016/j.compositesa. 2019.105692

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Multilayer WPU conductive composites with controllable electro-magnetic gradient for absorption-dominated electromagnetic interference shielding An Sheng1, Wei Ren1, Yaqi Yang1,2, Ding-Xiang Yan2,3 Hongji Duan1*, Guizhe Zhao1, Yaqing Liu1*, Zhong-Ming Li2,4 1

College of Materials Science and Engineering, Key Laboratory of Functional

Nanocomposites of Shanxi Province, North University of China, Taiyuan 030051, People’s Republic of China 2

State Key Laboratory of Polymer Materials Engineering, Sichuan University,

Chengdu 610065, People’s Republic of China 3

School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065,

People’s Republic of China 4

College of Polymer Science and Engineering, Sichuan University, People’s Republic

of China

Abstract A well-designed multilayered waterborne polyurethane (WPU) shielding composites with absorption-dominated shielding feature are realized by constructing a controllable electro-magnetic gradient. Using the layer-by-layer casting method with reasonable arrangement of Fe3O4@rGO and MWCNT nanofiller, an ordered multilayer shielding network can be constructed to provide the WPU composites with positive



Corresponding author. Tel./fax: +86 3513559669.

E-mail addresses: [email protected] (H. Duan); [email protected] (Y. Liu)

electrical conductivity gradient and negative magnetic gradient. Hence, the penetrating microwave would undergo a particular “absorption-reflection-reabsorption” process and interface polarization loss induced absorption process between impedance matching layer and high conductive layer, leading to rather low microwave reflection with effect electromagnetic interference shielding effeteness (EMI SE). With the increase of electro-magnetic gradient, the EMI SE of the Fe3O4@rGO/MWCNT/WPU composites reaches 35.9 dB, while the power coefficient of reflectivity can be significantly decreased to 0.27. This work offers a feasible strategy for designing absorption-dominated shielding material with tunable electromagnetic performance that suitable for next-generated smart electronic devices. Keywords: A: Polymer-matrix composites (PMCs); A: Nano particles; B: Electrical properties; Electromagnetic interference shielding

1. Introduction Nowadays, with the increasing necessity of electromagnetic shielding protection, conductive polymer composites (CPCs) served as electromagnetic interference (EMI) shielding materials have been extensively developed due to the great advantages for light weight, tunable conductivity, processability and corrosion resistance [1-9]. In order to obtain CPCs with more efficient EMI shielding effectiveness (SE), the pursuit of superior conductivity for CPCs has become the primary strategy due to the greatly enhancement of electromagnetic (EM) wave reflection that caused by intensified impedance mismatching [10-13]. In recent works, highly conductive fillers such as 2D transition-metal carbides (MXenes) [14, 15], silver nanowire (AgNW) [16, 17] and

silver nanoparticle (AgNP) [18] are employed in CPCs to realize superior EMI SE. For instance, the newly reported Ti3C2Tx MXene/PEDOT: PSS films exhibit a high EMI SE value of 42.1 dB with ultrahigh power coefficient of reflectivity (R) of 0.97, it means that 97 % of the incident electromagnetic waves are directly reflected back [19]. Obviously, highly conductive CPCs for superior EMI SE will inevitably result in excessive wave reflection, which would cause serious secondary electromagnetic wave pollution. To alleviate the secondary electromagnetic wave pollution, the magnetic nanoparticles, such as Fe3O4@rGO and Fe3O4@CNT hybrids, are introduced to promote the absorption feature of CPCs [20-24]. The magnetic nanoparticles can provide magnetic loss and the carbon nanofillers contribute dielectric loss, which can dissipate electromagnetic waves more effectively by means of absorption. Zhang et al. reported a Fe3O4@MWCNT/PMMA composite with an EMI SE of 25.11 dB in X band and the R value is 0.47 [25]. Sharif et al. introduced Fe3O4@rGO magnetic nanoparticle into PMMA matrix, revealing an EMI SE of 29.3 dB and R of 0.6 [26]. Although the composites with magnetic hybrid fillers can decrease reflection to some extent, such composites with homogeneous conductive network always exhibit uniform conductivity with fix impedance mismatch condition [27-30], which is useless to decrease the microwave reflection for shielding composites with high EMI SE. In order to avoid the contradiction between the low reflection and high EMI SE, in our previous work [31], a novel strategy of designing gradient conductive networks has been adopted to realize the low reflection feature for the high-efficient CPCs

