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Ion-electron conducting polymer composites: Promising electromagnetic interference shielding material Manoj Kumar Vyas, and Amita Chandra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05313 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on July 6, 2016

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ACS Applied Materials & Interfaces

Ion-electron conducting polymer composites: Promising electromagnetic interference shielding material

Manoj Kumar Vyas and Amita Chandra* Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India

ABSTRACT Polymer

nanocomposites

consisting

of

Poly

(vinylidenefluoride–co–

hexafluoropropylene) PVdF–HFP, inorganic salt (LiBF4), organic salt (EMIMBF4), multiwalled carbon nanotubes (MWCNTs) and Fe3O4 nanoparticles have been prepared as electromagnetic shield material. Improvement in conductivity and dielectric property due to the introduction of EMIMBF4, LiBF4 and MWCNTs has been confirmed by complex impedance spectroscopy. The highest conductivity obtained is ~1.86 mS/cm. This is attributed to the high ionic conductivity of the ionic liquids and the formation of a connecting network by the MWCNTs facilitating electron conduction. The total electromagnetic interference (EMI) shielding effectiveness has a major contribution to it due to absorption. While the total shielding effectiveness in the Ku band (12.4-18 GHz) of pure ion conducting system has been found to be ~19dB and that for the polymer composites which are mixed (ion + electron) conductors, it is ~46dB, the contribution to it due to absorption is ~16 dB and ~42 dB, respectively.

KEYWORDS Polymer composites, ionic liquid, transition metal oxide, MWCNTs, electromagnetic shielding effectiveness

ACS Paragon Plus Environment


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INTRODUCTION In the last two decades, large scale development and usage of electronic devices, wireless communication and instruments in military and civil area have made life easier for humans but this large dependency on electronic gadgets and equipment has increased a pollutant, namely, unwanted electromagnetic energy. This unwanted electromagnetic energy produced by electronic and/or electrical devices in the form of electric and magnetic field perturbs the operation of nearby devices due to electromagnetic interference (EMI). The efficiency and lifetime of commercial and military electronic devices are affected because of this.1,2 This problem of EMI is not only limited to the malfunctioning of electronic gadgets as it also affects the human organs. A regular long time exposure to unwanted electromagnetic energy increases the risk of childhood leukaemia, heart problems, cancer, asthma, migraine and even leads to miscarriage.3 Hence, to overcome or minimize the effect of EMI, one requires good EMI shielding materials which are effectively capable of blocking electromagnetic radiation (EMR).4 For attenuating EMR, reflection is the primary mechanism which depends on the mobile charge species (or conductivity, σ) of the shield material whereas absorption is an intrinsic property of the shield material which depends on the electric dipoles (permittivity, ε = ε′–jε″, dielectric loss or tangent loss, tan δ = ε″/ ε′) and magnetic dipoles (permeability, µ= µ′–jµ″) in the material. Besides these two, the thickness of the shield material also plays an important role in shielding effectiveness.5 Traditionally, metals and metallic composites have been the most widely used materials for shielding application because of their high conductivity. However, they have drawbacks like chemical resistance, poor flexibility, susceptibility of metals to oxidation and corrosion, heavy weight and difficulty in processing.6 Carbon based composite materials, polymer nanocomposites and dielectric/magnetic materials have replaced the heavy metals and metallic composites and are considered as suitable materials for EMI shielding application.7 Among the numerous options for obtaining EMI shielding material, polymer based composites have been extensively studied due to their many advantages such as light weight, resistance to corrosion, adjustable mechanical and functional properties. Electron conducting polymers such as poly (3, 4-ethylenedioxy thiophene) (PEDOT),8 polyaniline (PANI)9 and polypyrrole10 etc. have been most widely studied polymer nanocomposites for electromagnetic shielding application. Despite the various interesting properties of the conducting polymers, there have been problems in the processing at the commercial and industrial level. Recently, polymer composites based on carbon dispersion in poly(vinylidene


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fluoride) (PVdF)11-12 and poly(methylmethacrylate) PMMA13 have also attracted attention as alternate candidates for electromagnetic shielding due to their flexibility, resistance to corrosion, easy availability, processibility and cost effectiveness. Since, shielding effectiveness (SE) of the shield material depends on the electrical and magnetic properties of the material, therefore, light weight carbon fillers such as carbon nanofibers,14 carbon nanotubes,15 graphene16 etc. have been used for enhancing the conductivity of the ion as well as electron conducting polymers. Also, various types of magnetic materials like BaFe12O19,8 MnO2,17 Fe2O3,18 Fe3O4,19 γ-Fe2O39 and Co3O420 etc. have been dispersed for improving the magnetic property of the shield materials. Combination of both carbon nanofiller and magnetic particles (in an optimized amount) in the polymeric system should improve the reflection and absorption ability in the shielding materials. From the conductivity point of view, different types of polymer electrolytes (PEs) having ionic conductivity ranging from ~10-7 S.cm-1 to 10-2 S.cm-1 are available. The ionic conductivity of the polymer gel electrolyte is more than 10-4 S.cm-1 at ambient temperature. Polymer gel electrolyte is composed of a polar polymer matrix such as poly (ethylene oxide) PEO,21

poly

(vinylidenefluoride)

PVDF,22,23

Poly

(vinylidenefluoride–co–

hexafluoropropylene) PVdF–HFP24,25 and poly(methylmethacrylate) PMMA,13 an inorganic salt like LiBF4, LiPF6, LiN(CF3SO3)2, LiClO4, NH4ClO4 and an organic solvent such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl formamide (DMF), diethyl carbonate (DEC), dimethyl carbonate (DMC). Due to the presence of low molecular weight organic solvent which enhances the amorphicity of the system, the transport of ions is possible at higher temperatures also in the polymer matrix. Hence, ionic conductivity of gel electrolytes is higher as compared to other polymer electrolytes. However, improvement in ionic conductivity happens at the cost of mechanical stability.26 This problem is resolved by a new kind of polymer gel electrolyte in which the polymer electrolytes contain room temperature ionic liquids (RTILs). RTIL is an organic molten salt which constitutes of a large asymmetrical cation such as imidazolium, pyridinium, alkylammonium, alkylphosphonium, pyrrolidinium, guanidinium, etc. and a weakly coordinating small inorganic or organic anion such as tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), triflates (CF3SO3−), CF3COO−, or CH3COO− etc.27,28 RTILs have very high ionic conductivity up to their decomposition temperature due to the loosely coordinated bulky ions or lower lattice energy. Therefore, a



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large number of charge carriers are available for ionic transport in the polymer host matrix resulting in high conductivity in the polymer electrolyte system.29 Free standing gel polymer electrolyte films which are thin, light weight and flexible having conductivity ~ 10-3S.cm-1 at room temperature30,31 are thus obtained. Furthermore, RTIL's have other desirable features such as non–volatility, non–flammability, thermal stability (up to 300 oC), high polarity, wide electrochemical window (≥ 6V) along with being environment friendly.32 Apart from these properties, physical or chemical gelation of ILs have also attracted considerable attention. Inorganic nanoparticles,33 single-walled carbon nanotubes (SWNTs),34 multi-walled carbon nanotubes (MWCNT)15,22,35 and low-molecular weight compounds36 have been successfully used for the gelation of ILs. In the present work, polar polymer (PVdF-HFP, block copolymer) has been chosen as the host polymer because of its low glass transition temperature, high solubility in organic solvents and low crystallinity. In PVdF-HFP, the crystalline PVdF units provide the structural integrity as well as excellent chemical stability while the amorphous HFP units facilitate the migration of ions in the polymer matrix.37,38 Ionic conduction occurs due to the diffusion of ions through the free volume of the polymer using salts with low lattice energy. For the present study an organic salt, 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) due to its high ionic conductivity (1.24 x 10-2 S.cm-1)39 as well as an inorganic salt, lithium tetrafluoroborate (LiBF4) due to its low lattice energy have been chosen for making the gel polymer electrolytes. The simultaneous addition of LiBF4 along with EMIMBF4 having the same anion (BF-4) is expected to reduce the chances of formation of cross-contact ion pairs thereby enhancing ionic conductivity. Also, incorporation in small fractions of MWCNTs with transition metal oxides (Co3O4, Fe3O4, Mn3O4 and NiO) in the gel polymer electrolyte has been done for better shielding effectiveness of the polymer nanocomposites. EXPERIMENTAL 2.1 Materials For

preparing

the

polymer

nanocomposites,

Poly

vinylidenefluoride–co–

hexafluoropropylene (PVdF–HFP, Average Mw ~400,000, Average Mn ~130,000) with PDI 3.077,

1-Ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4, C6H11BF4N2, Mw ≈

197.97g/mol), lithium tetrafluoroborate (LiBF4) and MWCNTs (purity ≥ 98%) having


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dimensions 10nm ± 1nm x 4.5nm ± 0.5nm x 3~6 µm (O. D. x I. D. x L ) have been purchased from Sigma-Aldrich, Germany. 2.2 Synthesis 2.2.1 Synthesis of transition metal oxides nanoparticles The work has been initiated by the synthesis of magnetic nanoparticles. Transition metal oxides specifically, iron oxide (Fe3O4), nickel oxide (NiO), cobalt oxide (Co3O4) and manganese oxide (Mn3O4) have been synthesized. Fe3O4 nanoparticles have been prepared by chemical precipitation method in which FeCl2·4H2O and of FeCl3·6H2O have been dissolved in distilled water for 40 minutes at room temperature in 1:2 molar ratio. After that, 0.25 molar ammonia solution has been added to this solution at 80oC under vigorous stirring. The stirring has been done for 30 minutes and the reacted mixture has been cooled to room temperature. The obtained brownish black precipitate has been isolated in a uniform magnetic field for 2 hours and washed with water. Thereafter, 8 gm citric acid in 15 ml water has been introduced and the temperature has been raised up to 90oC for 15 minutes as described by M. Racuciu et al.40 For nickel oxide (NiO), nickel nitrate, nickel acetate, and nickel chloride have been used as nickel precursor to produce nickel oxide using the method described by Cai et al.41 Nickel nitrate and nickel acetate, both have been dissolved in distilled water in the same amount. At the same time, sodium carbonate solution of 0.25m has been added at a slow rate until the pH of the solution reached 10 at 80oC. Magnetic stirring has been continued for 5 hours. The solid precipitate has been filtered and dried overnight and calcined at 500oC for 4 hours. Cobalt oxide nanoparticles have been synthesized by thermal treatment of the precursor obtained via micro-chemical-reaction of Co(NO3)2.6H2O with NH4HCO3 by the method described by Yang et al.42 Co(NO3)2.6H2O(11.65gm) and NH4HCO3 (7.91gm) have been mixed and milled in 2:5 molar ratio until the colour of the milled powder remains unchanged. Now, this mixture has been washed with distilled water and dried at 100oC in air to obtain nanoparticles. The precursor has been calcined at 300oC in air for 2 hours.



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Ultrasonic-assisted synthesis at normal temperature and pressure described by Lei Shuijin et al.43 has been used to prepare nanosized colloidal manganese oxide (Mn3O4) particles. In this procedure, MnCl2.4H2O, 4.19gm (0.02mol) and 200ml of ethanol amine (EA) have been put in a beaker for ultrasonication for a few minutes till the MnCl2 completely dissolves in to the solution and the mixture forms a clear brown solution. Subsequently, 200ml distilled water has been added in to the solution and stirred at room temperature for 5 hours clear brown solution turns in to a dark brown suspension. This suspension has been centrifuged at 3000 rpm for 5 minutes to get Mn3O4 particles. 2.2.2 Preparation of polymer nanocomposites For preparing polymer nanocomposites, gel polymer electrolytes have been made by the solution cast procedure. First, PVdF-HFP has been dissolved in acetone by continuous stirring for 6 hours at 45oC. After that, LiBF4 has been introduced and the mixture has been stirred for another 6 hours. Thereafter, the IL (EMIMBF4) has been added and stirred continuously for 6-8 hours until a viscous solution is obtained. The resulting solution has then been casted in a petri dish and dried in room temperature for 3-4 days. The optimized weight ratio of the free standing film of PVdF-HFP:EMIMBF4:LiBF4 corresponding to the highest ionic conductivity has been found to be 1:1:0.5. In the second step, for better electromagnetic shielding properties, in the above solutions a small amount of ultrasonicated MWCNT (no pretreatment has been done) and Fe3O4 nanoparticles have been added (as Fe3O4 possesses highest magnetic saturation value among the transition metal oxide nanoparticles made in the lab). For well dispersed viscous solution, the Fe3O4 nanoparticles have been homogenised at 24,000 rpm. The films have been obtained by drying for 3-4 days at room temperature and then under vacuum for 4-5 hours. The samples prepared are enlisted in Table 1. 2.3 Characterization techniques X-ray diffraction (XRD) has been used for measurement of crystallite size as well as to distinguish between the crystalline and amorphous phases of the system at room temperature. The XRD patterns have been recorded using Bruker D8 discover X-ray diffractometer using Cu-Kα radiation of 0.15418 nm for the Bragg angle (2θ) range from 10° to 80°. ATR studies have been done by Perkin Elmer FTIR spectrometer in the absorbance mode between 4000 and 650 cm-1(having resolution 1cm-1). The electrical conductivity


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measurements have been done using stainless-steel electrodes at room temperature by Nova control impedance spectrometer operated in ~ 0.1 Hz to 107 Hz at one volt initial potential. The diameter of the samples taken was 13 mm and the thickness varied for different samples between 0.3 mm to 1.8 mm.

The Microsens ADE-Model EV9 vibrating sample

magnetometer (VSM) has been used for determining the magnetic saturation values of transition metal oxides and polymer nanocomposites as a function of the magnetic field. Ionic transport numbers have been measured by using CH I660-E Electrochemical work station. The characteristic EMI shielding parameters of the polymer nanocomposites have been measured using Agilent E 8362B PNA network analyser in the Ku band (12.4–18 GHz). The dimensions of the samples taken for shielding measurements were; length:28 mm, width:13 mm and thickness:2.6 mm.

RESULT AND DISCUSSION 3.1 XRD The XRD patterns of the TMO nanoparticles (Co3O4, Fe3O4, Mn3O4 and NiO) have been given in Figure 1(a). The characteristic peaks of the crystalline phases of Co3O4 have been observed at 2θ = 31.24°, 36.38°, 38.52°, 44.90°, 55.74°, 59.46°

and 65.28°

corresponding to the reflection planes [220], [311], [222], [400], [422], [511] and [440], respectively.44,45 The characteristic peaks of Fe3O4 have been observed at 2θ = 30.10°, 35.54°, 43.18°, 53.38°, 57.37°, and 62.80° corresponding to the reflections planes [220], [311], [400], [422], [511], and [440], respectively.46 The characteristic peaks of Mn3O4 have been observed at 28.96°, 32.36°, 36.10°, 38.17°, 44.46°, 50.88°, 58.52°, 59.98° and 64.64° corresponding to the reflection planes [112], [103], [211], [004], [220], [105], [321], [224]and [400]respectively.47,48 For NiO, the characteristic peaks have been observed at 2θ = 37.24°, 43.28°, 62.86°, 75.46° and 79.54° corresponding to the reflection planes [111], [200], [220], [311] and [222], respectively.49 The crystallite size of the nanoparticles have been calculated corresponding to the maximum intensity peak by using the Scherrer formula:

P =

(1)



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where, P is the mean size of the crystalline domains, k is the dimensionless shape factor (= 0.89), λ = 0.154nm, β is the full width at half maximum (FWHM) intensity (in radians) while θ is the Bragg angle. The crystallite size corresponding to Co3O4, Fe3O4, Mn3O4 and NiO nanoparticles has been found to be 15.47± 0.01nm, 15.96± 0.02nm, 25.15± 0.01nm, 18.26± 0.01nm, respectively. In the XRD pattern of the PVdF-HFP film, the characteristic diffraction peaks appear at 18.26°, 20.10°, 26.82°, and 39.4°, respectively assigned to the crystalline planes [100], [020], [110], and [021] of α-PVdF50 in Figure 1(b). These peaks indicate the crystalline nature of PVdF in the copolymer PVdF-HFP which has a semi-crystalline structure.51 When the IL EMIMBF4 has been added in PVdF-HFP, the crystalline peaks around 18.26° and 26.82° disappear and a broad hump appears indicating an enhanced amorphous nature of the film. Also, in addition to PVdF-HFP and EMIMBF4, when the salt LiBF4 has been introduced then the crystalline peak intensity further decreases and the FWHM becomes more broad. Therefore, XRD peak profiles of PVdF-HFP+IL with salt give less intense and broader halos in comparison to the pure PVdF-HFP. The decrement in the intensity and broadening of the peaks in the case of PVdF-HFP+IL with salt shows that the crystallinity of the PVdF-HFP continuously decreases by addition of the EMIMBF4 and LiBF4 which leads to the increment in the ionic conductivity of the PE. Calculated crystallinity from XRD peaks of PVdF-HFP, PE, PELi PELiC and PELiCFe15 are respectively, ~70%, ~57%, ~54%, ~68% and ~31%. Incorporation of MWCNT and Fe3O4 nanoparticles in the above gel electrolyte film further increases the amorphous phase. A peak arises in PELiC at 43.08° corresponding to the [100] plane due to MWCNT52,53 and in the PELiCFe15wt% system, crystalline peaks at 35.54° and 62.80°corresponding to the planes [311] and [400], respectively, occur due to the Fe3O4 nanoparticles.46 3.2 FTIR Figure 2 shows the ATR-IR transmittance spectrum of the pure PVdF-HFP, pure EMIMBF4, and pure LiBF4 along with different composites in the 650-4000 cm-1 range. The transmittance spectrum of PVDF-HFP shows the peaks at 762 cm-1,796 cm-1, 976 cm-1 and 1383 cm-1 which correspond to the crystalline (α) phase of PVdF-HFP and the peaks at 840 cm-1, 874 cm-1 and 1279cm-1 correspond to the β phase.54,55 The observed peaks around 1060 cm-1, 1148 cm-1 and 1178 cm-1 correspond to the symmetrical stretching mode of CF2 and the


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peak at 1203 cm-1 has been assigned to the asymmetrical stretching vibrations of the CF2 group in PVdF-HFP.24 Due to the addition of IL in the PVdF-HFP, peaks at 796 cm-1, 976 cm-1 and 1383 cm-1 corresponding to the crystalline phase of pure PVdF-HFP disappear, the peak at 762 cm1

wave number becomes weaker whereas the peak at 1178 cm-1 shifts to a lower wavenumber,

i.e., to 1168 cm-1. However, the peaks at 841 cm-1 and 879 cm-1 belonging to the amorphous phase (β phase) become more dominant in the gel electrolyte. Also, introduction of IL provides some additional peaks at 3126 cm-1 and 3165 cm-1corresponding to the complexion of IL's cation (EMIM4+1) with the polymer (PVdF-HFP) chain. The intense peak at 1576 cm-1 arises due to the C-C stretching of the imidazolium in EMIMBF4 ring.55 Effect of LiBF4 has also been observed. For pure Li salt, peaks have been observed at 1084 cm-1, 1305 cm-1 and 1638 cm-1. The peak at 1638 cm-1 shifts to 1636 cm-1 for the blends PELi, PELiC and PELiCFe15wt%. The presence of Li salt has also been found to shift the peak at 1168 cm-1 to 1170 cm-1 and gives a broad hump around 3432 cm-1 due to the formation of Li(OH).24,56 Shifting of the hydroxyl wave number at higher wave number is indicating weakening of the crystalline network in the semicrystalline PVdF-HFP. Table 2 gives the important modes of vibration due to the different functional groups for pure PVdFHFP and the polymer electrolytes. 3.3 Magnetic studies Field dependent room temperature magnetic properties of the synthesized TMO nanoparticles and polymer nanocomposites films have been studied with the help of VSM. Figure 3(a) gives the change in the magnetization (M) with the applied magnetic field (H) for the TMO nanoparticles. Negligible coercivity as well as retentivity (i.e., no hysteresis loop), indicates the superparamagnetic nature of the TMO nanoparticles. The saturation magnetization of Fe3O4 nanoparticles has been found to be the highest among all the TMO nanoparticles. The saturation magnetization for the Fe3O4 nanoparticles has been found to be ~45 emu/gm. As no hysteresis loop has been observed for Fe3O4 nanoparticles, it shows the soft ferrite nature of the nanoparticles. Figure 3(b) shows the magnetic behaviour of the polymer nancomposite films and Fe3O4 nanoparticles. In the M–H studies for Fe3O4 nanoparticles and the polymer composite films, a S-shaped curve at room temperature has been observed. For



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PELiC film, the saturation magnetization is almost zero and it increases with the increase in the Fe3O4 content in the PELiC film. The saturation magnetization value for PELiCFe5wt%, PELiCFe10wt% and PELiCFe15wt% composites has been found to be 1.68 emu/gm, 5.82 emu/gm and 21.73 emu/gm respectively. This is desirable for obtaining higher SE value in EMI shielding applications. 3.4 Electrical studies 3.4.1 Dielectric studies Dielectric constant or relative permittivity (ε) of a polymer electrolyte helps in understanding the polarization effect in the polymeric system and in characterizing stored charges and dielectric loss in a material.57 The relative permittivity is a complex quantity in which the ratio of the imaginary part to the real part represents the tangent loss (tanδ). Higher value of tangent loss is indicative of larger attenuation of the electromagnetic wave through the medium. The following equations give the relationship between the real part of the dielectric constant, dielectric loss and tangent loss

ε∗ ω = tanδ =

∗

= ε′ω − ε′′ω

(2) (3)

Variation of the dielectric constant of PVdF-HFP and its different composites at room temperature has been shown in Figure 4(a). Dielectric constant of PVdF-HFP has been found to be almost independent of the frequency whereas for the polymer electrolytes, it decreases with increasing frequency. Decrease in dielectric constant could be associated to the inability of the dipoles to follow the field variation at higher frequencies and also to the polarization effects.57,58 The migration and accumulation process of the charge carriers can be a reason for the large polarization and a very high dielectric constant at low frequencies.58,59 The enhanced dielectric permittivity of the polymeric gel electrolyte (larger than PVdF-HFP) suggests that the introduction of IL and salt provides a large number of free mobile ions due to which the stored charge in the samples increase.60 This increment in the dielectric permittivity of PELiC composite system can also be attributed to the homogeneous dispersion of MWCNT which gradually creates a micro-capacitor network in the PELiC system.61


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Inclusion of 5wt.% Fe3O4 nanoparticles in the PELiC slightly reduces the value of the dielectric constant. However, with increased weight percent (10% and 15%) of Fe3O4, the dielectric constant also increases which can be attributed to the strong interfacial polarization leading to charge accumulation at the interfaces. Dielectric loss of PVdF-HFP and its polymeric composites is shown in Figure 4(b). For PVdF-HFP, PE and PELi, the dielectric loss decreases with the frequency in the low frequency region and increases with the frequency at high frequencies. Due to the dipolar relaxation, the dielectric loss occurs at high-frequencies while at lower frequencies, reduction in dielectric loss is caused by the contributions of interfacial polarization. Nonlinear behaviour of the dielectric loss with frequency for PVdF-HFP, PE and PELi suggests that the conduction loss is not dominant in these systems. The linear slopes at lower and mid frequency region for PELiC and PELiCFe (5, 10 and 15wt.%) composites suggests that the conduction loss is dominant due to the presence of the free electrons at the surface of MWCNT.60,62 Dielectric loss in the composite films has been found to decrease as compared to PELiC due to the incorporation of Fe3O4 nanoparticles indicating the insulting behaviour of Fe3O4. Figure 4(c) represents the tangent loss of ion conducting (PVdF-HFP, PE and PELi) and ion/electron conducting system (PELiC, PELiCFe5wt%, PELiCFe10wt% and PELiCFe15wt%). For ion conducting polymeric system, the initial tangent loss decreases and then increases at higher frequencies due to the dipolar effect. Incorporation of MWCNT also improves the tangent loss due to leakage current flow on the surface of the MWCNT. Further enhancement in tangent loss occurs due to the inclusion of Fe3O4. Increased value of tangent loss corresponding to PELiCFe system has been attributed to the uniform dispersion of MWCNT and Fe3O4 in the presence of IL in the polymer host matrix.22,33,63 Figure 5(a) shows wrapping of PVdF-HFP chain (insulating) around MWCNTs (highly conducting) forming a micro-capacitor network to which the improvement in the dielectric constant of the polymer composite is attributed. The mobile ion's interaction with the polymer chain and their translational motion are shown in Figure 5(b). Rapid dipole orientations as well as translational motion are the main causes for the dielectric loss in the polymer nanocomposites. 3.4.2 Conductivity measurement



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The ionic conductivity (σ) of PVdF-HFP film and its composites has been measured by the electrochemical impedance spectroscopy (EIS) in which the samples have been sandwiched between two stainless steel electrodes. The measurement has been done in the frequency range 1Hz to 106 Hz. The ionic conductivity (σ) has been calculated from the bulk resistance (Rb) according to following equation:

σ =

(4)

Where, l is the thickness of the sample, r is the radius of the electrodes and Rb (bulk resistance) is measured from the Nyquist impedance plots. The observed ionic conductivity of pure PVdF-HFP has been found to be very low due to its semicrystalline nature which has been shown in Figure 6(a). When EMIMBF4 has been added in PVdF-HFP, mesoporous structure gets formed (shown in Figure 7a). These mesoporous structures provide the ionic diffusion of loosely coordinated ions of the IL through the pores or channels (free volume) of the polymer matrix by increasing the flexibility as well as the ionic mobility of the polymeric system. As the IL content increases, more number of ions are transported through the pores and hence, the ionic conductivity of the polymeric system further increases.64 Addition of EMIMBF4 in PVdF-HFP in 1:1 ratio has increased the conductivity to 0.276m S.cm-1 from 0.004m S.cm-1 for PVdF-HFP. Incorporation of LiBF4 in 5, 10, 15, 20 wt% further increases the conductivity. However, for more than 25wt % of LiBF4, the ionic conductivity decreases. Therefore, the optimized system corresponds to (1:1+ 20wt% = 40%+40%+20% or 1:1:0.5) giving highest conductivity of ~0.582m S.cm-1. This ionic conductivity has been attained due to the Li+ ion transport via the segmental motion of the polymer chains as shown schematically in Figure 7b. In the binary system (LiBF4 and EMIMBF4) of the polymer gel electrolyte, as the Li salt concentration is increased, the viscosity of the system also increases. After a specific concentration of the Li salt, the conductivity has been found to decrease due to the decreased mobility (due to high viscosity) of the gel electrolyte.65 This decrease in ionic conductivity after 20 wt % of Li salt has been attributed to the hindrance provided by the bulky ions (EMIM+ and BF4-) which restrict the ionic mobility and thereby the movement of Li+ ions.66 At high-frequencies, semicircle did not appear in the Nyquist plots of the PE and PELi in Figureure 6(b) due to the plasticizing effect of the ionic liquid in the polymer electrolyte.67,68


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For making a good shielding material, conductivity is a dominating factor. Therefore, MWCNT has been introduced in the above system with the composition PELiC in (1:1:0.5 + x wt%) where, x = 2, 4, 6, 8 and 10. The conductivity of the system has been found to increase up to 4wt% after which it decreases due to the aggregation of MWCNT in to bundles making the polymer composites brittle and mechanically unstable. The highest conductivity of ~1.86m S.cm-1 has been observed corresponding to 4wt% of MWCNT, Figure 6(c). This conductivity enhancement is attributed to the charge transfer on the surface of the MWCNT which provides a connecting path either by direct contact or by electron tunnelling. The film with PVdF-HFP:LiBF4+ MWCNT(80:20+4 wt % ratio) has also been prepared. The conductivity of this film has been found to be ~ 0.015 mS.cm-1, which is lower than the conductivity for films of PVdF-HFP:EMIMBF4 or PVdF-HFP:EMIMBF4:LiBF4 with MWCNTs. For improving the magnetic property of the shielding materials, 5wt%, 10wt% and 15wt% of Fe3O4 nanoparticles have been dispersed in the polymeric system. Due to the insulating nature of these nanoparticles, the conductivity of system has been found to decrease. The Nyquist plots of these systems have been given in Figure 6(c). It must be noted that while the conductivity of Fe3O4 and NiO dispersed composites is comparable, the saturation magnetisation of the Fe3O4 nanoparticles is several orders of magnitude higher and therefore, all studies have been conducted using Fe3O4 dispersed polymer composites. The conductivity and saturation magnetisation of the Co3O4 and Mn3O4 particles are far lesser than that of Fe3O4 and are thus, not being reported for EMI shielding effectiveness. 3.4.3 Transport number measurement For the polymer electrolyte system, ionic transport number (Ti) is the contribution to the total conductivity by the anionic/cationic charge carriers. If electron conducting material is also present in the system, it also contributes to the conductivity. The ionic transport number is obtained by using the following equation:

T" =

#$ %#&

(5)

#$

where, Ii = Ie+ Iion (initial value of current) and If = Ie (current due to electrons).



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Total ionic transport number for different polymeric systems has been measured by the amperometric technique shown in Figure 8. For pure PVdF-HFP, the ionic transport number is zero due to its insulating nature where as in the case PE and PELi, it is 0.9201 and 0.9854, respectively, due to the presence of IL and LiBF4. It must be noted that stainless steel electrodes have been used which are not reversible for Li ions. This non-ideal condition leads to ionic transport numbers less than 1. Introduction of MWCNT has been found to reduce the ionic transport number as it increases the electron transport number. The obtained ionic transport numbers corresponding to 2, 4 and 6 wt.% of MWCNT’s are 0.7793, 0.6964 and 0.57367, respectively. Further, presence of Fe3O4 in the polymeric gel electrolytes with 4 wt % of MWCNT initially increases the ionic transport number while decreasing the electronic transport number due to the insulating nature of the Fe3O4 particles. For higher values of Fe3O4 (10 wt% and 15 wt%), both the conductivity and the ionic transport number reduce due to the hindrance provided to the charge carriers by the insulating magnetic particles. Detailed transport and magnetic parameters for all the polymer composites are given in Table 3. 3.5 Electromagnetic shielding effectiveness: Theory and measurement Shielding effectiveness of a material is defined by the amount of the reduction in the incident radiation intensity after passing through a material (expressed in dB). Higher the value of EMI Shielding effectiveness, lesser the EMR transmitted through the material. The total shielding effectiveness (SET) is a combined effect of the reflection, absorption and multiple reflection of the incident wave. Multiple reflections are neglected when the thickness of the shield is more than the skin depth or when the SET is greater than 10 dB. The reflection mechanism contributes in conducting materials which consist of mobile charge carrier whereas the absorption mechanism requires both electric and magnetic dipoles. Mathematically, EMI SE is given by the following expression.69 4

SE) dB = −10 log12 3 5 6 = SE + SE8 + SE9 4 $

(6)

Where, Pt and Pi are, respectively, the transmitted and incident electromagnetic power, and SER, SEA and SEM are shielding effectiveness due to reflection, absorption and multiple reflections, respectively. If the effect of multiple reflections is negligible than the total SE is given by


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SE) dB = SE + SE8

(7)

SER and SEA in terms of dielectric constant (ε), conductivity (σ) and permeability (µ), are given by 1:;

=;

where, σ

6

(8)

?

6

(9)

= @2 εω

In a vector network analyser, the EMI SE is represented in terms of scattering parameters which are S11 (forward reflection coefficient), S12 (forward transmission coefficient), S21 (backward transmission coefficient), S22 (reverse reflection coefficient). The SET can be evaluated from the S parameters by using the following equations

SE) dB = 10 log12 A

1 1 1 B = 10 log A B = 10 log C D 12 12 > > T S1> S>1 (10)

SE dB = 10 log12 C

1

E1%F?? G E1%F?? G

SE8 dB = 10 log12 3

F?

1

D = 10 log12 31%6 6 = 10 log12 3

1% )

6

(11)

(12)

> > where, T = S1> and R = S11

In ideal conditions, without scattering, the sum of the reflectance (R), transmittance (T) and absorbance (A) is always equal to one.

R+T+A=1

(13)



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The individual roles of the various constituents used for EM shielding materials are as given below. In PVdF-HFP, the crystalline PVdF units provide structural integrity as well as excellent chemical stability while the amorphous HFP units help in the migration of the ions in the polymer matrix. The ionic liquid EMIMBF4 provides the high ionic conductivity as well as flexibility to the polymer composites. For further improving the ionic conductivity, the salt LiBF4 has been chosen because Li+ has small size and is highly mobile. Having same anion (BF4-) as the ionic liquid reduces the chance of formation of cross-contact ion pairs. Ions of the system enhance the storage charge which helps in improving the dielectric property of the polymer composites. The role of MWCNTs is to provide connecting pathways in the polymer composite system which helps in improving the electronic contribution. Formation of small micro-capacitors is also facilitated by the MWCNTs. Introduction of Fe3O4 nanoparticles improves the magnetization of the films which further helps in enhancing the shielding effectiveness. Figure 9 gives the variation of the EMI SE of PVdF-HFP and its polymeric complexes/composites as a function of frequency. It is observed that due to reflection and absorption, SE of ion conducting polymeric systems is almost independent of frequency while for mixed (electron and ions) conducting systems, SE due to reflection (SER) and absorption (SEA), slightly vary with frequency. It is observed that as the conductivity of the polymeric system increases, SE due to reflection (SER) also increases and its maximum value is in between 6dB to 7dB corresponding to PELiC system which has conductivity ~1.68 m S.cm-1. SE due to absorption (SEA) has been found to be much higher than the reflection (SER). It must be noted that SEA is almost zero in pure PVdF-HFP. Incorporation of IL in the polymer provides some mobile ions due to which SEA changes from zero to 8dB for the PE system. The observed value of SEA is nearly 16dB for ion conducting PELi system. This SEA indicates that induced polarisation is the main cause behind this SE. The total shielding effectiveness (SET) (Figure 10) of pure PVdF-HFP is almost 0.5dB signifying that it is transparent to the electromagnetic radiation in the measured frequency range (Ku band:12.4-18GHz). For PVdF-HFP and EMIMBF4 in 1:1 ratio, SET is around ~ 9dB whereas for the polymer electrolyte PELi, it is ~19dB. This SE is very close to the desirable value for shield materials (≥20dB) which are useful for commercial level


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applications. Therefore, this is a novel application for ion conducting PE systems which are otherwise being used and extensively studied for energy storage devices. For achieving high attenuation of EMR, connective network in the polymer host matrix is an essential requirement which is provided by the MWCNTs. Electrical conduction is mainly decided by the nanotube–nanotube distances and polymer–nanotube interactions. Direct contact of one CNT with another CNT gives a conducing path and results in a leakage current. However, electron tunnelling effect also leads to conduction if distance between CNTs is less than their diameter, thereby not necessitating direct contact between CNTs.70 Effect of MWCNT on shielding effectiveness can be observed in both SER and SEA values. MWCNT provides the high permittivity and also larger dielectric loss. The connecting path provided by the MWCNTs in the polymer host matrix gives leakage current and hence enhanced absorption loss. For PELiC which is a non-magnetic polymer nanocomposite the SET with value is ~43.48dB. Furthermore, the incorporation of Fe3O4 improves the magnetic saturation value in the polymer nanocomposite system at the cost of attenuating the electrical conductivity. The highest SEA value corresponding to PELiCFe15wt% has been found to be ~40dB while the SET is ~44.57dB at 15GHz. This composition has saturation magnetisation ~22 emu/gm and tangent loss ~52 (average value in the entire frequency range; 1 Hz to 106 Hz). The reason for such high value of SE is attributed to the combined effect of the various components of the polymer composite, viz., enhanced conductivity (due to the IL and salt), connecting network (due to the MWCNTs), high dielectric loss (due to the presence of multifunctional components having different conductivity existing within the same material and attenuated electric field intensity), superparamagnetic behaviour (of the Fe3O4 particles) and the space–charge polarization (at the MWCNT-polymer interface). CONCLUSION In summary, it can be said that a multi-component, lightweight, flexible thin film of a polymer nanocomposite material has been obtained for attenuating EMI pollution. It is found that the free ions provided by the Li-salt and the IL play an important role in shielding effectiveness. The MWCNTs further facilitate conductivity by forming connecting paths in the polymer composites due to which the SET of the nonmagnetic polymer composites reaches ~ 43.5 dB. The incorporation of Fe3O4 nanoparticles in the nonmagnetic polymer



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composites decreases the conductivity due to their insulating nature but improves the saturation magnetization to ~21.73 emu/gm and the SET to ~ 44.60dB in the Ku band.

AUTHOR INFORMATION *Corresponding author Tel: +91 1127662295; Fax: +91 1127667061 E-mail: [email protected] & [email protected] ACKNOWLEDGMENT The authors wish to thank Dr. S.K. Dhawan, Chief Scientist, CSIR-NPL, New Delhi, for providing the facility of Vector network analyser. They also gratefully acknowledge the financial support by the University of Delhi, India, through the R & D grant. REFERENCES 1. Kong, L. B.; Li, Z. W.; Liu, L.; Huang, R.; Abshinova, M.; Yang, Z. H.; Tang, C. B.; Tan, P. K.; Deng, C. R.; Matitsine, S. Recent Rogress in Some Composite Materials and Structures for Specific Electromagnetic Applications: EMI Shielding Methods and Materials—A Review, Int. Mater. Rev., 2013, 58, 203-259. 2. Geetha, S.; Satheesh Kumar, K. K.; Rao, C. R. K.; Vijayan M.; Trivedi, D. C. EMI Shielding: Methods and Materials—A Review. J. Appl. Polym. Sci., 2009, 112, 20732086. 3. Kheifets, L.; Afifi A. A.; Shimkhada, R. Public Health Impact of Extremely Low Frequency Electromagnetic Fields. Environ Health Persp., 2006, 114, 1532-1607. 4. Tong, X. C. Advanced Materials and Design for Electromagnetic Interference Shielding. CRC Press, London, New York, 2008. 5. Ott, H. W. Electromagnetic Compatibility Engineering. John Wiley & Sons, Inc., New York, 2009. 6. Kim, H. R.; Fujimori, K.; Kim B. S.; Kim, I. S. Lightweight Nanofibrous EMI Shielding Nanowebs Prepared by Electrospinning and Metallization: Compos. Sci. and Techn., 2012, 72, 1233-1239.


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53. Anjalin, F. M. Synthesis and Characterization of MWCNTs/PVDF Nanocomposite and its Electrical Studies. Der Pharma Chemica, 2014, 6, 354-359. 54. Du, C. H.; Zhu, B. K.; Xu, Y. Y. The Effects Of Quenching on the Phase Structure of Vinylidene Fluoride Segments in PVDF-HFP Copolymer and PVDF-HFP/PMMA Blends. J. Mater. Sci., 2006, 41, 417-421. 55. Shalu,; Chaurasia, S. K.; Singh, R. K.; Chandra, S. Thermal Stability, Complexing behavior, and Ionic Transport of Polymeric Gel Membranes Based on Polymer PVdFHFP and Ionic Liquid, [BMIM][BF4]. J. Phys. Chem. B, 2013, 117, 897-906. 56. Rajendran, S.; Kannan, R.; Mahendran, O. Ionic Conductivity Studies in Poly(Methylmethacrylate)–Polyethlene Oxide Hybrid Polymer Electrolytes with Lithium Salts. J. Power Sources, 2001, 96, 406-410. 57. Woo, H. J.; Majid, S. R.; Arof, A. K. Dielectric Properties and Morphology of Polymer Electrolyte Based On Poly(Ɛ-Caprolactone) and Ammonium Thiocyanate. Mater. Chem. Phys., 2012, 134, 755-761. 58. Li, Y.; Huang, X.; Hu, Z.; Jiang, P.; Li, S.; Tanaka, T. Large Dielectric Constant and High Thermal Conductivity in Poly(vinylidene fluoride)/Barium Titanate/Silicon Carbide Three-Phase Nanocomposites. ACS Appl. Mater. Interfaces, 2011, 3, 43964403. 59. Tareev, B. Physics of Dielectric Materials, MIR Publications, Moscow, 1979. 60. Buraidah, M. H.; Teo, L. P.; Majid, S. R.; Arof, A. K. Ionic Conductivity by Correlated barrier hopping in NH4I doped Chitosan Solid Electrolyte. Physica B: Condensed Matter, 2009, 404, 1373-1379. 61. Po¨tschkea, P.; Dudkinb, S. M.; Aligb, I. Dielectric Spectroscopy on Melt Processed Polycarbonate—Multiwalled Carbon Nanotube Composites. Polymer, 2003, 44, 50235030. 62. Dakin, T.W. Conduction and Polarization Mechanisms and Trends in Dielectric. IEEE Electr. Insul. M., 2006, 22, 11-28. 63. Sun, W.; Li, X.; Wang, Y.; Zhao, R.; Jiao, K. Electrochemistry and Electrocatalysis Of Hemoglobin on Multi-Walled Carbon Nanotubes Modified Carbon Ionic Liquid Electrode with Hydrophilic EMIMBF4 as modifier. Electrochim. Acta, 2009, 54, 41414148. 64. Sutto, T. E. Hydrophobic and Hydrophilic Interactions of Ionic Liquids and Polymers in Solid Polymer Gel Electrolytes. J. Electrochem. Society, 2007, 154, 101-107.



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65. Ye, H.; Huang, J.; Xu, J. J.; Khalfan, A.; Greenbaumb, S. G. Li Ion Conducting Polymer Gel Electrolytes Based on Ionic Liquid/PVDF-HFP Blends: J. Electrochem. Society, 2007, 154, 1048-1057. 66. Giroud, N.; Rouault, H.; Chainet, E.; Poignet, J.C. Properties of BMIBF4-LiBF4 Electrolytes for Lithium Ion Batteries. Electrochem. Society T., 2009, 16, 75-88. 67. Ramesh, S.; Arof, A.K. Ionic conductivity studies of plasticized poly(vinyl chloride) polymer electrolytes. Mater. Sci. Eng. B, 2001, 85, 11-15. 68. Rajendran, S.; Prabhu, M. R.; Rani, M. U. Ionic Conduction in Poly(Vinyl Chloride)/Poly(Ethyl Methacrylate)-Based Polymer Blend Electrolytes Complexed with different Lithium Salts. J. Power Sources, 2008, 180, 880-883. 69. Jaroszewski, M.; Ziaja, J.; Pospieszna, J. Polymer based Nanocomposites for Electromagnetic

Interference

(EMI) Shielding: EM Shielding- Theory and

Development of New Materials, 2012, Research Signpost, Trivandrum-695 023, Kerala, India. 70. Du, F. M.; Scogna, R. C.; Zhou, W.; Brand, S.; Fischer, J. E.; Winey, K. I. Nanotube Networks in Polymer Nanocomposites: Rheology and Electrical Conductivity. Macromolecules, 2004, 37, 9048-9055.


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Table 1. Sample compositions for different Polymer electrolytes/composites

Sample code

Constituents in weight percent

Polymer electrolyte/composites

PVdF-HFP

PVdF-HFP

100

PE

PVdF-HFP:EMIMBF4 in 1:1

50+50

PELi

PVdF-HFP:EMIMBF4:LiBF4 in 1:1:0.5

40+40+20

PELiC

PVdF-HFP:EMIMBF4:LiBF4+MWCNTs

38.4+38.4+19.2+4

in [(1:1:0.5) + 4wt%] PELiCFe5wt%

(PVdF-HFP:EMIMBF4:LiBF4+MWCNTs)

(36.48+36.48+18.24+3.8)+5

+ Fe3O4 in (1:1:0.5 + 4wt%) +5wt% PELiCFe10wt% (PVdF-HFP:EMIMBF4:LiBF4+MWCNTs)

(34.56+34.56+17.28+3.6)+10

+ Fe3O4 in (1:1:0.5 + 4wt%) +10wt% PELiCFe15wt% (PVdF-HFP:EMIMBF4:LiBF4+MWCNTs) + Fe3O4 in (1:1:0.5 + 4wt%) +15wt%


(32.64+32.64+16.32+3.4)+15


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Table 2. ATR-IR peak specifications for pure PVDF-HFP, EMIMBF4 and LiBF4

PVDF-HFP

762

CH2 rocking

83

796

CF2 stretching vibration of PVdF-HFP

83

976

Non polar trans−gauche−trans−gauche′ (TGTG′) conformation

83

α- phase

PVDF-HFP

1383 CH2 wagging

47

841

Mixed mode of CH2 rocking and CF2 asymmetric stretching

83

874

Due to combined CF2 and C-C symmetric stretching vibrations

83

β- Phase

PVDF-HFP

1060 CF2 symmetric stretching

47

PVDF-HFP


47

PVDF-HFP


47

PVDF-HFP

1203 CF2 asymmetric stretching

47

EMIMBF4

1576 C-C stretching of imdiazolium in EMIMBF4 ring

83

EMIMBF4

3126 C-H stretching of IL cation (EMIM+) ring in EMIMBF4

83

EMIMBF4

3165 C-H stretching of IL cation (EMIM+) ring in EMIMBF4

83

LiBF4

1084 Asymmetric stretching of BF4-

47

LiBF4

1638 Asymmetric stretching of BF4-

47

LiBF4

3432 Broad hump due to hydroxyl OH-


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Table 3. Electric and magnetic properties of different polymer complexes/composites

System

Tangent

Bulk

Conductivity

Ionic

Magnetic

loss

Resistivity

(σ) in

Transport

saturation

(m S.cm-1)

Number

in emu/gm

(Rb) in Ω

(tanδ)

PVdF-HFP

0.07

5755.76

0.004

-

-

PE

1.24

174.49

0.276

0.9201

-

PELi

7.89

107.18

0.582

0.9854

-

PELiC

12.61

25.56

1.857

0.6964

-

PELiCF5wt%

78.34

246.08

0.443

0.8019

1.55

PELiCF10wt%

51.32

1736.29

0.081

0.6068

5.90

PELiCF15wt%

52.62

1935.78

0.055

0.4280

21.94



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Figure Captions Figure 1. XRD patterns of (a) transition metal oxide nanoparticles and (b) PVdF-HFP, PE, PELi, PELiC and PELiCFe15wt% polymer composites. Figure 2. ATR-IR transmittance spectrum of (a) PVdF-HFP, (b) EMIMBF4, (c) PE, (d) LiBF4, (e) PELi, (f) PELiC and (g) PELiCFe15wt%. Figure 3. M-H curves for (a) TMO nanoparticles (inset shows magnetization curves for NiO, Co3O4 and Mn3O4) and (b) polymer nanocomposite films (PELiC, PELiCFe 5wt%, PELiCFe10wt% and PELiCFe15wt %) and Fe3O4 nanoparticles. Figure 4. (a) Dielectric constant, (b) dielectric loss and (c) tangent loss of PVdF-HFP and its composites. Figure 5. (a) Wrapping of PVdF-HFP polymer chain around MWCNTs forming microcapacitor network and (b) interaction of ions with polymer chain and translational motion (diagram not drawn to scale). Figure 6. Nyquist plots for (a) PVdF-HFP, (b) PVdF-HFP.EMIMBF4 (PE) and PVdFHFP.EMIMBF4.LiBF4 (PELi) and (c)PELiC,

PELiCFe5wt%, PELiCFe10wt% and

PELiCFe15wt%. Figure 7. Schematic diagramme for (a) formation of mesoporous structures in PVdF-HFP and (b) segmental motion of the polymer chain {initial chain position shown by (1) and (2) and after the jump of the Li ion, the chain is shown by (1') and (2')}. Figure 8. Amperograms for different polymeric systems. Figure 9. (a) SER due to reflection and (b) SEA due to absorption for PVdF-HFP and its polymeric complexes/composites. Figure 10. Total SE of PVdF-HFP and its polymeric complexes/composites.


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Figure 1. XRD patterns of (a) transition metal oxide nanoparticles and (b) PVdF-HFP, PE, PELi, PELiC and PELiCFe15wt% polymer composites.



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Figure 2. ATR-IR transmittance spectrum of (a) PVdF-HFP, (b) EMIMBF4, (c) PE, (d) LiBF4, (e) PELi, (f) PELiC and (g) PELiCFe15wt%.


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Figure 3. M-H curves for (a) TMO nanoparticles (inset shows magnetization curves for NiO, Co3O4 and Mn3O4) and (b) polymer nanocomposite films (PELiC, PELiCFe 5wt%, PELiCFe10wt% and PELiCFe15wt %) and Fe3O4 nanoparticles.



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Figure 4. (a) Dielectric constant, (b) dielectric loss and (c) tangent loss of PVdF-HFP and its composites.


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Figure 5. (a) Wrapping of PVdF-HFP polymer chain around MWCNTs forming microcapacitor network and (b) interaction of ions with polymer chain and translational motion (diagram not drawn to scale).



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Figure 6. Nyquist plots for (a) PVdF-HFP, (b) PVdF-HFP:EMIMBF4 (PE) and PVdFHFP:EMIMBF4:LiBF4 (PELi) and (c)PELiC, PELiCFe5wt%, PELiCFe10wt% and PELiCFe15wt%.


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Figure 7. Schematic diagramme for (a) formation of mesoporous structures in PVdF-HFP and (b) segmental motion of the polymer chain {initial chain position shown by (1) and (2) and after the jump of the Li ion, the chain is shown by (1') and (2')}.



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Figure 8. Amperograms for different polymeric systems.


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Figure 9. (a) SER due to reflection and (b) SEA due to absorption for PVdF-HFP and its polymeric complexes/composites.



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Figure 10. Total SE of PVdF-HFP and its polymeric complexes/composites.


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Graphical Abstract



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12x5mm (300 x 300 DPI)


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