Polyvinylidene Fluoride

Article pubs.acs.org/JPCC

Carbon Nanotube/Graphene Nanoribbon/Polyvinylidene Fluoride Hybrid Nanocomposites: Rheological and Dielectric Properties Mohammad Arjmand,† Soheil Sadeghi,† Maryam Khajehpour, and Uttandaraman Sundararaj*

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Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4 ABSTRACT: Results of the present study demonstrate the potential of graphene nanoribbon to induce giant synergistic effects in the broadband dielectric properties of multiwalled carbon nanotube/ graphene nanoribbon/polyvinylidene fluoride (MWCNT/GNR/ PVDF) nanocomposites. The nanocomposites were prepared using a melt-mixing technique at various nanofiller total contents and MWCNT/GNR weight ratios. Rheology coupled with AC conductivity measurements of the nanocomposites unearthed highly superior capability of MWCNT to neighbor or interlace compared to GNR; i.e., the MWCNT has higher ability to participate in a percolative network. Broadband dielectric spectroscopy demonstrated superior dielectric properties for MWCNT/GNR/PVDF ternary (hybrid) nanocomposites compared to the MWCNT or GNR binary nanocomposites. For instance, at 1.5 wt % and 1000 Hz, the ternary nanocomposite with an MWCNT/GNR ratio of 3:1 presented a real permittivity and dissipation factor of 41.4 and 0.91, surpassing the binary MWCNT nanocomposite with a real permittivity and dissipation factor of 39.3 and 86.7, respectively. We attribute this synergistic effect to the poor interlacing ability of GNRs, as secondary conductive nanofillers, acting as extra nanoelectrodes. In fact, the role of GNRs as extra nanoelectrodes in conjunction with their poor propensity to bridge MWCNTs led to effective nanocapacitor structures with low energy loss.

1. INTRODUCTION As anticipated by Moore’s law, the performance of electronic devices is increasing exponentially,1,2 and passive components constitute a major part of printed circuit boards (PCBs) in electronic devices. Passive components denote the type of electrical constituents that cannot generate power, including resistors, capacitors, and inductors. They possess a multibillion-dollar business, providing electronic products in automotive, telecommunication, computer, and aerospace industries, for both digital and analog-digital applications.3 The market shares of capacitors, resistors, and inductors in the U.S. are 76, 18, and 4%, respectively, and the rest belongs to circuit protection components.4 Hence, among all passive components, capacitors call for special attention. The current packaging technology is moving toward the passive components embedded into the substrate as a thin film layer rather than being surface mounted.5−7 Embedded passives provide superiorities to discrete components, such as decreased number of solder joints, improved design options, shorter leads, and lower inductance, all of which bring about enhanced electrical functionality. Embedment of polymer-based capacitors, as chief passive components, within the internal structure of PCBs leads to compatibility with polymers used as support of electronic circuits. Nonetheless, compared with inorganic materials, organic polymeric materials often present low real permittivity in the range of 2−5,8 hindering their introduction for advanced applications. Therefore, the key issue is to substantially raise the © 2016 American Chemical Society

real permittivity of polymers while retaining their excellent mechanical properties. In the past decade, high-real-permittivity (high-k) conductive filler/polymer nanocomposites (CPNs) have enticed wide attentions owing to giant real permittivities close to or above the percolation threshold.9−11 A nanocapacitor model is employed to elucidate high permittivity of CPNs. According to this model, a CPN can be envisioned as a network of nanocapacitors randomly distributed in a dielectric host, i.e., adjacent individual nanofillers as conductors with an insulating thin polymer layer in between as a nanodielectric.12 Electrical properties of CPNs with filler compositions near the percolation threshold follow a scaling law. According to the percolation theory, electrical properties of percolative composites can be described by the following laws13 σ ∝ (f filler − fc )t ε ∝ (fc − f filler )−q

for fc < f filler for fc > f filler

(1) (2)

where σ is the electrical conductivity, ε is the real permittivity, fc is the critical volume fraction at the percolation threshold, f f iller is the filler volume fraction, and t and q are the scaling constants. Received: October 25, 2016 Revised: December 8, 2016 Published: December 8, 2016 169

DOI: 10.1021/acs.jpcc.6b10741 J. Phys. Chem. C 2017, 121, 169−181

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The Journal of Physical Chemistry C

ratios and contents, and then compression molded. The morphology and crystallinity of MWCNT and GNR were compared using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and Raman spectroscopy. The morphological, rheological, and dielectric properties of the generated nanocomposites were also well-characterized. The results showed that the hybrid system of MWCNT/GNR/ PVDF showed hugely superior dielectric properties compared to the binary systems at some filler ratios, depending on nanofiller total content. The synergistic effect in the hybrid nanocomposite derives from the inherent nature of GNR, acting as extra nanoelectrodes with low tendency to bridge MWCNTs, and also reduced nanodielectrics thickness. Morphological discrepancies of the nanofillers and distinct morphological and rheological properties of the nanocomposites were used to describe the observed dielectric properties.

