Composites Part B 120 (2017) 118e127 Contents lists available at ScienceDirect Composites Part B journal homepage: www.elsevier.com/locate/compositesb Graphene nanoplatelets/carbon nanotubes/polyurethane composites as efficient shield against electromagnetic polluting radiations Meenakshi Verma a, Sampat Singh Chauhan a, S.K. Dhawan b, Veena Choudhary a, * a b Centre for Polymer Science & Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, 110016, India Polymeric & Soft Materials Section, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi, 110 012, India a r t i c l e i n f o a b s t r a c t Article history: Received 17 January 2017 Received in revised form 21 March 2017 Accepted 25 March 2017 Available online 5 April 2017 Hybrid nanocomposites are in the forefront of nanomaterials research owing to their unique ability to enhance the material property due to the existing synergistic effect of the fillers. Here, we report a ternary hybrid nanocomposite comprising of thermoplastic polyurethane as matrix and graphene nanoplatelets-carbon nanotubes hybrid (GCNT) as filled inclusion. The solution blending approach was used to prepare a series of polyurethane nanocomposites with GCNT loading ranging from 0 to 10 wt%. Due to the synergistic interaction of the two kinds of nanofillers, an electrical conductivity of the order of ~10 2 S/cm was achieved owing to the formation of a conducting network of CNTs bridging the gaps between graphene nanoplatelets throughout the electrically insulating polyurethane matrix. These hybrid nanocomposites exhibited excellent electromagnetic interference shielding up to 47 dB in the Ku-band of microwave frequency for 10 wt% loaded GCNT sample. In addition, the electromagnetic attributes, such as the real and imaginary permittivity of the nanocomposites as a function of frequency, were also investigated. The present studies, therefore, provides a new avenue for the preparation of hybrid carbon nanomaterials with unique structure and outstanding EMI shielding properties which make these materials as capable aspirants against electromagnetic polluting radiations. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction In today's electronic age, growing utilization of electrical and electronic equipments for industrial, military, household, medical/ biomedical, science and communication has become a serious cause of concern for modern society. Electromagnetic interference is an undesirable byproduct of proliferation of electronic devices and wireless communication technologies. These electromagnetic disturbances may often lead to malfunctioning of sensitive electronic equipments and interference with telecommunication as well as prolonged exposure to such radiations causes health threats to human beings [1e3]. The shielding or blocking of electromagnetic signals is one effective technique to meet working requirements of such devices [4]. Metals are effective materials widely applied in shielding industry but heavy weight, corrosion susceptibility and cumbersome processing methods makes these materials unsuitable for both the researchers and users [5]. Compared with the conventional metal based EMI shielding * Corresponding author. E-mail address: veenach@hotmail.com (V. Choudhary). http://dx.doi.org/10.1016/j.compositesb.2017.03.068 1359-8368/© 2017 Elsevier Ltd. All rights reserved. materials, nanocarbon based composite materials (graphene, graphene oxide, carbon nanotubes and other novel forms of carbon) are becoming attractive and potential replacements due to their tailor made properties, lightweight, structural flexibility, resistant to corrosion and ease of processing advantages [6e11]. Graphene, as a new member of carbon allotropes and with its exceptional electrical conductivity and structural flexibility, is a promising nanofiller to be employed in nanocomposites for many multi-functional applications [6,12e15]. Graphene oxide, as a precursor for graphene, with an abundance of versatile oxygen functionalities on its edges and basal planes has acquired tremendous consideration owing to its high processability and dispersibility in aqueous media. These ameliorating properties make GO an ideal choice for preparation of well dispersed nanocomposites with an engineered nanostructure. However GO suffers from poor electrical conductivity due to the disruption of conjugated carbon backbone by the