A Novel Polyaniline-Coated Bagasse Fiber Composite with Core

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A Novel Polyaniline-Coated Bagasse Fiber Composite with Core-Shell Heterostructure Provides Effective Electromagnetic Shielding Performance Yang Zhang, Munan Qiu, Ying Yu, BianYing Wen, and Lele Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11989 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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ACS Applied Materials & Interfaces

A Novel Polyaniline-Coated Bagasse Fiber Composite with Core-Shell Heterostructure Provides Effective Electromagnetic Shielding Performance Yang Zhang,*,† Munan Qiu,† Ying Yu,‡ Bianying Wen,*,† and Lele Cheng† † Department of Material Science and Engineering, Beijing Technology and Business University, Beijing 100048, China ‡ School of Environment, Beijing Normal University, Beijing 100875, China

ACS Paragon Plus Environment

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ABSTRACT：：A facile route was proposed to synthesize polyaniline (PANI) uniformly deposited on bagasse fiber (BF) via a one-step in situ polymerization of aniline in the dispersed system of BF. Correlations between the structural, electrical, and electromagnetic properties were extensively investigated. The SEM images confirm that the PANI was coated dominantly on the BF surface, indicating that the as-prepared BF/PANI composite adopted the natural and inexpensive BF as its core and the PANI as the shell. The FTIR spectra suggest significant interactions between BF and the PANI shell, and a high degree of doping in the PANI shell was achieved. The XRD results reveal that the crystallization of the PANI shell was improved. The dielectric behaviors are analyzed with respect to dielectric constant, loss tangent, and Cole–Cole plots. The BF/PANI composite exhibits superior electrical conductivity (2.01±0.29 S·cm-1), which is higher than the pristine PANI with 1.35±0.15 S·cm-1. The complex permittivity, electromagnetic interference (EMI) shielding effectiveness (SE) values, and attenuation constants of the BF/PANI composite were larger than these of the pristine PANI. The EMI shielding mechanisms of the composite were experimentally and theoretically analyzed. The absorption-dominated total EMI SE of 28.8 dB at a thickness of 0.4 mm indicates the usefulness of the composite for electromagnetic shielding. Moreover, detailed comparison of electrical and EMI shielding properties with respect to the BF/PANI, de-doped BF/PANI composite, and the pristine PANI indicate that the enhancement of electromagnetic properties for the BF/PANI composite was due to the improved conductivity and the core-shell architecture. Thus, the composite has potential commercial applications for high-performance electromagnetic shielding materials and also could be used as a conductive filler to endow polymers with electromagnetic shielding ability.

KEYWORDS: polyaniline, bagasse fiber, core-shell structure, composite, electromagnetic shielding


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1. INTRODUCTION The extensive use of electronic and communication devices generates severe electromagnetic irradiation. Electromagnetic interference (EMI) is not only unfavorable for highly sensitive precision electronic equipment but is also harmful to the living environment for human beings. This novel sort of pollution provides a strong motivation for developing highperformance EMI shielding materials.1-4 In addition to high EMI shielding ability, light weight, thinness, and cost-efficiency are other important technical requirements for practical and effective EMI shielding applications, especially in aerospace, aircraft, wearable devices, and other sophisticated electronic equipment applications.5-8 Generally, an EMI SE value of at least 20 dB is the target value required for commercial application in EMI shielding devices. The EMI shielding performance is critically dependent on the intrinsic electrical conductivity, complex dielectric constant, complex magnetic permeability, aspect ratio, dispersion, distribution, and content of conductive fillers.1, 3, 9-13 To obtain the desired electromagnetic shields, the current strategies mainly rely on increasing the materials thickness to prolong the microwave transmission routes or the loading amount of a conductive filler to increase the electrical conductivity. Unfortunately, these factors inevitably increase the cost and limit its scalability. Therefore, simple, effective, and versatile strategies for the development of thin materials with advanced electromagnetic shielding properties are highly desired. Among the various electromagnetic shields, materials with core-shell structures have received considerable interest due to the unique properties.14-17 Shui and Chung introduced nickelcoated carbon filaments with a core-shell structure into a polyether sulfone matrix, inducing a much higher conductivity and EMI SE than composites using fillers without coatings.18 De Rosa et al.