shielding film. The well-designed gradient absorption network can lead to a special “absorb-reflect-reabsorb” process when electromagnetic waves penetrating, hence resulting in an absorption dominated shielding mechanism. However, this composite film with “filler-density-induced” single-layered gradient has great limitations on both processing feasibility and structural controllability when constructing gradient shielding network. Therefore, seeking absorption-dominated EMI shielding CPCs with processable gradient shielding network structure and easy-tuned electrical-magnetic gradient remains a great challenge. In this work, we report a well-designed multilayered waterborne polyurethane (WPU) shielding composites with controllable electro-magnetic gradient for absorption-dominated shielding feature. The composite consists of three Fe3O4@rGO/ WPU absorption layers and one MWCNT/WPU reflection layer. Using the layer-bylayer casting (LbL-casting) method with reasonable arrangement of Fe3O4@rGO and MWCNT nanofiller, an ordered multilayer shielding network can be constructed to provide the WPU composites with positive electrical conductivity gradient and negative magnetic gradient. Thanks to the layered gradient structure design and easy-tuned electro-magnetic gradient realized by regulating the rGO content of Fe3O4@rGO in each Fe3O4@rGO/WPU layer, the impedance matching and shielding efficiency can be well-matched to give the WPU composites ideal EMI SE and very low reflection feature. With the regulation of rGO controlled electro-magnetic gradient, the R value of the composite can be decreased to 0.27 while the EMI SE reaches 35.9 dB, which means that 99.99 % of EM waves are blocked during penetrating but only 27 % of them are

reflected. The significant high absorption and low reflection characteristics demonstrate the absorption-dominated shielding mechanism of this Fe3O4@rGO/MWCNT/WPU composites. The multilayer design and gradient structure induced “absorptionreflection-reabsorption” shielding mechanism as well as the special interface polarization induced synergistic absorption mechanism between impedance matching layer and high conductive layer are comprehensively discussed in this wok.

2. Experimental 2.1 Materials Anionic aliphatic waterborne polyurethane (WPU) with a solid content of 35 wt% was supplied by Xincheng Engineering Plastics Co. Ltd. (Guangzhou, China). MWCNT with average diameter around 50 nm and length of 10-20 μm was obtained from Chinese Academy of Sciences (Chengdu, China). Graphene Oxide (GO) with 410 layers and lateral dimension of 0.5-30 μm was purchased from Tangshan Jianhua Technology Development Co., Ltd. (Hebei, China). FeCl3·6H2O, FeSO4·7H2O, NH3·H2O, hydrazine hydrate (80 wt%) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). All materials were directly used without further purification. 2.2 Synthesis of Fe3O4@rGO magnetic nanoparticles The Fe3O4@rGO magnetic nanoparticles was synthesize by the feasible coprecipitation method as the following process [31, 32]: 1.09 g FeSO4·7H2O and 0.6 g FeCl3·6H2O are firstly dissolved in deionized water to obtain Fe2+/Fe3+ solution, then the GO dispersion was slowly added into the solution. After vigorously stirring for

30 min under N2 atmosphere, the solution was heated to 90 ℃, followed by adding NH3·H2O and hydrazine hydrate. Subsequently, the reaction kept for 3 h at 90 ℃ to obtain Fe3O4@rGO dispersion, and then the black product was magnetically separated by a commercial magnet and washed with deionized water. Finally, the Fe3O4@rGO magnetic nanoparticles were obtained by freeze and drying, and denoted as Fe3O4@rGO-X, where X is the original additive amounts of GO. For example, Fe3O4@rGO-20 means the original GO additive amounts is 20 mg in its synthesizing process. 2.3 Preparation of Fe3O4@rGO/WPU and MWCNT/WPU aqueous dispersion The Fe3O4@rGO/WPU and MWCNT/WPU aqueous dispersion were prepared by simple mixing the nanoparticles with WPU emulsion. For the preparation of Fe3O4@rGO/WPU dispersion, a certain amount of Fe3O4@rGO nanoparticles were first added in WPU emulsion and then stirred, sonicated for 30 min in ultrasonic water bath to obtained the resultant dispersion. For MWCNT/WPU dispersion, the WPU emulsion was first diluted with deionized water, and then the MWCNT was ultrasonicated in the diluted WPU emulsion with 400 W ultrasonic treatment for 15 min and magnetic stirring for 5 h. All the dispersions were degassed in vacuum oven at room temperature for 12 h. 2.4 Preparation of Fe3O4@rGO/MWCNT/WPU composites The fabrication of the Fe3O4@rGO/MWCNT/WPU composites is illustrated in Fig.1. The composites prepared by the LbL-casting method are casted with the order of Fe3O4@rGO-X1/WPU,

Fe3O4@rGO-X2/WPU,

Fe3O4@rGO-X3/WPU

and

MWCNT/WPU from the top to bottom. For each Fe3O4@rGO-X/WPU layer, the loading of the Fe3O4@rGO particles is fix at 11.2 wt%, the difference is the gradually increased rGO content of Fe3O4@rGO that used in each layer, which would contribute to positive conductivity gradient and negative magnetism gradient. The final composite is designated as Fe3O4@rGO(X1-X2-X3)-nMWCNT, where the X is the original additive amounts of GO and X1-X2-X3 represent the gradient of Fe3O4@rGO layers, n is the mass fraction of MWCNT in MWCNT/WPU layer. For instance, Fe3O4@rGO(20-60-100)-30MWCNT means the Fe3O4@rGO layers gradient is 20-60100, and the MWCNT content of MWCNT/WPU layer is 30 wt%. Five Fe3O4@rGO magnetic nanoparticles with different GO addition amount are employed to prepare three kinds of graded Fe3O4@rGO/MWCNT/WPU composites, the Fe3O4@rGO(6060-60)-nMWCNT, Fe3O4@rGO(40-60-80)-nMWCNT and Fe3O4@rGO(20-60-100)nMWCNT.