oxygenated groups, necessitating an additional reduction step to restore its conductivity [16]. In order to fully utilize the potential of graphene as superior functional filler, individual single layer graphene sheet from their aggregates need to be assembled within the polymer matrix so that M. Verma et al. / Composites Part B 120 (2017) 118e127 the properties of composites could be tuned as per desired application. Recently, many attempts have been made to employ carbon nanotubes (CNT) as spacers between the graphene sheets which not only increases the inter layer spacing but also bridges the defects for electron transfer and thus a graphene-CNT hybrid material (GCNT) is obtained combining the synergistic effect of one dimensional CNT and two dimensional graphene [17]. GCNT hybrid nanostructures have been recently explored as anode materials for lithium-ion batteries [18], hydrogen storage materials [19], supercapacitors [17,20] microwave absorbers [21,22]and reinforcements in polymer composites [23e29]. A novel, water dispersible, three dimensional GCNT hybrid prepared by the direct reduction of GO sheets in the presence of acid treated CNTs had been reported by Zhang et al. They reported high performance poly(vinyl alcohol)/ GCNT nanocomposites with improved mechanical properties and thermal degradation temperature suggesting a potential flame retardant effect of GCNT hybrid [30]. Lee et al. investigated the use of electrostatic force driven randomly stacked three dimensional hybrid electrocatalysts of multiwalled carbon nanotubes hybridized with reduced graphene oxide to maximize the utility of oxygen during the oxygen reduction reactions [31]. Kim et al. demonstrated the effect of GCNT hybrid filler prepared by thermal chemical vapor deposition technique on the dielectric performance of cyanoethyl pullulan polymer and a dielectric constant of 32 with a dielectric loss of 0.051 at 100 Hz for 0.062 wt% loaded GCNT sample [32]. Vinayan et al. described the synthesis of a hybrid nanomaterial comprising of poly(diallyl dimethyl ammonium chloride) modified solar exfoliated graphene and negatively surface charged CNTs for being used as an anode for lithium ion batteries [33]. Kamalia et al. reported a novel process for large scale synthesis of 3D network of CNT pillared graphene nanostructures (GCNT) using chemical vapor deposition for bio-functional optical properties suggesting the potential application of GCNT in optoelectronics, biomedical and ultrafast optical sensing [34]. Ding et al. studied the electromagnetic wave absorbing properties(frequency range of 8.2e12.4 GHz) of a novel absorber based on poly(vinyl pyrrolidone) @GCNT prepared by ultrasonication infiltration method and achieved a maximum reflection loss of 26.5 dB at 11.29 GHz [35]. In the present study, we demonstrate the effect of hybridization of graphene nanoplatelets and CNTs on the enhancement of attenuation of electromagnetic radiations for polyurethane based composite materials. The mechanism involving the formation of GCNT hybrid has been proposed. Composites of GCNT (at varying loading) and thermoplastic polyurethane were prepared and explored for their potential as an effective and light weight EMI shielding material in the frequency range of 12.4e18 GHz (Ku band). The mechanism of shielding was systematically assessed by evaluating the contribution of reflection and absorption to the total shielding. To further investigate the reasons behind the observed increase in shielding effectiveness, the electromagnetic attributes (complex permittivity) were also evaluated. 119 2.1. Synthesis of acid functionalized MWCNT Acid functionalized MWCNTs were prepared by refluxing the asproduced MWCNTs in concentrated nitric acid for 48 h under constant stirring according to a previously described procedure [38]. The treated material was washed with DI water until pH 7 was attained and dried in a vacuum oven at 100  C for 12 h. These functionalized MWCNTs were designated as FCNT. 2.2. Synthesis of GCNT hybrids of GO and FCNT Graphene oxide (GO) was synthesized from natural graphite powder using an improved Hummers method. In a typical synthesis, 3 g (1 wt eq.) natural graphite powder was added to the reaction flask containing a homogeneous mixture of H2SO4 (360 ml) and o-H3PO4 (40 ml) under vigorous stirring, immersed in an ice bath. After achieving uniform dispersion