Electromagnetic interference shielding of layered

Materials and Design 95 (2016) 97–106

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Electromagnetic interference shielding of layered linen fabric/polypyrrole/nickel (LF/PPy/Ni) composites Hang Zhao, Lei Hou, Yinxiang Lu ⁎ Department of Materials Science, Fudan University, Shanghai 200433, China

a r t i c l e

i n f o

Article history: Received 3 November 2015 Received in revised form 18 January 2016 Accepted 20 January 2016 Available online 21 January 2016 Keywords: Linen fabric Electroless nickel plating In-situ polymerization Polypyrrole Shielding effectiveness

a b s t r a c t In this paper, layered linen fabric/polypyrrole/nickel (LF/PPy/Ni) composites were successfully fabricated by combining a chemical in-situ polymerization approach with an electroless plating method. Owing to the multiple characteristics of both wave-absorption and wave-reflection, the resultant composites showed great potential as electromagnetic interference (EMI) shielding material to realize electromagnetic compatibility. Various experimental conditions including initiator concentration, polymerization time and synthesis cycles were optimized in the in-situ doping polymerization process. Under the optimized conditions, the coating amount of PPy was adequate for generating the inter-fiber connection. Afterwards, Ni layer was deposited on the as-made PPy coated LF (LF/PPy). X-ray diffraction (XRD) analysis indicated that the Ni layer had a characteristic face-centered cubic (FCC) crystalline structure. Vibrating sample magnetometry (VSM) analysis revealed that the resultant LF/PPy/ Ni composites exhibited strong magnetic properties. Furthermore, comparative study of shielding effectiveness (SE) among nickel plated LF (LF/Ni), LF/PPy and LF/PPy/Ni was conducted by using a Spectrum analyzer at a frequency range of 30 to 1000 MHz. The result indicated that the highest SE was obtained in the LF/PPy/Ni sample, which had already achieved civilian product standard. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Electromagnetic (EM) waves from electronic instruments have an adverse effect on the performance of neighboring equipment and even can cause mutual malfunction [1–5]. Hence, EM compatibility of an electronic circuit at broadband EM wave is of great importance in the design of integrated circuits. External EM radiations should not interfere with the internal circuit. Meanwhile, the internal circuit should not radiate EM energy to disturb other neighboring circuits. The most efficient method to realize such an EM compatibility is to house the circuit in an enclosure utilizing electromagnetic interference (EMI) shield, which can attenuate the radiations from the internal circuit and prevent external radiations interfering with the internal circuit [6]. Conductive textile emerging as a burgeoning material have been applied extensively in EMI shield due to their advantages such as prominent flexibility, high EMI shielding effectiveness (SE), good electrostatic discharge and light weight. To manufacture a conductive fabric featuring with high SE, a variety of conductive components has been introduced, such as metals and intrinsically conducting polymers (ICPs) [7–12]. Herein, SE is a notion to characterize the effectiveness of EMI shield in terms of decibels (dB). When an incident EM wave encounters the conductive fabric, it will be blocked via three forms, namely

⁎ Corresponding author. E-mail address: [email protected] (Y. Lu).

http://dx.doi.org/10.1016/j.matdes.2016.01.088 0264-1275/© 2016 Elsevier Ltd. All rights reserved.

reflection, absorption and internal multiple reflections. Therefore, the EMI SE can be expressed by the following equation: SE ¼ A þ R þ B

ð1Þ

where A, R and B denote absorbed wastage, reflected wastage and multiple internal reflected wastage, respectively [12]. ICPs such as polyaniline (PAN), polyacetylene (PA) and polypyrrole (PPy), are typically EM wave-absorption dominated material due to their tunable electrical/dielectric properties and non-transparency toward EM radiations [13–15]. Conversely, metals are EM wave-reflection dominated material due to their abundant mobile charge carriers [16–19]. To transform the textile from insulating to conductive, various methods have been proposed [20–22], such as a mixture of metal powders during melt or wet spinning process, twisting or wrapping insulating fibers with metallic wires during mechanical spinning process and selecting ICPs for chemical spinning, etc. However, those aforementioned methods are always sophisticated and involve in numerous problems, such as importing conductive components during spinning will result in filaments or yarns with poor flexibility, selecting ICPs for spinning is difficult to form homogeneous and mechanically strong filaments. Be different from those spinning-stage methods, directly forming metal or ICPs coating on textile production (fiber, yarn, fabric) may be more suitable due to their low cost, labor-saving and simplicity of processing. Qin et al. successfully manufactured copper coated polyester fabric by autocatalytic copper plating process using glyoxylic acid as a reducing agent.

