Cellulose Nanofiber-Based Polyaniline Flexible ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Cellulose Nanofiber-Based Polyaniline Flexible Papers as Sustainable Microwave Absorbers in the X‑Band Deepu A. Gopakumar,†,‡ Avinash R. Pai,† Yasir Beeran Pottathara,†,‡ Daniel Pasquini,§ Luís Carlos de Morais,∥ Mereena Luke,† Nandakumar Kalarikkal,† Yves Grohens,‡ and Sabu Thomas*,† †

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686560, India ‡ Laboratoire d’Ingénierie des Matériaux de Bretagne, Centre de Recherche, Rue Saint Maude-BP 92116, F-56321 Lorient Cedex, France § Chemistry Institute, Federal University of Uberlandia-UFU, Campus Santa Monica-Bloco1D-CP 593, 38400-902 Uberlandia, Brazil ∥ Institute ICENE, Federal University of Triângulo Mineiro (UFTM), Av. Doutor Randolfo Borges, 1400, Campus Univerdecidade, 38064-200 Uberaba, Minas Gerais, Brazil S Supporting Information *

ABSTRACT: A series of flexible, lightweight, and highly conductive cellulose nanopapers were fabricated through in situ polymerization of aniline monomer on to cellulose nanofibers with a rationale for attenuating electromagnetic radiations within 8.2− 12.4 GHz (X band). The demonstrated paper exhibits good conductivity due to the formation of a continuous coating of polyaniline (PANI) over the cellulose nanofibers (CNF) during in situ polymerization, which is evident from scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction analysis. The free hydroxyl groups on the surface of nanocellulose fibers promptly form intermolecular hydrogen bonding with PANI, which plays a vital role in shielding electromagnetic radiations and makes the cellulose nanopapers even more robust. These composite nanopapers exhibited an average shielding effectiveness of ca. −23 dB (>99% attenuation) at 8.2 GHz with 1 mm paper thickness. The fabricated papers exhibited an effective attenuation of electromagnetic waves by a predominant absorption mechanism (ca. 87%) rather than reflection (ca. 13%), which is highly desirable for the present-day telecommunication sector. Unlike metal-based shields, these demonstrated PANI/CNF papers have given a new platform for designing green microwave attenuators via an absorption mechanism. The prime novelty of the present study is that these robust PANI/CNF nanopapers have the ability to attenuate incoming microwave radiations to an extent that is 360% higher than the shielding effectiveness value reported in the previous literature. This makes them suitable for use in commercial electronic gadgets. This demonstrated work also opens up new avenues for using cellulose nanofibers as an effective substrate for fabricating conductive flexible papers using polyaniline. The direct current conductivity value of PANI/CNF nanopaper was 0.314 S/cm, which is one of the key requisites for the fabrication of efficient electromagnetic shields. Nevertheless, such nanopapers also open up an arena of applications such as electrodes for supercapacitors, separators for Li−S, Li−polymer batteries, and other freestanding flexible paper-based devices. KEYWORDS: cellulose nanopapers, polyaniline, in situ polymerization, EMI shielding, microwave suppression Lightweight, flexible polymer nanocomposite materials have been a key interest for microwave suppression and electromagnetic absorption due to many benefits over traditionally used metal sheets.2−4 Conducting polymers such as polyaniline (PANI), polypyrrole, and polyacetylene are popular for EMI shielding. Among them, PANI has gained considerable interest due to the controllable electrical conductivity via simple

1. INTRODUCTION In our modern society, the electromagnetic (EM) shielding of devices functioning in radio frequency band remains to be a severe concern. For the safety of the sensitive circuits, lightweight electromagnetic interference (EMI) shielding materials are required to shield the workspace from radiations transmitted out from computers and telecommunication equipment. Electromagnetic waves from the sensitive electronic equipments are reflected or absorbed by the human body. Thus, misdirected radiations emitted out from the electronic devices may have an adverse effect on human health.1 © XXXX American Chemical Society