However, high-k CPNs always exhibit very high dielectric loss, due to the insulator−conductor transition occurring at the percolation threshold. This could bring huge energy wastage and low reliability in service.14 Therefore, the methodology to decrease the dielectric loss of traditional high-k CPNs is a premise for practical applications. Accordingly, several methods have been devised to cut off the direct contact between adjoining conductive fillers to reduce the dielectric loss. These techniques include, but not limited to, coating and surface modification of fillers,15 filler alignment by electrospinning16 and injection molding,17 incorporation of a secondary insulative filler,18−23 using an interparticle barrier layer,24 modulation of topological structure,25 alignment of fillers by cell growth during polymer foaming,26−28 and surface oxidation of metallic nanowire.29 Moreover, other techniques such as nitrogen doping of nanofillers,30,31 hybrid fillers,32 and multilayered dielectrics10,33,34 have been developed to increase the real permittivity in the insulative region. Despite some success, researchers are still actively exploring more effective and viable methods. Different types of conductive fillers have been employed to develop dielectric CPNs, among which carbonaceous fillers, such as carbon black, multiwalled carbon nanotube (MWCNT), and graphene have been scrutinized intensely.10−20 However, to the best of our knowledge, a rare number of papers have demarcated the dielectric properties of CPNs with graphene nanoribbon (GNR).35,36 GNRs, thin elongated strips of sp2 bonded carbon atoms, can be fabricated by unzipping CNTs.37−39 The outstanding electronic and spin transport properties of GNRs in conjunction with their huge surface area make them attractive materials in a broad spectrum of device applications.39−43 GNRs have large surface area, nominating them as outstanding materials for improving the mechanical properties of polymers due to easy adherence of polymer layer onto the flat geometry of GNRs.44,45 In terms of electrical properties, employing tight-binding band calculations, Nakada et al.46 indicated that three or four zigzag sites per sequence are adequate to show an edge state in GNRs. Fujita et al.47 and Son et al.48 claimed that normalized density of states decline drastically with the increase in GNR width. This signifies the superior electronic structure of GNRs compared to graphene and puts them forward as appealing nanofillers for electrical applications. As yet, a few studies have been devoted to the electrical properties of GNR nanocomposites.35,36,49,50 Performing electrical and rheological characterizations, detailed later in this study, we found out that GNR has much inferior interlacing ability compared to MWCNT (GNR has inferior ability to participate in percolative network). We also know that the insulator−conductor transition is a major challenge in coping with CPNs for charge storage applications. These bring up the idea of employing a ternary (hybrid) system of MWCNT/GNR/polymer nanocomposites as efficient dielectric materials. Inferior interlacing ability of GNR can be employed as a positive point to reduce the dielectric loss in the hybrid system. On the other hand, GNRs can play the role of additional nanoelectrodes, enhancing the real permittivity of the hybrid system. In this regard, GNR, as secondary conductive filler, could outshine the secondary insulative fillers, which solely play the role of insulative barriers. In order to verify the aforementioned statement, MWCNT and GNR were melt mixed with a polyvinylidene fluoride (PVDF) matrix using a miniature melt mixer at various fillers

