Excellent, Lightweight and Flexible Electromagnetic Interference Shielding Nanocomposites Based on Polypropylene with MnFe2O4 Spinel Ferrite Nanoparticles and Reduced Graphene Oxide

nanomaterials Article Excellent, Lightweight and Flexible Electromagnetic Interference Shielding Nanocomposites Based on Polypropylene with MnFe2O4Spinel Ferrite Nanoparticles and Reduced Graphene Oxide Raghvendra Singh Yadav 1,* , Anju 1, Thaiskang Jamatia 1, Ivo Kuˇritka 1, Jarmila Vilˇcáková1, David Škoda 1, Pavel Urbánek 1, Michal Machovský1, Milan Masaˇr 1, Michal Urbánek 1, Lukas Kalina 2and Jaromir Havlica 2 1Centre of Polymer Systems, University Institute, Tomas Bata University in Zlín, Trida Tomase Bati 5678, 760 01 Zlín, Czech Republic; [email protected] (A.); [email protected] (T.J.); [email protected] (I.K.); [email protected] (J.V.); [email protected] (D.Š.); [email protected] (P.U.); [email protected] (M.M.); [email protected] (M.M.); [email protected] (M.U.) 2 Materials Research Centre, Brno University of Technology, Purkyˇnova 464/118, 61200 Brno, Czech Republic; [email protected] (L.K.); [email protected] (J.H.) *Correspondence: [email protected]; Tel.: +42-0576031725 Received: 18 November 2020; Accepted: 7 December 2020; Published: 10 December 2020   Abstract: In this work, various tunable sized spinel ferrite MnFe 2 O 4 nanoparticles (namely MF20, MF40, MF60 and MF80) with reduced graphene oxide (RGO) were embedded in a polypropylene (PP) matrix. The particle size and structural feature of magnetic filler MnFe 2 O 4 nanoparticles were controlled by sonochemical synthesis time 20 min, 40 min, 60 min and 80 min. As a result , the electromagnetic interference shielding characteristics of developed nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP were also controlled by tuning of magnetic/dielectric loss. The maximum value of total shielding effectiveness (SE T ) was 71.3 dB for the MF80-RGO-PP nanocomposite sample with a thickness of 0.5 mm in the frequency range (8.2–12.4 GHz). This lightweight, flexible and thin nanocomposite sheet based on the appropriate size of MnFe 2 O 4 nanoparticles with reduced graphene oxide demonstrates a high-performance advanced nanocomposite for cutting-edge electromagnetic interference shielding application. Keywords: spinel ferrite; nanocomposites; electromagnetic interference shielding; magnetic loss; dielectric loss 1. Introduction Extensive practice of electronic and communication devices, liberating electromagnetic (EM) waves, generates EM radiation pollution [ 1 ]. Electromagnetic interference (EMI) does not only affect the working and life of electronic devices but also is harmful to human health [ 2 ]. This noble type of EM radiation pollution delivers a solid motivation to develop efficient EMI shielding materials [ 3 ]. Lightweight, thinness and cost efficiency are other additional necessities of high-performance EMI shielding materials for operational applications [ 4 ]. Polymer-based EMI shielding composite materials are lightweight, resistant to corrosion, flexible and simple in preparation [ 5 ]. The performance of polymer-based EMI shielding materials depends on the intrinsic electrical conductivity, aspect ratio, and concentration of the fillers [ 6 ]. Graphene has received considerable attention as nano-fillers due to their excellent electrical and thermal conductivities, and ultrahigh mechanical characteristics [ 7 ]. Additionally, spinel ferrite nanoparticles as nanofillers have been established as potential magnetic absorbers due to their outstanding magnetic loss, good stability and cost-effectiveness [8,9]. Nanomaterials 2020,10, 2481; doi:10.3390/nano10122481 www.mdpi.com/journal/nanomaterials Nanomaterials 2020,10, 2481 2 of 23 The particle shape and size of nanoparticles have a vital impact on the microwave absorption and electromagnetic interference shielding characteristics of nanoparticles and their nanocomposites [10]. In recent years, researchers have noticed the influence of particle size on microwave absorption and electromagnetic shielding performance [ 11 ]. Yi-Jun Liang et al. [ 12 ] noticed the size-dependent microwave absorption performance of Fe 3 O 4 nanoparticles prepared by the rapid microwave-assisted thermal decomposition method. Niandu Wu et al. [ 13 ] observed particle size-dependent microwave absorption characteristics of carbon-coated nickel nanocapsules. A correlation of particle size with electromagnetic parameters can benefit us in better control of electromagnetic interference shielding performance. Our research group [ 14 ] also noticed that the particle size of NiFe 2 O 4 nanoparticles correlates with the electromagnetic interference shielding performance of nanocomposites. Efficient electromagnetic interference shielding nanocomposite material having a feature of lightweight, flexible and excellent shielding characteristics are highly essential. Here, lightweight, flexible and excellent EMI-shielding nanocomposites with control of magnetic loss/dielectric loss through the control of particle size of MnFe 2 O 4 spinel ferrite embedded in polypropylene matric with reduced graphene oxide have been developed. Various sized MnFe 2 O 4 spinel ferrite nanoparticles were synthesized by sonochemical synthesis at different sonication times. 2. Materials and Methods 2.1. Materials The reagents manganese nitrate, iron nitrate and sodium hydroxide were procured from Alfa Aesar GmbH and Co KG (Karlsruhe, Germany). Potassium permanganate and graphite flakes were acquired from Sigma-Aldrich, (Munich, Germany). Sodium nitrate was obtained from Lach-Ner (Brno, Czech Republic). The utilized polypropylene (Vistamaxx 6202) was procured from Exxon Mobil (Machelen, Belgium). The reducing agent Vitamin C (Livsane) was obtained from Dr. Kleine Pharma GmbH, (Bielefeld, Germany). 2.2. Preparation of Nanoparticles Various sized MnFe 2 O 4 spinel ferrite nanoparticles were prepared by the sonochemical synthesis approach as reported in our previous report [ 15 ]. A schematic illustration of the preparation of MnFe 2 O 4 spinel ferrite nanoparticles by the sonochemical synthesis approach is shown in Figure 1. Further, the synthesis condition for the preparation of these MnFe 2 O 4 nanoparticles by the sonochemical method is tabulated in Table 1. For the preparation, manganese nitrate and iron nitrate was mixed with deionized water in a beaker. This solution was stirred on a magnetic stirrer for 5 min at room temperature. To this prepared mixed solution, sodium hydroxide aqueous solution was added and the whole mixed solution was placed under sonication (Ultrasonic homogenizer UZ SONOPULS HD 2070 (Berlin, Germany) (frequency: 20 kHz and power: 70 W)) for 20 min. The precipitate was collected and then washed with deionized water and ethanol and finally dried at 40 ◦ C. Further, the increased particle