Fig.1 Schematic of the fabrication of Fe3O4@rGO/MWCNT/WPU composites, the insets are (a) the Fe3O4@rGO magnetic nanoparticles and (b) Fe3O4@rGO/MWCNT/WPU composite sample.

2.5 Characterization The X-ray diffraction (XRD, Haoyuan 2700B) using Cu Kα (k = 0.1546 nm) was used to determine the structure of Fe3O4@rGO magnetic nanoparticles. A scanning electron microscopy (SEM, JEOL JSM-6510) was used for morphology observations at the accelerated voltage of 30 kV, the specimens for SEM observations were cryofractured in liquid nitrogen and the freshly rupture surfaces were sputter-coated with gold before observation. Energy dispersive spectrometer (EDS) was employed to testify the distribution of fillers in the film. The high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F) at an accelerating voltage of 200 kV was carried out to investigate the morphology of Fe3O4@rGO nanoparticles. Magnetic properties were measured at room temperature using vibrating sample magnetometer (VSM, MicroSense EZ-7). Tensile property of the flexible composites film was investigated by electronic universal testing machine (Gotech, AI-7000-SGD) at room temperature. The size of the test samples is 0.8 mm in thickness and 10 mm in width. Five samples were tested to calculate the average value and standard deviation. The volume resistivity value of Fe3O4@rGO/MWCNT/WPU composites was detected via a DC resistance meter (Tonghui TH2516B). EMI SE of the composites were examined by a vector network analyzer (VNA, Agilent N5232A) with a coaxial test cell (APC-7 connector) in conjunction according to ASTM ES7-83. All the samples were cut to discs with 15 mm diameter and placed in the specimen holder. The measured scattering parameters (S11 and S21) were used to calculate the EMI SE. From the measured scattering parameters (S11 and S21), the total EMI SE (SETotal), shielding effectiveness

of the absorption (SEA), reflection (SER), multiple internal reflections (SEM) and power coefficients of reflectivity (R), transmissivity (T), and absorptivity (A) could be calculated according to the following equations [33]: R = |S11|2

(1)

T = |S21|2

(2)

A = 1−R−T

(3)

SER = −10 lg(1−R)

(4)

SEA = −10 lg(T/(1–R))

(5)

SETotal = SER + SEA + SEM

(6)

3. Results and discussion 3.1 Characterization of Fe3O4@rGO nanoparticles The pristine GO shows a typical laminar structure with huge specific surface area as presented in Fig.2a. MWCNT with large aspect ratio can be clearly observed from Fig.S1, the tube wall thickness is about 15 nm. Fig.2b shows the HRTEM image of Fe3O4@rGO magnetic nanoparticles prepared by one-step co-precipitation method. After co-precipitation, the Fe3O4 magnetic nanoparticles have been uniformly decorated on the surface of rGO sheets. The HRTEM image in Fig.2c exhibits the lattice fringes with measured interplanar distance about 0.253 nm for Fe3O4 magnetic nanoparticles, which correspond to the (311) plane of Fe3O4 [34, 35]. The XRD pattern of Fe3O4@rGO nanoparticles exhibited in Fig.2d shows the characteristic diffraction peaks of Fe3O4, (220), (311), (400), (422), (511) and (440), which can be indexed to the cubic inverse spinel structure of magnetite according to JCPDS card No. 19-0629 [36]. The results

above confirm the successful synthesis of Fe3O4@rGO magnetic nanoparticles. Magnetic hysteresis loops of Fe3O4@rGO nanoparticles with different rGO content are demonstrated in Fig.2e. The saturation magnetization of Fe3O4@rGO nanoparticles declined with the increase of rGO content. Fe3O4@rGO-20 nanoparticles with lowest rGO content possess maximum saturation magnetization (71.8 emu/g), while the Fe3O4@rGO-100 nanoparticles exhibit a minimum value (59.8 emu/g). In addition, the hysteresis loops show no significant remanence or coercivity, which suggesting a superparamagnetic feature of Fe3O4@rGO magnetic nanoparticles. This specific superparamagnetic feature indicates that the Fe3O4@rGO nanoparticles could be easily actuated by a magnetic field with a fast response, which is benefit for microwave absorption ability [36, 37]. As presented in Fig.2f, the electrical conductivity of Fe3O4@rGO-X nanoparticles is significantly improved with the increase of rGO content. Consequently, once the Fe3O4@rGO nanoparticles with different rGO content were added into WPU with ordered arrangement, a dual magnetic and electrical gradient would be automatically formed in the composite, and easily tuned by simply adjusting the adding strategy of Fe3O4@rGO nanoparticles.

Fig.2 TEM images of (a) pristine GO and (b) Fe3O4@rGO nanoparticles, (c) high resolution TEM image of Fe3O4 nanocrystals embedded in the rGO matrix, (d) XRD patterns, (e) hysteresis loops and (f) conductivity of Fe3O4@rGO nanoparticles with various rGO content.