of graphite powder, 18 g (6 wt eq.) KMnO4 was slowly added into the reaction flask producing a slight exotherm to 35e40  C. The slurry formed was then heated to 50  C and stirred for 20 h. The solution was diluted in ice cold DI water (400 ml) followed by addition of H2O2 (10 ml) at room temperature. The mixture was washed repeatedly by centrifugation till pH 7 was attained. To remove the metal ions, HCl was used during centrifugation. The final product was recovered with ethanol and dried at 60  C in a vacuum oven. As-synthesized GO (200 mg) was suspended in DI water (200 ml) to yield a yellowbrown homogeneous dispersion through ultrasonication for 4 h to achieve exfoliation of GO sheets. Using a similar method, a dispersion of FCNT (200 mg) in DI water was also prepared. Then the FCNT dispersion was added to the as-prepared GO sheet dispersion and the mixture was sonicated for 2 h. Subsequently, the mixture was reduced chemically using 10 ml of hydrazine hydrate at 95  C for 4 h to obtain a hybrid of graphene nanoplatelets and FCNT [39]. The sample was filtered and dried at 80  C in vacuum oven. The final sample was denoted as GCNT hybrid. 2.3. Fabrication of GCNT-TPU nanocomposites Nanocomposite films with different GCNT loadings i.e. 0, 0.5, 1, 2, 3, 5 and 10 wt% in TPU matrix were prepared by the solution blending method as shown in Fig. 2. The TPU nanocomposite samples were designated as PUGCNT x where ‘x’ stands for wt% of GCNT in TPU matrix. TPU granules were dissolved in DMF using magnetic stirring for 4 h and GCNT powder was uniformly dispersed in DMF in another beaker for 4 h by ultrasonication. GCNT dispersion in DMF was added to the polymer solution and thoroughly mixed using a magnetic stirrer for 6 h. The resultant solution was then poured in a petridish (diameter 13.5 cm) and placed in a vacuum oven at 80  C for 12 h. PUGCNT nanocomposite films were then peeled off from the petridish and samples of the desired thickness were prepared by compression molding at 150  C. 2.4. Characterization 2. Experimental section Multiwalled carbon nanotubes (MWCNT) were synthesized by using chemical vapor deposition method using toluene as a carbon source and ferrocene as a catalyst precursor according to the procedure described elsewhere [36]. Graphene oxide (GO) was prepared from natural graphite powder using an improved Hummer's method [37]. Deionized (DI) water was used throughout the experiments. Commercially available thermoplastic polyurethane (Desmopan, 385S) was procured from Bayer Material Science, India. All the other reagents were acquired from Fisher Scientific, India and used as received without further purification. Morphological properties of GO, FCNT, GCNT and PUGCNT nanocomposites were imaged by using a scanning electron microscope (SEM, Zeiss EVO-50) operated at 1 kV. Nanocomposite films were broken in liquid nitrogen to obtain cryogenically fractured surfaces and coated with gold in order to limit the charging effects under investigation. High resolution transmission electron microscopy (HRTEM), operating at an accelerating voltage of 300 kV and having a point resolution of 0.2 nm, was carried out using a Tecnai G2 F20, USA. Raman spectra were recorded using a Renishaw in Via reflex Raman spectrometer, UK, with an excitation source of 785 nm. The electrical conductivity of the composite films 120 M. Verma et al. / Composites Part B 120 (2017) 118e127 was measured by d.c. four probe contact method using a Keithley 224 programmable current source for providing current. The voltage drop was measured by Keithley 197A auto ranging digital microvolt meter. The values reported in text are averaged of at least five readings of voltage drop at different positions on the sample. The room temperature EMI shielding properties of PUGCNT nanocomposites were measured by recording the scattering parameters on a vector network analyzer (VNA E8263B Agilent Technologies) in the frequency range of12.4e18 GHz (Ku band) using a two port measurement technique. The measured scattering parameters were S11 (the forward reflection co-efficient), S21 (the forward transmission coefficient), S12 (the reverse transmission co-efficient) and S22 (the reverse reflection co-efficient). The unit for S parameters is decibel (dB). The VNA was calibrated prior to each measurement sequence to minimize the error. The compression molded rectangular pellets with dimensions 15.8  7.9 mm2of thickness 3 mm were inserted in a sample holder connected between the waveguide flanges of a network analyzer. 