Mechanically Durable, Highly Conductive, and Anticorrosive

Mar 7, 2019 - Metal-based materials have been widely used for the electromagnetic interference (EMI) shielding due to their excellent intrinsic conduc...
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Mechanically Durable, Highly Conductive, and Anticorrosive Composite Fabrics with Excellent Self-Cleaning Performance for High-Efficiency Electromagnetic Interference Shielding Junchen Luo,† Ling Wang,† Xuewu Huang,† Bei Li,† Zheng Guo,† Xin Song,† Liwei Lin,† Long-Cheng Tang,‡ Huaiguo Xue,† and Jiefeng Gao*,† †

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, China

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/10/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Metal-based materials have been widely used for the electromagnetic interference (EMI) shielding due to their excellent intrinsic conductivity. However, their high density, poor corrosion resistance, and poor flexibility limit their further application in aerospace and flexible electronics. Here, we reported a facile means to prepare lightweight, mechanically durable, superhydrophobic and conductive polymer fabric composites (CPFCs) with excellent electromagnetic shielding performance. The CPFC could be fabricated by three steps: (1) the polypropylene (PP) fabric was coated by a polydopamine (PDA) layer; (2) PP/PDA adsorbed the Ag precursor that was then chemically reduced to Ag nanoparticles (AgNPs); (3) PP/PDA/AgNPs fabrics were modified by one layer of polydimethylsiloxane (PDMS). The contact angle (CA) of the CPFCs could reach ∼152.3° while the sliding angle (SA) was as low as ∼1.5°, endowing the materials with excellent self-cleaning performance. Thanks to the extremely high conductivity of 81.2 S/cm and the unique porous structure of the fabric, the CPFC possessed outstanding EMI shielding performance with the maximum shielding effectiveness (SE) of 71.2 dB and the specific shielding effectiveness (SSE) of 270.7 dB cm3 g−1 in the X band. The interfacial adhesion is remarkably improved owing to the PDMS layer, and the superhydrophobicity, conductivity and EMI SE of CPFCs are almost maintained after cyclic abrasion and winding test. Also, the CPFCs can be used in a harsh environment, due to their excellent water proof property. KEYWORDS: Composite fabric, Conductivity, Superhydrophobic, Polydimethylsiloxane, Electromagnetic interference shielding

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melting or solution processing for preparation of the CPC.10,11 As known, the electrical conductivity is one of the factors that influence the EMI shielding performance. Generally, the higher the conductivity, the larger the shielding effectiveness (SE).6,12,13 To obtain the CPC with a high conductivity, a high nanofiller concentration is usually indispensable. However, a high nanofiller content would inevitably cause the nanofiller aggregation, deteriorating material performance such as the flexibility and mechanical strength.14,15 As an alternative,

INTRODUCTION In recent years, with the fast development of electronic devices, electromagnetic pollution has become a serious issue, threatening people’s health or interrupting the performance of the nearby electronic apparatus.1−5 Therefore, it is very urgent and desirable to explore high performance electromagnetic interference (EMI) shielding materials. Compared with traditional metal based materials,6 conductive polymer composite (CPC) has now attracted more and more attention from both academia and industry due to its unique properties such as lightweight, flexibility and excellent processability.7−9 The conductive nanofillers such as carbon and metal nanomaterials are usually mixed with polymer matrix through © XXXX American Chemical Society

Received: December 20, 2018 Accepted: February 25, 2019

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DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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are reliable and durable even after cyclic mechanical deformation and exposure in harsh conditions.