3.2 Microstructure of Fe3O4@rGO/MWCNT/WPU composites SEM observation are conducted to investigate the microstructure and morphology of WPU composites. As presented in the SEM image of the upper surface of Fe3O4@rGO/MWCNT/WPU composite (Fig.3b), numerous Fe3O4@rGO magnetic nanoparticles (bright parts) are densely embedded in WPU matrix, which could provide effective microwaves absorption and reduce reflection. At the bottom surface (Fig.3c), plentiful MWCNT fillers have constructed a compact 3D conductive network to provide effective EM wave reflection. In order to elucidate the gradient structure of Fe3O4@rGO/MWCNT/WPU composites more visually, EDS elemental mappings at cross section of the composites are obtained to reveal the distribution of fillers. As shown in Fig.3d-f, the Fe element from Fe3O4@rGO is mainly distributed in a gradient at the upper part, while the C element get denser from top to bottom. The results indicate

that an ordered gradient structure has been successfully formed in WPU matrix by means of LbL-casting. Fig.3g-i shows the SEM images of fractured surfaces of the Fe3O4@rGO/ MWCNT/WPU composites with different MWCNT content (SEM images of Fe3O4@rGO/MWCNT/WPU composites with 30 wt% MWCNT content are exhibited in Fig.S2). Four layers (three Fe3O4@rGO distributed layers and one MWCNT distributed layer) with clear boundaries are well-defined in these images. At the junction of neighboring layers, there is no breakage, hole or other defect, which means that the four layers of WPU composite film are tightly bonded during LbL-casting process. By strictly controlling the casting volume of WPU dispersion, the thickness of every layer is about 0.2 mm with a negligible fluctuation. From the high magnification images (Fig.3j-l and Fig.S2b) of the interfaces between Fe3O4@rGO/WPU layer and MWCNT/WPU layer, it is clearly seen that the WPU matrixes of two layers are fully integrated with no crevices. Such perfect interfacial adhesion should be attributed to the excellent film-forming performance and adhesive property of WPU as well as the autonomous penetration and diffusion of WPU macromolecular chain during the LbLcasting process [38, 39]. Besides, some MWCNT fillers penetrate into Fe3O4@rGO/WPU regions and further strengthen the interlayer bonding due to the “suture lines” effect [40]. While for the MWCNT/WPU layer, the MWCNTs uniformly disperse in WPU matrix even at higher content without aggregation, which is benefited from high power ultrasound and sufficient shearing action. With the increase of MWCNT content, the MWCNT network is gradually dense and improved. This perfect

MWCNT network is conducive to improve the conductivity and strength, which are necessary to achieve high EMI SE and excellent mechanical performance of the composite.

Fig.3 (a) The flexibility and gradient multilayered structure of Fe3O4@rGO/MWCNT/WPU composites, SEM images of (b) upper surface (the inset is high magnification image) and (c) bottom surface (the inset is high magnification image), (d-f) Fe and C elemental EDS mappings at fracture surface of Fe3O4@rGO/MWCNT/WPU composites. (g-i) SEM images of fracture surface of Fe3O4@rGO(20-60-100)-nMWCNT composites and (j-l) corresponding interfaces between Fe3O4@rGO/WPU and MWCNT/WPU with different MWCNT content: (g, j) 15 wt%, (h, k) 45

wt%, (i, l) 60 wt%.

3.2 Electro-magnetic gradient of Fe3O4@rGO/WPU/MWCNT/WPU composites Magnetic hysteresis loops of Fe3O4@rGO/WPU layer with different Fe3O4@rGO nanoparticles are demonstrated in Fig.4a. The saturation magnetization of Fe3O4@rGO/WPU layer decreases with the improved rGO content (from 6.7 emu/g to 4.9 emu/g), indicating the successful construction of negative magnetic gradient in the composite. The electrical conductivity of Fe3O4@rGO/WPU layer with various Fe3O4@rGO-X nanoparticles are presented in Fig.4b. Due to the different initial conductivity of Fe3O4@rGO particles that controlled by rGO content, the conductivity of Fe3O4@rGO-100/WPU reaches 1.2710-1 S/m, while the conductivity of Fe3O4@rGO-20/WPU is only 610-5 S/m. The conductivity of three Fe3O4@rGO/WPU layers in the composite differs by four orders of magnitude, which demonstrates the successful construction of conductivity gradient structure in Fe3O4@rGO/WPU layers. For the MWCNT/layer, with the increase of MWCNT loading, the conductivity rises rapidly (Fig.4c) from only 5.1 S/m (15 wt% MWCNT content) to 1145.8 S/m (60 wt% MWCNT content). The conductivity of MWCNT/WPU is 4~8 orders of magnitude higher than that of Fe3O4@rGO/WPU layers, which is mainly attributed to the perfect 3D MWCNT conductive network that constructed with easily connected MWCNT of large aspect ratio. This high conductivity further amplifies the conductive gradient in Fe3O4@rGO/MWCNT/WPU composites, which would not only provide a powerful guarantee for EMI shielding performance but also be promising to enhance microwaves absorption ability.