3. Results and discussion 3.1. Morphology and microstructure of GCNT hybrid The strategy for synthesis of GCNT hybrid is illustrated in Fig. 1. Graphene oxide was prepared from commercially available graphite powder using improved Hummer's method [37], followed by sonication for exfoliation of graphite oxide to graphene oxide. Due to the hydrophilic oxygen functionalities present on the basal plane and edges of GO, a brown colloidal suspension of GO sheets can be easily obtained under sonication in water. Similarly, acid treatment introduces oxygen-containing groups such as carboxyl groups, hydroxyl groups to the CNT tips and sidewalls, leading to a fine dispersion of FCNT in water. Once these two carbon allotropes were mixed in aqueous solution, the oxygen functionalities on the basal planes of GO served as surfactant to allow strong interactions with FCNT. Besides the p-p interaction between FCNTs surface and GO basal planes, hydrogen bonds between GO and FCNT also helped FCNT to strongly adsorb to the basal planes of GO sheets [39]. Then the chemical reduction of GO was carried out in the presence of FCNTs using hydrazine hydrate to obtain GCNT hybrid. In order to demonstrate the hybridization of GO sheets with FCNTs, SEM and TEM micrographs were taken as shown in Fig. 2. SEM micrograph of GO shows the flaky structure overlapped with each other like folds of tissue slice whereas CNTs are long, fibrous and stringy in nature. SEM micrograph of GCNT hybrid reveals the formation of hybrid nanostructure with tubular networks of CNTs adsorbed on the graphene nanoplatelets via non covalent p- p stacking interaction [40]. Both the laminated structure of GO and tubular structure of CNTs are retained in the hybrid. In hybrid nanostructure, it is evident that the FCNT get entangled with graphene nanoplatelets forming physical interactions with them. The TEM and HRTEM images further validated the thin wrinkled sheet structure of graphene nanoplatelets being covered with randomly arranged CNTs. The formation of GCNT was further characterized by X-ray diffraction and Raman spectroscopy. XRD patterns of GO, FCNTs and GCNT hybrid are shown in Fig. 3(a). XRD of GO shows a distinct peak at 2q ¼ 12.15 (001 plane) corresponding to a d-spacing (an interlayer distance between sheets) of approximately 0.727 nm due to the oxygen functionalities present between the hydrophilic graphene oxide sheets [37]. However, after reduction of GO, a much broader diffraction peak at 2q ¼ ~24e27 appears suggesting a reduced and exfoliated nanostructure which is significantly different from the pristine graphite (as reported previously in our work, [41]). The diffraction peak of FCNTs observed at 2q ¼ 26.1 (002 plane) is originated from the hexagonal graphitic structure of CNTs. The XRD curve of the GCNT hybrid may be considered as a simple combination of the XRD curves of both reduced GO and CNTs. It can be seen that the characteristic diffraction peak of GO disappeared as a result of the effective removal of oxygen containing groups (see Fig. 3). Fig. 3(b) shows the Raman spectra of GO, FCNT and GCNT hybrid. It can be seen that the Raman spectra of GO, FCNTs and GCNT hybrid displayed characteristic features which includes D-band (disorder due to sp3 carbon bonds), G-band (stretching in graphitic lattice) and 2G modes. The intensity ratio of D to G peaks obtained (ID/IG) was used to provide information of the structural defects present in the hybrid. The ID/IG ratios of GO, CNT and GCNT hybrid are 0.93, 1.06 and 1.40 respectively. The increase in ID/IG ratio of GCNT hybrid could be attributed to the increase in the number of domains of aromatic structures responsible for the D band. The Fig. 1. Schematic representation of the synthesis procedure adopted for synthesizing graphene nanoplatelets-CNT hybrid. M. Verma et al. / Composites Part B 120 (2017) 118e127 121 Fig. 2. SEM, TEM and HRTEM micrographs of (a and d) GO, (b and e) FCNT and (c and f) GCNT. Fig. 3. XRD diffraction patterns (a) and Raman spectra (b) of GO, CNT and GCNT. broadening of the 2D band along with a decrease in intensity revealed the creation of structural defects and lattice distortions on the GCNT hybrid. 