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H. Zhao et al. / Materials and Design 95 (2016) 97–106

The copper coated polyester fabric possessed SE of 40–50 dB at the frequency of 6 GHz to 18 GHz. However, the metal coated fabric had reflection as its major shielding form, which limited its development in SE [23]. Parveen et al. proposed composite absorbers based on conducting fabrics, which possess moderate conductivity and dielectric/magnetic properties. The absorbers were prepared by in situ incorporation of nanoparticles of BaTiO3 or Fe3O4 within coated polyaniline matrix. The specific SE value of the composite absorber was 17–20 dB cm3/g, which demonstrated that these fabrics exhibited great potential as promising microwave-shielding material [24]. Although metal plated fabrics or ICPs coated fabrics for EMI shielding have been studied recently, researches about combination of them two to manufacture conductive fabrics have not been revealed to date. If the contribution of absorbance or reflectance to the total EMI SE can be controlled by the appropriate array of ICPs and metal, an extremely high SE will certainly be achieved theoretically. Inspired by the aforementioned analysis, ICPs and metal were successively deposited on the natural textile to fabricate EMI shielding materials in this paper. In the present work, linen fabric (LF) was selected as the substrate to manufacture layered linen fabric/polypyrrole/nickel (LF/PPy/Ni) composites via a PPy deposition followed by an electroless nickel plating. LF/PPy/Ni composites were synthesized in two successive steps: (I) PPy layer was constructed on LF substrate by selfpolymerization of pyrrole monomers. (II) Nickel film was grown on 3aminopropyltrimethoxysilane (APTMS) self-assembled monolayers (SAMs) modified LF/PPy by electroless nickel plating. In step (I), pyrrole was used as monomer, Ferric chloride (FeCl3) was designed as oxidant, and sodium p-toluenesulfonate (STS) was selected as dopant. Prior to the step (II), LF/PPy was modified by APTMS. The APTMS used here was a kind of trifunctional alkoxy silanes (R′Si(OR)3, where R and R′ are alkyl groups), which acted as a molecular bridge between organics (PPy film) and inorganics (Ni layer) [25]. It was confirmed that the APTMS-SAMs could enhance the adhesive strength between metal layer and substrate by chemical sorption instead of the physical sorption in the conventional sensitizing-activation method [26–29]. In step (II), electroless nickel plating was performed by multistep processes including modification, activation and nickel deposition followed by rinsing and drying. The synthetic route of the LF/Ni/PPy composites was shown schematically in Fig. 1.

Optimization study was conducted in order to achieve the optimum experimental conditions in the polymerization process. Herein, the optimum PPy layer was evaluated in terms of SE (30–1000 MHz). Fourier transform infrared spectroscopy (FTIR) measurement was conducted to investigate the interaction mechanism between LF substrate and PPy. The surface morphologies of the samples in each step were investigated by scanning electron microscopy (SEM), and the crystal structures of the external nickel layer were detected by X-ray diffractometer (XRD) measurement. Surface resistance (Rs) was measured by the four probe method described in ASTM F 390. Magnetic properties of the resultant LF/PPy/Ni composites were investigated by vibrating sample magnetometry (VSM). Peel test and tensile test were utilized to evaluate the reliability of the composites. Additionally, SE values of LF/Ni, LF/PPy and LF/PPy/Ni were recorded and compared in this study by using a Spectrum analyzer. 2. Experimental 2.1. Materials The linen fabrics (45 × 45 count/cm2, 24 mg/cm2) were purchased from Taicang Biqi Novel Material Co., Ltd., and were precisely cut into rectangular patch (6 × 6 cm or 5 × 15 cm). Pyrrole, sodium ptoluenesulfonate (STS), 3-aminopropyltrimethoxysilane (APTMS) and hexahydrate ferric chloride (FeCl3·6H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. Particularly, pyrrole was distilled under reduced pressure and stored at 4 °C for the following usage. All other reagents were of analytical grade and were used without further purification unless otherwise mentioned. Moreover, the distilled water was purified by a Milli-Q system (Milford, MA, USA). 2.2. In-situ polymerization of pyrrole The pristine LFs were degreased in alkaline solution (60 g/L NaOH + 30 g/L Na3PO4 + 15 g/L Na2CO3) to remove oils and other organic chemicals, followed by rinsing with distilled water until the pH value reached neutral and drying in an oven at 50 °C. Afterwards, a two-stage bath process was performed for the in-situ polymerization of pyrrole. In the first stage, LFs were soaked in the pyrrole monomer

Fig. 1. The synthetic route of the layered LF/PPy/Ni composites (APTMS and STS are the abbreviations of 3-aminopropyltrimethoxysilane and sodium p-toluenesulfonate, respectively).