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. A semicrystalline PVDF polymer (11008/0001) was obtained in pellet form, from 3M Canada, with an average density of 1.78 g/cm3 and melting point of 160 °C. GNR was supplied by AZ Electronic Materials, Branchburg, USA. GNR was synthesized according to the Na/ K alloy intercalation method from the parent MWCNT described elsewhere.51,52 The chemical vapor deposition (CVD) technique was used to synthesize MWCNT employed in this study. This MWCNT was different than the parent MWCNT used for the synthesis of supplied GNR. We employed our synthesized MWCNT due to its superior compressed powder conductivity compared to the parent MWCNT (900 vs 10 S·m−1). The synthesis catalyst was prepared by the incipient wetness impregnation of catalyst precursor, iron(III) nitrate nanohydrate, Baker Analyzed ACS grade, dissolved in water, on an aluminum oxide support (Sasol Catalox Sba-200), followed by drying, calcination, and reduction. The catalyst precursor was dried at ambient temperature for 24 h and at 100 °C for 2 h. The calcination was then carried out at 350 °C under an air atmosphere with a flow rate of 100 sccm for 4 h. The precursor was finally reduced by hydrogen gas at a flow rate of 100 sccm at 400 °C for 1 h to attain the alumina-supported metal catalyst. Source gases were conveyed over the alumina-supported metal catalyst in a quartz tubular reactor with an inner diameter of 4.5 cm. MWCNT was synthesized using a mixture of ethane, hydrogen, and argon. The synthesis temperature, synthesis time, catalyst mass, and gas flow rate were kept at 650 °C, 2 h, 0.6 g, and 150 sccm, respectively. Further information regarding the MWCNT synthesis procedure is detailed elsewhere.53,54 MWCNT/GNR/PVDF nanocomposites were prepared via a melt-mixing method using an Alberta Polymer Asymmetric Minimixer (APAM) at 240 °C and 235 rpm. Further information regarding the APAM mixer can be found elsewhere.55 First, the PVDF matrix was masticated for 3 min, and then the nanofillers were inserted into the chamber and mixed for an additional 14 min. For the hybrid nanocomposites, the nanofillers were dry mixed in a vial before insertion into the chamber. The nanocomposites were prepared at different concentrations of 0.0, 0.5, 1.0, 1.5, 2.0, and 2.7 wt % (MWCNT and GNR in total) with various MWCNT-to-GNR weight ratios of 1:0, 3:1, 1:1, 1:3, 0:1. Then, the nanocomposites were molded with a Carver compression molder (Carver Inc., Wabash, IN) at 220 °C under 38 MPa pressure for 10 min to make disk samples with a 2.5 cm diameter and 0.5 170


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Figure 1. HRTEM images of (a) CVD-synthesized MWCNT and (b) as received GNR. (c) and (d) show schematics of fully and partially longitudinally opened MWCNT, respectively.

cuts (5 μm thickness) of the compression-molded samples, prepared with a Leica EM UC6 (Leica Biosystems, Germany) ultramicrotome at room temperature. A glass knife with a cut angle of 35° was employed for the sample preparation. An Olympus BX60 optical microscope (Olympus Corporation, Japan) equipped with an Olympus DP80 camera was used to capture images with dimensions of 600 × 800 μm2 from different cut sections. The software Stream Motion (Olympus) was used to analyze the images. The agglomerate area ratio (in %) was defined by dividing the spotted area of nondispersed nanofillers (with equivalent circle diameter > 5 μm, area > 19.6 μm2) over the whole sample area (15 cuts, ca. 7.2 mm2). Mean value and standard deviation, demonstrating the differences between the cuts, and thus heterogeneity, were reckoned. The relative transparency of the cuts provided added information about the amount of dispersed nanofillers in the samples. The relative transparency was quantified by dividing the transparency of the cut over the transparency of the glass slide/cover glass assembly. Ten various areas per sample were used to obtain mean values and standard deviations. Further information on employing LM to evaluate the microdispersion state of nanofillers within nanocomposites is presented elsewhere.53,54 2.2.4. Transmission Electron Microscopy of Nanocomposites. Ultrathin sections of the samples were cut using an ultramicrotome EM UC6/FC6 setup with an ultrasonic diamond knife at room temperature. Thickness of the sections was 60 nm, and the speed of sectioning was 0.4 mm/s. The sections were floated off water, and thereafter transferred on carbon-filmed TEM copper grids. TEM measurements were carried out on a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA), at 200 kV acceleration voltage, with a standard single-tilt holder. 2.2.5. Rheology. Rheological measurements were performed using an Anton-Paar MCR 302 rheometer at 240 °C using 25 mm cone-plate geometry with a cone angle of 1° and a