size MnFe 2 O 4 spinel ferrite nanoparticles were prepared for sonication time 40 min, 60 min and 80 min. The reaction temperature was 65 ◦ C, 74 ◦ C, 85 ◦ C and 93 ◦ C, after sonication time 20 min, 40 min, 60 min and 80 min, respectively. The synthesized MnFe 2 O 4 nanoparticles were designated as MF20, MF40, MF60 and MF80 related to different sonication times 20 min, 40 min, 60 min and 80 min, respectively. Further, graphene oxide was prepared by the modified Hummer’s method [ 16 ]. Furthermore, graphene oxide (GO) was converted into reduced graphene oxide (RGO) by utilizing vitamin C as a reducing agent. Nanomaterials 2020,10, 2481 3 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 23 Figure 1. Schematic illustration of the preparation of MnFe2O4 nanoparticles by the sonochemical synthesis method. Table 1. Synthesis condition for preparation of MnFe2O4 nanoparticles by the sonochemical method. Sample Concentration of Mn (NO3)2 4H2O Concentration of Fe (NO3)2 9H2O Concentration of NaOH Sonication Time Reaction Temperature MF20 0.17 M 0.36 M 1.66 M 20 min 65 °C MF40 0.17 M 0.36 M 1.66 M 40 min 74 °C MF60 0.17 M 0.36 M 1.66 M 60 min 85 °C MF80 0.17 M 0.36 M 1.66 M 80 min 93 °C 2.3. Preparation of Nanocomposites A schematic illustration of the preparation of polypropylene (PP) based nanocomposites embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) is shown in Figure 2. Nanocomposites of PP (50 wt %) with MnFe2O4 nanoparticles (40 wt %) and RGO (10 wt %) as nanofillers were developed by using the melt-mixing method. Four nanocomposite samples, namely (i) MF20-RGO-PP, (ii) MF40-RGO-PP, (iii) MF60-RGO-PP and (iv) MF80-RGO-PP were prepared. The rectangle-shaped sheet of a 22.86 × 10.16 × 0.5 mm3 dimension of prepared nanocomposites was developed by the hot-press approach. A representative digital photograph of PP nanocomposite embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) as nanofillers is shown in Figure 3. Figure 2. Schematic illustration of the preparation of polypropylene (PP) based nanocomposites embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO). Figure 1. Schematic illustration of the preparation of MnFe 2 O 4 nanoparticles by the sonochemical synthesis method. Table 1. Synthesis condition for preparation of MnFe2O4nanoparticles by the sonochemical method. Sample Concentration of Mn (NO3)2 4H2O Concentration of Fe (NO3)2 9H2O Concentration of NaOH Sonication Time Reaction Temperature MF20 0.17 M 0.36 M 1.66 M 20 min 65 ◦C MF40 0.17 M 0.36 M 1.66 M 40 min 74 ◦C MF60 0.17 M 0.36 M 1.66 M 60 min 85 ◦C MF80 0.17 M 0.36 M 1.66 M 80 min 93 ◦C 2.3. Preparation of Nanocomposites Aschematicillustrationofthepreparation of polypropylene(PP)basednanocompositesembedded with MnFe 2 O 4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) is shown in Figure 2. Nanocomposites of PP (50 wt %) with MnFe 2 O 4 nanoparticles (40 wt %) and RGO (10 wt %) as nanofillers were developed by using the melt-mixing method. Four nanocomposite samples, namely (i) MF20-RGO-PP, (ii) MF40-RGO-PP, (iii) MF60-RGO-PP and (iv) MF80-RGO-PP were prepared. The rectangle-shaped sheet of a 22.86 × 10.16 × 0.5 mm 3 dimension of prepared nanocomposites was developed by the hot-press approach. A representative digital photograph of PP nanocomposite embedded with MnFe 2 O 4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) as nanofillers is shown in Figure 3. Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 23 Figure 1. Schematic illustration of the preparation of MnFe2O4 nanoparticles by the sonochemical synthesis method. Table 1. Synthesis condition for preparation of MnFe2O4 nanoparticles by the sonochemical method. Sample Concentration of Mn (NO3)2 4H2O Concentration of Fe (NO3)2 9H2O Concentration of NaOH Sonication Time Reaction Temperature MF20 0.17 M 0.36 M 1.66 M 20 min 65 °C MF40 0.17 M 0.36 M 1.66 M 40 min 74 °C MF60 0.17 M 0.36 M 1.66 M 60 min 85 °C MF80 0.17 M 0.36 M 1.66 M 80 min 93 °C 2.3. Preparation of Nanocomposites A schematic illustration of the preparation of polypropylene (PP) based nanocomposites embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) is shown in Figure 2. Nanocomposites of PP (50 wt %) with MnFe2O4 nanoparticles (40 wt %) and RGO (10 wt %) as nanofillers were developed by using the melt-mixing method. Four nanocomposite samples, namely (i) MF20-RGO-PP, (ii) MF40-RGO-PP, (iii) MF60-RGO-PP and (iv) MF80-RGO-PP were prepared. The rectangle-shaped sheet of a 22.86 × 10.16 × 0.5 mm3 dimension of prepared nanocomposites was developed by the hot-press approach. A representative digital photograph of PP nanocomposite embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) as nanofillers is shown in Figure 3. Figure 2. Schematic illustration of the preparation of polypropylene (PP) based nanocomposites embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO). Figure 2. Schematic illustration of the preparation of polypropylene (PP) based nanocomposites embedded with MnFe2O4spinel ferrite nanoparticles and reduced graphene oxide (RGO). Nanomaterials 2020,10, 2481 4 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 23 Figure 3. Digital photograph of PP nanocomposite embedded with MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) as nanofillers. 2.4. Characterization Techniques The EMI shielding effectiveness of prepared nanocomposite (MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP) sheets of dimension 22.86 × 10.16 × 0.5 mm3 was studied with a vector network analyzer (Agilent N5230A) at 8.2–12.4 GHz (the so-called X-band) frequency range using a waveguide sample holder. X-ray powder diffraction (Rigaku Corporation, Tokyo, Japan) characterization tool was employed to analyze the crystal structure of nanocomposites. A field emission scanning electron microscope (FEI NanoSEM450) was employed to observe the morphology and presence of MnFe2O4 nanoparticles and reduced graphene oxide in the polypropylene matrix. Raman spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used for Raman spectra of prepared RGO, PP, and its nanocomposites. A vibrating sample magnetometer (VSM 7407, Lake Shore) was employed to study magnetic hysteresis curves of prepared nanocomposite (MF20-RGOPP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP). The FTIR spectrometer (Nicolet 6700, Thermo Scientific) was utilized to achieve the FTIR spectra of prepared nanocomposites. Thermogravimetric analyses of prepared nanocomposites were performed on a Setaram LabSys Evo with TG/DSC sensor in an atmosphere of air (heating ramp 5 °C min–1, up to 1000 °C, and air flow 60 mL min–1). Mechanical properties of prepared polypropylene based nanocomposites were measured on a Testometric universal-testing machine of type M 350–5CT (Testometric Co. Ltd., Rochdale, UK). 3. Results 3.1. XRD Study XRD pattern of polypropylene (PP) and its prepared nanocomposites MF20-RGO-PP, MF40RGO-PP, MF60-RGO-PP and MF80-RGO-PP is shown in Figure 4. The X-ray diffraction peaks indexed with (220), (311), (222), (400), (422), (511) and (440) confirm the presence of cubic spinel structure of MnFe2O4 nanoparticles in prepared nanocomposites [17]. It is noticeable in Figure 4 that the diffraction peak intensity of MnFe2O4 spinel ferrite nanoparticles was increased with the increase of sonication time, which signified an increase of crystallite size also [15]. The X-ray diffraction peaks at 14.2°, 16.8°, 18.2°, 21.1° and 21.9°, which is associated with (110), (040), (130), (111) and (131) + (041), respectively, crystal plane of the α-form of polypropylene [18]. Further, no diffraction peak associated with reduced graphene oxide was observed because of the low XRD intensity of RGO in prepared nanocomposites [19]. Figure3. Digital photograph of PP nanocomposite embedded with MnFe 2 O 4 spinelferrite nanoparticles and reduced graphene oxide (RGO) as nanofillers. 2.4. Characterization Techniques The EMI shielding effectiveness of prepared nanocomposite (MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP) sheets of dimension 22.86 × 10.16 × 0.5 mm 3 was studied with a vector network analyzer (Agilent N5230A) at 8.2–12.4 GHz (the so-called X-band) frequency range using a waveguide sample holder. X-ray powder diffraction (Rigaku Corporation, Tokyo, Japan) characterization tool was employed to analyze the crystal structure of nanocomposites. A field emission scanning electron microscope (FEI NanoSEM450) was employed to observe the morphology and presence of MnFe 2 O 4 nanoparticles and reduced graphene oxide in the polypropylene matrix. Raman spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used for Raman spectra of prepared RGO, PP, and its nanocomposites. A vibrating sample magnetometer (VSM 7407, Lake Shore) was employed to study magnetic hysteresis curves of prepared nanocomposite (MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP). The FTIR spectrometer (Nicolet 6700, Thermo Scientific) was utilized to achieve the FTIR spectra of prepared nanocomposites. Thermogravimetric analyses of prepared nanocomposites were performed on a Setaram LabSys Evo with TG/DSC sensor in an atmosphere of air (heating ramp 5 ◦ C min −1 , up to 1000 ◦ C, and air flow 60 mL min −1 ). Mechanical properties of prepared polypropylene based nanocomposites were measured on a Testometric universal-testing machine of type M 350–5CT (Testometric Co. Ltd., Rochdale, UK). 3. Results 3.1. XRD Study XRD pattern of polypropylene (PP) and its prepared nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP is shown in Figure 4. The X-ray diffraction peaks indexed with (220), (311), (222), (400), (422), (511) and (440) confirm the presence of cubic spinel structure of MnFe 2 O 4 nanoparticles in prepared nanocomposites [ 17 ]. It is noticeable in Figure 4that the diffraction peak intensity of MnFe 2 O 4 spinel ferrite nanoparticles was increased with the increase of sonication time, which signified an increase of crystallite size also [ 15 ]. The X-ray diffraction peaks at 14.2 ◦ , 16.8 ◦ , 18.2 ◦ , 21.1 ◦ and 21.9 ◦ , which is associated with (110), (040), (130), (111) and (131) +(041), respectively, crystal plane of the α -form of polypropylene [ 18 ]. Further, no diffraction peak associated with reduced graphene oxide was observed because of the low XRD intensity of RGO in prepared nanocomposites [19]. Nanomaterials 2020,10, 2481 5 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 23 10 20 30 40 50 60 70 0 1000 2000 3000 10 20 30 40 50 60 70 500 1000 10 20 30 40 50 60 70 600 900 1200 10 20 30 40 50 60 70 300 600 900 1200 10 20 30 40 50 60 70 600 900 1200 α (111) α (301) + (041) α (130) α (040) α (110) PP 2 θ (Degree) α (301) + (041) α (301) + (041) (440) (511) (422) (400) α (111) α (130) α (110) α (040) Intensity (cps) MF20-RGO-PP (440) (511) (422) (400) (222) (311) (220) α (111) α (130) α (040) α (110) α (110) (311) (220) MF40-RGO-PP α (301) + (041) (440) (511) (422) (400) (222) (222) (311) (220) α (111) α (130) α (040) MF60-RGO-PP α (301) + (041) (440) (511) (422) (400) (222) (311) (220) α (111) α (130) α (040) α (110) MF80-RGO-PP Figure 4. XRD pattern of polypropylene (PP) and prepared nanocomposites MF20-RGO-PP, MF40RGO-PP, MF60-RGO-PP and MF80-RGO-PP. 3.2. FE-SEM Study Field emission scanning electron microscopy (FE-SEM) was utilized to investigate morphology of prepared nanocomposites. FE-SEM image of cross-sections of prepared MF60-RGO-PP and MF80RGO-PP nanocomposites is shown in Figure 5. Images display the existence of MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide in the polypropylene matrix system. Further, FE-SEM image of prepared MF20-RGO-PP and MF40-RGO-PP nanocomposites is shown in Figure 6a,c, respectively. The presence of MnFe2O4 nanoparticles and reduced graphene oxide can be noticed in the polypropylene matrix. In addition, energy dispersive X-ray spectrum (EDX) of the MF20-RGOPP (Figure 6b) and MF40-RGO-PP (Figure 6d) showed the existence of C, O, Mn and Fe. (a) (b) RGO MF60 PP MF60 RGO PP Figure 4. XRD pattern of polypropylene (PP) and prepared nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP. 3.2. FE-SEM Study Field emission scanning electron microscopy (FE-SEM) was utilized to investigate morphology of prepared nanocomposites. FE-SEM image of cross-sections of prepared MF60-RGO-PP and MF80-RGO-PP nanocomposites is shown in Figure 5. Images display the existence of MnFe 2 O 4 spinel ferrite nanoparticles and reduced graphene oxide in the polypropylene matrix system. Further, FE-SEM image of prepared MF20-RGO-PP and MF40-RGO-PP nanocomposites is shown in Figure 6a,c, respectively. The presence of MnFe 2 O 4 nanoparticles and reduced graphene oxide can be noticed in the polypropylene matrix. In addition, energy dispersive X-ray spectrum (EDX) of the MF20-RGO-PP (Figure 6b) and MF40-RGO-PP (Figure 6d) showed the existence of C, O, Mn and Fe. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 23 10 20 30 40 50 60 70 0 1000 2000 3000 10 20 30 40 50 60 70 500 1000 10 20 30 40 50 60 70 600 900 1200 10 20 30 40 50 60 70 300 600 900 1200 10 20 30 40 50 60 70 600 900 1200 α (111) α (301) + (041) α (130) α (040) α (110) PP 2 θ (Degree) α (301) + (041) α (301) + (041) (440) (511) (422) (400) α (111) α (130) α (110) α (040) Intensity (cps) MF20-RGO-PP (440) (511) (422) (400) (222) (311) (220) α (111) α (130) α (040) α (110) α (110) (311) (220) MF40-RGO-PP α (301) + (041) (440) (511) (422) (400) (222) (222) (311) (220) α (111) α (130) α (040) MF60-RGO-PP α (301) + (041) (440) (511) (422) (400) (222) (311) (220) α (111) α (130) α (040) α (110) MF80-RGO-PP Figure 4. XRD pattern of polypropylene (PP) and prepared nanocomposites MF20-RGO-PP, MF40RGO-PP, MF60-RGO-PP and MF80-RGO-PP. 3.2. FE-SEM Study Field emission scanning electron microscopy (FE-SEM) was utilized to investigate morphology of prepared nanocomposites. FE-SEM image of cross-sections of prepared MF60-RGO-PP and MF80RGO-PP nanocomposites is shown in Figure 5. Images display the existence of MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide in the polypropylene matrix system. Further, FE-SEM image of prepared MF20-RGO-PP and MF40-RGO-PP nanocomposites is shown in Figure 6a,c, respectively. The presence of MnFe2O4 nanoparticles and reduced graphene oxide can be noticed in the polypropylene matrix. In addition, energy dispersive X-ray spectrum (EDX) of the MF20-RGOPP (Figure 6b) and MF40-RGO-PP (Figure 6d) showed the existence of C, O, Mn and Fe. (a) (b) RGO MF60 PP MF60 RGO PP Figure 5. Cont. Nanomaterials 2020,10, 2481 6 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 23 Figure 5. (a,b) FE-SEM image of cross-sections of the MF60-RGO-PP sample and (c,d) FE-SEM image of cross-sections of the MF80-RGO-PP sample. 0246810 0 5,000 10,000 15,000 20,000 (b) Fe Fe Mn Mn Fe Mn O CMF20-RGO-PP I n t ens it y ( cps ) Energy (keV) 0246810 0 2,000 4,000 6,000 8,000 10,000 12,000 (d) MF40-RGO-PP Fe Fe Mn Mn Mn Fe O C I n t ens it y ( cps ) Energy ( keV ) Figure 6. (a) FE-SEM image of cross-sections of MF20-RGO-PP, (b) EDX spectrum of MF20-RGO-PP, (c) FE-SEM image of cross-sections of MF40-RGO-PP and (d) EDX spectrum of MF40-RGO-PP. (c) (d) MF80 RGO PP MF80 RGO PP MF20 RGO PP (a) RGO MF40 PP (c) Figure 5. ( a , b ) FE-SEM image of cross-sections of the MF60-RGO-PP sample and ( c , d ) FE-SEM image of cross-sections of the MF80-RGO-PP sample. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 23 Figure 5. (a,b) FE-SEM image of cross-sections of the MF60-RGO-PP sample and (c,d) FE-SEM image of cross-sections of the MF80-RGO-PP sample. 0246810 0 5,000 10,000 15,000 20,000 (b) Fe Fe Mn Mn Fe Mn O CMF20-RGO-PP I n t ens it y ( cps ) Energy (keV) 0246810 0 2,000 4,000 6,000 8,000 10,000 12,000 (d) MF40-RGO-PP Fe Fe Mn Mn Mn Fe O C I n t ens it y ( cps ) Energy ( keV ) Figure 6. (a) FE-SEM image of cross-sections of MF20-RGO-PP, (b) EDX spectrum of MF20-RGO-PP, (c) FE-SEM image of cross-sections of MF40-RGO-PP and (d) EDX spectrum of MF40-RGO-PP. (c) (d) MF80 RGO PP MF80 RGO PP MF20 RGO PP (a) RGO MF40 PP (c) Figure 6. ( a ) FE-SEM image of cross-sections of MF20-RGO-PP, ( b ) EDX spectrum of MF20-RGO-PP, (c) FE-SEM image of cross-sections of MF40-RGO-PP and (d) EDX spectrum of MF40-RGO-PP. Nanomaterials 2020,10, 2481 7 of 23 3.3. Raman Spectroscopy Figure 7shows the Raman spectra of polypropylene (PP), reduced graphene oxide (RGO) and prepared nanocomposite MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP samples. The crystal structure and presence of MnFe 2 O 4 in nanocomposites were confirmed through the measurement of A 1g , E g and T 2g peak positions in the Raman spectrum. In Figure 7, the existence of characteristics Raman bands, i.e., E g mode (296 cm −1 ), T 2g mode (242 cm −1 , 355 cm −1 and 580 cm −1 ) and A 1g mode (604 cm −1 and 657 cm −1 ) of spinel ferrite can be noticed [ 20 ]. The appearance of two characteristics peaks of RGO at 1338 cm −1 and 1594 cm −1 corresponds to the D-band and G-band of RGO, respectively [ 21 ]. Additionally, the other Raman peaks in the nanocomposites are associated with the chemical group of polypropylene [22]. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 23 3.3. Raman Spectroscopy Figure 7 shows the Raman spectra of polypropylene (PP), reduced graphene oxide (RGO) and prepared nanocomposite MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP samples. The crystal structure and presence of MnFe2O4 in nanocomposites were confirmed through the measurement of A1g, Eg and T2g peak positions in the Raman spectrum. In Figure 7, the existence of characteristics Raman bands, i.e., Eg mode (296 cm−1), T2g mode (242 cm−1, 355 cm−1 and 580 cm−1) and A1g mode (604 cm−1 and 657 cm−1) of spinel ferrite can be noticed [20]. The appearance of two characteristics peaks of RGO at 1338 cm−1 and 1594 cm−1 corresponds to the D-band and G-band of RGO, respectively [21]. Additionally, the other Raman peaks in the nanocomposites are associated with the chemical group of polypropylene [22]. 500 1000 1500 2500 3000 3500 MF80-RGO-PP MF60-RGO-PP MF40-RGO-PP MF20-RGO-PP RGO PP G band D band T 2g (2)E g T 2g (1) T 2g (3) A 1g (2) A 1g (1) Raman Intensity ( a.u. ) Raman Shift ( cm − 1 ) Figure 7. Raman spectrum of (a) polypropylene (PP), (b) reduced graphene oxide (RGO), (c) MF20RGO-PP, (d) MF40-RGO-PP, (e) MF60-RGO-PP and (f) MF80-RGO-PP. 3.4. FTIR Spectroscopy Figure 8 displays the FTIR spectra of polypropylene (PP) and developed nanocomposite samples MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP. The presence of characteristic FTIR peaks of MnFe2O4 spinel ferrite nanoparticles and polypropylene can be noticed in the prepared nanocomposites, as shown in Figure 8. In spinel ferrite, the infrared bands noticed between 100 and 600 cm−1 indicate the formation of single phase spinel ferrite material. The absorption band at 565 cm−1 was associated with the intrinsic stretching vibration of metals at tetrahedral sites in MnFe2O4 nanoparticles [23]. The absorption peak at 840 cm−1 was associated with C–CH3 stretching vibration in PP. The peak 972 cm−1, and 1165 cm−1 were associated with –CH3 rocking vibration. The absorption peak at 1375 cm−1 and 2952 cm−1 were related to symmetric bending vibration of the –CH3 group and –CH3 asymmetric stretching vibration. The absorption peak at 1455 cm−1, 2838 cm−1 and 2917 cm−1 were related to –CH2-symmetric bending, –CH2-symmetric stretching, and –CH2-asymmetric stretching, respectively [24]. In amalgamation with Raman and FTIR spectroscopy results, the presence of MnFe2O4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) in the polypropylene (PP) were confirmed. Figure 7. Raman spectrum of polypropylene (PP), reduced graphene oxide (RGO), MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP. 3.4. FTIR Spectroscopy Figure 8displays the FTIR spectra of polypropylene (PP) and developed nanocomposite samples MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP. The presence of characteristic FTIR peaks of MnFe 2 O 4 spinel ferrite nanoparticles and polypropylene can be noticed in the prepared nanocomposites, as shown in Figure 8. In spinel ferrite, the infrared bands noticed between 100 and 600 cm −1 indicate the formation of single phase spinel ferrite material. The absorption band at 565 cm −1 was associated with the intrinsic stretching vibration of metals at tetrahedral sites in MnFe 2 O 4 nanoparticles [ 23 ]. The absorption peak at 840 cm −1 was associated with C–CH 3 stretching vibration in PP. The peak 972 cm −1 , and 1165 cm −1 were associated with –CH 3 rocking vibration. The absorption peak at 1375 cm −1 and 2952 cm −1 were related to symmetric bending vibration of the –CH 3 group and –CH 3 asymmetricstretchingvibration. Theabsorptionpeak at 1455cm −1 , 2838 cm −1 and2917 cm −1 were related to –CH 2 -symmetric bending, –CH 2 -symmetric stretching, and –CH 2 -asymmetric stretching, respectively [ 24 ]. In amalgamation with Raman and FTIR spectroscopy results, the presence of MnFe 2 O 4 spinel ferrite nanoparticles and reduced graphene oxide (RGO) in the polypropylene (PP) were confirmed. Nanomaterials 2020,10, 2481 8 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 23 500 1000 1500 2500 3000 3500 4000 2952cm −1 2917 cm −1 2838 cm −1 1455 cm −1 1375 cm −1 1165 cm −1 840 cm −1 972 cm −1 565 cm −1 MF80-RGO-PP MF60-RGO-PP MF40-RGO-PP MF20-RGO-PP PP Absorbance ( a.u. ) Wavenumber (cm − 1) Figure 8. FTIR spectrum of (a) polypropylene (PP), (b) MF20-RGO-PP, (c) MF40-RGO-PP, (d) MF60RGO-PP and (e) MF80-RGO-PP. 3.5. Thermogravimetric Analysis (TGA) Figure 9 depicts the TGA curves of polypropylene (PP) and its prepared MF20-RGO-PP, MF40RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites under air atmosphere. It can be noticed that the PP had lower degradation temperature in comparison with its prepared nanocomposites. Further, nanocomposites exhibited higher thermal stability as compared to PP, which is associated with the result of an interaction between PP, MnFe2O4 nanoparticles and RGO [25]. Furthermore, the oxidative residues at 1000 °C are 37%, 39%, 46% and 49.2% for MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP, respectively, with 50% nanofillers loading [26]. The slightly lower residue values especially for MF20-RGO-PP and MF40-RGO-PP sample than the corresponding actual residues (i.e., loaded nano-fillers) were mainly due to the evaporation of surface impurities/chemical functional group attached on surface of small sized nanoparticles MF20 and MF40 [27]. 100 200 300 400 500 600 700 800 900 1000 0 20 40 60 80 100 Weight (%) Temperature ( oC) PP MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP Figure 8. FTIR spectrum of polypropylene (PP), MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP. 3.5. Thermogravimetric Analysis (TGA) Figure 9depicts the TGA curves of polypropylene (PP) and its prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites under air atmosphere. It can be noticedthatthePP hadlower degradationtemperatureincomparison withitspreparednanocomposites. Further, nanocomposites exhibited higher thermal stability as compared to PP, which is associated with the result of an interaction between PP, MnFe 2 O 4 nanoparticles and RGO [ 25 ]. Furthermore, the oxidative residues at 1000 ◦ C are 37%, 39%, 46% and 49.2% for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively, with 50% nanofillers loading [ 26 ]. The slightly lower residue values especially for MF20-RGO-PP and MF40-RGO-PP sample than the corresponding actual residues (i.e., loaded nano-fillers) were mainly due to the evaporation of surface impurities/chemical functional group attached on surface of small sized nanoparticles MF20 and MF40 [27]. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 23 500 1000 1500 2500 3000 3500 4000 2952cm −1 2917 cm −1 2838 cm −1 1455 cm −1 1375 cm −1 1165 cm −1 840 cm −1 972 cm −1 565 cm −1 MF80-RGO-PP MF60-RGO-PP MF40-RGO-PP MF20-RGO-PP PP Absorbance ( a.u. ) Wavenumber (cm − 1) Figure 8. FTIR spectrum of (a) polypropylene (PP), (b) MF20-RGO-PP, (c) MF40-RGO-PP, (d) MF60RGO-PP and (e) MF80-RGO-PP. 3.5. Thermogravimetric Analysis (TGA) Figure 9 depicts the TGA curves of polypropylene (PP) and its prepared MF20-RGO-PP, MF40RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites under air atmosphere. It can be noticed that the PP had lower degradation temperature in comparison with its prepared nanocomposites. Further, nanocomposites exhibited higher thermal stability as compared to PP, which is associated with the result of an interaction between PP, MnFe2O4 nanoparticles and RGO [25]. Furthermore, the oxidative residues at 1000 °C are 37%, 39%, 46% and 49.2% for MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP, respectively, with 50% nanofillers loading [26]. The slightly lower residue values especially for MF20-RGO-PP and MF40-RGO-PP sample than the corresponding actual residues (i.e., loaded nano-fillers) were mainly due to the evaporation of surface impurities/chemical functional group attached on surface of small sized nanoparticles MF20 and MF40 [27]. 100 200 300 400 500 600 700 800 900 1000 0 20 40 60 80 100 Weight (%) Temperature ( oC) PP MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP Figure 9. TGA curves of polypropylene (PP) and its prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites under air atmosphere. Nanomaterials 2020,10, 2481 9 of 23 3.6. Magnetic Property MagneticpropertiesofpreparedMF20-RGO-PP,MF40-RGO-PP,MF60-RGO-PPandMF80-RGO-PP nanocomposites were investigated by using a vibrating sample magnetometer. The magnetic hysteresis curves of MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites are shown in Figure 10. Ferromagnetic behavior can be noticed in magnetic hysteresis curves as depicted in Figure 10 for MF20-RGO-PP (H c =33.2 Oe, M r =0.003 emu/g, M s =0.45 emu/g), MF40-RGO-PP ( Hc=43.57 Oe ,M r =0.008 emu/g, M s =0.55 emu/g), MF60-RGO-PP (H c =61.0 Oe, M r =1.57 emu/g, Ms=14.6 emu/g ) and MF80-RGO-PP (H c =45.9 Oe, M r =2.03 emu/g, M s =24.8 emu/g) nanocomposites. The ferromagnetic behavior of nanoparticles MF20 (M s =1.9 emu/g, H c =45.0 Oe, M r =0.12 emu/g), MF40 (M s =2.5 emu/g, H c =42.0 Oe, M r =0.13 emu/g), MF60 (M s =30.2 emu/g, H c =34.0 Oe, Mr=2.27 emu/g ) and MF80 (M s =52.5 emu/g, H c =32.0, M r =4.50 emu/g) was noticed, as mentioned in our previous report [ 15 ]. The high-frequency resonance in terms of anisotropy constant (K), anisotropy energy (H a ) and resonance frequency (f r ) has the following interrelationship with coercivity (H c ) and saturation magnetization (Ms) [28]: K=uoMsHc 2(1) Ha=4|K| 3uoMs(2) 2πfr=rHa(3) where µo is the universal value of permeability in free space (4 π× 10 −7 H/m) and r is the gyromagnetic ratio. The correlation of the above equations signifies that the value of H c and M s can influence the magnitude of K,Haand frand consequently electromagnetic properties of nanocomposites [29]. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 23 Figure 9. TGA curves of polypropylene (PP) and its prepared MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP nanocomposites under air atmosphere. 3.6. Magnetic Property Magnetic properties of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80RGO-PP nanocomposites were investigated by using a vibrating sample magnetometer. The magnetic hysteresis curves of MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites are shown in Figure 10. Ferromagnetic behavior can be noticed in magnetic hysteresis curves as depicted in Figure 10 for MF20-RGO-PP (Hc = 33.2 Oe, Mr = 0.003 emu/g, Ms = 0.45 emu/g), MF40-RGO-PP (Hc = 43.57 Oe, Mr = 0.008 emu/g, Ms = 0.55 emu/g), MF60-RGO-PP (Hc = 61.0 Oe, Mr = 1.57 emu/g, Ms = 14.6 emu/g) and MF80-RGO-PP (Hc = 45.9 Oe, Mr = 2.03 emu/g, Ms = 24.8 emu/g) nanocomposites. The ferromagnetic behavior of nanoparticles MF20 (Ms = 1.9 emu/g, Hc = 45.0 Oe, Mr = 0.12 emu/g), MF40 (Ms = 2.5 emu/g, Hc = 42.0 Oe, Mr = 0.13 emu/g), MF60 (Ms = 30.2 emu/g, Hc = 34.0 Oe, Mr = 2.27 emu/g) and MF80 (Ms = 52.5 emu/g, Hc = 32.0, Mr = 4.50 emu/g) was noticed, as mentioned in our previous report [15]. The high-frequency resonance in terms of anisotropy constant (K), anisotropy energy (Ha) and resonance frequency (fr) has the following interrelationship with coercivity (Hc) and saturation magnetization (Ms) [28]: 𝐾=µ𝑀𝐻 2 (1) 𝐻=4|𝐾| 3µ𝑀 (2) 2𝜋𝑓=𝑟𝐻 (3) where µo is the universal value of permeability in free space (4π × 10−7 H/m) and r is the gyromagnetic ratio. The correlation of the above equations signifies that the value of Hc and Ms can influence the magnitude of K, Ha and fr and consequently electromagnetic properties of nanocomposites [29]. -10,000 -5,000 0 5,000 10,000 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 -10,000 -5,000 0 5,000 10,000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -10,000 -5,000 0 5,000 10,000 -30 -20 -10 0 10 20 30 -10,000 -5,000 0 5,000 10,000 -30 -20 -10 0 10 20 30 (a) MF20-RGO-PP Magnetization ( emu g − 1 ) Magnetic Field ( Oe ) (b) MF40-RGO-PP Magnetization ( emu g −1 ) Magnetic Field ( Oe ) (c) MF60-RGO-PP Magnetization ( emu g − 1 ) Magnetic Field ( Oe ) (d) MF80-RGO-PP Magnetization ( emu g − 1 ) Magnetic Field ( Oe ) Figure 10. Magnetic hysteresis curves of prepared nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP samples. Figure 10. Magnetic hysteresis curves of prepared nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP samples. Nanomaterials 2020,10, 2481 16 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 16 of 23 9G 10G 11G 12G 0.00 0.02 0.04 0.06 0.08 (c) tan δ ε Frequency (Hz) MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP 9G 10G 11G 12G -0.04 0.00 0.04 0.4 0.6 0.8 (d) tan δμ Frequency (Hz) MF 20-RGO-PP MF 40-RGO-PP MF 60-RGO-PP MF 80-RGO-PP Figure 14. Frequency-dependent (a) real part of permeability (µ’), (b) imaginary part of permeability (µ″), (c) dielectric loss (tanδε) and (d) magnetic loss (tanδµ) of prepared MF20-RGO-PP, MF40-RGOPP, MF60-RGO-PP and MF80-RGO-PP composite samples. Additionally, the dielectric loss tangent (tanδε = ε″/ε’) was utilized to calculate the loss capability against the stored capacity for electric energy. Figure 14c depicts the frequency dependence variation of dielectric loss (tanδε) of developed nanocomposites. Noteworthy, the trend of value of dielectric loss (tanδε) of developed nanocomposites was similar to the trend of the imaginary part of permittivity (ε″). The value of dielectric loss (tanδε) was 0.042–0.075, 0.037–0.077, 0.035–0.068 and −0.008–0.038 for prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples, respectively. Moreover, the magnetic loss ability of the prepared nanocomposites can be evaluated by the magnetic tangent loss (tanδµ = µ″/µ’). Figure 14d depicts the frequency dependence changes in magnetic loss (tanδµ) of prepared nanocomposites. The value of magnetic loss (tanδµ) was −0.014– 0.053, −0.021–0.037, −0.031–0.021 and 0.331–0.853 for prepared MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP, respectively. Generally, the magnetic loss is attributed to the magnetic resonance (natural resonance and exchange resonance), eddy current loss, magnetic hysteresis loss and domain wall resonance [55]. The magnetic hysteresis loss had no appearance in the weak electromagnetic field, whereas the domain wall resonance had occurrence only at 1–100 MHz. The magnetic resonance and the eddy current effect induced the magnetic loss in the range of GHz frequency. When the magnetic loss was associated with the eddy current loss, the value of µ″(µ’)-2f-1 should be constant with the variation of the frequency [56]. The eddy current can be calculated by using the following relation [57]: 𝐶= 𝜇󰆒󰆒(𝜇′) 𝑓  =2𝜋𝜎𝜇𝑑/3 (16) where µo is the permeability of the vacuum, σ is the electric conductivity and d is the thickness of the material. As shown in Figure 15a, the value of Co was constant at a lower frequency range from 8.2 to 8.8 GHz for MF20-RGO-PP, MF40-RGO-PP and MF60-RGO-PP nanocomposites, which implies that the magnetic loss in this frequency range was eddy current loss. Further, for these nanocomposites, the value of µ″(µ’) −2f−1 varied at a higher frequency from 8.8 to 12.4 GHz, which suggests that the magnetic loss was not only induced by eddy current effect but also natural ferromagnetic resonance. Furthermore, for MF80-RGO-PP composite sample, the value of µ″(µ’)−2f−1 was not constant throughout the whole frequency range. Figure 14. Frequency-dependent ( a ) real part of permeability ( µ ’), ( b ) imaginary part of permeability ( µ ”), ( c ) dielectric loss (tan δε ) and ( d ) magnetic loss (tan δµ ) of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples. Additionally, the dielectric loss tangent (tan δε = ε ”/ ε ’) was utilized to calculate the loss capability against the stored capacity for electric energy. Figure 14c depicts the frequency dependence variation of dielectric loss (tan δε ) of developed nanocomposites. Noteworthy, the trend of value of dielectric loss (tan δε ) of developed nanocomposites was similar to the trend of the imaginary part of permittivity ( ε ”). The value of dielectric loss (tan δε ) was 0.042–0.075, 0.037–0.077, 0.035–0.068 and − 0.008–0.038 for prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples, respectively. Moreover, the magnetic loss ability of the prepared nanocomposites can be evaluated by the magnetic tangent loss (tan δµ = µ ”/ µ ’). Figure 14d depicts the frequency dependence changes in magnetic loss (tan δµ ) of prepared nanocomposites. The value of magnetic loss (tan δµ ) was − 0.014–0.053, − 0.021–0.037, − 0.031–0.021 and 0.331–0.853 for prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Generally, the magnetic loss is attributed to the magnetic resonance (natural resonance and exchange resonance), eddy current loss, magnetic hysteresis loss and domain wall resonance [ 55 ]. The magnetic hysteresis loss had no appearance in the weak electromagnetic field, whereas the domain wall resonance had occurrence only at 1–100 MHz. The magnetic resonance and the eddy current effect induced the magnetic loss in the range of GHz frequency. When the magnetic loss was associated with the eddy current loss, the value of µ ”( µ ’) −2 f −1 should be constant with the variation of the frequency [56]. The eddy current can be calculated by using the following relation [57]: Co=µ00 (µ0)−2f−1=2πσµod2/3 (16) where µo is the permeability of the vacuum, σ is the electric conductivity and d is the thickness of the material. As shown in Figure 15a, the value of C o was constant at a lower frequency range from 8.2 to 8.8 GHz for MF20-RGO-PP, MF40-RGO-PP and MF60-RGO-PP nanocomposites, which implies that the magneticloss inthisfrequency rangewaseddy current loss. Further, for thesenanocomposites, thevalue of µ ”( µ ’) −2 f −1 varied at a higher frequency from 8.8 to 12.4 GHz, which suggests that the magnetic loss was not only induced by eddy current effect but also natural ferromagnetic resonance. Furthermore, for MF80-RGO-PP composite sample, the value of µ ”( µ ’) −2 f −1 was not constant throughout the whole frequency range. Nanomaterials 2020,10, 2481 17 of 23 Nanomaterials 2020, 10, x FOR PEER REVIEW 17 of 23 9G 10G 11G 12G -0.005 0.000 0.005 0.02 0.03 0.04 0.05 0.06 (a) μ "f − 1 ( μ ' ) − 2(ns) Frequency (Hz) MF 20-RGO-PP MF 40-RGO-PP MF 60-RGO-PP MF 80-RGO-PP 9G 10G 11G 12G 0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 (b) δ ( μ m) Frequency (Hz) MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP 9G 10G 11G 12G 0.01 0.02 0.03 0.04 0.05 (c) Impedance Matching Ratio, Z Frequency (Hz) MF 20-RGO-PP MF 40-RGO-PP MF 60-RGO-PP MF 80-RGO-PP 9G 10G 11G 12G 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 (d) Attenuation Constant, α Frequency (Hz) MF 20-RGO-PP MF 40-RGO-PP MF 60-RGO-PP MF 80-RGO-PP Figure 15. Frequency-dependent (a) eddy current loss, (b) skin depth, (c) impedance matching ratio and (d) attenuation constant of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80RGO-PP composite samples. The superior value of EMISE was associated with the low skin depth of the prepared nanocomposites. The skin depth (δ) is the depth where the incident power of the EM waves fell to 1/e of its value at the surface. It can be given by the following relation [58]: 𝛿=(𝜋𝑓𝜇𝜎)  ⁄ (17) where f is the frequency, µ is the permeability of the material and σ is the electrical conductivity. Figure 15b depicts the variation of skin depth (δ) of prepared MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP composite samples. The skin depth varied from 0.003 to 0.007 μm, 0.008 to 0.036 μm, 0.012 to 0.048 μm and 0.0012 to 0.0013 μm for MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP nanocomposites, respectively. It was noticed that the value of the skin depth of nanocomposites was much lower than their thickness, which leads to a high EMI SE [59]. Further, in general, the material with the shallowest skin depth exhibits high absorption loss [60]. In general, to achieve a large role of electromagnetic absorption, the shielding material should exhibit a large impedance matching ratio (Z) to free space [61]. The impedance matching ratio (Z) can be evaluated from the following relation [62]: 𝑍=𝑍𝑍 ⁄=(𝜇𝜀 ⁄)  ⁄ (18) where, Z1 is the impedance matching of the electromagnetic wave absorber material, and Zo is the impedance in free space. As shown in Figure 15c, the impedance matching ratio was increased with Figure 15. Frequency-dependent ( a ) eddy current loss, ( b ) skin depth, ( c ) impedance matching ratio and ( d ) attenuation constant of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples. The superior value of EMISE was associated with the low skin depth of the prepared nanocomposites. The skin depth ( δ ) is the depth where the incident power of the EM waves fell to 1/e of its value at the surface. It can be given by the following relation [58]: δ=(πfµσ)−1/2(17) where fis the frequency, µ is the permeability of the material and σ is the electrical conductivity. Figure 15b depicts the variation of skin depth ( δ ) of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples. The skin depth varied from 0.003 to 0.007 µ m, 0.008 to 0.036 µ m, 0.012 to 0.048 µ m and 0.0012 to 0.0013 µ m for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites, respectively. It was noticed that the value of the skin depth of nanocomposites was much lower than their thickness, which leads to a high EMI SE [ 59 ]. Further, in general, the material with the shallowest skin depth exhibits high absorption loss [60]. In general, to achieve a large role of electromagnetic absorption, the shielding material should exhibit a large impedance matching ratio (Z) to free space [ 61 ]. The impedance matching ratio (Z) can be evaluated from the following relation [62]: Z=Z1/Zo=(µr/εr)1/2(18) Nanomaterials 2020,10, 2481 18 of 23 where, Z 1 is the impedance matching of the electromagnetic wave absorber material, and Z o is the impedance in free space. As shown in Figure 15c, the impedance matching ratio was increased with the increase of nanoparticle size of MnFe 2 O 4 spinel ferrite in developed MF20-RGO-PP, MF40-RGO-PP and MF60-RGO-PP nanocomposites, whereas it was increased more at a lower frequency and decreased more at a higher frequency in case of the MF80-RGO-PP nanocomposite sample. The other important electromagnetic parameter for electromagnetic interference shielding nanocomposites is the electromagnetic wave attenuation, and the attenuation constant ( α ) can be evaluated by the following relation [63]: α=√2πf c"(µ00ε00 −µ0ε0)+q(µ00ε00 −µ0ε0)2+(µ0ε00 +µ00 ε0)2#1/2 (19) Figure 15d depicts the attenuation constant of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP composite samples. In general, a large value of attenuation constant ( α ) indicates a good attenuation ability, which reveals the great dissipation characteristics of materials [ 64 ]. It can be observed that the nanocomposites MF20-RGO-PP, MF40-RGO-PP and MF60-RGO-PP had a very similar attenuation constant ( α ) value in the frequency range 8.2–12.4 GHz, whereas there was a noticeable gap in the attenuation constant ( α ) value of MF80-RGO-PP composite samples, especially in the low-frequency range. These results indicate that the total loss ability of MF80 spinel ferrite nanoparticles based nanocomposites displayed high magnetic loss in comparison with other samples. Moreover, a clear design of the electromagnetic wave shielding mechanism as reflected above is illustrated in Figure 16. Nanomaterials 2020, 10, x FOR PEER REVIEW 18 of 23 the increase of nanoparticle size of MnFe2O4 spinel ferrite in developed MF20-RGO-PP, MF40-RGOPP and MF60-RGO-PP nanocomposites, whereas it was increased more at a lower frequency and decreased more at a higher frequency in case of the MF80-RGO-PP nanocomposite sample. The other important electromagnetic parameter for electromagnetic interference shielding nanocomposites is the electromagnetic wave attenuation, and the attenuation constant (α) can be evaluated by the following relation [63]: 𝛼= √ 2𝜋𝑓 𝑐 󰇣(µ󰆒󰆒𝜀󰆒󰆒 −µ󰆒𝜀󰆒 )+  (µ󰆒󰆒𝜀󰆒󰆒−µ󰆒𝜀󰆒)+(µ󰆒𝜀󰆒󰆒+ µ󰆒󰆒𝜀󰆒)󰇤 ⁄ (19) Figure 15d depicts the attenuation constant of prepared MF20-RGO-PP, MF40-RGO-PP, MF60RGO-PP and MF80-RGO-PP composite samples. In general, a large value of attenuation constant (α) indicates a good attenuation ability, which reveals the great dissipation characteristics of materials [64]. It can be observed that the nanocomposites MF20-RGO-PP, MF40-RGO-PP and MF60-RGO-PP had a very similar attenuation constant (α) value in the frequency range 8.2–12.4 GHz, whereas there was a noticeable gap in the attenuation constant (α) value of MF80-RGO-PP composite samples, especially in the low-frequency range. These results indicate that the total loss ability of MF80 spinel ferrite nanoparticles based nanocomposites displayed high magnetic loss in comparison with other samples. Moreover, a clear design of the electromagnetic wave shielding mechanism as reflected above is illustrated in Figure 16. Figure 16. Schematic illustration of the electromagnetic interference shielding mechanism in prepared nanocomposites. 3.9. Mechanical Properties In general, the variation in mechanical properties is associated with particle size, morphology and loading amount of fillers in polymer matrix [65,66]. Figure 17a depicts representative strainstress curves of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites. The extracted mechanical parameter tensile strength of prepared nanocomposites is depicted in Figure 17b. The value of tensile strength was 4.84 MPa, 4.32 MPa, 5.56 MPa and 6.42 MPa for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Further, Figure 17c depicts the extracted mechanical parameter elongation at break for prepared MF20-RGOPP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites. The value of elongation at break was 711%, 486%, 598% and 699% for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80RGO-PP, respectively. Furthermore, the extracted mechanical parameters Young’s modulus is shown in Figure 17d. The value of Young’s modulus was 15.2 MPa, 17.2 MPa, 17.8 MPa and 8.7 MPa for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Figure 16. Schematic illustration of the electromagnetic interference shielding mechanism in prepared nanocomposites. 3.9. Mechanical Properties In general, the variation in mechanical properties is associated with particle size, morphology and loading amount of fillers in polymer matrix [ 65 , 66 ]. Figure 17a depicts representative strain-stress curves of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites. The extracted mechanical parameter tensile strength of prepared nanocomposites is depicted in Figure 17b. The value of tensile strength was 4.84 MPa, 4.32 MPa, 5.56 MPa and 6.42 MPa for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Further, Figure 17c depicts the extracted mechanical parameter elongation at break for prepared MF20-RGO-PP, Nanomaterials 2020,10, 2481 19 of 23 MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites. The value of elongation at break was 711%, 486%, 598% and 699% for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Furthermore, the extracted mechanical parameters Young’s modulus is shown in Figure 17d. The value of Young’s modulus was 15.2 MPa, 17.2 MPa, 17.8 MPa and 8.7 MPa for MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP, respectively. Nanomaterials 2020, 10, x FOR PEER REVIEW 19 of 23 0 20 40 60 80 100 120 140 160 180 200 0 2 4 6 8 10 12 14 16 18 (a) Stress ( MPa ) Strain (%) MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP 0 1 2 3 4 5 6 (b) Tensile Strength ( MPa) MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP 0 100 200 300 400 500 600 700 800 900 (c) Elongation at Break (%) MF20-RGO-PP MF40-RGO-PP MF60-RGO-PP MF80-RGO-PP 0 5 10 15 20 (d) Young's Modulus ( MPa) Figure 17. Mechanical behavior of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites: (a) representative strain–stress curves, (b) the tensile strength, (c) elongation at break and (d) Young’s modulus. 4. Conclusions We developed electromagnetic interference shielding nanocomposites based on polypropylene (PP) matrix with reduced graphene oxide (RGO) and MnFe2O4 spinel ferrite nanoparticles as nanofillers. Different sized magnetic filler MnFe2O4 (namely MF20, MF40, MF60 and MF80 samples) nanoparticles were prepared by the sonochemical approach at sonication synthesis time 20, 40, 60 and 80 min. It was noticed that the electromagnetic interference shielding performances of designed nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP were also controlled with the tuning of dielectric/magnetic loss. The maximum value of total shielding effectiveness (SET) was 71.3 dB for MF80-RGO-PP nanocomposite with a thickness of 0.5 mm in the frequency range (8.2–12.4 GHz). The excellent electromagnetic interference shielding properties with a lightweight, flexible and thinness sheet of developed nanocomposites was realized. Author Contributions: A. and T.J. performed the experiments; D.Š., P.U., M.M. (Michal Machovsky), M.M. (Milan Masar), M.U., and L.K. performed the characterizations; R.S.Y., I.K., J.V. and J.H. analyzed the data and wrote the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: We thank the financial support by the Czech Science Foundation (GA19–23647S) project at the Centre of Polymer Systems, Tomas Bata University in Zlin, Czech Republic. Conflicts of Interest: The authors declare no conflict of interest. Figure 17. Mechanical behavior of prepared MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP nanocomposites: ( a ) representative strain–stress curves, ( b ) the tensile strength, (c) elongation at break and (d) Young’s modulus. 4. Conclusions We developed electromagnetic interference shielding nanocomposites based on polypropylene (PP) matrix with reduced graphene oxide (RGO) and MnFe 2 O 4 spinel ferrite nanoparticles as nanofillers. Different sized magnetic filler MnFe 2 O 4 (namely MF20, MF40, MF60 and MF80 samples) nanoparticles were prepared by the sonochemical approach at sonication synthesis time 20, 40, 60 and 80 min. It was noticed that the electromagnetic interference shielding performances of designed nanocomposites MF20-RGO-PP, MF40-RGO-PP, MF60-RGO-PP and MF80-RGO-PP were also controlled with the tuning of dielectric/magnetic loss. The maximum value of total shielding effectiveness (SE T ) was 71.3 dB for MF80-RGO-PP nanocomposite with a thickness of 0.5 mm in the frequency range (8.2–12.4 GHz). The excellent electromagnetic interference shielding properties with a lightweight, flexible and thinness sheet of developed nanocomposites was realized. Author Contributions: A. and T.J. performed the experiments; D.Š., P.U., M.M. (Michal Machovsk ý ), M.M. (Milan Masar) , M.U., and L.K. performed the characterizations; R.S.Y., I.K., J.V. and J.H. analyzed the data and wrote the manuscript. All authors have read and agreed to the published version of the manuscript. 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