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solution for 5 min. Herein, pyrrole was dissolved in 50 mL distilled water to form a milky emulsion solution under ultrasonication vibration and the concentration of pyrrole was set as 1.0 mol/L. In the second stage, the in-situ polymerization was conducted by dipping the pyrrole-absorbed LFs into oxidant solutions for various times (5, 10, 15 and 20 min) at an appropriate polymerization temperature. Oxidant solutions were prepared by dissolving FeCl3·6H2O together with STS into the distilled water, and the concentrations of FeCl3 were designed at four levels (0.2, 0.4, 0.6 and 0.8 mol/L). The PPy coated fabrics were sequentially washed with distilled water to remove blank PPy powders, and dried in an oven for 30 min at 60 °C. Chemical in-situ polymerization, as described above, was repeatedly performed for several cycles (n = 1, 2, 3 and 4) to obtain diverse coating amounts of PPy. The coating amount (WA) was defined as the following equation: WA ¼ ðW−W0 Þ=W0 100%

ð2Þ

where W was the mass (mg) of LF/PPy composites, and W0 was the mass (mg) of pristine LF. All the samples were weighed on an electronic microbalance (FA 1104, Shanghai, China) to a resolution of 0.1 mg. 2.3. Electroless nickel plating Electroless nickel plating was carried out by multistep processes, which included modification, activation and electroless deposition followed by rinsing and drying. Surface modification was conducted by immersing the LF/PPy into the acetone solution with 0.125% APTMS for 5 min, and then heated at 120 °C for 10 min to form selfassembled monolayers (SAMs). Dipping and heating of samples were carried out for 3 cycles. Afterwards, the APTMS-SAMs modified LFs were rinsed with distilled water and activated through immersion into Pd colloid solution for 24 h. The preparation of Pd colloid solution was described as S2.2 in the supplementary information. Finally, the Pd-activated LF/PPy samples were dipped into the electroless nickel plating bath at 60 °C for 30 min. Nickel nucleated on these catalytically active surface, then further nickel reduction and growth occurred. Formulations and operation conditions of the electroless nickel plating were listed as Table S1 in the supplementary information. After plating, the LF/PPy/Ni composites were carefully rinsed with distilled water, ethanol and then dried in an oven for 1 h at 60 °C.

Fig. 2. Schematic drawing of the spectrum analyzer for electromagnetic interference shielding effectiveness.

2.5. Peel test and tensile test The adhesion between the nickel coating and the substrate was assessed qualitatively by Peel test. Briefly, a tape (Scotch®-Ruban Magic Tape™/MC (3M, Minnesota, USA)) was carefully adhered to the composites and subsequently removed by peeling it off parallelly. The adhesion strength was evaluated by checking if any Ni scraps were peeled off from the substrate or not. As for tensile test, all the samples for test (dimension of the fabric sample was 5 × 15 cm) were conditioned at 65 ± 2% relative humidity and room temperature for 24 h to relieve any localized load caused by handling during preparation. Breaking force of the samples was characterized by electronic tensile testing machine (HANDPI Instruments Co., LTD, YueQing, China) together with a digital force gauge (HP-100) based on the ISO 13934-1:1999 Standard (65 ± 2% relative humidity, room temperature) [30–31]. The samples were loaded between a pair of clips with a displacement rate of 100 mm/min until rupture. The gauge length of the counter-clips was set as 10 cm. For breaking force values reported here, at least ten sample measurements were performed and averaged to ensure the reliability of the results.