3.2. Characterization of PUGCNT composites 3.2.1. Morphological analysis The extent of dispersion and interfacial interactions between the hybrid filler and the polyurethane matrix were investigated using scanning electron microscopy of the cryo-fractured surfaces of the PUGCNT nanocomposites. As can be clearly seen pure polyurethane shows a continuous morphology with a clean and smooth rupture surface. Analysis of the fractured surfaces of PUGCNT nanocomposites revealed the good dispersion state of GCNT hybrid inside the polyurethane matrix i.e. a homogeneous dispersion can be seen in the TPU matrix. The PUGCNT nanocomposites displayed a much rougher surface and the pull out mechanism of the CNTs is observed. At high loading of GCNT, layered structures of graphene nanoplatelets can be clearly seen (Fig. 4). 3.2.2. X-ray diffraction analysis Fig. 5 shows the XRD patterns of the PUGCNT nanocomposites with varying loading of GCNT. Pure polyurethane exhibits a strong broad diffraction peak at about 2q ¼ 20 of (110) reflection plane 122 M. Verma et al. / Composites Part B 120 (2017) 118e127 Fig. 4. SEM micrographs of the fractured surfaces of (a) PUGCNT 0, (b) PUGCNT 0.5, (c) PUGCNT 1, (d) PUGCNT 3, (e) PUGCNT 5, (f) PUGCNT 10. CNTs are marked by yellow circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 5. XRD diffraction patterns of PUGCNT composites. with an interchain spacing of 0.442 nm. The peak broadens and a shift is observed with the addition of GCNT, which may be ascribed to the fact that GCNT significantly affects the micro-structural phases of the polyurethane matrix. A small peak around 26.5 (002 reflection plane) which corresponds to the characteristic peak of GCNT starts appearing at 1 wt% GCNT loading in PUGCNT composites. The intensity of this peak increases as the GCNT loading is increased. The increased intensity recorded at higher GCNT loading could be attributed to the presence of higher number of GCNT nanostructures in the nanocomposites. 3.2.3. Raman spectroscopic studies Raman spectroscopy has become a key technique for the characterization of carbon based materials and their composites. It has also been used to probe the interactions between the polymer chains and the fillers in carbon nanomaterials based composites. Fig. 6 displayed the Raman spectrum of pure polyurethane (sample PUGCNT 0). The fundamental Raman scattering peaks of TPU are observed at 2922, 1724, 1614, 1538, 1436, 1307, 1250 and 1182 cm 1 which can be assigned to the eCH2 asymmetric stretching vibrations, C¼O stretching of ester and urethane amide I, stretching vibrations of aromatic ring, C-N stretching of urethane amide II, N¼C¼O symmetric stretching, N-H stretching of urethane amide III respectively. After the addition of GCNT, the characteristic polyurethane bands become less obvious and finally disappear at higher loading of GCNT which shows that GCNT dominates over polyurethane. The intensity ratio of D and G bands (ID/IG) which is used as indicator of the quality of the carbon based system was also investigated. The ID/IG increases with increasing GCNT loading upto 1 wt.%, which suggests the creation of more defect sites in the hybrid nanocomposites. Further, at loadings 2e10 wt.%, the ID/IG was found to decrease suggesting the existence of p-p interactions between the aromatic rings of TPU and the sp2 domains of the GCNT. The decrease in ID/IG ratio can also be explained by the fact that although some sp2 carbon atoms in the GCNTs were transformed to sp3 carbon atoms, the amount of sp3 carbon atoms was still less than that of sp2 carbon atoms in the hybrid nanocomposites. Therefore, while comparing the ratio of ID/IG for GCNT and PUGCNT nanocomposites, it is observed that the nanocomposites have the lower ratio due to p-p interactions of the polymer matrix with the high surface area of graphene nanoplatelets as well as with the passivation of dangling bonds in MWCNT. This behavior was found to be similar to graphene/carbon nanotubes/polystyrene nanocomposites reported by Patole et al. [23]. 