the nanofiller can be controlled at the interphase of polymer blocks instead of in the whole polymeric substrate, that is, construction of a segregated structure, which has been well proved to be an effect way to lower the percolation threshold while increase the conductivity of the CPC.16,17 Nevertheless, delicate interface manipulation is usually required, in order to prevent the nanofiller diffusion into the polymer during the hot press, and hence the matrix used for the formation of the segregated structure is usually limited to the polymer with a high melt viscosity. In addition, in many cases, the segregated structure usually gives rise to a weak interfacial interaction between the nanofillers and the polymeric substrate, thereby decreasing the mechanical properties of the polymer.18,19 Instead of creating a segregated structure by melting processing, the nanofillers (metal in many cases) can be coated onto the skeleton of fabrics for fabrication of conductive polymer fabric composites (CPFCs),20−22 and the nanofiller decorated fibers possess large aspect ratio, facilitating the formation of a conductive network. Furthermore, the porous structure can repeatedly reflect the electromagnetic microwaves, thereby improving the SE.2,23,24 Although some work has been done to prepare the CPFCs as the efficient EMI shielding materials, the interfacial interaction between the conductive fillers and the polymer fibers is usually weak.25 As a result, the conductivity and thus the EMI shielding performance may dramatically decline when the CPFCs undergo external force such as friction and bending. In addition, the metal is easily oxidized or even corroded when the material is exposed to a harsh environment including moisture, acid and other corrosive conditions.26,27 These above-mentioned drawbacks greatly limit the CPFCs in practice use. Thus, it is high of importance to develop the CPFC for EMI shielding materials with excellent flexibility, durability, corrosive resistance, and shielding effectiveness.13 In this study, we proposed a facile bioinspired method to fabricate a flexible and superhydrophobic CPFC with excellent EMI shielding performance. Polydopamine (PDA) was decorated on the surface of polypropylene (PP) fibers by self-polymerization of dopamine, which promoted the subsequent Ag precursor adsorption. After the Ag+ was entirely reduced to Ag nanoparticles (AgNPs), a conductive shell was formed on the PDA layer.28 Then, the conductive fabric experienced polydimethylsiloxane (PDMS) modification, and finally the superhydrophobic CPFC was obtained. The contact angle (CA) and sliding angle (SA) for CPFCs could reach ∼152.3° and ∼1.5°, respectively. It was worth noting that Ag precursors could be quickly attached onto the PDA layer and then the AgNPs immobilization, while PDMS served as a “glue” to tightly stick the AgNPs on the fiber surface. It was found that the superhydrophobicity and conductivity could almost be maintained even after the CPFC undergoed cyclic mechanical deformation. Thanks to the PDMS layer, a very high AgNP density could be deposited on the fiber surface, and the conductivity of the obtained CPFC could be as high as 81.8 S/cm, corresponding to a very large SE and SSE of ∼71.2 dB and ∼270.7 dB cm3 g−1, respectively. The SE was almost kept at its initial value after the CPFC experienced cyclic abrasion and winding tests. In addition, the PDMS could be regarded as a protective layer, preventing the AgNPs from being oxidized. Also, this layer is waterproof and can effectively resist the invasion of water or even corrosive solutions. The bioinspired superhydrophobic CPFC is promising as high performance and multifunctional EMI shielding materials that