2.4. Apparatus

3. Results and discussion

Before characterization, the as-made samples were conditioned for 24 h under laboratory conditions of 20 ± 2 °C and 65 ± 2% relative humidity. Infrared (IR) spectra were recorded by FTIR spectrometer (Nicolet Nexus 470) in diffuse reflectance mode. Particularly, the IR spectrum of PPy powders was conducted using KBr pellets. X-ray diffraction (XRD) patterns were recorded with a D/max-2500 diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 0.154 nm) over the angular ranging from 10° to 90°. Scanning electron micrographs were obtained using HITACHI SU1510 electron microscope. The coaxial transmission line method as described in ASTM D 4935–99 was used to investigate the SE of the conductive fabrics with the aid of a Spectrum analyzer (ATTEN AT5011, China). Each sample was cut into 6 × 6 cm in dimension and placed between the flanges in the middle of the cell, as shown in Fig. 2. The magnetic properties of the resultant LF/PPy/Ni composites were studied by using a LDJ 9600-1 vibrating sample magnetometry (LDJ Electronics Inc., Tory, MI. USA) at room temperature with the field range of −20 to 20 KOe. The surface resistance of the samples was measured by digital four-point probe ST2258C (Suzhou, China). Rs is considered to be the resistance of a square sample, and the unit of Rs is expressed as Ω/sq. or KΩ/sq. For Rs and SE reported here, at least ten sample measurements were performed and averaged.

3.1. Optimization of polymerization conditions In the in-situ polymerization process, reactant concentrations and synthesis parameters played an important role in the formation of a homogeneous PPy layer. With the aim to obtain a high-quality PPy layer, a series of experimental parameters including initiator concentration, polymerization time and synthesis cycles were optimized separately in our work. The quality of the PPy layer was evaluated in terms of both EMI SE and Rs. In general, materials with low Rs value will have high EMI SE and vice versa [32]. As we have known, the quality of the PPy layer is largely determined by the quantity of initiator. To obtain a LF/PPy composite with the prime EMI SE, in other words, with the lowest surface resistivity, the concentration of FeCl3 was investigated in the range from 0.2 mol/L to 0.8 mol/mL. As shown in Fig. 3(a), when the concentration of FeCl3 reached 0.4 mol/L, the LF/PPy composites exhibited optimum SE and lowest Rs. With the concentration increasing, SE decreased while Rs increased simultaneously. The result could be ascribed to the over-oxidation of the linen fiber, which leaded to the formation of carbonyl defects on the PPy backbone. Carbonyl defects caused interruptions in the Π conjugation and acted as electron withdrawing groups, inhibiting charge transfer and thus increasing

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was used as the initiator concentration in the following optimization work. After being soaked in the pyrrole monomer solution for 5 min, some pyrrole molecules had penetrated into the microspores or cracks of linen fibers. Hence, appropriate polymerization time was an essential condition to assure the sufficient contact between pyrrole and FeCl3. The effect of polymerization time on EMI SE in the range of 5–20 min was shown in Fig. 3(b). The results indicated that the EMI SE of the LF/ PPy composites increased with an extending time from 5 min to 10 min. As the time prolonging to 15 min, there was no significant improvement due to the rapid polymerization of pyrrole monomers. Therefore, 10 min was selected as the optimized polymerization time. The last experimental parameter need to be optimized was the synthesis cycles, which made a significant difference to the coating amount of PPy. The more cycles the chemical polymerizations repeated, the more PPy will be added on the LF substrate. Appropriate coating amount could result in high-quality EMI shielding textiles, while excessive coating amount not only made the process sophisticated but also increased the corresponding cost. Hence, a variety of synthesis cycles in range from 1 to 4 times was studied in this work. The effect of synthesis cycles on EMI SE was shown in Fig. 3(c), and the corresponding data of coating amount and Rs were summarized in Table 1. According to the results, homogeneous PPy film was difficult to form due to the low coating amount at n = 1 and 2, which resulted in high Rs and low EMI SE. When the synthesis cycles reached n = 3, the Rs of the PPy layer reduced to 11.03 Ω/sq., which had already achieved the same magnitude of the reported Rs of bulk PPy. Once prolonged to n = 4, no significant improvement in EMI SE occurred. Moreover, the coating amount at n = 4 was 29.21%, which would certainly result in a thick PPy film on LF substrate. As we have known, the thick film easily becomes brittle due to the lack of mechanical strength. Hence, n = 3 was identified as the optimal synthesis cycles for LF/PPy composites, which got excellent EMI SE (13–20 dB) with relatively less coating amount (24.22%). In summary, the optimized conditions were as follows: initiator concentration, 0.4 mol/L; polymerization time, 10 min; synthesis cycles, 3 times. Additionally, it could also be found that almost all the LF/PPy group samples showed a decrease in SE at high frequency. This phenomenon could be explained as follows: for EMI shielding, any holes or meshes in the shield must be significantly smaller than the wavelength of the radiation; EM wave at high frequency certainly possess short wavelength. Therefore, when an incident EM wave featuring with high frequency is shorter than the aperture size of the meshes in the LF/PPy composite, the EMI SE of the barrier will suffer a significant decline. Meshes in the shield force current to flow around them, so that EM fields passing through the meshes cannot excite enough opposing electromagnetic fields to cancel the incident field. Hence, for textilebased conductive materials, appropriate coating amount is of great importance to reduce the meshes. 3.2. FTIR and XPS analysis