mm thickness for various characterizations. Moreover, the dried powders of the nanofillers were compressed into a rectangular cavity at the same pressure and time, and then their conductivity was measured to obtain a rough estimation about the inherent electrical conductivity of the nanofillers. 2.2. Materials Characterization. 2.2.1. Transmission Electron Microscopy of Nanofillers. The overall microstructural features of MWCNT and GNR were studied with high-resolution transmission electron microscopy (HRTEM). The HRTEM was carried out on a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA), at 200 kV acceleration voltage, with a standard single-tilt holder. The images were captured with a Gatan UltraScan 4000 CCD camera (Gatan, Pleasanton, California, USA) at 2048 × 2048 pixels. Typically, less than 1.0 mg of the nanofiller powder was suspended in 10 mL of ethanol and bath sonicated for 15 min. A drop of the suspension was placed on the carbon side of a standard TEM grid covered with an ∼40 nm holey carbon film (EMS, Hatfield, Pennsylvania, USA) and placed on a filter paper to quickly dry. Measurement of the geometrical dimensions of the nanofillers was conducted for more than 100 individual ones using the MeasureIT software (Olympus Soft Imaging Solutions GmbH). 2.2.2. Raman Spectroscopy and TGA. The structural defects of the nanofillers were investigated using Raman spectroscopy. The Raman spectra were recorded with a WITec alpha 300 R Confocal Raman Microscope (WITec GmbH, Germany) with a laser radiation of 532 nm, integration time of 50 s, 10× objective, and a laser power of 24 mW. The thermal stability of the nanofillers was evaluated using a Thermogravimetric Analyzer (TA Instruments − TGA Q500). The samples were heated under an air atmosphere (Praxair AI INDK) from room temperature to 950 °C at a ramp rate of 10 °C/min, and their mass loss was recorded. 2.2.3. Light Microscopy of Nanocomposites. The microdispersion state of the nanofillers within the PVDF matrix was enumerated using light transmission microscopy (LM) on thin 171


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Figure 2. (a) TGA, (b) Raman spectra, and (c) powder conductivity of MWCNT and GNR.

Waals interactions would hamper flattening and exfoliation of GNR layers.52 The schemes in Figure 1c,d show a fully and partially longitudinally opened MWCNT. The longitudinal opening of MWCNT increases the edge-to-basal plane carbon ratio, leading to an increased normalized density of states and superior performance in energy applications.46,47 As depicted in Figure 2, discrepancies in the thermal stability, structural defects, crystallinity, and inherent conductivities of MWCNT and GNR were studied using TGA, Raman spectroscopy, and powder conductivity measurement, respectively. As it can be identified in the TGA results (Figure 2a), the inflection temperatures for MWCNT and GNR were 636 and 506 °C, respectively. The huge difference in the onset degradation and inflection temperatures between MWCNT and GNR (∼130 °C) is attributable to the presence of multifaceted structures, and abundant edges in the GNR structures, which acted as defects, facilitating the reaction with oxygen at high temperatures, and thereby leading to inferior thermal stability of GNR. It was also observed that the remaining residues after the TGA tests for MWCNT and GNR were 14.3% and 1.3%, respectively. These residues were essentially catalyst particles and their substrate.57,58 A catalyst substrate is insulative, and catalyst particles have much lower surface area than synthesized nanofillers. Their surface area reduced further due to a sintering phenomenon at the synthesis temperatures.59,60 Thus, nanofillers with higher purity are more desirable for electrical applications. Figure 2b compares the Raman spectra of MWCNT and GNR. In the Raman spectra of carbonaceous materials, the Gband (the tangential mode), D-band (the defects-related mode), and G′-band are three substantial characteristic peaks, offering invaluable information with respect to the structural defects and electronic properties of nanofillers.61,62 The G-band (∼1600 cm−1) corresponds to the stretching of the C−C bond in graphitic materials, and its intensity is commonly employed to normalize the intensity of the other bands for comparison purposes. The D-band (∼1400 cm−1) derives from lattice

truncation of 47 μm. The thermal stability of the prepared samples was verified by performing small-amplitude oscillatory shear measurements before and after the long-time exposure of the samples to elevated temperatures. 2.2.6. Broadband Dielectric Properties. The broadband dielectric properties of the nanocomposites were measured for three replicates with a Bio-Logic Impedance Analyzer (SP-200 EIS) in the frequency range of 102 to 107 Hz. The impedance analyzer was connected to a Solartron 12962 sample holder with an electrode diameter of 10 mm. The amplitude of the applied voltage was 100 mV (Vrms ∼ 70 mV). Prior to the measurements, the electrodes were painted on the samples using silver paste.

3. RESULTS & DISCUSSION 3.1. Nanofiller Characterization. Figure 1 shows the HRTEM images of the MWCNT synthesized with the CVD technique and as-received GNR with an unzipped structure. The average length and diameter of MWCNT were 1.7 μm and 15 nm, respectively. GNR had an average length of 2.7 μm, and an average width of 110 nm. The standard deviation of the average nanofiller dimensions was about 10% of the average values. On the basis of the HRTEM micrograph shown in Figure 1a, the MWCNT had an open-channel structure, fairly consistent inner and outer diameters, and the same number of walls all through the channel with a methodical crystalline structure. The unzipped structure of GNRs (Figure 1b) had many more edges along the length of the nanoribbon compared to MWCNT. In addition to the GNR basal plane, these edges could provide additional available surface area for interaction with polymer chains in nanocomposites.56 In order to have a good interaction between the polymer matrix and GNR, ideally, GNRs must be dispersed fully longitudinally opened with an exfoliated morphology. However, the resultant 3D GNR structures were observed to be generally stacks of the fewlayer GNRs (the inset in Figure 1b). It is worth noting that the robust C−C bonds at the curled areas and interlayer van der 172