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EXPERIMENT Materials and Reagents. Polypropylene (PP) nonwoven fabric (unit weight: 80 g m−2 and the thickness: ∼0.35 mm) was kindly supplied by Shenxin manufacturer (Zhejiang, China). Ethanol (99.7%) and n-heptane (98.5%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Silver trifluoroacetate (STA, 98%) and Hydrazine hydrate (N2H4· H2O, 80%) were provided from Energy Chemical of Sam Chemical Technology Co., Ltd. (Shanghai, China). Tris(hydroxymethyl) aminomethane hydrochloride (Tris, ≥ 98.0%) was purchased from Accela ChemBio Co., Ltd. Dopamine hydrochloride (DA, 98%) was provided by SigmaAldrich., Co., St Louis, MO. Polydimethylsiloxane (PDMS) and the curing agent were bought from Dow Corning corporation Midland, MI. Preparation of PP/PDA Composite Fabric. To obtain a DA solution with a pH value of 8.5, a certain number of Tris and DA powders with a weight ratio of 3:5 were dissolved in 200 mL water. Then, PP nonwoven fabric with the size of 4.0 × 4.0 cm (length × width) was dipped into the as-prepared DA solution for 12 h at 20 °C. Afterward, the modified fabric was sonicated with deionized water for 5 min, then cleaned with deionized water. At last the PP fabric modified with PDA was dried in a drying oven at 40 °C for 1 h. Preparation of PP/PDA/AgNPs Composite Fabric. STA solutions were prepared by mixing a certain number of STA into the ethyl alcohol. The PP fabric modified with PDA was immersed into the STA solution for 10 min, and then immersed into a 50 wt % N2H4·H2O solution for 30 min, in order to reduce the STA to AgNPs. The conductivity and EMI SE of the composite fabrics were controlled by changing the immersion time (10 to 50 min) and STA concentrations varying from 5 wt % to 25 wt %. After rinsed with deionized water, the AgNPs coated PP/PDA composite fabric was finally obtained. The dipping time in various STA solution is fixed at 40 min unless otherwise specified. Preparation of PP/PDA/AgNPs/PDMS Composite Fabric. We dissolved 0.1 g of polydimethylsiloxane and 0.01 g of curing agent into 9.9 g of n-heptane while stirring for 5 min to obtain a 1 wt % homogeneous PDMS solution. The asprepared fabric was first immersed into the prepared PDMS solution (1 wt %) and then put in a drying oven at 80 °C for 1 h. By adjusting the dipping time in the PDMS solution, the hydrophobicity of fabric surface could be controlled. In this work, PP/PDA/AgNPs-C/PDMS-T represents the superhydrophobic and conductive fabric material, in which C is the concentration (wt %) of STA solution and T is the time (min) of the composite fabric dipped in the PDMS solution, respectively. Characterization. Field emission scanning electron microscopy (Zeiss SUPRA55, Germany) was used to characterize the morphology and microstructure of the composite fabrics at an accelerating voltage of 5 kV. The optical contact angle (CA) measuring device (OCA20, Germany) was utilized to measure the CAs of the composite fabric. The X-ray photoelectron spectroscopy (ESCALAB 250Xi, U.S.) was employed to analyze surface chemical composition of the composite fabric. The X-ray diffractometer (XRD, D8 Advance, Germany) with Cu Kα radiation was used to identify the crystalline structure of the AgNPs. The 2θ angle ranged from10 to 80°during the B

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic for the fabrication of the superhydrophobic and EMI shielding PP/PDA/AgNPs/PDMS fabric.

hydrazine for a period of time, the STA is completely transformed to AgNPs, wrapping the fibers. These nanoparticles connect to each other, forming conductive network inside the fabric. Although, the PDA layer enhances the interaction between the AgNPs and the PP fabrics, the interfacial adhesion between the AgNPs is relatively weak, and thus the nanoparticles easily fall off the fabrics especially at a high AgNPs concentration, which could lead to the decline of the conductivity and hence the EMI shielding performance. To further improve the interfacial bonding between AgNPs and also between the AgNPs and the PDA layer, the PDMS is diffused inside the fabric and served as a “glue” after curing that is able to tightly stick the AgNPs together with the fibers. More importantly, the PDMS layer with a low surface energy can endow the fabric surface with superhydrophobicity, and this protection layer could prevent the AgNPs from oxidation and corrosion, making it possible to be used in harsh environment.35 The surface topology of PP/PDA is barely changed (SI Figure S1c d), compared with that of the pristine PP fibers (SI Figure S1a and b). As above-mentioned, AgNPs density on the PP/PDA surface strongly depends on the concentration of the Ag precursor where the PP/PDA is dipped. At a low concentration of 5 wt %, only a few Ag nanoparticles are distributed on the surface of PP fiber, leaving some surface intact without nanoparticle decoration (Figure 2a). With the successive increase of the STA concentration, more and more AgNPs are observed, and the AgNPs density becomes increasingly large. The fiber surface is almost completely covered by the AgNPs at a STA concentration of 12 wt %, forming a conductive shell (Figure 2b). Furthermore, the diameter of the AgNPs also becomes bigger and even large aggregates are generated with the STA concentration of 20 wt % (Figure 2c and SI Figure S2c). There are evident interfacial gaps between the silver shell and the surface of PP fiber, as

XRD measurement. The thermogravimetric analyzer (Pyris 1 TGA, U.S.) with a heating rate of 10 °C min−1 from ambient temperature to 800 °C in an N2 atmosphere was exploited to measure the weight percentage of Ag nanoparticles of the composite fabrics. A 4-probe resistivity meter (RTS-9, China) was employed to measure the conductivity of the composite fabrics. A vector network analyzer (Agilent N5230, U.S.) was employed to evaluate the scattering parameters (S11 and S21) of the composite fabrics with a circular shape (diameter: 13 mm) in the frequency of 8.2−12.4 GHz (X-band), and the detailed calculation of SEtotal, SER, SEM, and SEA can be found in the Supporting Information (SI).