Fig. 3. The optimization of (a) FeCl3 concentrations (0.2, 0.4, 0.6 and 0.8 mol/L), (b) polymerization time (5, 10, 15 and 20 min) and (c) synthesis cycles (n = 1, 2, 3 and 4). The inset figures were fabricated by plotting surface resistance (y) versus corresponding FeCl3 concentrations (x), polymerization time (x) and synthesis cycles (x), respectively.

resistivity. This could explain why the EMI SE decreased at 0.6 mol/L or 0.8 mol/L of the FeCl3 concentration. All in all, 0.4 mol/L was considered to be the optimized concentration of FeCl3, and such optimized level

FTIR analysis was conducted to investigate the possible interaction mechanism between LF and PPy, and the result was shown in Fig. 4. In the FTIR spectrum of pristine LF (Fig. 4(a)), the broad and intense absorption peak at 3458 cm−1 was ascribed to the O–H stretching vibrations of cellulose and absorbed water. The peak observed at 2903 cm−1 could be attributable to the C–H stretching vibrations of methyl, methylene and methoxy groups, which corresponded to the carbon chain of the cellulose. The peak at 1643 cm−1 was due to the C_C stretching which could be attributed to the presence of aromatic rings. The vibration at 1425 cm−1 could be ascribed to aliphatic and aromatic groups in the plane deformation vibrations of methyl and methylene groups. The band around 1073 cm−1 could be assigned to the C–O stretching vibration of alcohols. These assignments were in agreement with that reported earlier for cellulose-based textile materials [33]. In the FTIR spectrum of PPy powders (Fig. 4(b)), the band at 1535 cm−1 was associated to the pyrrole C_C ring stretching vibration. The vibrations at


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Table 1 Coating amount, surface resistance (Rs) and color of the samples in each step. Sample

Coating amount Rs (Ω/sq) Color

LF

– – White

LF/PPy

LF/PPy/Ni

n=1

n=2

n=3

n=4

5.43% 324.51 ± 8.01 Gray

10.26% 117.72 ± 6.03 Black

24.22% 11.03 ± 2.54 Black

29.21% 10.22 ± 1.63 Jet black

29.59% 0.97 ± 0.12 Argentate

The values after the mark “±” are the standard deviations.

1446 cm−1 and 1300 cm−1 were assigned to C–C stretching and C–N stretching, respectively. The band at 1088 cm−1 was the characteristic vibration of STS dopant, being attributable to the vibration of the aryl sulfonate salt. Other bands at 1161 cm−1, 1033 cm−1, 887 cm−1 and 777 cm−1 were attributed to the characteristic bending (C-H) vibration of PPy. The band at 964 cm−1 was assigned to a fraction of PPy free from the influence of STS dopant. As for LF/PPy composites (Fig. 4(c)), all bands at 1535 cm−1 (–C_C PPy ring stretching vibration), 1446 cm−1 (–C–C stretching vibration), 1300 cm−1 (–CN stretching vibration), 1161, 1043, 964, 896 and 784 cm−1 were present in the spectrum. It could be found that the intensity of peaks corresponding to LF substrate decreased substantially after PPy coating, and only bands at 2903 cm−1 (C–H stretching vibrations) and 3458 cm−1 (O–H stretching) retained, which could be attributed to considerable coating amount of PPy. Additionally, new band appeared at 2242 cm−1, which might be ascribed to the chemical interaction (hydrogen bond) between PPy and LF substrate, as plotted in Fig. 5. Notably, in the chemical polymerization