Fig.4 (a) Hysteresis loops and (b) conductivity of Fe3O4@rGO-X/WPU layer with various Fe3O4@rGO particles, (c) conductivity of MWCNT/WPU layer with various MWCNT content.

3.3 The EMI shielding performance of Fe3O4@rGO/MWCNT/WPU composites The EMI SE values in X-band for Fe3O4@rGO/MWCNT/WPU composites with different gradient (the MWCNT/WPU layers of all the composites have a MWCNT content of 60 wt%) are demonstrated in Fig.5a. The Fe3O4@rGO(20-60-100)60MWCNT composite owns the highest EMI SE of 35.9 dB, while the EMI SE value of Fe3O4@rGO(40-60-80)-60MWCNT and Fe3O4@rGO(60-60-60)-60MWCNT are 32.5 and 30.2 dB, respectively. The result indicates that the larger the electro-magnetic gradient, the higher the shielding effectiveness of the Fe3O4@rGO/MWCNT/WPU composites. It is worth mentioning that the improvement in EMI SE is mainly attributed to the elevated EM waves absorption rather than reflection. As shown in Fig.5b, the SER of three gradient Fe3O4@rGO(X1-X2-X3)-60MWCNT composites are almost the same. But interestingly, the average value of SEA is significantly improved from 28.4 to 34.4 dB with the increased gradient. The absorption of Fe3O4@rGO/MWCNT/WPU composite with maximum gradient contributes 95.8 % to the total shielding effectiveness. Moreover, the R value decreases from 0.32 to 0.27 with the increase of gradient (Fig.5c), which means the low reflection characteristic can be more significant when the multilayer composite has larger gradient. This result shows that the multilayer

EMI shielding composites with designed electro-magnetic gradient can possess strong EM wave absorbing ability, and increase the gradient can further strengthen this valuable high absorption and low reflection characteristic.

Fig.5 (a) EMI SE, (b) SER, SEA and (c) R value of the Fe3O4@rGO(X1-X2-X3)-60MWCNT composites in X band with different gradient. (d) EMI SE, (e) SER, SEA and (f) R value of the Fe3O4@rGO(X1-X2-X3) layers with different gradient.

In order to further explain the above result, the EMI SE and R value of the Fe3O4@rGO/WPU layers are shown in Fig.5d-f. As the gradient increases, the trends of EMI SE and R value of the Fe3O4@rGO/WPU are the same with the Fe3O4@rGO/MWCNT/WPU composites. It demonstrates that the increased gradient of the Fe3O4@rGO/WPU layers contributes to the higher EMI SE and lower reflection, hence the improved EMI SE completely depends on the enhanced absorption attenuation. This phenomenon should be ascribed to the changes of the complex permittivity and permeability of the gradient of Fe3O4@rGO/WPU layers. According to Debye theory [35, 41], the complex permittivity (=′-j′′) and complex permeability

(μ=μ′-jμ′′) determine the microwave absorption ability, the real part of complex permittivity and permeability (′ and μ′) represent the storage ability of electric and magnetic energy while the imaginary part (′′ and μ′′) represent the dissipation. Dielectric loss tangent (tan δε = ′′/′) and magnetic loss tangent (tan δμ = μ′′/μ′) are calculated from complex permittivity and permeability. The former represents the ability to convert external electric field energy into heat while the latter is relevant to the energy loss inside a magnetic material [22]. The electromagnetic parameters of the Fe3O4@rGO/WPU layers with various gradient in the frequency of 8.2-12.4 GHz are revealed in Fig.S3. As the gradient changes from 60-60-60 to 20-60-100, the increased difference of interlayer conductivity induces stronger interface polarization and thus leads to higher tan δε (Fig.6a). With the increase of gradient, the enhancement of tan δμ (Fig.6b) is not significant, suggesting that the improved waves absorption mainly depends on dielectric loss. The tan δε and tan δμ of single Fe3O4@rGO/WPU layer are also shown in Fig.6 c, d (the electromagnetic parameters are exhibited in Fig.S4). With the increase of rGO content, the tan δε has a significant increase, which contributes to the excellent dielectric loss. Therefore, the Fe3O4@rGO/WPU layers with higher gradient tend to show the stronger dielectric loss. The above results confirm that Fe3O4@rGO/WPU layers with larger gradient own higher absorption abilities. Consequently, the Fe3O4@rGO/MWCNT/WPU composites with larger gradient exhibit higher EMI SE and more significant low reflection characteristic.

Fig.6 (a) Dielectric loss tangent and (b) magnetic loss tangent of Fe3O4@rGO/WPU layers with different gradient in X-band, (c) dielectric loss tangent and (d) magnetic loss tangent of single Fe3O4@rGO/WPU layer, EMI SE of every (e) Fe3O4@rGO/WPU layer and (f) MWCNT/WPU layer.