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Figure 3. LM micrographs, agglomerate area, and relative transparency of binary and hybrid nanocomposites with 2.7 wt % nanofiller content.

fillers,69−71 which are out of the focus of the present study. It is of significance to mention that the inherent conductivity of nanofillers is a key factor in determining the final dielectric properties of nanocomposites. 3.2. Nanocomposite Morphological Characterization. Besides the inherent conductivity of nanofillers, the quality of their dispersion state within the polymer matrix also influences the network formed, and thus the rheological and electrical/ dielectric properties of CPNs.53,54,72−74 This necessitates the significance of having a clear, multiscale image of the nanofillers dispersion state in the nanocomposites. In this context, by using a combination of LM and TEM imaging, it is possible to provide an image of the dispersion state in three different length scales. To study the microdispersion state of the nanofillers, LM can be used to quantify what portion of the nanofillers formed visually observable agglomerates (>5 μm in diameter), which was denoted as agglomerate area ratio in Figure 3. Furthermore, the relative light transmittance of the thin sections can be computed to obtain information about the amounts of the agglomerates with sizes equal to or slightly larger than the wavelength of visible light, ca. 400−700 nm, but smaller than visually recognizable agglomerates. In fact, lower transmittance of the sample indicates higher population of small-sized agglomerates scattering incident visible light. Finally, TEM images provide information on the quality of the nanodispersion state, wherein individually dispersed nanofillers can be perceived. Further information can be found in our preceding studies.53,54 Figure 3 depicts the LM micrographs of the binary and hybrid nanocomposites with a total concentration of 2.7 wt % of MWCNT and GNR. As depicted and quantified in Figure 3, the number of large agglomerates in the GNR binary nanocomposites is greater than the MWCNT binary nano-

distortion, and hence the incidence of structural defects, such as pentagon−heptagon pairs, vacancies, heteroatoms, and impurities, touches its behavior.63,64 Therefore, the intensity ratio of D- and G-bands (ID/IG) is an indication of the structural defects in carbonaceous materials. The G′-band (∼2700 cm−1) originates from the electronic properties of graphitic materials, and thus could be in association with the structural defects.65,66 The MWCNT powder featured three obvious peaks at 1361, 1586, and 2712 cm−1. The D-band, G-band, and G′-band were slightly blue-shifted in the GNR powder compared to MWCNT. The D-band to G-band signal intensity ratios, ID/ IG, were 0.66 and 0.77 for MWCNT and GNR, respectively. Higher ID/IG observed for GNR denoted inferior crystallinity and abundance of edge atoms compared to MWCNT, and is in well agreement with the TGA results. IG′/IG ratios obtained from the Raman spectroscopy were 0.97 and 0.92 for MWCNT and GNR, respectively. Some studies showed that G′-band intensity could be a measure of metallicity of carbonaceous fillers.61,67,68 In order to verify this relationship for the employed nanofillers, their powder conductivity was measured (Figure 2c). Measurement of the compressed powder conductivity of the nanofillers provides a rough estimation of their inherent conductivity. The results showed that the electrical conductivities of the compressed powders of MWCNT and GNR were 900 and 40 S·m−1, respectively. This could say that the inherent conductivity of MWCNT was higher than that of GNR. This is in line with the greater IG′/IG ratio obtained for MWCNT. The discrepancy in electrical conductivity of the compressed powders can be contemplated as a result of the morphological and structural dissimilarities of MWCNT and GNR. Experimental techniques, such as scanning tunneling spectroscopy, would provide more detailed information on the electronic properties of nano173


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Figure 4. TEM images of (a) MWCNT/PVDF, (b) MWCNT/GNR/PVDF (ratio 3:1), and (c) GNR/PVDF nanocomposites with 2.7 wt % nanofiller content.