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RESULTS AND DISCUSSION

The multifunctional PP/PDA/AgNPs/PDMS fabric is fabricated through a multiple coating method, as schematically shown in Figure 1. As known, there is only carbon and hydrogen in PP macromolecular chain. To make the fiber surface chemically active, PDA is attached onto the PP fiber by the self-polymerization of dopamine under an alkaline condition.29−31 In fact, dopamine can be adhered onto almost all substrate surface (either hydrophobic or hydrophilic).32 The functional groups of PDA including carboxyl, quinone, catechol, amino, and imino groups could provide a versatile platform for the further reaction. In this study, the interaction between the Ag+ and catechol groups, that is, the chelation, promotes the Ag precursor migration onto the PDA layer.33 In addition, there exists ion-dipole interactions between the hydroxyl groups (−OH) of PDA and trifluoroacetate anions (CF3COO−).33,34 The multiple interactions guarantee the adsorption of STA and then the AgNPs immobilization on the PDA modified PP surface. The STA adsorption capacity that finally determines the AgNPs concentration could be controlled by the STA concentration and the time of the fabrics dipped in the STA solution. After reduction by the C

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Morphologies of the surface of (a) PP/PDA/AgNPs-5%, (b) PP/PDA/AgNPs-12%, (c) PP/PDA/AgNPs-20%, (d) PP/PDA/AgNPs5%/PDMS-40, (e) PP/PDA/AgNPs-12%/PDMS-40, (f) PP/PDA/AgNPs-20%/PDMS-40, (g) Elemental mapping of PP/PDA/AgNPs-25%/ PDMS-40.

Figure 3. Surface morphology of PP/PDA/AgNPs-25%/PDMS fabricated with different dipping time in 1 wt % PDMS solution. (a) 20 min, (b) 40 min, (c) 60 min, and (d) 90 min.

D

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Variation of electrical conductivity (a) and CAs (b) of the composite fabrics prepared under different STA concentration. Conductibility (c) and CAs (d) of the composite fabrics with the immersion time in the STA solution. Conductibility (e) and CAs (f) of PP/PDA/AgNPs-25% as a function of the dipping time in 1 wt % PDMS solution.

displayed from red arrows in Figure 2a−c, whereas they become invisible after PDMS modification (see the red dotted circles in Figure 2d−f), indicating the improved interfacial adhesion. The detailed information about the interface between the AgNPs and the fibers for the PP/PDA/AgNPs and PP/PDA/AgNPs/PDMS can be observed from the magnified SEM images in SI Figure S2a−c and d−f, respectively. We have obtained the coating thickness by observation of the cross section of the composite fabric. SI Figure S3 shows the change of thicknesses of the Ag layers with different STA concentration. The AgNPs layer covered on the PP fiber was discontinuous with a very thin thickness of around 0.18 μm at a low STA concentration of 5 wt % (SI Figure S3a). The Ag shell thickness further increases to 0.29 and 0.52 μm (SI Figure S3b and c) with the increase of the STA concentration to 12 and 25 wt %, respectively. The elemental mapping is used to analyze the chemical composition for the multiple layers on the fiber surface of PP/PDA/AgNPs-25%/ PDMS-40, as displayed in Figure 2g. It is found that the Ag, Si, O, C, and N are uniformly distributed on the fabric surface. In addition to improvement of interfacial adhesion, the PDMS introduction could tightly stick AgNPs together on the surface of the fibers. The surface morphology of the PP/PDA/AgNPs/ PDMS depends on the dipping time in PDMS solution. A very thin PDMS layer is deposited onto the AgNPs surface after the

fabrics are dipped for 20 min (Figure 3a). When the dipping time in PDMS solution is increased to 40 and 60 min, the PDMS layer becomes thicker, making the interface between AgNPs vague (Figure 3b and c). When the dipping time reaches 90 min, all the AgNPs are bonded together, as shown in Figure 3d. The crystalline structure for the AgNPs is examined by the XRD pattern (SI Figure S4). The sharp characteristic peaks of PP/PDA/AgNPs are located at 38.5°, 43.9°, 64.3°, and 77.1°, corresponding to 111, 200, 220, and 311 crystal planes of the Ag nanoparticles,36,37 respectively. It is also found that the PDMS decoration does not change the crystal structure of the AgNPs. The AgNPs content in the composite fabric could be determined from the TGA curves, as shown in SI Figure S5. After thermal degradation in the nitrogen atmosphere at 800 °C, the weight percentage for the residuals of PP/PDA and PP/PDA/AgNPs-25% is about 2.1% and 17.3%, respectively. Based on the method reported in our previous work,38 the AgNPs concentration is calculated to be 15.5 wt %. It is clear that the surface wettability and conductivity of the composite fabrics are influenced by the surface composition and its geometry. Figure 4a shows the conductivity variation of the composite fabrics with the concentration of the Ag precursor. Naturally, the lager the precursor concentration, the larger the Ag nanoparticles density and hence the higher the E

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) The photograph showing the surface resistance of the composite fabric (PP/PDA/AgNPs-25%/PDMS-40). (b) Digital photo images displaying the composite fabric in a circuit, and (c) the LED light in the circuit maintaining its original brightness during the bending. (d−h) Photographs of contact, press, and departure processes of a 5 μL water droplet on the composite fabric surface. (i) CAs of various corrosive solution droplets on the composite surface. (j) Photograph of the liquid droplets of acid (dyed in red), alkali (dyed in pink) and salt (dyed in yellow) solution sitting on fabric surface. (k−n) Photograph exhibiting the self-cleaning behavior of the fabric surface. Note that the composite fabric used in all the tests is PP/PDA/AgNPs-25%/PDMS-40.

superhydrophobicity. In addition to the high CAs, low SAs could also be achieved for the composite fabrics. SI Table. S1. shows the detailed information on the sliding angles (SA) of PP/PDA/AgNPs-C/PDMS-40. With a STA concentration of 20 wt %, the SA for the composite fabric is 8°, while it further decreases to as low as 1.5° at the STA concentration of 25 wt %, indicating the excellent self-cleaning performance which will be discussed in the following section. Apart from the precursor concentration, the time of the fabric dipped in STA solution can also affect the STA adsorption and hence the surface conductivity and CA, which are shown in Figure 4c and d. Only a slight increase for the conductivity is observed for the PP/PDA/AgNPs and PP/PDA/AgNPs/PDMS-40 when the dipping time is prolonged from 10 to 30 min, while the conductivity greatly rises and reaches the maximum value of 130.0 and 75.5 S/cm, respectively for the two composite fabrics dipped in STA solution for 40 min. Then it slightly declines to 119.0 and 63.2 S/cm with further extension of the dipping time to 50 min. Similarly, the contact angle raises from 144.9° to 153.1° with increasing the dipping time from 10 to 40 min, and then slightly drops to 152.1° at the dipping time of 50 min. More Ag precursor is adsorbed onto the fiber by prolonging the immersion time until the fibers surface is saturated, and a larger number of Ag nanoparticles are accordingly formed, leading to the enhancement of both the conductivity and CA. The PDMS decoration onto the AgNPs surface could not only influence the surface composition but

EMI SE. The conductivity exhibits a dramatic increase by increasing the STA concentration for the PP/PDA/AgNPs, and it reaches as high as 131.7 S/cm with the STA concentration of 25 wt %. On the other hand, the introduction of PDMS could decreases the conductivity of the PP/PDA/ AgNPs. As known, both the percolation theory (contact of conductive nanofillers) and tunnel current theory (noncontact of conductive nanofillers) are responsible for the conduction in the conductive polymer composite. As shown in Figure 3, AgNPs have been modified with the PDMS in the composite fabric, and the tunnel current contributes to the conduction even when the AgNPs are wrapped by an ultrathin layer of PDMS, although the insulating PDMS could, to a certain degree, decrease the composite conductivity. Our result is consistent with that reported in the literature.39 Hence, the conductivity of PP/PDA/AgNPs/PDMS grows much more slowly with the STA concentration than that of PP/PDA/ AgNPs. Even so, the conductivity of PP/PDA/AgNPs/PDMS can reach 81.8 S/cm at the precursor concentration of 25 wt %. In addition to improving the interfacial adhesion, the PDMS could decrease the surface energy of the fabrics, resulting in the increase of the CA. It is found in Figure 4b that the CA increases monotonically with the STA concentration, because higher density of AgNPs stacking on the fiber surface form more microbumps, bringing about a larger surface roughness (see SI Figure S2d−f) and thus a higher CA. The CA arrives at 150.3° with the precursor concentration of 20 wt %, displaying F