For the Fe3O4@rGO(20-60-100)-60MWCNT composites with largest gradient, the value of R is 0.27, i.e. nearly 73 % of the incident EM waves are attenuated by absorption. So the composites reveal a conspicuous absorption feature and absorption-

dominated shielding mechanism. It is worth noting that the R value in this work is lower than many other reported results in recent literatures (Fig.7), and the detailed information are listed in Table S1. However, the absorption-dominated shielding mechanism not only relies on the Fe3O4@rGO/WPU layers but also results from the MWCNT/WPU layer. On one hand, the Fe3O4@rGO/WPU layer cannot provide ideal EMI SE, while the highly conductive MWCNT/WPU can effectively prevent the transmission of incident EM wave and ensure a high EMI SE of the composites (Fig.6e, f). On the other hand, the existence of MWCNT/WPU layer can cause a unique “absorbreflect-reabsorb” process when microwaves penetrate into the composite. The incident EM waves would be first attenuated in Fe3O4@rGO/WPU layers, then reflected back by highly conductive MWCNT layer and reabsorbed in Fe3O4@rGO/WPU layers. If the EM waves incident from the MWCNT/WPU layer, although the EMI SE of the composite is nearly unchanged, the R is significantly increased to 0.9 (Fig.S5), indicating a reflection-dominated shielding mechanism. Therefore, the postpositional MWCNT/WPU layer and the unique “absorb-reflect-reabsorb” process are essential to realize the absorption-dominated shielding mechanism.

Fig.7 Comparison of R of different structural composites reported in recent literature with our work.

To confirm the effect of MWCNT/WPU layer on the high absorption and low reflection characteristics of Fe3O4@rGO/MWCNT/WPU composites, we use the ratio of A and R (A/R) to quantitatively compare the absorption and reflection properties of the composites (Fig.8c). An absorption-dominated EMI shielding material can be certified when the value of A/R over 1. It is easy to understand that higher value of A/R represent higher absorption and lower reflection. The A/R value of Fe3O4@rGO/WPU layers without gradient (Fe3O4@rGO(60-60-60)) is less than 1, which means that reflection is the primary shielding mechanism. However, for Fe3O4@rGO/WPU layers with gradient, the value of A/R is higher than 1, representing an obviously absorptiondominated shielding mechanism. It proves that the gradient structure is the key factor for designing the low reflection shielding materials. By comparing the different power coefficients of Fe3O4@rGO/MWCNT/WPU and Fe3O4@rGO/WPU, we find that the A/R values of Fe3O4@rGO/MWCNT/WPU are much larger than that of Fe3O4@rGO/WPU. All the A/R values of Fe3O4@rGO/MWCNT/WPU are higher than 2, demonstrating that the microwaves power attenuation caused by absorption is more

than twice that caused by reflection. Hence, the existence of highly conductive MWCNT/WPU not only guarantees an effective EMI SE but also essential for the absorption-dominated

shielding

performance

of

Fe3O4@rGO/MWCNT/WPU

composites.

Fig.8 (a) The average values of R, A and T for Fe3O4@rGO/WPU layers with different gradient, (b) the average values of R and A for Fe3O4@rGO/MWCNT/WPU composites with different gradient, (c) the A/R value for Fe3O4@rGO/WPU and Fe3O4@rGO/MWCNT/WPU composites.

The influence of MWCNT content on EMI SE and power coefficients of the composites with the same absorption gradient of Fe3O4@rGO(20-60-100) is further investigated, and the result is shown in Fig.9. When the MWCNT content in MWCNT/WPU is 15 wt%, the EMI SE of Fe3O4@rGO/MWCNT/WPU is only 9.1 dB (Fig.9a). With the increase of MWCNT loading, the MWCNT conductive network become denser, which is beneficial to enhance shielding performance. As the MWCNT content increases to 60 wt%, the average EMI SE of the composite reaches 35.9 dB. Even at a wider frequency range, the composites can also show an effective and stable EMI SE. As presented in Fig.S6, the EMI SE of Fe3O4@rGO(20-60-100)-60MWCNT sample exhibits weak frequency dependence at 2-18 GHz, and the average EMI SE is 35.2 dB. Fig.9b presents the SET, SEA, SER of the Fe3O4@rGO/MWCNT/WPU

composite with various MWCNT content. With the increase of MWCNT content, the SEA rises significantly due to the improved conductivity. However, the value of SER has little changed, which is extremely different from common CPCs shielding materials with homogeneous conductive network structure [27, 42-44]. Generally, SER is proportional to conductivity since higher conductivity would increase the impedance mismatching and thus enhance the wave reflection. The sharply increased SEA and little changed SER indicate that the improvement of SET is mainly relied on absorption rather than reflection. Moreover, an interesting result should be emphasized that the SER and R (the R value as a function of frequency in X-band is shown in Fig.S7) rise at the initial stage but then decrease with the increased MWCNT content (the inset of Fig.9b, and Fig.9c).Initially, the Fe3O4@rGO(20-60-100)-15 MWCNT composite exhibits a minimum R of 0.25. When the MWCNT content increases to 30 wt%, the R value rises to 0.34. Then, the R value decreases to 0.27 with a MWCNT content of 60 wt%. In contrast, the A value monotonously increases from 0.63 to 0.73 with the increase of MWCNT content.