Figure 5. (a) Shear-stress growth (τ12+(t)) of binary and hybrid nanocomposites at a total concentration of 2.7 wt % as a function of time upon startup of a shear flow at 240 °C and shear rate of 0.1 s−1. The inset shows the failure stress as a function of GNR ratio content. Data points are the averages of three measurements. (b) Temporal variation of shear stress upon an instantaneous reduction in shear rate from 0.1 to 10−4 s−1 normalized with respect to the minimum shear stress at different MWCNT:GNR ratios with 2.7 wt % nanofiller content.

and further propagation. These defective sites are responsible for the shortening of GNRs during the melt-mixing process. In brief, TEM imaging reveals that all the nanocomposites had a relatively decent state of nanodispersion. 3.3. Rheological Behavior. Figure 5 depicts the shearstress growth (τ12+(t)) normalized with respect to its steadystate value (τ∞) as a function of time for nanocomposites with 2.7 wt % nanofiller content. These results were obtained from the start-up experiment, where a constant shear rate of 0.1 s−1 at 240 °C was applied from rest, and the subsequent stress response was monitored for MWCNT, GNR, and MWCNT/ GNR nanocomposites. The temporal sequence of the response initiates with a linear increase in shear stress reaching a maximum value τmax at tmax, and then decreases toward a steady-state value. The emergence of the stress overshoot in step rate experiments has been observed for soft glassy materials, such as emulsions,75 colloidal suspensions,76 foams,77 microgels,78 immersed granular materials,79 and polymer nanocomposites.80−83 For polymer nanocomposite melts, the stress overshoot is considered as a signature for transition from pseudo-solid-like behavior under nearly at-rest condition to liquid-like behavior above a characteristic shear stress. This transition is attributable to the failure of the nanofiller superstructure and orientation of individual nanofillers. The corresponding failure stress is denoted as τmax in the inset in Figure 5. For polymer nanocomposites, it is widely reported that the onset of the stress overshoot is scaled with accumulated strain and nanofiller loading.81 It is suggested that, for polymer nanocomposites, the temporal stress response is dictated by hydrodynamic forces

composites, and the hybrid nanocomposite exhibited an intermediate value. For instance, the agglomerate area ratios in the MWCNT, hybrid (3:1), and GNR nanocomposites with 2.7 wt % nanofiller content were 1.01, 1.62, and 2.01%, respectively. Moreover, the relative transparencies of the MWCNT, hybrid (3:1), and GNR nanocomposites with 2.7 wt % nanofiller content were 49, 51, and 55%, respectively. These results show that the MWCNT nanocomposites had slightly better microdispersion state compared to the GNR nanocomposites. Figure 4 shows the TEM micrographs of ultrathin sections of the PVDF nanocomposites containing 2.7 wt % nanofiller. These images are the representatives of nanodispersion states of MWCNT and GNR in the PVDF matrix. The TEM micrograph of the MWCNT/PVDF sample (Figure 4a) shows lightly clustered MWCNT assemblies as well as individually dispersed nanotubes. Unlike GNRs, MWCNTs were less defective and more tolerant to the shear forces, as confirmed by TGA and Raman spectroscopy; thus, they had more resistance to breakage during the melt-mixing process. The TEM micrograph for the hybrid sample at a total content of 2.7 wt % and MWCNT-to-GNR ratio of 3:1 (Figure 4b) depicts that GNRs and MWCNTs can coexist in close proximity of each other (see the inset in Figure 4b) and no segregation was observed. It is observable that GNRs are fully open at the ends, while they are folded onto themselves in the middle (Figure 4c). The folding of GNRs can adversely impact their performance for dielectric properties. There are usually some structural defects in the sp2 network of GNRs, shown by yellow arrows, which are mainly due to the initiation sites of unzipping 174


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Figure 6. Broadband electrical conductivity of (a) MWCNT/PVDF and (b) GNR/PVDF binary nanocomposites.

GNR ratios of 3:1 and 1:1, MWCNT/GNR hybrids eventually reached a stress value of ∼1.8 × τmin, which is comparable with MWCNT nanocomposites. Nonetheless, at 1:3 MWCNT/ GNR ratio, GNR nanofillers presented a significantly hindered networked reformation process in the hybrid nanocomposite. This is in conformity with the result of the step-rate test, demonstrating that, as the MWCNT-to-GNR ratio decreases, the presence of GNR deteriorates the structural stability of the MWCNT microstructure and coherence of nanotubes domains movement. In fact, on the basis of the results presented in Figure 5a,b, it can be concluded that, in hybrid samples, the ability of the MWCNT network superstructure to recover from shear-induced anisotropy significantly decreased as the MWCNT-to-GNR ratio decreased. This could be ascribed to decreased network interconnectivity and prevented direct tube−tube contact due to the presence of GNR. 3.4. Broadband Dielectric Properties. 3.4.1. Broadband Electrical Conductivity of Binary Nanocomposites. Primarily, leakage current of a dielectric under an alternating field derives from its electrical conductivity. Hence, it is imperative to obtain a profound perception of the electrical conductivity of CPNs as dielectrics under an AC field. The broadband electrical conductivity is commonly termed as σ = ω·ε0·(ε″ + iε′), where σ is electrical conductivity, ω is angular frequency, ε0 is the permittivity of free space, ε″ is imaginary permittivity, and ε′ is real permittivity.7,85 For CPNs, conductivity is described as σ = σDC + iσAC, where σDC is the direct-current (DC) conductivity, brought about by the movement of electrons in phase with the applied electric field, and it is usually independent of frequency. σ AC is the component of conductivity engaged with the alternating current, and is generated by the reorientation of electric dipoles in each half cycle of alternating field.14,86,87 In a CPN, σDC is commensurate with the number of nomadic charges and level of conductive network (ε″), whereas σAC is proportionate to the number and magnitude of electric dipoles (ε′).3,31,88 In insulative materials, owing to high electrical resistance, there is no substantial in-phase current flow and the current (charge over time) increases with frequency; thus, AC conductivity follows a rising trend with frequency. This is due to higher current density coming from electric dipole reorientation at higher frequencies. In conductive materials, nevertheless, the current arising from nomadic charges, which is in phase with the applied electric field, takes over the effect of electric dipole reorientation, and thus σAC becomes negligible with respect to σDC.11,89,90 Accordingly, the broadband electrical conductivity of conductive materials presents a frequency independent behavior. In semiconductive materials, there is a