noted that nickel-coated carbon fibers/polyester composites presented superior electromagnetic absorption properties due to their special core-shell structure.19 Zhu et al. reported the synthesis of one-dimensional core-shell barium titanate (BaTiO3)@multi-walled carbon nanotube (MWCNT) composites, whose absorption properties were significantly improved compared with both pure MWCNTs and BaTiO3.20 From the above reported results, it can be concluded that these core-shell heterostructures present a significantly better EMI shielding performance than the pure core or shell materials alone due to their confinement effects, interfacial polarization, and synergetic behavior. 21 As a typical intrinsically conducting polymer, PANI has gained increasing attentions


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because of its easy synthesis, tunable conductivity, low density, low cost, and superior environmental and thermal stability.22-23 PANI was most often used as an effective component for electromagnetic shielding purposes.24-26 By considering the advantages of core-shell heterostructures, there is also an increasing interest in constructing core-shell composites by using PANI as shells to further improve their microwave shielding properties. For example, Wang et al. deposited PANI on the MoO3 to fabricate core-shell nanorods, and the enhanced microwave absorption was confirmed to be linked with their structural characteristics.

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and co-workers reported that the SE values of PANI-coated MWCNT composites were in the range of 27.5 to 39.2 dB with a thickness 2.0 mm, and the high shielding performance was attributed to the synergistic effect of the two complementary components.28 Though a quantity of studies are available on the EM shielding abilities of these core-shell composites, further research is needed to describe the effects of core materials on the electrical conductivity of the PANI shell and their underlying EMI shielding mechanisms, which are important issues influencing the EMI shielding properties. In addition, we have chosen bagasse fiber (BF) as the core material because the electrically insulating nature of BF makes it a desirable material to regulate the conductivity of the composite. Moreover, BF combines low cost, low density, renewability, and biodegradability.29-30 Herein, we report a facile method to synthesize core-shell structured composites with BF cores and PANI shells. The BF/PANI composite with superior EMI SE performance has been realized through a one-step in situ polymerization strategy. The microstructural features, electrical conductivity, complex permittivity, and EMI SE were systematically investigated. Due to the hierarchical core-shell architecture, BF/PANI composite exhibits multifunctional and unique properties. Moreover, a comprehensive study of the fundamental shielding mechanism has also been performed.


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2. EXPERIMENTAL SECTION 2.1. Materials. Sugarcane bagasse was provided from a sugar cane factory of Guangxi province. The BF was screened through a 300-µm mesh net to remove large debris. Aniline and ammonium peroxydisulfate (APS) were purchased from Aldrich Chemical Company, Inc. 98% H2SO4 was obtained from the National Medicine Group Chemical Reagent Co., Ltd., China. Deionized water was used in this investigation. Aniline was freshly distilled before use. The other chemicals were of analytical grade and used as obtained.

2.2. Fabrication of BF/PANI composite. The BF/PANI composite was chemically polymerized as follows. A total of 4 mL of aniline and 0.75 g of BF were added to 40 mL of 1 M H2SO4 under ultrasonication. Then, 11.52 g APS (dissolved in 40 mL of 1 M H2SO4) was slowly dropped into the above solution with mechanical stirring. The polymerization was allowed to proceed for 4 h at ~0 °C. The solid product was obtained by filtration, washed with deionized water and methanol until the washing solution was completely colorless, dried at 50 °C for several hours. In contrast to the BF/PANI composite, pristine PANI was prepared using a similar procedure.

2.3. Characterizations. The chemical constituents of BF/PANI composites were measured by a Fourier transform infrared spectrometer (FTIR, Nicolet iZ 10, Thermo Fisher Scientific) at a resolution of 8 cm-1 using KBr as a reference. The crystal structure was determined using a PANalytical X'Pert3 Powder X-ray diffraction instrument (XRD) with a Cu Kα radiation source at 40 kV and 40 mA. The detailed morphologies of the BF/PANI composites were investigated with a scanning electron microscope (SEM, Quanta FEG 250, FEI) at 10 kV. For the crosssectional analysis, the samples were cut with a scalpel blade. All the samples were sputter-coated with gold before examination. Compressed pellets 0.4 mm in thickness were fabricated for electrical and EMI shielding measurements. Typically, the powder samples were pelletized at a pressure of 20 MPa for 3 min using a hydraulic press. The electrical conductivity was measured using a standard four-point probe resistivity measurement system (Four Probes Tech, RTS-9, Guangzhou, China) at ambient temperature. The spacing of two adjacent probes is 1.59 mm. The EMI SE and complex permittivity of the samples were measured using the wave-guide method in 8.2–12.4 GHz (X band) by a vector network analyzer (VNA, Agilent technologies, E5071C). The wave-guides were linked to the VNA with two cables. The calibration of the VNA was made