3.2.4. Electrical conductivity Fig. 7 depicts the electrical conductivity of PUGCNT composites measured by four probe contact method which is plotted as a function of GCNT content. The conductivity of neat polyurethane (3.9  10 11 S/cm) is consistent with the value reported in the literature [42]. It was observed from Fig. 7 that the electrical conductivity increased exponentially at low GCNT contents, followed by a slow increase at higher loadings. The electrical conductivity jumped by almost 5 orders of magnitude when 0.5 wt% GCNT was added which may be accredited to the uniform dispersion of GCNT nanostructures within the matrix. A further increase in GCNT loading beyond 2 wt% resulted in a rather saturated conductivity. The maximum electrical conductivity obtained for the PUGCNT 10 M. Verma et al. / Composites Part B 120 (2017) 118e127 123 Fig. 6. Raman spectra of pure polyurethane (a) and PUGCNT composites (b). The reflection is related to the impedance mismatch between air and the absorber; the absorption results from the energy dissipation of the EM radiations and the multiple reflections are induced by the scattering effect of the inhomogeneity in the material [47]. The multiple reflection term (SEM) can be ignored in cases where the contribution of absorption to EMI SE is more than 10 dB or if the shield is thicker than the skin depth [48]. In general, the SE of a shielding material is commonly expressed in decibels (dB). A higher SE value indicates less energy transmitted through the shielding material. Thus, SE can be expressed as, SE ðdBÞ ¼ SEA ðdBÞ þ SER ðdBÞ Fig. 7. Effect of GCNT content on the electrical conductivity of TPU. composite is in the order of 9.5  10 2 S/cm. The integration of FCNTs provides the electronic conductivity thereby allowing the electronic contact of graphene nanoplatelets by bridging the gaps between graphene sheets. The network so formed by the GCNT nanostructures within the matrix renders new conductive platform and increases the number of charge transport path. Nevertheless, an electrical conductivity of order of ~10 2 S/cm is considered to be sufficient for many practical applications such as conductive thin films and coatings, sensors, electrostatic dissipation and EMI shielding [41,43,44]. 3.2.5. EMI shielding performance of PUGCNT nanocomposites The total electromagnetic interference shielding effectiveness (SET) is the ability of a material to attenuate incident electromagnetic radiation and is defined by the following equation [45,46]. SE ¼ 10 log ðPt =Pi Þ ¼ 20 log ðEt =Ei Þ ¼ 20 log ðHt =Hi Þ (1) where the symbols P, E and H stand for power, electric and magnetic field intensity respectively. The subscripts‘t’ and ‘i'are used for the transmitted and incident wave on the shield, respectively. EMI shielding consists of three different mechanisms, namely reflection, absorption and multiple-reflection. The EMI shielding effectiveness (SET) of a shielding material is the sum of the shielding effectiveness due to reflection (SER), absorption (SEA) and multiple reflections (SEM). SET ¼ SER þ SEA þ SEM (2) (3) Fig. 8(a) shows the frequency dependence of total EMI shielding effectiveness value for PUGCNT nanocomposites with varying loadings of GCNT hybrid filler over the frequency range of 12.4e18 GHz. The results revealed that the pure polyurethane matrix is almost transparent towards incident electromagnetic radiation displaying a poor EMI shielding response (i.e. SET ¼ 3 dB). However, incorporation of GCNT causes a systematic improvement in the shielding effectiveness for PUGCNT nanocomposites and an EMI SE of 47 dB (corresponding to a blocking of more than 99.99% of the incident EM radiations) was obtained for PUGCNT 10 nanocomposite. This improvement can be ascribed to the formation of a conducting network of FCNTs bridging the gaps between graphene nanoplatelets throughout the electrically insulating polyurethane matrix. The EMI shielding value obtained in this study is considerably high which is mainly accredited to the synergism originated as a result of hybridization of graphene nanoplatelets with FCNT. Table 1 provides a comparison of EMI shielding performance of recently published results on CNT or graphene based polyurethane composites. It is clearly evident from the table that the SET value of 47 dB of PUGCNT composites in the present work is the highest among the reported values of SET for polyurethane based composites. The EMI shielding effectiveness for PURGOCNT (prepared by 1:1 physical mixture of RGO and CNT and loading level is 10 wt%) was lower i.e. 32 dB as compared to 47 dB for PUGCNT 10 which can be attributed to the synergistic effects originated as a result of hybridization of graphene nanoplatelets with FCNT. We postulate that this high value of SET is the result of the synergistic effect originated from the formation of an extended conjugation network with the FCNTs bridging the gaps between the graphene nanoplatelets and inhibiting the face to face aggregation of graphene nanoplatelets. Moreover, the minimizing of the stacking effect and aggregation of graphene nanoplatelets increases the polymer contact area and interfacial interactions leading to a synergistic improvement in nanocomposite properties. 124 M. Verma et al. / Composites Part B 120 (2017) 118e127 Fig. 8. (a) Variation in EMI shielding effectiveness with frequency for PUGCNT nanocomposites, (b) Variation in SET, SEA and SER with GCNT loading at 15.2 GHz, (c) variation in absorption efficiency, frequency dependence of (d) dielectric constant and (e) dielectric loss and dielectric tangent loss for PUGCNT nanocomposites. Table 1 Electromagnetic attenuation analysis of graphene or carbon nanotubes based polyurethane nanocomposites. Filler Filler Concentration (wt.%) Frequency (GHz) Shielding Effectiveness (dB) Reference SWCNT SWCNT MWCNT MWCNT MWCNT MWCNT Acid functionalized MWCNT Polydopamine coated graphene Hydrogen iodide reduced graphene Covalently modified graphene Non covalently modified graphene Reduced graphene oxide Graphene nanoplatelets/CNT hybrid RGO þ CNT 20 5 10 10 9 22 10 4.75 vol.% 8.2e12.4 2e18 8.2e12.4 8.2e12.4 13e16 8.2e12.4 8.2e12.4 1.2 8.2e12.4 8.2e12.4 8.2e12.4 8.2e12.4 12.4e18 12.4e18 17 22 41.6 21.8 35 20 29 17.6 34 38 32 21 47 32 Liu et al. [49] Liu et al. [50] Gupta et al. [51] Ramoa et al. [28] Jin et al. [52] Hoang et al. [53] Gupta et al. [38] Yang et al. [54] Hsiao et al. [55] Hsiao et al. [56] Hsiao et al. [57] Verma et al. [41] Present work Present work 7.7 5.5 vol% 10 10 Although total EMI shielding effectiveness is an important parameter used to quantify the efficiency of a shield, it does not provide the information on contributions of each of the shielding mechanisms i.e. reflection and absorption. Therefore, the mechanism of EMI shielding was analyzed by plotting SET, SEA and SER of PUGCNT nanocomposites as a function of the GCNT loading. Fig. 8(b) shows that both SEA and SER increase with increase in GCNT concentration; however SEA increases more rapidly compared to the corresponding SER component. With increase in GCNT concentration, more mobile charge carriers are available which can form more conductive filler networks that can attenuate the penetrating electromagnetic wave [58]. This behavior suggests that the absorption is the main mechanism for PUGCNT nanocomposites in EMI shielding and a similar growth trend in SE was also seen. An incident electromagnetic wave penetrating through the shield decays inside it by conductive dissipation. The high electrical conductivity and the formation of conductive networks within the matrix play an important role in the EMI shielding performance of the composites. The residual defects present in GCNT are helpful in impedance matching and defect polarization, which serve to improve the electromagnetic wave absorption. Also, the large surface and interface areas of GCNT in the composite increase the probability of more interaction with the incident EM wave. Therefore, it is difficult for the incident EM wave to escape from the PUGCNT composite. These EM waves are absorbed and transformed into thermal energy by the continuous GCNT networks. The developed PUGCNT 10 composite exhibited an absorption M. Verma et al. / Composites Part B 120 (2017) 118e127 125 Fig. 9. Schematic representation of the proposed EMI shielding mechanism in PUGCNT nanocomposites. efficiency of more than 99.9% (Fig. 8(c)), which means that most of the EM energy incident on the shield attenuates and dissipates in the form of heat energy. To expatiate more on the shielding performance of the PUGCNT composites, electromagnetic parameters, such as the complex permittivity of composites, were evaluated using experimental scattering parameters (S11 and S21) applying the theoretical calculations given by Nicholson and Ross and Weir algorithms [59,60]. The real part of the complex permittivity symbolizes the intensity of polarization or the electrical energy storage ability of a material and is also known as the dielectric constant [61]. The imaginary part of the complex permittivity represents the energy loss during activation by an electromagnetic wave and is known as the dielectric loss. The dielectric performance of the material depends on ionic, electronic, orientational (arising due to the presence of bound charges) and space charge polarization (due to the heterogeneity in the system). In a heterogeneous system, the accumulation of virtual charges at the interface of two media having different dielectric constants and conductivities leads to interfacial polarization [61]. Here, GCNT hybrid was incorporated in the polyurethane matrix and thus different conductivities and dielectric constants show interfacial polarization in the applied frequency range. Fig. 8(def) shows the real part of permittivity (ε0 ), imaginary part of permittivity (ε00 ) and tangent loss (tan d ¼ ε00 /ε0 ) as a function of frequency for the PUGCNT nanocomposites at a thickness of 3 mm. The results show that the real and imaginary part of permittivity of the nanocomposite with 10 wt % GCNT hybrid filler lies in the range of 35.1e41.4 and 14e21.1 respectively, while that the pure polyurethane had a dielectric constant and dielectric loss of 3.2 and 0.5 respectively. It is proposed that the observed increase in ε00 and ε0 with increasing GCNT loading level may be attributed to the increase in the electrical conductivity and space charge polarization during activation by the electromagnetic wave. The ratio of imaginary to real part is ‘dissipation factor’, which is represented by tan d, where d is called the “loss angle”, denoting the angle between the voltage and the charging current. Fig. 8(f) shows the plots of tan d vs. frequency for the PUGCNT nanocomposites as a function of GCNT loading. Since the tan d value indicates the absorptive property of a material, i.e., the ability of a material to convert applied energy into heat, materials with a high tan d value are used as microwave absorbing materials and in stealth technology [41]. The tan d values of PUGCNT composites increased with increasing GCNT content and a tan d value of 0.35e0.59 was observed for the PUGCNT 10 composite. It is proposed that the humps observed in the dielectric properties (at a higher loading of GCNT) are suggestive of two main phenomena responsible for dielectric losses. These are due to the interfacial polarization and electron hoping between FCNTs and graphene nanoplatelets and between the GCNT sheets and the polyurethane matrix and high anisotropy energy of the nanocomposites [62e65]. These phenomena appear in heterogeneous systems due to the accumulation of charge at interface and the formation of large dipoles [66]. The proposed interaction mechanism of electromagnetic waves with GCNT hybrid is shown in Fig. 9 through a schematic representation. From all the above mentioned discussion, it is concluded that the polyurethane filled GCNT composites are capable aspirants as versatile and lightweight shielding materials against electromagnetic polluting radiations. 4. Conclusion In summary, one dimensional fibrous FCNTs and two dimensional planar graphene nanoplatelets were combined to achieve a three dimensional graphene nanoplatelets/FCNT hybrid nanostructure that exhibited a synergistic effect in the enhancement of EMI shielding response of polyurethane nanocomposites. The novel PUGCNT nanocomposites were prepared using solution blending technique and their EMI shielding response was investigated in the frequency range of 12.4e18 GHz. The synergism in the GCNT hybrid nanostructure originates from the FCNTs bridging the gaps between the nanoplatelets which leads to the improved electrical conductivity and enhanced EMI shielding properties of PUGCNT nanocomposites. The maximum electrical conductivity has been achieved up to 9.5  10 2 S/cm for 10 wt% PUGCNT composite, which is 9 orders of magnitude higher than pure insulating polyurethane. This significant improvement in electrical conductivity is thus responsible to achieve up to 47 dB EMI shielding effectiveness in Ku-band frequency range (12.4e18 GHz). It can be inferred from the results that the developed nanocomposites open up a new avenue to design promising futuristic lightweight EMI shielding materials. Acknowledgement The authors thank the Ministry of Human Resource Development (MHRD), India for providing financial assistance to one of the authors Mrs. Meenakshi Verma and IIT Delhi for providing all the facilities. 126 M. 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