and particle−particle short-range interactions, making Brownian relaxation a nonmajor contributor.81,83,84 As shown in the inset in Figure 5, the different values observed for the maximum stress (τmax) are mainly due to the formation of dissimilar network structures with substantially various levels of resilience to yielding in the MWCNT, GNR, and hybrid nanocomposites. Accordingly, in the present work, it was observed that GNR nanocomposites had relatively smaller failure stress compared to the MWCNT nanocomposite. It is inferable that, at MWCNT/GNR ratios of 3:1, 1:1, and 1:3, MWCNT and GNR formed a mutual network structure with a failure stress proportional to the MWCNT-toGNR ratio. The other noticeable feature is the intensity of the stress overshoot (τmax/τ∞), signifying the extent of structural changes as a result of shear field application. Higher observed overshoot intensities for the hybrid samples and GNR binary nanocomposite indicate that the shearing action extensively affected the nanofillers at-rest microstructure in these samples. This is, however, not the case for the MWCNT binary nanocomposite sample as the intensity of the stress overshoot is not significant. Figure 5b shows the temporal variation of shear stress upon a step reduction in shear rate from 0.1 to 10−4 s−1. The immediate monotonic decrease corresponds to viscoelastic stress relaxation reaching a nonzero residual stress (τmin) at t ∼ 0.06 s. After the initial reduction, at longer times, the stress response increased, reaching a new steady-state value. The MWCNT binary nanocomposites and hybrid samples at MWCNT-to-GNR ratios of 3:1 and 1:1 reached a steadystate value in the first 10 s after the motor-speed change. On the contrary, the GNR binary nanocomposite after partial stress relaxation and reaching a minimum value showed a weak subsequent increase in the shear stress in the time frame of the experiment. The other hybrid nanocomposite (nanofiller ratio of 1:3) showed a hindered recovery behavior and reached a steady-state stress value close to that of the GNR nanocomposite. Disorientation of anisotropic nanofiller clusters and elastic recovery of deformed nanofiller assemblies play major roles in the short term response.80,83 However, the increase in captured shear stress at longer times is an indication for structural recovery and reforming of the nanofiller superstructure, stemming from the thixotropic nature of polymer nanocomposites.80,83 This confirms very poor ability of GNR nanofillers to effectively recover from flow-induced anisotropy. All MWCNT/GNR hybrid nanocomposites demonstrated weaker stress build-up at longer times compared to the MWCNT binary nanocomposite. Noticeably, at MWCNT/ 175


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Figure 7. Broadband (a) imaginary permittivity and (b) real permittivity of binary and hybrid nanocomposites of MWCNT and GNR at different MWCNT:GNR weight ratios with filler total content of 2.7 wt %.