Received: March 20, 2018 Accepted: May 14, 2018

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DOI: 10.1021/acsami.8b04549 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

magnetic radiations specifically in a small bandwidth of the microwave frequency region (8.2−12.4 GHz). It is also noteworthy that extensive studies on the usage of polyaniline-based cellulose nanopapers as effective EM attenuator in modern flexible electronic devices is limited. The only report is by Marins et al., where they made use of bacterial cellulose (BC) membranes in situ polymerized with aniline monomer to form a coating of PANI on the surface of BC membranes.28 The overall shielding effectiveness (SE) of such membranes was found to be −5 dB (nearly 63.38% attenuation) at a thickness of about 0.080 mm in the X band region. These membranes cannot be used for commercial device applications. The key novelty of the present work is that the demonstrated PANI/CNF papers have achieved an electromagnetic shielding effectiveness of −23 dB (>99% of shielding), which makes them suitable for commercial device applications in the field of flexible electronics. Moreover, the absorption-dominated shielding mechanism in the fabricated PANI/CNF papers makes these nanopapers potential candidates for suppressing electromagnetic pollution. To the best of our knowledge, this is the first report on PANI/CNF flexible nanopapers with an effective total shielding value of >20 dB, which makes them suitable for commercial device applications with a prominent suppression of electromagnetic waves by absorption mechanism.

protonation/deprotonation process, good environmental stability.5 Thus, various applications like sensors, biomedical devices, optoelectronics, electrochromic devices, and fuel cells have been explored for PANI.6−11 Furthermore, PANI possesses a fascinating electromagnetic interference shielding mechanism through absorption, making it a tremendous material especially for military purposes.5 PANI occurs in various forms and particular forms find diverse specific technological applications. The doping of the emeraldine form results in the conducting PANI, which has been used in various applications like sensors, EMI shielding, and electrochromic devices. Unlike other conducting polymers like polypyrrole, polythiophene, and polyfuran, the hetro atoms in the PANI contributes to the πband formation, thereby become conductors. This makes PANI a unique conductive polymer among the conductive polymers.12 However, the existence of conjugated system in PANI enhances the stiffness of the material, which results in deprived film-forming capability. This is due to the fact that such polymers are not capable to form H bonds, so it is challenging to produce films by processes like casting and drying. To fix this issue, extensive work has been done in recent years to fabricate various PANI composites by blending diverse polymers with excellent mechanical properties and processability. Wide range of polymeric materials like cellulose, rubbers, plastics, and textiles with tremendous physical properties such as tensile strength, toughness, flexibility, and process ability have been used as worthy substrates for PANI.13−16 Among them, cellulose nanofibers (CNFs) as abundant renewable environment-friendly materials have gained considerable attention.13,17 Cellulose nanofibers as substrate have been proposed as a solution for this obstacle. Cellulose nanofiber-based papers or cellulose nanofiber suspensions give adequate H-bonding, so that films may be easily produced from the mixture of conducting polymers and cellulose nanofibers. By combining the cellulose nanofibers with PANI, the poor formability and frangibility of PANI can be alleviated to produce a conducting composite that is capable of shielding electromagnetic radiations. Cellulose nanopapers are a class of promising functional constructs that can be exploited for varying technological applications ranging from flexible optoelectronic devices18−20 to nonwoven porous membranes for water purification21,22 and as fire-retarding gas barrier films.23,24 Such applications are made possible due to superior mechanical properties, high aspect ratio, thermal stability, and chemical resistance of nanocellulose fibers, all of which can be sensibly fine-tuned as per the application. Electromagnetic interference (EMI) is a serious concern and need of the hour for present-day electronics and telecommunication sector. Electromagnetic (EM) pollution deteriorates the performance and life of electronic gadgets and adversely affects the human health. Hence, research works are ongoing to develop novel flexible polymeric lightweight materials, which can attenuate these unwanted radiations.25−27 Conventionally, metals were used to shield EM waves but are highly undesirable due to their inherent drawbacks such as higher weights, corrosive nature, and difficulty of processing into intricate shapes. Conductive cellulose nanopapers can offer a potential green feasible solution to this problem. Herein, we intend to make use of a series of conductive cellulose nanopapers fabricated via in situ polymerization with polyaniline (PANI) to attenuate electro-