Fig.9 (a) EMI SE and (b) SET, SER, SEA for Fe3O4@rGO(20-60-100)-nMWCNT composite with various MWCNT content, (c) average values of R, A and (d) T of Fe3O4@rGO(20-60-100)nMWCNT composite with various MWCNT content.

This interesting phenomenon should be attributed to the following aspects: first, when the MWCNT content is relatively low (15 wt%), the EM waves can easily enter into the interior of the materials and pass through the material without too much reflection because of the favorable impedance matching of the front Fe3O4@rGO layer (Fig.S8) and inferior waves reflection property of MWCNT/WPU layer. As presented in Fig.9d, the T of Fe3O4@rGO(20-60-100)-15MWCNT is 0.13, which means that as many as 13 % of the incident EM waves can penetrate the material without being reflected or absorbed. So the Fe3O4@rGO(20-60-100)-15MWCNT composite with lowest R value also show a lowest A. Once the MWCNT content exceeds 15 wt%, the

sharply enhanced conductivity endows the MWCNT/WPU layer a strong reflection to block almost all of the incident microwaves. Therefore, with the increase of MWCNT loading, the MWCNT/WPU layer can reflect more microwaves back to absorption layer and thus strengthen the “absorb-reflect-reabsorb” process. So, the R value decreases while the A value increases when the MWCNT content improves from 30 wt% to 60 wt%, suggesting that an effective improvement of EMI SE and reduction of microwave reflectivity can be achieved simultaneously with the increased MWCNT content. Secondly, interfacial polarization is considered as the dominant polarization mode during dielectric loss process for microwave absorption materials [45]. At the interface of Fe3O4@rGO/WPU and MWCNT/WPU layers, lots of free charges spontaneous accumulate at the heterointerfaces between WPU-MWCNT, Fe3O4-MWCNT and rGOMWCNT to cause macroscopic dipole moments that produce Debye relaxation to attenuate electromagnetic wave [46]. Therefore, with the increased interface contact induced by gradually dense MWCNT network in MWCNT/WPU layer, more and more heterointerfaces and free charges can be generated to reinforce the polarization relaxation loss. Besides, conductive loss is undoubtedly another important factor for wave attenuation due to the existence of conductive MWCNT, which could convert electromagnetic energy into heat dissipation. The time-varying electromagnetic field induced currents could be quickly dissipated in the 3D MWCNT conductive network and transformed into thermal energy [47], hence the Fe3O4@rGO/MWCNT/WPU composites with high MWCNT loading could exhibit a more significant decay of EM wave. Therefore, the MWCNT/WPU layer also contributes to the high absorption and

low reflection characteristics by polarization relaxation loss and conductive loss. The results demonstrate that improving reflection ability of highly conductive layer can effectively strengthen the absorption and lead to low reflection characteristic for this layered gradient Fe3O4@rGO/MWCNT/WPU composites. As the Fe3O4@rGO/WPU layers also play a key role for absorption-dominated EM shielding mechanism, the cooperation of Fe3O4@rGO/WPU and MWCNT/WPU layers endows the composites superb high absorption and low reflection EM performance. To intuitively scheme this particular

shielding

course,

the

EMI

shielding

mechanism

of

the

Fe3O4@rGO/MWCNT/WPU composite is illustrated in Fig.10.

Fig.10 (a) EMI shielding mechanism of the Fe3O4@rGO/MWCNT/WPU composite, (b) polarization relaxation loss mechanism of the interface between Fe3O4@rGO/WPU and MWCNT/WPU.

As shown in Fig.10a, the incident EM waves can easily penetrate into the interior of the composites due to the well impedance matching, and only few of them are reflected at the surface. The absorption layers with positive conductivity gradient and

negative magnetism gradient firstly provide ideal absorption by strong magnetic hysteresis loss and dielectric loss. Three Fe3O4@rGO/WPU layers with different electrical and magnetic properties can induce multiple reflections at each interface, which also contribute to the absorption attenuation. Then, the penetrating microwaves are reflected back to absorption layer by highly conductive reflection layer. The reflected microwaves are absorbed by Fe3O4@rGO/WPU once again, causing a reabsorption process. Moreover, as illustrated in Fig.10b, a large amount of Fe3O4@rGO magnetic nanoparticles and MWCNT huddle together at the interface of Fe3O4@rGO/WPU and MWCNT/WPU layers, producing plentiful heterointerfaces between MWCNT-WPU, Fe3O4-MWCNT and rGO-MWCNT and thus causing polarization relaxation loss to further dissipate EM waves. Consequently, depending on the layered gradient structure design, the Fe3O4@rGO/MWCNT/WPU composites with effective EMI SE exhibit excellent high absorption and low reflection EMI performance. 3.5 The stability of EMI shielding performance of Fe3O4@rGO/MWCNT/WPU composites For practical application, the durability under mechanical deformation is essential for the flexible Fe3O4@rGO/MWCNT/ WPU composites. Fig.11 shows the stability of EMI shielding properties of Fe3O4@rGO/MWCNT/WPU composites, which is evaluated by repeated bending to the angle of 180 ° for 1000 times. The EMI SE shows only a slight decline from 35.4 to 33.8 dB (Fig.11a), and the retention is as high as 95.5 %. The high EMI SE retention rate is mainly attributed to the dense MWCNT conductive network, which maintains good integrity after deformation. Besides, the