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according to the Through-Reflect-Line method. For the EMI SE measurement, the samples were clamped between two coupling flanges of the input and output wave guides. For complex permittivity investigation, in order to obtain the precise dimensions (22.86 mm×10.16 mm) to fit the internal cavity of the standard copper X-band waveguide, the prepared BF/PANI samples were first homogenously mixed with paraffin at 80 °C under stirring. Then, the mixture was cast into a mold. Finally, the sample was taken out and accurate dimensions were obtained by gently scraping the excess part with the edge of a scalpel blade. To obtain more accurate data, the high content of PANI in the paraffin was used. The mass ratio of BF/PANI to paraffin was 40:60. In addition, the input power used in the present research was 0 dBm, which corresponds to 1 mW.

3. RESULTS AND DISCUSSION

Figure 1. (a) Schematic representation of BF/PANI composite synthesis. Photos of (b) a pristine BF sample and (c) the BF/PANI composite. The synthetic strategy for the BF/PANI is illustrated in Figure 1a. The obtained BF/PANI composite was a fine dark green powder. The colors of the BF after treatment changed from light yellow (Figure 1b) to dark green (Figure 1c). This change in color should arise substantially from the considerable deposition of doped PANI on the BF surface.


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Figure 2. SEM images for surfaces of (a, b) neat BF, (c, d) the BF/PANI composite, and (e) cross section of the BF/PANI composite. The inset of (d) is a high-resolution SEM image of the PANI shell. The morphological characteristics of neat BF and the BF/PANI composite were investigated by SEM, as shown in Figure 2. The raw BF is constituted by parallel stripes and presents a rigid and compact structure (Figure 2a). The outer diameter of BF ranged from a few tens to a few hundreds of microns. From a high magnification SEM image (Figure 2b), it is observed that the surface of BF is smooth. After being covered by PANI, distinct morphologies are observed. The surface of BF is rough and looks granular (Figure 2c), suggesting that the surface of BF was coated uniformly and randomly with interconnected PANI. Hence, the composite shows a coreshell structure. The BF was the core and PANI was the shell material. A careful examination of the high magnification image, as shown in the inset of Figure 2d, revealed that the surface of the PANI microparticles is composed of small agglomerated spherical particles. The diameter of these nanoparticles is measured as 37±7 nm. This result demonstrates that the nanometer-scale PANI is deposited on a micron-scale BF. This leads to a hierarchical structure of the BF/PANI composites. In addition, it should be noted that there was a slightly larger fiber diameter after the deposition process. This can be interpreted by the successful deposition of PANI on the BF surface. The thickness of coated PANI layers is measured as 2.5±0.7 µm. A slightly larger fiber diameter also demonstrates that the typical bagasse bundles were not dismantled and there were


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no fibers detached from the others, even though the synthesis was performed in acidic conditions. This is because the fiber structure is a lignified tissue, which was relatively resistant to acidic environments.30

Figure 3. FTIR spectra of pristine PANI, neat BF, and the BF/PANI composite. FTIR analysis was conducted in order to provide insight on the interactions between PANI and BF. The FTIR spectra of pristine PANI, neat BF, and the BF/PANI composite were shown in Figure 3. The PANI spectrum presents the characteristic peaks of PANI at approximately 1574 cm-1 and 1493 cm-1, which are attributed to the C=C stretching vibrations of quinonoid and benzenoid rings, respectively.31 The band at 1300 cm-1 corresponds to C-N stretching of the secondary aromatic amine. The band at 1117 cm-1 corresponds to the vibrational mode of the – NH+= structure. This band is associated with the high degree of electron delocalization in conducting PANI. Hence, the strong peak observed at 1117 cm-1 accounts for the conductive nature of the PANI.32 The peak located at 881 cm-1 was indicative of the formation of branched and/or substituted phenazine-like segments. The main features of the BF spectrum are attributed to the presence of the natural components of lignocellulose fibers, including lignin, hemicellulose and cellulose. The band located at 1737 cm-1 corresponds to the acetyl groups in hemicellulose, which indicates the C=O stretch in non-conjugated ketones, carbonyls, and ester groups.33 The bands at 1635 cm-1 is attributed C=C skeletal vibrations in the lignin aromatic structure. The bands at 1045 cm-1 and 1160 cm-1 are assignable to the primary and secondary OH groups of BF.34 BF/PANI exhibits similar peaks as the PANI and neat BF. Compared with the corresponding


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peaks of pure PANI, the peak corresponding to conductive nature of PANI (1117 cm-1) shifts to a lower frequency of 1110 cm-1 for the BF/PANI composite, suggesting a high degree of doping in the PANI shell in the composite.31 Meanwhile, the peaks related to the stretching vibration of quinoid and benzenoid rings were shifted slightly to 1582 and 1488 cm-1, respectively. The acetyl group signal at 1737 cm-1 and the primary and secondary OH groups at 1045 cm-1 and 1160 cm-1 almost disappeared in the BF/PANI FTIR spectrum compared to that of the BF sample. These phenomena indicate that interfacial interactions occur between the BF and PANI.35 In addition, the small size of the BF with high specific surface area provides a large number of sorption sites to the aniline monomer. Therefore, these monomers were easily attached to the BF surface and could polymerize to form a coating over the BF. The structural features of the BF, PANI, and the BF/PANI composite were characterized using X-ray diffraction patterns. Figure 4 indicates that PANI has some degree of crystallinity. Three diffraction peaks at approximately 15.3◦, 20.5◦, and 25.3◦ correspond to the (010), (100), and (011) planes of the doped form of PANI.36 These peaks can be ascribed to the periodicity parallel and perpendicular to the polymer chains of PANI. In addition, the peak centered at 25.3◦ suggests the presence of doped polyaniline, because this peak is the characteristic of the highly doped emeraldine salt of polyaniline. 31

Figure 4. X-ray diffraction pattern of pristine PANI, neat BF, and the BF/PANI composite. The neat BF exhibits typical cellulose diffraction peaks. The diffraction peaks appear at 2θ angles of 15.6◦ and 21.7◦, which correspond to the (101) and (002) crystallographic planes of cellulose, respectively.29 The powder XRD patterns of the produced BF/PANI composite not only have the characteristic diffraction peaks of the pristine PANI but also those of the intrinsic


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diffraction peaks for the BF. The intensities of the BF characteristic peaks are all reduced. This may be attributed to the coating of PANI chains onto the BF surface. The full widths at halfmaximum of the peaks at approximately 25.3◦ were 1.25◦ and 1.84◦ for the BF/PANI composite and PANI, respectively. In other words, the XRD pattern of the BF/PANI samples at 2θ = 25.7◦ is relatively sharper and narrower than the PANI. This phenomenon indicates the presence of a better crystalline structure of PANI in the BF/PANI composite. Therefore, the BF core has some positive effect on the crystallization performance of the PANI shell. The relative complex permittivity, permeability, and conductivity are key factors in determining the electromagnetic properties of the shielding materials. Because the present systems are non-magnetic, the relative complex permittivity was first measured, and the results are shown in Figure 5. Generally, the real part of the complex permittivity (ε′, known as dielectric constant) symbolizes the amount of polarization occurring in the materials or the storage capacity of the electrical energy, while the imaginary part of complex permittivity (ε′′, known as loss factor) signifies the dissipated electrical energy. Figures 5a and b show the ε′ and ε′′ for the paraffin matrix composites containing 40 wt.% pristine PANI or BF/PANI in the Xband. The overall average values for ε′ are found to be ~5.3 and 6.6, while these values for ε′′ are ~2.6 and 3.8 of PANI and BF/PANI, respectively. In addition, a slightly decreasing trend of ε′ and ε′′ values with frequency is observed. With increasing the frequency, the dipoles present in this system cannot reorient themselves fast enough to respond to the applied electric field; therefore, the dielectric constant decreases. BF/PANI shows enhancement in the values of ε′ and ε′′, suggesting that BF/PANI has higher efficiency in storing and attenuating the electrical energy. The higher ε′ and ε′′ values of BF/PANI may be due to higher conductivity of the BF/PANI nanocomposites compared to PANI.


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Figure 5. Complex permittivity spectra for PANI- and BF/PANI-paraffin composites: (a) the real and (b) the imaginary parts of permittivity. The Debye theory has been widely used to describe the dielectric properties of materials, which is generally used to explain the relaxation phenomenon of dipoles. Based on the Debye theory for dielectric loss behavior,29 the ε′ and ε′′ values can be written as: ε ε (1)

ε

ωτ

(2)

in which εs and ε∞ are the static and infinite frequency dielectric constants, ω = 2πf is the angular frequency, τ is the relaxation time, and σ is the conductivity. Both the ε′ and ε′′ are functions of ωτ. So ε′ and ε′′ are not mutually independent with each other. If we ignore the contribution of σ to ε′′, by eliminating ωτ, the relationship of ε′ and ε′′ can be further deduced as:

ε

ε

（3）

On the basis of equation (3), we can find that the plot of ε′′ versus ε′ should be a semicircle, which can be defined as the Cole-Cole semicircle.37 Figure 6 shows the Cole-Cole plots for pristine PANI- and BF/PANI-paraffin composites in the frequency range from 8.2 GHz to 12.4 GHz. Both of the above systems simply produce randomly fluctuating trends, which is due to the non-applicability of Cole-Cole equation and forced fitting of data to the same. It is well-known that relaxation is usually caused by a delay in polarization relative to changing electrical field. In the present research, there were no obvious Cole–Cole semicircles observed, which suggests that the relaxation makes little contribution to the dielectric property.37


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Figure 6. The typical Cole-Cole plot for the (a) PANI-paraffin and (b) the BF/PANI-paraffin composite. To check the effect of BF core on electrical conductivity of the PANI shell, a room temperature electrical conductivity measurement was performed. The conductivity of the BF/PANI composite is 2.01±0.29 S·cm-1. Apparently, the PANI shell is the most important charge carrier in the BF/PANI composite. The PANI shell improves the electrical conductivity of the composite. In addition, although BF is electrically insulating, the conductivity of the BF/PANI composite is even increased when compared with the pristine PANI (1.35±0.15 S·cm1

). The results for the BF/PANI composites show an interesting contradiction to the general rule

that the electrical conductivity of composites should decrease when the amount of the nonconductive component increases. We supposed that the conductivity of the BF/PANI composite is closely related to the high doping degree of PANI shell, just like the pristine PANI.38-39 In order to confirm this hypothesis, the conductivities of the core-shell composites with different degrees of doping were tested. The low degrees of doping to the PANI shell were synthesized by changing the 1M H2SO4 to 0.5M and 0.3M. These composites were denoted as BF/PANI0.5 and BF/PANI0.3, respectively. The conductivity of BF/PANI0.5 composite is 0.85±0.08 S·cm-1, which is higher than that of the BF/PANI0.3 (0.63±0.06S·cm-1) but lower than that of the BF/PANI (2.01±0.28 S·cm-1). Thus, it can be concluded that the high doping degree of PANI shell will result in the high electrical conductivity. From the above results, this abnormal phenomenon of BF/PANI with a larger electrical conductivity than the pristine PANI could be explained as follows. First, a high degree of doping to the PANI shell is formed on BF surface. Second, there is a strong correlation between the


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crystallinity of the PANI and its electrical conductivity.40-41 Due to the better crystalline structure of PANI in the BF/PANI composites, a more ordered PANI chain arrangement is beneficial for electron transport. Furthermore, BF acts as an interconnecting bridge, which increases the interchain charge transport between the conducting PANI shells. This increased connectivity may increase long-range charge transport and accelerate charge transfer processes. Thus, the BF core plays a positive role in the electrical conductivity, and the high conductivity of the BF/PANI composite is obtained. The EMI SE value represents the ability of shielding materials to attenuate the propagating electromagnetic energy. The EMI SE is defined as the logarithm of the ratio of the transmitted power (PT) to the incident power (PI) and is given by: #

SE dB 10log $ #%

(4)

When the EM wave is incident on a shielding material, some part of the incident wave is absorbed, some part is reflected and the rest is transmitted through the material.

Figure 7. EMI SE as a function of frequency for PANI and the BF/PANI composites. Figure 7 illustrates that the variation of SE with frequency for pristine PANI and the BF/PANI composites in the X band. The BF/PANI composite with a thickness of 0.4 mm exhibits excellent EMI SE in the range of 27.9 to 30.4 dB. The EMI SE of the composite shows almost stabilized shielding behavior over the X-band. Meanwhile, the PANI with the same thickness shows SE values in the range of 19.4 to 22.7 dB. It is apparent that the EMI shielding properties of the BF/PANI composites are superior to those of pristine PANI. In addition, the


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total EMI SE of the BF/PANI composite is greater than the recommended limit of SE (20 dB) for commercial applications. Therefore, the as-fabricated BF/PANI composite could serve as an effective replacement for conventional metallic shields.

Figure 8. (a) The dielectric loss and (b) the attenuation constant of PANI- and BF/PANI-paraffin samples versus frequency. The dielectric loss (tan δ* ε /ε ) is normally responsible for energy attenuation in nonmagnetic shielding materials. The dielectric loss is also calculated in order to explain why the BF/PANI composites have high effective shielding properties. The results are shown in Figure 8a. The tan δ* value of the BF/PANI composite is approximately 0.57, indicating an effective contribution of dielectric loss over the whole frequency range. Interestingly, the BF/PANI composite possesses higher dielectric loss characteristics than the pristine PANI. The differences in dielectric loss can be derived from the BF/PANI composite presents higher electrical conductivity than the PANI. In addition to the dielectric loss, the attenuation constant (α) also provides a clear understanding about the ability of a shield to suppress the incident electromagnetic radiation. The attenuation constant defines the rate at which the fields of the wave are attenuated as the wave propagates. An electromagnetic wave propagates in an ideal (lossless) medium without attenuation (α= 0). The attenuation constant was estimated according to equation (5):42

α

√./ 0

1 2 μ′′ε′′ μ′ε′ 5 μ′′ε′′ μ′ε′ μ′ε′′ μ′′ε′ (5)

Figure 8b depicts the attenuation constant as a function of frequency for the paraffin matrix composites containing 40 wt.% PANI and BF/PANI. The attenuation constant is increased in the BF/PANI composite over the PANI. The enhanced attenuation constant can be easily associated with the total EMI SE. The increased EMI SE and attenuation constant may be attributed to the


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enhanced electrical conductivity and the unique structure of the BF/PANI. A good EMI shielding material exhibits maximum attenuation mainly through the reflection and/or absorption mechanisms. Toward a more comprehensive understanding of the electromagnetic shielding properties of the BF/PANI composite, power data were first analyzed. The PT and reflected power (PR) were directly measured by the EMI SE characterization setup used in this research. Because that constant incident power (PI = 1 mW) was used, the absorbed power (PA) can be calculated according to equation (6). PA = PI – (PR + PT) = 1 mW- (PR + PT)

(6)

Figure 9. Dependence of (a) PA, (b) PR, and (c) PT of pristine PANI and the BF/PANI composite as a function of X-band frequencies. It should be noted that in addition to the power that has been reflected from the external surface, the measured PR also involves the positive contributions of internal surface reflections and multiple reflections.43 Figure 9 shows the calculated power values of the pristine PANI and the BF/PANI composite in the X-band. The results reveal that the BF/PANI composite provides negligible transmission. According to the plots as exhibited in Figure 9, the PA values were observed in the range of 0.20~0.30 and 0.14~0.25 for PANI and BF/PANI, respectively. The PR values were in the range of 0.69~0.79 and 0.75~0.86 for PANI and BF/PANI, respectively. Moreover, the PT values were in the range of 0.0054~0.012 and 0.00091~0.0016 for PANI and BF/PANI, respectively. The low PT values indicate that the BF/PANI composite is an effective microwave attenuator. Although the PA value of PANI is higher than that of BF/PANI, the PR value of PANI is much lower than that of BF/PANI. In other words, more EM waves are consumed by the BF/PANI composite, which leads to a more significant decrease of the PT values. Obviously, the amount of energy that is blocked by reflection is greater than by absorption. However, it should be stressed that this observation does not indicate that reflection is the dominant shielding mechanism, because EMI shielding effectiveness is a relative quantity


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and it is not closely related to the absolute power values.43-44 Actually, the absorbed power is a value that describes the ratio of the power attenuation by a shield to the overall incident power. The lower power that is blocked by absorption is due to the lower amount of power transmitted into a shielding material as a result of the better reflection. The contribution of absorption to the overall shielding should be based on the ability of the material to attenuate the power that has been transmitted into the samples. The total electromagnetic SE is a sum of the reflection loss (SER), absorption loss (SEA), and multiple reflections loss.43 The SER and SEA can be calculated from experimental power data as:

SE6 10 log SE9 10 log

7%

7% 78

(7)

7% 78 7$

(8)

Figure 10. Variations in (a) SER and (b) SEA as a function of frequency for the pristine PANI and the BF/PANI composite. Figure 10 shows the dependences of the SER and SEA as a function of frequency. The results suggest that the SER and SEA of the BF/PANI composite are 7.3 and 21.5 dB, respectively, indicating that absorption contributes signiﬁcantly (74.7%) to the overall EMI SE than reflection (25.3%). Thus, it is evident that absorption is the dominant shielding mechanism. One more observation is that both SER and SEA of the BF/PANI composite are larger than those of the pristine PANI. To further understand the electromagnetic shielding mechanism, we attempted to conduct a theoretical analysis. Unfortunately, there is no existing theory for the predictions of EMI SE based on the heterogeneous composite as presented in this paper. Some insights can be gained


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from the existing theories for conventional shielding systems. The contribution of SER and SEA to the overall EMI SE can be quantified according to the equations (9) and (10).45 SE6 39.5 10log (9) ./>

SE9 8.7 d5ABCD

(10)

where f is the frequency, µ is the magnetic permeability (µ = µ0µr), µ0 = 4π×10-7 H/m, µr is the relative magnetic permeability, d is the thickness. µr is assumed to be 1 in the present study, because both BF and PANI are non-ferromagnetic materials; thus, the magnetic permeability equals µ0.45 Table 1 shows comparisons of EMI SE values between the experimentally obtained results and the theoretically calculated predictions of the BF/PANI composite at the X-band. It is found that, experimentally, the total EMI SE and SEA values are larger than the theoretical predictions. For instance, the experimental SE and SEA values are approximately 28.8 and 21.5 dB, respectively, while the corresponding theoretical prediction values are approximately 23.2 and 9.7 dB, respectively. In contrast, the SER value of the experimental result (7.3 dB) is smaller than that of the theoretical prediction (13.5 dB).


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Table 1. Comparisons between EMI SE Data Obtained from the Experiment and the Theoretical Calculation for the BF/PANI Composites. Frequency (GHz)

SE (dB)

SER(dB)

SEA(dB)

Exp.

Theory

Exp.

Theory

Exp.

Theory

8.5

29.41

23.32

7.17

14.27

22.70

9.05

9.0

28.45

23.33

8.28

14.03

20.16

9.30

9.5

28.11

23.35

8.36

13.79

19.74

9.56

10.0

28.33

23.38

8.09

13.57

20.24

9.81

10.5

27.93

23.41

7.63

13.35

20.29

10.05

11.0

28.30

23.44

6.56

13.15

21.79

10.29

11.5

28.38

23.48

6.01

12.96

23.37

10.52

12.0

29.70

23.52

6.03

12.78

23.66

10.74

12.4

29.55

23.56

6.17

12.64

23.38

10.92

Generally, equations (9) and (10) are applicable for a monolithic conductive material. Because the multi-facets are available for reflection and multiple reflections in the BF/PANI core-shell composites, shielding mechanism is expected to be more complicated than those for conventional homogeneous materials. Thus, the discrepancy between theoretical and experimental EMI SE values was mainly caused by assumptions taken in derivation of used simplified equations. Although the theoretical predictions are unreliable, equations (9) and (10) can still be used as a criterion for predicting the general effects of various parameters on the electromagnetic shielding abilities of conductive materials. This is critical to providing technical guidance for the design of high-performance shielding materials.