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Aniline (pure, d = 1.022 g/mL, Mw = 93.13 g/mol), ammonium persulfate (APS, 98% purity, Mw = 228.20 g/mol), and hydrochloric acid (35%) were purchased from SigmaAldrich, Brazil. The cellulose nanofibers (CNF) in water suspension in the range of 20−30 nm in diameters and several micrometers in length were supplied from SUZANO, Brazil and extracted from eucalyptus bleached Kraft pulp. 2.2. In Situ Polymerization of Aniline on CNFs. In this work, oxidative synthetic methodology was employed for the synthesis of polyaniline. To prepare a set of PANI/CNF ratios of 0.2:1, 0.3:1, 0.5:1, 0.8:1, 0.9:1, and 1:1, different amounts of aniline monomer were dissolved in 40 g of CNF suspension (1.15 mg/mL) at room temperature for 1 h in a final liquid volume of 1 L. Then, the prepared solution was cooled down at 1 °C in a bath with controlled temperature, after which the APS solution (dissolved in 40 mL of 1 M HCl) in APS/aniline ratio of 1.25 w/w was added. This proportion is equivalent to 0.0055 mol of APS and 0.0107 mol of aniline, which means a mole ratio of 0.51. After 3 h, PANI/CNF suspension became dark green and suspended. Then, the resulting aqueous mixture/ suspensions were filtered and the solid fraction was washed with deionized water and acetone to eliminate the oligomers and oxidized components. Then, the solid fraction was redispersed in 500 mL of 0.25 M HCl and kept for 120 min to guarantee the doping again of PANI on CNF. 2.3. Fabrication of PANI/CNF Flexible Composite Papers. Vacuum filtration technique was used for the fabrication of PANI/ CNF flexible composite papers. A filtration assembly with a cellulose ester membrane (47 mm in diameter and 0.45 μm pore size) was employed for the vacuum filtration of the PANI/CNF suspension. After this, the fabricated paper was washed several times with 1 M HCl to remove all the contaminants. Lastly, the fabricated PANI/CNF composite papers were peeled off from the filter and dried in hot press at 80 °C for 5 min. 2.4. Electrical Conductivity Measurements of the Fabricated Papers. Four-probe methods using a Keitheley 2400 source meter equipped with a gold probe was employed for the direct current (DC) conductivity measurements for the fabricated PANI/CNF paper. The measurements were made in accordance with the van der Pauw method so as to minimize the error in the DC conductivity value. The tests were performed at the ambient atmospheric conditions. B


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Figure 1. Strategy for the in situ polymerization of aniline monomers onto cellulose nanofibers.

Figure 2. Interaction between CNF and PANI inset photographs of PANI/CNF paper. 2.5. Dielectric Measurements. The dielectric analysis of the PANI/CNF papers was done using a WAYNE KERR-6500B Precision Impedance Analyser with a frequency range of 30 Hz to 30 MHz. High-frequency dielectric measurements were done using a Novocontrol Alpha High-Resolution Dielectric analyzer with 10 and 15 GHz resonator having the sample dimension of 3 cm × 3 cm. 2.6. Scanning Electron Microscopy (SEM). The morphology of the CNF and PANI/CNF flexible papers surface and cross section were examined using a scanning electron microscope (TESCAN VEGA 3 SBH). The cross-sectioned samples were prepared by fracturing the CNF and PANI/CNF flexible papers in a liquid nitrogen bath for 90 s and kept in desiccator. The dried samples were sputtered with gold coating of 3 nm thickness in an argon atmosphere at 20 mA for 2 min. The flexible papers were observed in the SEM at an acceleration voltage of 20 kV. 2.7. X-ray Diffraction Analysis (XRD). X-ray diffraction patterns for prepared CNF, PANI, and PANI/CNF samples were obtained with SHIMADZU XRD-6000. The X-ray diffractograms were obtained at room temperature within in a 2θ range from 5 to 40° at a scan rate of 2° min−1. 2.8. Fourier Transform Infrared (FTIR) Spectroscopy. Fourier transform infrared (FTIR) spectra of CNF, PANI/CNF, and PANI

samples were investigated. IR prestige-21, Fourier transform infrared spectrophotometer, Shimadzu, was employed for recording the spectra of all the samples. The FTIR spectra of all the samples were obtained in the range of 400−4000 cm−1. The studies were carried out with a resolution of 2 cm−1 and a total of 15 scans for each sample. 2.9. EMI Shielding Performance of Fabricated PANI/CNF Papers. The EMI shielding values of fabricated PANI/CNF paper was evaluated using an Agilent E8362B (10 MHz to 20 GHz) two-port PNA Network analyzer coupled to a Keycom waveguide to measure the scattering parameters at the frequency range of 8.2−12.4 GHz (X band). The scattering parameters (S11, S12, S22, and S21) were recorded and the SEtotal of the PANI/CNF nanopapers were calculated with standard equations.

3. RESULTS AND DISCUSSION The scheme in Figure 1 illustrates the in situ polymerization of aniline on the surface of the cellulose nanofibers and thereby the formation of PANI/CNF flexible composite papers. As shown in Figure 1, the aniline hydrochloride molecules could be uniformly coated on the surface of the cellulose nanofibers due to the secondary forces of interaction via hydrogen C


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Figure 3. Diagram depicting the fabrication of PANI/CNF flexible nanopaper.

those at 2902, 1430, 1371, 1058, and 897 cm−1 were ascertained to −CH stretching, −CH2 and −OCH in-plane bending, −CH bending, C−O−C stretching, and vibration of anomeric carbon (C1), respectively.29,30 In the case of virgin PANI, typical bands around 3262 cm−1 were attributable to the N−H stretching.31 The CC stretching vibration of quinine ring shows at 1567 cm−1 and that of benzene ring presents at 1478 cm−1. The existence of benzenoid and quinoid entities demonstrates that prepared PANI is in its emeraldine form. The FTIR spectrum of PANI/CNF composite papers seems like the entire spectrum of PANI. Compared with the pristine CNF paper, the absorption peaks at 3305, 2902, 1430, 1058, and 897 cm−1 disappeared in the case of PANI/CNF composite papers. Moreover, the FTIR spectrum of PANI/CNF composite paper has the characteristic transmission peaks at 1292 and 784 cm−1 for C−N stretching and C−H bending at C1 and C4 of the benzene ring, indicating that CNFs formed a strong interfacial hydrogen bond with PANI. 3.2. XRD Studies. The XRD patterns of pristine PANI, CNF, and PANI/CNF paper are illustrated in Figure 5. The XRD spectra of CNF shows diffraction peaks around 2θ = 16.3 and 22.6°, corresponding to (110) and (200) planes, respectively, which typically are attributed to cellulose type I structure.32 The spectra of pure PANI exhibited a broad background with crystalline peaks at 2θ 9.73, 18.63, 20.24, 21.39, 23.37, 24.96, and 26.81°, corresponding to (001), (011), (020), (121), (022), (200), and (121) reflections of polyaniline in its emeraldine salt form.33 The main reflection peaks at 2θ = 21.39 and 23.37° are ascribed to the parallel and perpendicular periodicities of polyaniline.34 The XRD spectra of PANI/CNF composite paper demonstrated the combination of PANI and CNF diffraction profiles. 3.3. Morphological Studies of PANI/CNF Flexible Composite Papers. Figure 6a illustrates the SEM image of pristine CNF paper, and Figure 6b illustrates the SEM of in situ polymerized PANI on CNF paper. By comparing both figures,

bonding between the amine groups of aniline and the hydroxyl groups of cellulose nanofibers, as shown in Figure 2. The aniline monomers are polymerized on the surface of cellulose nanofibers upon the addition of the ammonium persulfate (APS) oxidant. This resulted in the formation of dark black PANI/CNF suspensions and composite papers as shown in Figure 3. Aqueous suspensions of PANI/CNF composite papers with different PANI ratios were then obtained by washing with distilled water. 3.1. FTIR Studies. To investigate the structure of PANI, CNF, and PANI/CNF papers, the FTIR studies have been done. As shown in Figure 4, pristine CNF shows the absorption peak at 3305 cm−1, which was due to −OH stretching, whereas

Figure 4. FTIR spectra of PANI powder, PANI/CNF, and CNF flexible nanopapers. D


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bonding inside the amino groups of PANI and hydroxyl groups of CNF. Individual CNFs could not be distinguished and the coating of PANI can be seen on the surface of the PANI/CNF paper. Additionally, the FTIR spectra of CNF and PANI/CNF flexible papers were used to illustrate the possible interactions between CNF and PANI in the flexible composite paper. Figure 7a,b shows the cross section of the PANI/CNF flexible paper and pristine CNF paper, respectively. From Figure 7a, it is clearly evident that CNF paper shows a layered structure due to the hydrogen bonding between the cellulose nanofibers. Figure 7b shows that the PANI/CNF paper demonstrates a similar layered structure, with white dots depicting the fracture ends of the CNFs, thereby indicating a good bonding between CNFs and PANI. 3.4. DC Conductivity of the Fabricated PANI/CNF Paper. The formation of a continuous network of PANI on the surface of the cellulose nanofibers even at low concentration of PANI (0.2:1) is a key requirement for the transport of mobile charge carriers.35,36 The high aspect ratio of cellulose nanofibers and the meticulous use of the secondary forces via H bonding have resulted in a high value of conductivity. The DC conductivity value of the cellulose nanopapers with varying concentration of PANI is given in Table 1. The fabricated PANI/CNF papers exhibited a highest value of 0.314 S/cm at 1:1 ratio (PANI/CNF). Figure 8a shows the digital image of the flexible PANI/CNF paper, and Figure 8b demonstrates an intuitive evidence of conductivity of demonstrated PANI/CNF (1:1) paper (see Video S1). 3.5. Dielectric Properties of PANI/CNF Paper. According to Maxwell’s equations,37 a material’s response to electromagnetic radiation is determined by the following parameters: electrical conductivity (σ), electrical permittivity (ε), and magnetic permeability (μ). The real part of permittivity represents the stored energy between the material

Figure 5. XRD patterns of PANI/CNF, PANI, and CNF.

it could be seen that during in situ polymerization, the aniline monomer was coated uniformly and polymerized to PANI on the surface of the cellulose nanofibers, which resulted in the high conductivity of the samples. During the vacuum infusion process, individual CNF fibers fused together to form larger bundles of fibers due to the strong intermolecular hydrogen bonding. This was clearly evident from Figure 6a. Figure 6b shows the SEM image of the PANI/CNF papers. From Figure 6b, it is clearly seen that the PANI/CNF paper has the similar morphology; however, the surface is rougher and the diameters of the cellulose nanofibers were larger with an increased PANI content due to the polymerization of polyaniline on the CNFs surface. It is clearly shown that the fibers were well bonded with each other, indicating the existence of a strong hydrogen

Figure 6. SEM images of (a) pristine CNF paper and (b) fabricated PANI/CNF (1:1) paper. E


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Figure 7. SEM images of the cross section of PANI/CNF (1:1) flexible paper (a) and pristine CNF paper (b).

of polymer nanocomposites, the dielectric constant mainly arises from matrix filler incongruity. Maxwell−Wagner−Sillars (MWS) theory declares that this mismatch results in the interfacial polarization at the interface.39−41 Frequency-dependent dielectric constant and loss factor of PANI/CNF papers with varying PANI ratios are shown in Figure 9. The dielectric constant of all PANI/CNF papers (Figure 9a) decreases with increase in frequency. In the case of conducting polymer composites, the frequency dependency is related to the dissipation of charge at the filler−matrix interface. A material exhibits its real and intrinsic properties in higher frequency regions rather than at lower frequencies, which mainly affects the presence of surface layers.42 As shown in Figure 9a, PANI/ CNF (1:1) composite paper shows the maximum ε′ value compared to other compositions. At 30 MHz, PANI/CNF (0.2:1) composite paper displayed ε′ value of about 28, whereas

Table 1. DC Conductivity of PANI/CNF Papers with Varying PANI Ratios PANI/CNF ratios

DC conductivity (S/cm)

1.0:1 0.9:1 0.8:1 0.5:1 0.3:1 0.2:1

0.314 0.291 0.12 0.06 0.002 0.00045

and the fields, whereas the imaginary parts indicate the dissipated energy. Materials with a high dielectric constant can store a substantial amount of electric charge. A material with a high dielectric constant and dielectric loss values would show high electromagnetic shielding effectiveness.38 In the case

Figure 8. Images of (a) fabricated PANI/CNF paper and (b) conductivity demonstration of PANI/CNF paper. F


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Figure 9. Frequency dependence of (a) dielectric constant (ε′), (b) dielectric loss (ε″), and (c) alternating current (AC) conductivity of PANI/CNF papers with varying PANI ratios.

PANI/CNF (1:1) composite paper showed ε′ value of 36. This enhancement may be attributed to the decrease in the gap between fillers due to the high filler loading and because of the higher charge accumulation at the MWS interface of PANI/ CNF (1:1) paper. Figure 9b shows that the imaginary permittivity, ε″, increases with increase in PANI loading. Generally, in the case of nanocomposites, the conductive nanofiller networks act as charge carries, and increase in these conductive networks results higher loss factor for nanocomposites. As expected, PANI/CNF (1:1) paper display a higher value of imaginary permittivity with an ε″ value of 5.9 at 30 MHz. The AC electrical conductivity, which is directly related to the loss factor (ε″), is given by the following equation σAC = 2πfε0ε

significant amount of free charges present in the PANI/CNF (1:1) paper contributes more to the conductivity by hopping mechanism than lesser-amount PANI incorporated papers.43 This may lead to the formation of conducting network among the PANI chains. At 30 MHz, PANI/CNF (1:1) composite paper shows σAC value of 0.009 S/m. Because the frequency of interest for the cellulose nanopapers was mostly in the X band region, the dielectric response of the CNF/PANI papers was also studied at 10 and 15 GHz. The dielectric properties of PANI/CNF (1:1) composite paper was further evaluated at higher frequencies of 10 and 15 GHz (Figure 10). The ε′ value (1.66 for 10 GHz and 1.26 for 15 GHz) shows the same decreasing trend with respect to frequency dispersion, whereas ε″ value (0.47 for 10 GHz and 0.58 for 15 GHz) increases with respect to frequency. At the highest frequency of electromagnetic waves, the dipoles cannot follow the field variations. This will decrease the permittivity because of the decreased charge accumulation. The CNF/PANI paper has shown good permittivity values even in the high-frequency region of 15 GHz, which makes it an ideal candidate for shielding EM waves in the X band region.

(1)

where f is the frequency in hertz and ε0 is the permittivity of free space (ε0 = 8.854 × 10−12 F/m). As shown in Figure 9c, σAC increases with increase in frequency and PANI content. In contrast to DC electrical conductivity, AC conductivity arises from the charge accumulation and interfacial polarization. A G


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placing 30 mm × 30 mm samples inside a waveguide and measurements taken after calibrating the VNA. The impedance mismatch is yet another factor that enhances the attenuation of incident electromagnetic energy.45 The output impedance profiles of 0.2:1, 0.5:1, and 1:1 PANI/CNF nanopapers is shown in Figure 11. It is worth comparing the

Figure 10. Dielectric properties of CNF/PANI (1:1) flexible paper at a frequency of 10 and 15 GHz.

3.6. EMI Shielding of the Fabricated PANI/CNF Paper. The extent of attenuation of incident EM radiation is analyzed by measuring the scattering parameters and thereby calculating the total shielding effectiveness (SEtotal), which is usually expressed as decibels. As per the classical EMI shielding theories, the main criterion for effective microwave attenuation is the high conductivity values preferably in the range of 10−1− 1 S/m.44 However, along with high conductivity values, formation of a continuous network and presence of electrical dipoles in PANI/CNF also contributes to the total shielding effectiveness. It is well established that SEtotal is a summation of three different shielding mechanisms, i.e., shielding by absorption (SEA), shielding by reflection (SER), and shielding due to multiple reflections (SEMR). SEtotal (dB) = SEA + SE R + SEMR

Figure 11. Output impedance profiles of 0.2:1, 0.5:1, and 1:1 PANI/ CNF nanopapers in the X band.

impedance profiles of PANI/CNF nanopapers. The output impedance values of PANI/CNF nanopapers are in good agreement with the conductivity values. The least conductive PANI/CNF nanopapers exhibited higher impedance values and vice versa. The variation in the total shielding effectiveness and the corresponding shielding by absorption and reflection components of the fabricated PANI/CNF papers in the X band (8.2− 12.4 GHz) are given in Figures 12 and 13. A total SE value of ca. −23 dB was observed for PANI/CNF paper at 1.00 mm thickness. In all the EMI measurements, PANI/CNF ratio was 1:1, as this composition has shown the maximum DC conductivity value of 0.314 S/cm. However, it was also observed that as the PANI ratio was increased, nanopapers exhibited a poor flexibility and were found to be brittle.46−48 It is also evident that the effect of forming a continuous network of PANI on the surface of cellulose nanofibers has played a pivotal role in enhancing the electrical conductivity and attenuating the electromagnetic radiations. Nevertheless, the secondary interactions such as the intermolecular hydrogen bonding between hydroxyl groups of the nanocellulose and the nitrogen of the PANI has been attributed as the key factor for achieving >99% attenuation even at a very low thickness of 1.0 mm. The basic mechanism of EM wave attenuation of PANI is by absorption, but its application as EM shields is very limited because of its poor film-forming characteristics. Thus, the combined effects of cellulose nanofibers and PANI has resulted in the formation of flexible and robust nanopapers, which could be used as EM shields inside electronic devices such as terrestrial communication systems, aerospace communications, and radar that operate mostly in the X band frequencies. Figure 14 clearly shows that the PANI/CNF composite paper exhibited a predominant attenuation of the microwave radiation by absorption mechanism. The flexible PANI/CNF composite papers demonstrated an attenuation of 87% by absorption and

(2)

It is also well known that SEMR can be neglected if the absorption component is ≥10 dB. Then, the total shielding effectiveness can be re-expressed as 3 SEtotal (dB) = SEA + SE R

(3)

The total shielding effectiveness, shielding due to absorption and shielding due to reflection can also be expressed in terms of scattering parameters as given in 4−6 1 1 SEtotal (dB) = 10 log = 10 log 2 |S21| |S12|2 (4) ⎡ (1 − S ) 2 ⎤ 11 ⎥ SEA = 10 log10⎢ ⎢⎣ S12 2 ⎥⎦

(5)

⎤ ⎡ 1 ⎥ SE R = 10 log10⎢ 2 ⎣ (1 − S11) ⎦

(6)

where S21 and S12 are the forward transmission coefficient and the reverse transmission coefficients, respectively, and S11 is the forward reflection coefficient, all of which were obtained from the vector network analyzer (VNA). The SEtotal values of the PANI/CNF nanopapers of varying thickness were measured by H


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Figure 12. EMI shielding effectiveness of the PANI/CNF flexible paper at (a) 0.25 mm and (b) 0.50 mm thickness in the X band (8.2−12 GHz).

Figure 13. EMI shielding effectiveness of the PANI/CNF flexible paper at (a) 0.75 mm and (b) 1.0 mm thickness in the X band (8.2−12 GHz).

papers were placed in a hot air oven at 80 °C for 72 h and the total shielding effectiveness of unaged and aged PANI/CNF papers were compared. Šeděnková et al. studied the effect of thermal oxidation on PANI films aged at 80 °C and have reported that structural changes via cross-linking occurs during the ageing process.52 In the present work, the total shielding effectiveness of aged and unaged PANI/CNF papers of 1 mm thickness were found to be −23 and −18 dB at 8.2 GHz. This decrease in the total shielding effectiveness of PANI/CNF papers can be attributed to the thermal oxidation of PANI coated over the cellulose nanofibers. The total shielding effectiveness of aged PANI/CNF papers was found to be quiet stable throughout the X band region, as shown in Figure 16. 3.7. Skin Depth Analysis of PANI/CNF Papers. The intensity of the electromagnetic radiations impinging on the surface of the shield decreases as the waves traverse through the thickness of the shield. This is yet another important parameter

13% by reflection mechanism at 8.2 GHz, which is highly desirable, as it prevents any further electromagnetic pollution and provides a noise-free workspace. The plausible shielding mechanisms of the fabricated PANI/CNF nanopapers is illustrated in Figure 15. The incident EM waves are simultaneously reflected and absorbed by the PANI/CNF nanopapers with an enhanced shielding by absorption mechanism in the X band region. Table 2 shows some of the works reported on the cellulosebased EMI shielding materials and their corresponding shielding effectiveness, thickness, and operating frequency. It is also evident from these data that achieving the >20 dB shielding effectiveness in cellulosic substrates is quite a remarkable value, which makes the fabricated PANI/CNF nanopapers as commercially viable shields. The effect of thermal ageing and oxidation on PANI/CNF papers were also studied to elucidate its influence on the total shielding effectiveness of the PANI/CNF papers. The nanoI


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Figure 14. Shielding effectiveness (SEtotal, SEA, SER) of fabricated PANI/CNF nanopapers of varying thickness at 8.2 GHz.

Figure 16. Effect of thermal oxidation on aged and unaged PANI/ CNF flexible papers in the X band (8.2−12.4 GHz).

papers was found to be 0.71, 0.53, and 0.36 mm, respectively. Figure 17 shows the variation in the skin effect of PANI/CNF papers in the X band. The skin effect was more prominent in the 1:1 PANI/CNF nanopapers.

Figure 15. Schematic illustration of the EMI shielding interference of the demonstrated PANI/CNF flexible paper.

that governs the attenuation and is known as skin depth, as given in 7. Mathematically, it is described as the thickness at which the intensity of the electromagnetic radiation reduces to 1/e or 33% of the incident electromagnetic energy53 1 t skin depth (δ) = = −8.68 SEA πfμσ (7)

Figure 17. Variation in skin depth for 0.2:1, 0.5:1, and 1:1 PANI/CNF nanopapers in the X band.

It is also evident from the above equation that the skin depth decreases with the increasing conductivity and SEA. Herein, the PANI/CNF nanopapers exhibited more SEA, which is a bulk phenomenon, and all the compositions have portrayed an effective attenuation by a predominant absorption mechanism. The skin depth at 8.2 GHz for 0.2:1, 0.5:1, and 1:1 PANI/CNF

4. CONCLUSIONS We reported herein a facile approach for the fabrication of highly processable PANI/CNF suspensions containing various

Table 2. Shielding Effectiveness Data for Some Cellulose-Based Nanocomposites for EMI Shielding Application Sr no.

polymer/filler combination

SEtotal (dB)

thickness (mm)

frequency (GHz)

references

1 2 3 4 5 6

cellulose/CNT composite paper bacterial cellulose/PANI membrane cellulose/CNT aerogel cellulose/CNT aerogel CNF/PANI paper CNF/PANI paper

−20 −5 −20 −19.4 −16.3 −23

0.45 0.080 2.5

15−40 8.2−12.4 8.2−12.4 8.2−12.4 8.2−12.4 8.2−12.4

49 28 50 51 this work this work

0.75 1.0 J


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ACS Applied Materials & Interfaces aniline ratios. The PANI/CNF flexible composite papers exhibited a high DC conductivity value of 0.314 S/cm with a PANI/CNF ratio of 1:1. It was clearly evident from the SEM images that in situ polymerization of PANI formed a continuous network on the surface of the cellulose nanofibers. This formation of continuous network was attributed to the secondary forces of interaction via hydrogen bonding, which was confirmed from the FTIR studies. Nevertheless, the PANI coating on the cellulose nanofibers is of paramount importance in attenuating the incident EM waves. The total shielding effectiveness of the composite nanopapers was found to be −16.3 and −23 dB at 0.75 and 1.0 mm paper thickness, respectively, at 8.2 GHz. The attenuation of EM waves was governed by a predominant absorption mechanism (ca. 87%) and very less reflection mechanism (ca. 13%). Such nanopapers have given a new arena for designing green microwave attenuators that can shield the incoming radiation by absorption-dominated mechanism. Additionally, the demonstrated PANI/CNF flexible paper can be potentially used as material for plethora of applications such as flexible electrodes, sensors, and electrically conductive and flexible films and other flexible paper-based devices. The EMI shielding measurements of the demonstrated paper confirmed that the PANI/CNF flexible composite paper was a good promising candidate for attenuating electromagnetic radiations. We firmly believe that this study will be an effective platform for developing green microwave attenuators in the future.

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des Materiaux de Bretagne, Centre de Recherche Rue Saint Maude, France.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04549. Intuitive evidence of conductivity of demonstrated PANI/CNF nanopapers (S1) (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sabu Thomas: 0000-0003-4726-5746 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from India-Brazil Bilateral Scientific Cooperation MCTICNPq/DST (India, project: 401051/2013-7), Visvesvaraya PhD scheme for Electronics and IT, Media Lab Asia, MeitY, Government of India (Ref No. PhD-MLA/4(58)/2015-16). This is a collaboration research project of members of the Rede Mineira de Quimica (RQ-MG) supported by Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) (project CEX-RED-00010-14). The authors also like to thank Prof. P Mohanan, Department of Electronics, CUSAT, for VNA measurements and Prof. S Jayalekshmi, Department of Physics, CUSAT, for the four-probe measurements. The authors gratefully acknowledge the support from International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, India Federal University of Uberlandia, Brazil, and Laboratoire d’Ingenierie K


Research Article


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