average value of R changes from 0.27 to 0.30 (Fig.11b), indicating that the absorptiondominated EMI shielding characteristic is unaffected by repeated deformation. Such super durability under mechanical deformation comes from the layered structure design. By using the LbL-casting method, perfect interfacial adhesion greatly enhances the stability of the gradient shielding network in the composites. Thus, the flexible Fe3O4@rGO/MWCNT/WPU composites can afford effective and absorptiondominated EMI shielding performance under mechanical deformation. This illustrates that the flexible WPU composites own great potential as ideal shielding materials in next-generation flexible electronics.

Fig.11 EMI SE (a) and R (b) of the Fe3O4@rGO/MWCNT/WPU composites before and after repeated bending to the angle of 180 ° for 1000 times.

3.6 The mechanical property of Fe3O4@rGO/MWCNT/WPU composites Aside from the remarkable EMI shielding performance, mechanical performance is also vital for the practical application of the Fe3O4@rGO/MWCNT/WPU composites. Fig.12a presents the stress-strain curves of pristine WPU and Fe3O4@rGO/MWCNT/ WPU composites. Particularly, mechanical properties of the composites fabricated by LbL-casting and overall-casting methods are compared. Comparing with the pristine

WPU, the Young՚s modulus of the Fe3O4@rGO/MWCNT/WPU composites are enhanced while the tensile strength and elongation at break get decreased with the addition of functional fillers. Interestingly, the tensile property of LbL-casting Fe3O4@rGO/MWCNT/WPU composite is superior to that of overall-casting composite. As showed in Fig. 12b, the Young՚s modulus of LbL-casting composite is 20.78 MPa, which is much higher than that of overall-casting composites (12.50 MPa) and pristine WPU (9.22 MPa). The tensile strength of LbL-casting composite is 7.49 MPa, only 8 % lower

than

that

of

pristine

WPU.

However,

the

overall-casting

Fe3O4@rGO/MWCNT/WPU composite has a much lower tensile strength (4.48 MPa) than the pristine WPU or LbL-casting composite. It means that the LbL-casting method is benefit to keep the mechanical performance of WPU matrix. To interpret this effective enhancement in mechanical property that induced by LbL-casting method, SEM observation of the tensile fracture surfaces of the composites are showed in Fig.12c, d. The rough tensile fracture surface of LbL-casting composite suggests that the crack deflection plays an important role for the tensile properties. This means that during crack propagation the crack deflection, LbL-casting composite could generate larger total fracture surface area, leading to greater energy absorption compared with overall-casting composite [48,49]. Thus, the LbL-casting composite exhibits better tensile strength than overall-casting composite, demonstrating that the LbL-casting method can greatly alleviate the damage of high fillers loading on mechanical performance. Since the LbL-casting method leads to a more uniform dispersion of the fillers, the LbL-casting composite also shows higher Young՚s modulus than overall-

casting composite. This significant advantage in mechanical property guarantees the stability of shielding network structure in the Fe3O4@rGO/MWCNT/WPU composites during repeating beading and folding deformations, leading to the excellent durability and EMI shielding stability of the composite film.

Fig.12 (a) Stress-strain curves, (b) the tensile strength, and Young՚s modulus of pristine WPU and Fe3O4@rGO/MWCNT/WPU composites fabricated by LbL-casting and overall-casting. tensile fracture

surfaces

morphologies

of

(c)

overall-casting

and

(d)

LbL-casting

Fe3O4@rGO/MWCNT/WPU composites.

4. Conclusions In this work, a well-designed multilayer Fe3O4@rGO/MWCNT/WPU composite with controllable electro-magnetic gradient was successfully fabricated via LbL-casting method. The EMI shielding characteristic can be easily tuned by this rGO controlled electro-magnetic gradient. Since the increased gradient in absorption layer induce more magnetic hysteresis loss and dielectric loss, lower wave reflection and higher EMI SE

were obtained with the increase of gradient. Moreover, the improvement of shielding efficiency in reflection layer strengthened the “reabsorption effect” and interface polarization loss, which further enhanced the EMI SE and reduced the EM wave reflection. For the gradient Fe3O4@rGO/MWCNT/WPU composite with maximum gradient and electrical conductivity, the R value as low as 0.27 and the EMI SE reached 35.9 dB. This special design of the layered gradient structure indicated a valid method for fabricating EMI shielding material with absorption-dominated shielding mechanism and high EMI SE, which exhibited potential applications in next-generation wearable electronic devices. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21704070), Natural Science Foundation of Shanxi Province (Grant No. 201701D221089; 201801D121109) and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme2018-4-35).

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: