PVDF was used as polymer matrix with a molecular weight of 100,000 g.mol and it was purchased from Alfa Aesar. MWCNTs were synthesized by chemical vapor deposition technique. MNTs were prepared by hydrothermal route and N,N-dimethyl formamide was used as the solvent for carbon nanotubes and MnO₂ nanotubes. Laboratory grade acids, bases, and organic solvents were used.

MWCNTs were synthesized by chemical vapor deposition technique using misch metal (approximately 50% cerium and 25% lanthanum, with small amounts of neodymium and praseodymium)-based AB₃ alloy hydride catalysts 32. The as-grown MWCNTs not only contain pure MWCNTs but also amorphous carbon, fullerenes, and other metal catalysts. In order to remove these catalytic impurities and amorphous carbon, air oxidation was performed at 350°C for 4 h followed by acid treatment in concentrated HNO₃. After purification, MWCNTs were functionalized with 3:1 ratio of H₂SO₄ and HNO₃ at 60°C for 6 h in order to impart hydroxyl and carboxyl functional groups over the side walls.

MNTs were prepared by hydrothermal route 33. Briefly, 0.608 g of KMnO₄ and 1.27 ml of HCl (37 wt.%) were added to 70 ml of de-ionized water with continuous stirring to form the precursor solution. After stirring, the solution was transferred to a teflon lined stainless steel autoclave with a capacity of 100 ml. The autoclave was kept in an oven at 140°C for 12 h and then cooled down to room temperature. The resulting brown precipitate was collected, rinsed, and filtered to a pH 7. The as-prepared powders were then dried at 80°C in air.

MNTs and f-MWCNTs reinforced polymer matrix composites were prepared by mixing the respective composite solutions at high-speed rotations per minute followed by solvent casting. Here, we describe the method of preparation of the composites. Initially, 10 mg of MNTs and 990 mg of polymer were dispersed separately in dimethylformamide (DMF) with the help of an ultrosonicator for 1 h at room temperature for the preparation of 1 wt.% MNTs in polymer matrix. These two solutions were mixed by sonicating together for 1 h and the composite solution was transferred to a melt mixer and stirred at room temperature at 4,000 rpm for 2 h and at 80°C for 30 min. The resulting solution was transferred into the beaker and kept in an oven to remove the solvent. Finally, dried thin films were put in a mold and pressed to form 1-mm thick structures. A similar procedure was followed for the preparation of functionalized multiwalled carbon nanotubes (f-MWCNTs)/PVDF composite films. For the preparation of f-MWCNTs/MNTs/PVDF composite, fixed amount of MNTs, f-MWCNTs, and PVDF were added to DMF separately for a desired composition, and the above-mentioned procedure was followed to prepare the composite films. A series of composites were prepared in a similar way by varying the amount of polymer, MNTs, and MWCNTs.

The direct current (DC) volume electrical conductivity of the composites was measured at room temperature using homemade resistivity setup with the help of Keithley 2400 sourcemeter and 2182 nanovoltmeter. The high resistance of the films was measured with a 617 programmable electrometer and a 6517B high-resistance electrometer. The EMI shielding measurement was performed with an Agilent E8362B vector network analyzer using a 201-point averaging in the frequency range of 8 to 12 GHz (X-band). Figure 1 shows the pictorial representation of the experimental setup for measuring the shielding effectiveness of the composite materials. Here, we followed the transmission line technique using an X-band waveguide sample holder for measuring scattering parameters of the composites. Samples of dimensions 22.84 × 10.16 mm² were prepared and kept inside the waveguide. The EMI shielding effectiveness is defined as the ratio of incoming (P_i) to outgoing power (P_o) of radiation. Shielding effectiveness (SE) = 10 log (P_i/P_o) and is defined in decibels (dB). The higher the value in decibels, the less energy passes through the material. When electromagnetic radiation falls on the shielding material, reflection, absorption, and transmission are observed. The corresponding reflectivity (R), absorptivity (A), and transmissivity (T) are according to the equation A + R + T = 1. R and T can be calculated from the measured scattering coefficients, from the relations S₁₂ = 10 log T and S₁₁ = 10 log R. The cross-sectional morphology of the composites were observed using field emission scanning electron microscope (FESEM, QUANTA 3 D, FEI) and transmission electron microscope. X-ray elemental mapping was also performed using EDX genesis software. Powder X-ray diffraction (XRD) studies were carried out using X'Pert PRO, PANalytical diffractometer with nickel filter Cu K_α radiation as the X-ray source. The samples were scanned in steps of 0.016° in the 2θ range 10 to 80. For the determination of functional groups, a Fourier transform infrared spectrum was acquired using Perkin Elmer FTIR spectrometer from 400 to 4,000 cm^-1. The chemical resistance of the composites in different acids, bases, alkanes and organic solvents was estimated by measuring the weight of the sample before and after treatment with these chemicals using METTLER TOLEDO XS 105 weighing balance.

The crystal structure of polymer, MNTs, and f-MWCNTs has been investigated by powder X-ray diffraction. Figure 2 shows the XRD pattern of the PVDF, f-MWCNTs, and MNTs. Figure 2a shows the XRD pattern of f-MWCNTs in which the peaks are indexed to the reflections of hexagonal graphite. The absence of additional peaks corresponding to the catalytic impurities confirms that the impurities have been removed by the acid treatment. The XRD spectrum of the as-synthesized MNT is shown in Figure 2b. All the diffraction peaks can be indexed according to the α-MnO₂ phase, and no other characteristic peaks from any impurity are observed. This establishes the high purity of the sample. In Figure 2c, it can be seen that pure PVDF membrane is crystalline in nature with visible peaks at 18.65° and 20.09°. The sharp peak at 20.09° can be attributed to the presence of β-polymorph.

Figure 3 shows the FTIR spectra of purified and functionalized MWCNTs (f-MWCNTs). The broad absorption band at 3,438 cm^-1 is attributed to the hydroxyl group (νOH). The asymmetric and symmetric stretching of CH bonds are observed at 2,927 and 2,853 cm^-1, respectively and the stretching of C = O of the carboxylic acid (-COOH) group is observed at 1,734 cm^-1. The stretching of C = C, O-H bending deformation in -COOH and CO bond stretching in the f-MWCNTs are observed at 1,635 cm^-1; 1,436 cm^-1; and 1,073 cm^-1; respectively indicating that carboxyl and hydroxyl functional groups were attached to the surface of MWCNTs.

Figure 4 shows the Raman spectra of purified and functionalized MWCNTs. The spectra consists of three main peaks. The peak at 1,343 cm^-1 is assigned to the defects and disordered graphite structures, while the peaks at 1,586 cm^-1 and 2,693 cm^-1 are attributed to the graphite band which is common to all sp² systems and second-order Raman scattering process, respectively. Intensity ratio of defect band and graphite band is a signature of the degree of functionalization of the MWCNTs. As seen from Figure 4, I_D/I_G of pure carbon nanotubes is 0.868 whereas that for functionalized carbon nanotubes is 0.928 indicating the more defective nature of f-MWCNTs.

Morphology is an important factor which affects the EMI SE of the composites. Figure 5a, b, c, d, e, f shows the FESEM images of polymer, nanofillers and nanofiller reinforced polymer composites. The corresponding images are (a) pure PVDF, (b) f-MWCNTs, (c) pure MNTs, (d) 1 wt.% MNTs-PVDF composite, (e) 2 wt.% MNTs-PVDF composite, and (f) high resolution image of 2 wt.% MNTs-PVDF composite. As shown in the Figure 5b andc, MWCNTs are 30 to 40 nm in diameter and approximately 10 μm in length and MNTs are 50 to 70 nm in diameter and in micron length. It can be observed that MWCNTs are entangled with each other because of Van der Waals interactions, whereas manganese dioxide nanotubes were straight and rigid and PVDF shows smooth surface as shown in the Figure 5a. f-MWCNTs and MNTs were homogeneously distributed and embedded in the PVDF matrix as shown in Figure 6a, b, c, d, e, f due to ultrasonication and shear mixing of the solutions at high rpm in the formation of composite films. Figure 6d, e, f indicates that the space between filler aggregates in carbon nanotube-PVDF composites is much smaller than that of MNTs-PVDF composites. Figure 6e shows the FESEM image of 5 wt.% MNTs filled PVDF composite along with 1 wt.% MWCNTs. It is observed that a very good microstructure has been formed, and f-MWCNTs were uniformly dispersed and embedded between the MNTs throughout the PVDF matrix. This good network can increase the number of inter nanostructure connections, and hence provide better EMI SE. Further, to confirm the homogeneity of the composites, we have performed X-ray elemental mapping over the sample surface to visualize the atomic elements of manganese, oxygen, carbon, and fluorine. Figure 7 shows the EDX spectra of PVDF-based MNTs and f-MWCNTs composite. It confirms the presence of manganese and oxygen from MnO₂, carbon from f-MWCNTs, and fluorine from the PVDF polymer. Figure 8 shows the elemental mapping of the 5 wt.% MNTs-1 wt.% f-MWCNTs-PVDF composite. As can be seen from the figures, all the elements were distributed homogeneously in the polymer matrix.

The percentage of chemical resistance of the composites in different acids, bases, organic solvents, and alkanes are shown in the Table 1. It indicates that all the polymer composites are highly resistant towards the chemicals. The MNT-MWCNTs-PVDF composite shows 95% to 100% resistance towards chemicals which indicates the potentiality of the present composite. For comparison, the chemical resistances of MWCNT-PVDF, PVDF, and MNT-PVDF composites were also measured.

Electrical conductivity is of utmost importance for effective EMI shielding material. As shown in the Figure 9, the conductivity of the PVDF is about 10^-16S/m. As the concentration of the MNTs increases in the PVDF matrix, electrical conductivity increases, and it follows percolation behavior. Conductivity of the 1 wt.% MNTs/PVDF composite was found to be approximately 10^-6S/m, which indicates that there is a drastic improvement in electrical conductivity. An increase of about ten orders of magnitude of electrical conductivity was observed which can be attributed to the high aspect ratio and efficient dispersion of the MNTs in the PVDF matrix. Similar trend is observed in the case of electrical conductivity of the f-MWCNTs/PVDF composites as shown in Figure 9b. The possible mechanism for the increment in the electrical conductivity of the composites can be the tunneling effect of the electrons from one nanotube to the other. The effect of f-MWCNTs content on the electrical conductivity of the MNTs/PVDF composites was studied. Incorporation of 1 wt.% f-MWCNTs in 5 wt.% MNT/PVDF composites increases the conductivity from 10^-5S/m to approximately 10^-1S/m which can be attributed to the high aspect ratio, homogeneous dispersion, and high electrical conducting nature of the f-MWCNTs.

The EMI SE of MNTs/PVDF composites with various mass fractions of MNTs as a function of frequency are presented in Figure 10a. The results show that EMI shielding effectiveness of pure PVDF is almost 0.3 dB indicating that it is transparent to the electromagnetic radiation throughout the measured frequency. This is probably due to its electrically insulating nature. It is observed that EMI SE starts increasing with the addition of MNTs to the insulating PVDF matrix. The EMI SE for 1 wt.% MNTs filled PVDF composite is found to be 2.27 dB and it increases further to 5.14 and 11 dB at higher loading of MNTs of 3 and 5 wt.%, respectively. Hence, it is clear that the major contribution to the EMI shielding comes from the addition of semiconducting MNTs to the PVDF matrix. This increment in EMI SE can be attributed to the formation of conductive and connective network in the PVDF matrix, which is in accordance with the high-resolution FESEM image of MNTs filled PVDF composite (Figure 5f). Since electrical conductivity of the MNTs is two orders less compared to that of carbon nanotubes, there is a limit over the highest obtainable conductivity of the total composite. This limits the EMI SE to approximately 12 dB for 5 wt.% MNTs/PVDF composite. These results suggest that the MNTs/PVDF composites can be used for electrostatic discharge applications. In order to make it suitable for EMI shielding applications, a small amount of (1 wt.%) f-MWCNTs have been incorporated in MNT/PVDF matrix. With this, 1 wt.% f-MWCNTs in 5 wt.% MNTs/PVDF composite, we could achieve an EMI SE of 18 to 22 dB. For comparison, the EMI SE of 7 wt.% f-MWCNTs/PVDF composites alone in the same frequency region has been measured, and in this case, an EMI SE of 18 dB has been obtained as shown in Figure 10c. Table 2 shows the overall EMI SE of different composites and their electrical conductivities. It is clear that 5 wt.% MNTs-1 wt.% f-MWCNTs-PVDF composite can be a better and low-cost EMI shielding material.

It is well reported that reflection is the most prominent EMI shielding mechanism in CNT-polymer composites 34. In the present case, EMI shielding in f-MWCNTs reinforced PVDF composites has been studied and from the measured scattering parameters reflectivity, transmissivity, and absorptivity were derived using the formulae mentioned in the experimental section. For 5 wt.% f-MWCNT-PVDF composites, the transmissivity, reflectivity, and absorptivity are 0.177, 0.601, and 0.222, respectively and the corresponding parameters for 7 wt.% f-MWCNT-PVDF composites are 0.131, 0.794, and 0.075. From these results, we can conclude that reflection is the major EMI shielding mechanism in the present f-MWCNT-PVDF composites. This may be due to the presence of conjugated π electrons on the surface of f-MWCNTs. In the case of MNTs/PVDF composites, the chances of absorbing incident radiation are more due to the presence of electric dipoles. Table 3 gives a comparison of the reflectivity and absorptivity of various composites. It is observed that f-MWCNTs/MNTs/PVDF composites and MNTs/PVDF composites exhibit more absorption than reflection. For 5 wt.% MNTs/1 wt.% f-MWCNTs/PVDF composite, the absorptivity, transmissivity, and reflectivity values are respectively 0.78, 0.01, and 0.210. Based on the measured fundamental properties of MNTs/PVDF, f-MWCNTs/PVDF, and MNTs/f-MWCNTs/PVDF composites, the present composites can be engineered for reflection to absorption of the incoming EM radiation by varying the amount of carbon nanotubes and MnO₂ nanotubes in the polymer matrix. The incorporation of MNTs in f-MWCNT-PVDF composite helps in overcoming the Van der Waals forces between f-MWCNTs while utilizing the high aspect ratio of them. Another advantage of the addition of MNTs is that it could decrease the amount of f-MWCNT loading in PVDF matrix.