process, p-toluene sulfonate ions could replace Cl− ions which had already been intercalated inside the planar PPy backbones. Incorporation of p-toluene sulfonate anion enhanced environmental stability of PPy because it was difficult to be removed from PPy backbone due to its larger size in comparison with Cl− ion. Fig. 6 displayed typical XPS wide-scan spectra of the LF/PPy, APTMSSAMs modified LF/PPy and Pd activated LF/PPy, which indicated that C1s, N1s and O1s were the dominant signals and occurred at 281.5 eV, 396.4 eV and 528.5 eV, respectively. The peak at 164.02 eV corresponding to sulfur was also seen in all spectra, which derived from STS and behaved as the dopant. Comparing curve (b) to curve (a), the presence of silicon on APTMS-SAMs modified LF/PPy was detected from their characteristic emissions at 104.74 eV and 154.91 eV, which indicated that APTMS molecules had been grafted on the LF/PPy composites. After an exposure to the palladium colloid, an additional peak at 340.2 eV corresponding to palladium element was detected. This implied that Pd nanoparticles had been absorbed on the surface of APTMS-SAMs modified LF/PPy. During N1s deconvolution of LF/PPy composites (Fig. 7(a)), four components (BE = 398.3 eV, 400.6 eV, 401.86 eV and 403.3 eV) were proposed. The one at lower BE corresponded to nitrogen in imine state (_N− structure). The second peak had the highest intensity and located at 400.6 eV, which corresponded to secondary amine nitrogen (− NH− structure) and acted as a component of the PPy backbone. Finally, two signals with higher BE (401.86 and 403.3 eV) were assigned to positively charged nitrogen (−NH+ (polaron) and _NH+ (bipolaron)), which were commonly in the doped PPy ideal structure. Consequently, these XPS spectra confirmed that PPy films incorporating with STS dopant had been obtained from the oxidation of pyrrole monomers. After being modified with APTMS-SAMs (Fig. 7(b)), an additional signal at 401.44 eV was shown, which could be attributed to − NH2 groups originating from APTMS molecules. This result also verified that APTMS had been grafted on the LF/PPy composites. 3.3. Crystal structure, morphology and magnetism analysis

Fig. 4. IR spectra of (a) pristine LF, (b) LF/PPy composites and (c) PPy powders (the synthesis of LF/PPy powders was shown as S1 in the supplementary information).

The typical XRD patterns for pristine LF, LF/PPy and LF/PPy/Ni were represented in Fig. 8(A). Because the intensity of PPy was too weak to be discerned in the wide spectrum, a narrow XRD pattern for LF/PPy within the angular interval of 10° to 20° was shown in Fig. 8(B). In all wide spectra, the diffraction peaks at 2θ = 14.81°, 16.51°, 22.61° and 34.24° were attributable to the LF substrates. After being coated with PPy layer, a weak peak was detected at 11.11° which was due to the short range arrangement of PPy chains. The result revealed that the PPy layer obtained was amorphous in nature. It was noteworthy that the broad characteristic peak of PPy at 2θ angles from 20° to 30° did not show it presence, which might be covered up by the strong characteristic diffraction peak of LF substrate at 22.61°. As shown in curve (c), three peaks shown their presence at 2θ = 44.67°, 51.69° and 76.77° after electroless nickel plating, which were attributed to (1 1 1), (2 0 0) and (2 2 2) planes of face-centered cubic nickel. It can be seen that the sites and intensity of those diffraction peaks were consistent with the standard pattern for JCPDS Card No. 04–0836. Comparing curve (c) to curve (a, b), the characteristic peaks intensity diffracted from LF substrate decreased significantly, which indicated that the covering

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Fig. 5. Scheme of chemical in-situ polymerization to form PPy onto LF surface.

degree of the outer nickel coating was high. According to the Debye– Scherrer formula, the crystallite size Dhkl of the sample can be given by the following equation: Dhkl ¼ 0:89λ=ðB cosθÞ

ð3Þ

where B is the full-width at half-maximum (FWHM) of XRD diffraction lines, λ is the X-ray wavelength corresponding to Cu Kα radiation (0.154056 nm) and θ is the half diffraction angle of 2θ. The particle size was determined by taking the average of the sizes at the peaks (D111, D200 and D222), and the calculated particle size was found to be 22.05 nm. The morphologies of the samples in each step were investigated by SEM. As shown in Fig. 9(a), impurities were clearly seen on the pristine LF surface, which may influence the adhesion properties with PPy in the chemical polymerization process. After an exposure to alkaline solution for 2 h (Fig. 9(b)), no impurities were shown on the LF surface due to the cleaning effect of the alkaline solution. It can be found that some cracks propagated along the fiber axial direction, which would certainly result in lower tensile strength in comparison with pristine fiber. After PPy deposition (Fig. 9(c,d,e)), the surface of LF/PPy became rougher and some

Fig. 6. XPS spectra of (a) LF/PPy, (b) APTMS-SAMs modified LF/PPy, (c) Pd activated LF/ PPy.

Fig. 7. N1s core-level spectra of (a) LF/PPy and (b) APTMS-SAMs modified LF/PPy.


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Fig. 8. XRD patterns for A: (a) the pristine LF, (b) LF/PPy and (c) LF/PPy/Ni; B: an extended portion in the region of 10°–20° to show clear picture of LF/PPy sample.

globular PPy agglomerations emerged. Moreover, the increase of the inter-fiber connection or network was also observed in LF/PPy group samples, and the LF/PPy (n = 3) exhibited the prime inter-fiber connection effectiveness. In general, EMI shield requires not only low surface

resistance but also high connectivity of the conduction path in the conductive textiles. Conductive fabric with coherent conductive fibers has better EMI SE than those with discontinuous conductive PPy coated fibers. Therefore, the LF/PPy (n = 3) was selected as the optimal

Fig. 9. SEM photographs of (a) pristine LF, (b) alkaline treated LF, (c, d, e) LF/PPy (n = 1, 2, 3), (f) LF/PPy/Ni (n = 3).

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Fig. 10. The magnetization hysteresis loops of LF/PPy/Ni composites with the field between −20 and 20 KOe (a) and an extended portion in the region of −100 Oe–100 Oe (b).

substrate to perform the following electroless nickel plating. As for the resultant LF/PPy/Ni composites (Fig. 9(f)), nickel coating was uniformly covered and the ball-shaped nickel grains were well dispersed on the substrate. The magnetization curve of the resultant LF/PPy/Ni composites was measured at room temperature using a super conducting quantum interference device by cycling the field between − 20 and 20 KOe, as shown in Fig. 10(a). The magnetic hysteresis loop was S-like curve, and the specific saturation magnetization (Ms) was 7.66 emu/g. Notably, such Ms. value was obtained just with 4.14% magnetite (Ni) content. Moreover, remnant magnetization (1.77 emu/g) and coercivity (65 Oe) were also present in the extended graph as shown in the Fig. 10(b). The results indicated that the LF/PPy/Ni composites exhibited strong magnetic properties. As we have known, the best materials for EMI shielding should possess both high conductivity and excellent magnetic behaviors. Consequently, the as-made LF/PPy/Ni composites showed great potential as electromagnetic interference (EMI) shield on account of their low Rs (as shown in Table 1) and outstanding magnetic behaviors.

3.4. Tensile properties and peeling strength Physical properties of pristine LF, LF/PPy, LF/Ni and LF/PPy/Ni were evaluated by tensile test. The typical load-time curves were plotted in Fig. 11. In comparison, with pristine LF, the breaking force of LF/PPy suffered a significant decline. This phenomenon can be explained as follows: (I), PPy coating has limited ability to strengthen the LF substrate due to its brittleness in nature; (II), The soaking in alkaline

solution may damage the surface of LF, thus resulting in a lower tensile strength in comparison with pristine LF (as Fig. S1 in the supplementary information). On the contrary, LF/Ni had a higher breaking force in comparison with pristine LF, which could be ascribed to both the high strengthen ability of nickel film and the absence of alkaline-treatment. For the LF/PPy/Ni sample, the tensile strength at break (157.5 N) was within the interval of LF/PPy (102.2 N) and LF/Ni (180.0 N), and the breaking force was still higher than that of pristine LF (135.9 N), which was sufficient for most usage. It was worth noting that pretreatment of the LF substrate with alkaline aqueous solution was necessary to realize successful PPy-coating. Alkaline-pretreatment could improve the affinity between LF substrate and PPy layer. Without this treatment, plentiful blank PPy powders would be found everywhere in the polymerization process, and the PPy film would exhibit a poor adhesion to the LF substrate. As we have known, after being treated with alkaline solution, more microspores and cracks would generate on the textile material surface. Afterwards, polypyrrole penetrated into the yarn structure through those microspores or cracks and certainly resulted in a high affinity between LF substrate and PPy layer. As shown in Fig. 12(c), the cross-section of the LF/PPy composites was in absolutely black color, which indicated that a deep penetration of PPy into the yarn had occurred. The high bonds between LF and PPy layer laid a reliable foundation for the following growth of nickel film. In this paper, the adhesion between the nickel coating and the LF/PPy composites was assessed qualitatively by Peel test. As represented in Fig. 12(d), the nickel film never failed in the Peel test, revealing that the adhesion strength of the external nickel film was reliable. The commendable adhesion properties of the resultant LF/PPy/Ni composites may be attributed to both alkalinepretreatment and APTMS-SAMs modification. 3.5. Comparative study of SE among LF/Ni, LF/PPy and LF/PPy/Ni

Fig. 11. Tensile load-time curves for the pristine LF (a), LF/PPy (b), LF/Ni (c) and LF/PPy/Ni (d), respectively. The LF/Ni composites used here was fabricated following the synthesis route shown as S2 in the supplementary information.

The results of the EMI SE of LF/PPy, LF/Ni and LF/PPy/Ni were shown in Fig. 13. In comparison, with LF/PPy or LF/Ni, the LF/PPy/Ni composite had a higher SE of 20.22–43.51 dB, which could be ascribed to its multiple characteristics of both EM wave-absorption and EM wave-reflection. As for LF/PPy/Ni composites, PPy and Ni were successively deposited on LF substrate. The internal PPy layer is EM wave-absorption dominant, while the external Ni film is EM wave-reflection dominant. The shielding mechanism of the LF/PPy/Ni composites was illustrated in Fig. 14. When an EM radiation encounters the external Ni film, a large proportion of EM will be attenuated through reflection-formation (as for Ni film, the loss of EM radiation via absorption or multiple internal reflection account for a tiny part) and then the rest of the wave is transmitted. Afterwards, the subdued EM radiation encounters the internal PPy layer, most of it will be shielded through absorption-formation (as for PPy layer, the loss of EM radiation via reflection or multiple internal reflection account for a small portion), and the transmitted EM wave is too weak to break in electromagnetic compatibility. Requirements for


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Fig. 12. Pictures of (a) LF/PPy, (b) LF/PPy/Ni, (c) cross-section of LF/PPy and (d) tape peel test of LF/PPy/Ni (the background of a, b or c is LF).

EMI textiles were classified by the Committee for Conformity Assessment of Accreditation and Certification on Functional and Technical Textiles, as shown in Table S2 in the supplementary information. Based on the classification, the LF/PPy/Ni composite was between ‘AAAA’ and ‘AAAAA’ grade for general use, which indicated that more than 99.99% EM could be shielded by such conductive fabric. Hence, the resultant LF/PPy/Ni composites can be used as EMI shield in consumptive electronic products such as radio receivers, television sets, video recorders, DVD players, digital cameras, camcorders, personal

Fig. 13. The shielding effectiveness of (a) LF/PPy, (b) LF/Ni and (c) LF/PPy/Ni, respectively.

computers, video game consoles, telephones and mobile phones, etc. Herein, most consumptive electronic products are based on radio broadcasting technology. Therefore, the LF/PPy/Ni composite material described in our work can be made into radio frequency (RF) shielding products to realize EM compatibility.

4. Conclusions In this work, the layered LF/PPy/Ni composites were successfully synthesized. Due to the excellent characteristics of EM waveabsorption and EM wave-reflection, the conductive textile exhibited extremely high EMI SE. Variety of experimental parameters influencing the quality of PPy layer were studied, which included FeCl3 concentration, polymerization time and synthesis cycles. The optimized conditions were as follows: FeCl3 concentration, 0.4 mol/L; polymerization time, 10 min; synthesis cycles, 3 times. Afterwards, the nickel films were compactly deposited on the as-made LF/PPy, which could be confirmed by XRD and SEM measurements. Comparative study of SE among LF/Ni, LF/PPy and LF/PPy/Ni was conducted, and the result revealed that multi-layers (both PPy and Ni) could lead to a higher SE (20.22–43.51 dB) than monotypic layer (PPy or Ni). Peel test indicated that resultant multi-layers were firmly adhered to the LF substrates for LF/PPy/Ni composites. Tensile test manifested that the breaking force of LF/PPy/Ni was between that of LF/PPy and LF/Ni, but still higher than that of pristine LF, which was adequate for usage in most cases. Based on those results, an efficient alternative combining chemical polymerization with electroless plating to manufacture conductive textiles was established.

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Fig. 14. Shielding mechanism of LF/PPy/Ni composites (Second reflection occurred between PPy layer and Ni film, and the shielding effect of LF substrate were not indicated in the figure).

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