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Figure 11. Frequency dependence of EMI SE for the de-doped BF/PANI, BF/PANI0.5, and the mixture. These results indicate that in addition to the intrinsic characteristics of PANI, the incorporation of a core-shell structure may also be essential for achieving excellent EMI shielding ability. To eliminate the impact of conductivity and further clarify the effect of the core-shell structure on its electromagnetic shielding properties, the BF/PANI composite with low electrical conductivity was prepared. The electrical conductivity of PANI can be affected by doping and de-doping processes on exposure to an acidic or basic environment. The conductivity of the doped PANI is larger than de-doped PANI.46 Thus, the BF/PANI composite was exposed to the vapor of ammonia water, and then, the de-doping process occurs according to the previous research.47 The conductivity of the obtained de-doped BF/PANI composite was 0.90±0.13 S•cm1

, which is lower than that of pristine PANI. But the EMI SE value of the de-doped BF/PANI

composite is higher than pristine PANI (Figure 11). Meanwhile, it is noteworthy that the EMI shielding performance of BF/PANI0.5 is almost the same as that of pristine PANI, even though the conductivity of BF/PANI0.5 (0.85±0.08 S•cm-1) is lower than that of PANI (1.35±0.15 S•cm1

). In addition, a mixture of BF and PANI with the same component of the BF/PANI composite

was also prepared. The preparation process was as follows. The BF and pristine PANI were mixed under stirring for about 0.5 h. Then, the mixture was compressed to 0.4 mm for the conductivity and EMI shielding measurements. The conductivity of the mixture was just 0.91±0.04 S•cm-1, which is lower than both BF/PANI core-shell composite and pristine PANI. The mixture shows weak EMI shielding abilities with SE values only varying from 15.6 to 17.4 dB (Figure 11). Thus, a simple mixture of BF with PANI cannot increase the SE values. These


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results definitely indicate that besides the conductivity, the core-shell structure was also one of the major factors leading to the superior EMI shielding performance.

Figure 12. Schematic EMI shielding mechanism for the BF/PANI composite. Based on the above discussion, a potential mechanism can be proposed. In this present research, the in situ grown PANI on the BF surface constructs a binary hierarchical architecture. When an electromagnetic wave is incident on the conducting BF/PANI surface (including the internal surface between BF and PANI), the electron cloud near the surface becomes distorted and generates an electric field opposite to the applied electric field.25 Because of impedance mismatch, the combined opposite electric fields cause reflection and/or absorption of the microwaves from the surface and the internal surface rather than penetration through the materials. Multiple reflections and scattering of microwaves could be induced in the composite. Then, the transmission routes of microwaves are effectively enlarged by this unique structure. Thus, the absorption capacity and the total EMI SE could be significantly intensified, as shown in Figure 12.

4. CONCLUSIONS In summary, a hierarchical BF/PANI composite with a core-shell structure has been successfully synthesized by a simple technique. The BF core plays an important role in inducing a high degree of doping and a high crystalline phase of PANI shell in the final composite, so the electrical conductivity of the composite was larger than the pristine PANI. The thin BF/PANI composite exhibits electromagnetic shielding performance (28.8 dB) with a thickness of only 0.4 mm. The superior electromagnetic shielding properties are mainly attributed to the increased


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electrical conductivity and the effective multiple reflections induced by the special core/shell structure. The dominant mechanism of EMI shielding for the BF/PANI composite is absorption. The method developed is versatile and cost-effective. In addition, this facile strategy is expected to allow the highly efficient utilization of low-cost natural materials in EMI shielding applications. Our results demonstrate that the PANI-coated BF core-shell composites with low thickness and strong shielding ability are attractive candidates for new types of electromagnetic shielding materials.


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AUTHOR INFORMATION Corresponding Authors *Tel.: +86-10-6898 5531. E-mail: [email protected]. *Tel.: +86-10-6898 5480. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21274007 and 51473007), the Science and Technology Development Project of Beijing Municipal Commission of Education (SQKM201610011001), Innovative Research Team of Polymeric Functional Film of Beijing Technology and Business University (19008001071), and the Two Sections Cultivation Fund of Beijing Technology and Business University (LKJJ201623).


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GRAPHICAL ABSTRACT:


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A Novel Polyaniline-Coated Bagasse Fiber Composite with Core

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