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) EMI SE of the PP/PDA/AgNPs-C/PDMS-40 in the X band. (b) SE total, SE absorption and SE reflection of the PP/PDA/AgNPsC/PDMS-40 at 8.2 GHz. (c) The photo of wireless power transfer that can light a LED. (d) The photograph showing the shielding performance of the composite fabric (the red rectangle in Figure.6d) on the wireless power transfer.

also the surface topology. There is a sustainable decrease for the conductivity with the increase of the immersion time of PP/PDA/AgNPs in PDMS solution (Figure 4e), because the increasing number of insulating PDMS is detrimental to the electron transportation. What is noteworthy is that the PP/ PDA/AgNPs possesses a relatively low CA of about 109.0°. With extension of the immersion time to 10 and 20 min, the contact angle largely adds to about 139.5° and 151.9°, respectively, due to the decoration of the low surface energy material onto the AgNP surface. However, the CA decreases gradually from 151.9° to 143.8° when the immersion time further increases from 20 to 120 min (Figure 4f), and the slightly decreased CAs may be caused by the reduced surface roughness resulting from the filling of the polymer to the interparticle gap. The superhydrophobic composite fabric (PP/PDA/Ag25%/PDMS-40) possesses an ultralow surface resistance of around 0.36 Ω, as demonstrated in Figure 5a. Figure 5b shows that when the highly conductive fabric is connected with the wire in a current circuit, the LED is lighted, demonstrating that the composite fabric can be applied for an excellent electronic conductor. Interestingly, the LED keeps burning even the material is undergoing multiple bending (Figure 5c), and the detailed information can be found in SI Movie S1. The stability and durability for the conductivity of the composite fabric under different mechanical deformation will be further revealed in the following sections. As described, the PDMS layer endows the fabric with superhydrophobicity. As exhibited in Figure 5d, one water droplet suspended on the nozzle of a metallic needle slowly approaches the composite fabric, and finally touches the material surface (Figure 5e). Surprisingly, the water droplet can keep its original shape and does not adhere on the surface even the water droplet undergoes squeezing (Figure 5f). When the needle is lifted, the water droplet is quickly detached from the fabric surface (Figure 5g

and 5h), displaying outstanding water proof property. Furthermore, the PDMS with a cross-linking structure is anticorrosive. As shown in Figure 5i, the CAs of different droplets including water, acid, alkaline and salt solution on the composite fabric are all larger than 150°. The solution droplets with different pH values from 1 to 13 can stand well on the composite fabric surface, keeping their original spherical shapes (Figure 5j). Apart from the high CA, the PP/PDA/AgNPs/ PDMS has an extremely low sliding angle of almost 0°, which means the water droplets are able to roll off the fabric surface even when the material is placed on a flat support. As a result, the superhydrophobic fabric possesses excellent self-cleaning performance, which is demonstrated in SI Movie S2. The carbon black (CB) powders served as a model contaminant is deposited on the composite fabric surface (Figure 5k). Subsequently, the water droplets are dripped on the material surface (Figure 5l), and they can easily roll back and forth on the fabric surface under the assistance of a syringe needle, during which the CB powders are gradually adsorbed onto the water droplets (Figure 5m). Finally, all the CB powders are removed from the fabric surface, and these water droplets wrapped by the CB can still roll out the material surface (Figure 5n). EMI shielding effectiveness (SE) is generally used to evaluate the EMI shielding performance, which is defined as the logarithmic ratio of incident to transmitted power, reflecting the capability to attenuate electromagnetic interference. Generally, three mechanisms, namely absorption (SEA), the reflection (SER) and multiple reflections (SEM), are responsible for the attenuation of the electromagnetic waves. SEA is related with the energy dissipation of electromagnetic waves, while SER and SEM originate from the impedance mismatch between air and the shield. It was reported that SEM could be neglected when SEtotal is beyond 15 dB.40 Figure 6a shows the EMI SE of the composite fabrics prepared by G

DOI: 10.1021/acsami.8b22212 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) The CA and conductivity variation of PP/PDA/AgNPs-25%/PDMS-40 with the abrasion times. (b) EMI SE of the fabrics before and after the fabric experiences the 50 times abrasion. (c) The CA and conductivity variation of PP/PDA/AgNPs-25%/PDMS-40 with the winding times. (d) EMI SE of the fabrics before and after 50 times winding at a radius of curvature (ROC) of 3.5 mm. (e) The contact angles and conductivity of PP/PDA/AgNPs-25%/PDMS-40 vs the corrosion time in an acid solution with the pH of 1. (f) EMI SE of the fabrics before and after corrosion in the acid solution for 20 h. Note that the insets in panels b, d, and f are photographs showing the composite fabrics subject to abrasion, winding, and treatment in the acid solution, respectively.

immersion the PP/PDA in STA solution with different concentrations. It is clear that the SE values of all the composite fabrics fluctuate around a stable value in the whole X band, which is similar to that reported in the previous work.27,41 It is also found that the higher the conductivity, the larger the SE. The SE increases from 1.9 dB for PP/PDA/ AgNPs-5%/PDMS-40 to 71.2 dB for PP/PDA/AgNPs-25%/ PDMS-40. To reveal the EMI shielding mechanism of the conductive fabrics, the relationship between the SE (including SET, SEA, and SER) and the Ag precursor concentration is calculated, as displayed in Figure 6b. It can be found that both the SEA and SER increase with the Ag precursor concentration, but the SEA exhibits a greater increase than SER, indicating the adsorption contributes more than reflection to the attenuation of electromagnetic waves. For instance, at the Ag precursor concentration of 25 wt %, the SEA of the composite fabric at 8.2 GHz is 56.5 dB, much higher than the SER (12.8 dB). As a proof of concept for the excellent shielding performance, the composite fabric was used to shield a wireless power

transmission system. Figure 6c shows a typical wireless power transmission system consisting of a DC power, transistors, transmitter coil, receiver coil, and light-emitting diode (LED) lights. After a current is generated in the circuit, it flows through the power transmitter coil, creating an electromagnetic field. Subsequently, the receiver coil receives electromagnetic waves from the transmitter coil, and the LED is accordingly lighted (Figure 6c). When the composite fabric is inserted between the transmitter coil and the receiver coil, the electromagnetic transmission is blocked, leading to the elimination of the current in the receiver coil. As a result, the LED is turned off (Figure 6d). The detailed information about the shielding performance is shown in SI Movie S3. The influence of electrical conductivity on the EMI shielding performance can be interpreted from the view of dielectric and magnetic properties of materials. There exists a “skin depth” (δ) for electrically conductive materials that refers to the thickness below the outer surface of material where the H

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Table 1. Comparison of the EMI Shielding Performance of the CPFCs in this Study and Various Silver Based Conductive Polymer Composite Materials Reported in the Literature sample 52

PLA/Ag nanocomposites Ag@C hybrid sponges27 Ag@FRGO/WPU37 PPy/PDA/AgNWs composites53 AgNWs/MWCNT/cellulose hybrid papers54 Ag@CFs nonwoven fabrics55 AgNWs/PI Foams56 CA/AgNW/PU Film57 Ag/RGO coated PET fabrics12 PP/PDA/AgNPs/PDMS (this study)

Ag filler

frequency (GHz)

density (g cm−3)

EMI SE (dB)

SSE (dB cm3 g−1)

Ag particles AgNW AgNP AgNW AgNW Ag particles AgNW AgNW Ag particles AgNP

8.2−12.4 8.2−18 8.2−12.4 8.0−12.0 0.5−1.0 8.2−12.4 0.03−1.5 8.2−12.4 1−18 8.2−12.4

>1.25 0.00382

50 70.1 35 48.4 23.8 111