critical frequency ( fc) below which the DC current of in-phase charges becomes dominant, while, beyond such frequency, the AC current coming from electric dipole reorientation conquers. Therefore, the overall conductivity becomes frequency independent in the lower frequency ranges and frequency dependent at higher frequency ranges.20,28,30 Figure 6 depicts the broadband electrical conductivity of the binary nanocomposites of MWCNT and GNR. As shown in Figure 6a, pure PVDF and 0.5 wt % MWCNT nanocomposite demonstrated an ascending trend with frequency, denoting their insulative nature. It was observed that the MWCNT nanocomposite with 1.0 wt % nanofiller content showed a semiconductive behavior, signifying the percolation region. MWCNT nanocomposites at 2.0 and 2.7 wt % exhibited a conductive behavior, confirming the formation of a wellestablished conductive network. For GNR nanocomposites, adding 0.5 and 1.0 wt % nanofiller indicated an insulative nature. However, it was observed that incorporating 2.0 and 2.7 wt % GNR presented the behavior of a semiconductive material. These results unearth that GNR nanocomposites had a semiconductive behavior over a wide range of nanofiller content (2.0−2.7 wt %) without any momentous enhancement in electrical conductivity. This can be attributed to inferior capability of GNR to contact each other and form a conductive network, as endorsed by rheology. Increase in critical frequency, shown by black arrows, with GNR content implies that GNRs were in closer proximity at higher nanofiller contents. Given the comparable dispersion states of the MWCNT and GNR binary nanocomposites, as validated by LM and TEM, the lower percolation threshold and higher AC conductivity of MWCNT imply its stronger ability to interlock and form an interconnected network. It is also observable that the DC conductivity of MWCNT and GNR binary nanocomposites at high filler loading (2.7 wt %) was considerably lower than the conductivity of their compressed powders. The powder conductivity for MWCNT and GNR was 900 and 40 S·m−1, whereas the DC conductivity of MWCNT and GNR binary nanocomposites with 2.7 wt % nanofiller content was 1.6 × 10−3 and 3.3 × 10−8 S·m−1, respectively. This huge difference can be ascribed to the presence of the polymer layer between conductive nanofillers, thwarting their direct contact. It is believed that the transference of electrons across a conductive CPN, due to its heterogeneous structure, comprises three mechanisms of conduction, hopping, and tunneling.91−94 Conduction is materialized by direct physical contacts between nanofillers. Hopping and tunneling mechanisms occur when there is a thin

layer of polymer in-between nanofillers. On the basis of the quantum mechanics, once the insulative gap between fillers reaches below 1.8 nm, electron tunneling is so likely.93,95,96 3.4.2. Broadband Dielectric Properties of Hybrid Nanocomposites. The capacitance of a capacitor is defined as the following C=

εA d

(3)

where C is capacitance, ε is real permittivity, A is surface area, and d is thickness of the capacitor. The AC conductivity results revealed that MWCNTs had a higher tendency to be in close proximity of each other compared to GNRs. This led to lower thickness of nanodielectrics, and thereby higher electric field and capacitance. Furthermore, higher innate electrical conductivity and crystallinity of MWCNT than GNR, as certified by the powder conductivity measurement, TGA, and Raman spectroscopy, facilitated the storage of more nomadic charges at the interface of MWCNT and the PVDF matrix. On the other side, inferior capability of GNR to form a conductive network proposes them as promising materials to develop low-loss nanocapacitors. Therefore, it can be contemplated that the hybrid system of MWCNT/GNR/PVDF nanocomposite outperforms the binary nanocomposites due to synergy between MWCNT and GNR. In order to validate this postulation, the dielectric properties of the hybrid nanocomposites at MWCNT/GNR weight ratios of 3:1, 1:1, and 1:3 were investigated and compared with those of the binary nanocomposites. The dielectric properties of the hybrid nanocomposites were investigated at nanofiller total contents of 1.0, 1.5, 2.0, and 2.7 wt %. We very nearly observed the same trend for all the concentrations; thus, herein we solely depict the dielectric properties of 2.7 wt %. Figure 7 depicts the imaginary permittivity and real permittivity of the binary and hybrid nanocomposites with 2.7 wt % nanofiller content. Imaginary permittivity represents Ohmic loss, arising from the dissipation of electrical energy by nomadic charges moving throughout a dielectric.17,26 As the frequency decreases, the nomadic charges are given more time to travel and thus more electrical energy is dissipated, leading to an increase in imaginary permittivity. When the conductive network is more well-established, the mean free paths of nomadic charges rise, and thus Ohmic loss and consequently the imaginary permittivity increase. Figure 7a shows that the imaginary permittivity of the MWCNT binary nanocomposite was higher than its GNR counterpart. This was owing to the superior capability of MWCNT to found a conductive network in conjunction with 176


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Table 1. Real Permittivity and Dissipation Factor of Binary and Hybrid Nanocomposites of MWCNT and GNR with Various Nanofiller Contents at a Frequency of 1000 Hza 1.0 wt % MWCNT hybrid 3:1 hybrid 1:1 hybrid 1:3 GNR a

1.5 wt %

2.0 wt %

2.7 wt %

ε′

tan δ

ε′

tan δ

ε′

tan δ

ε′

tan δ

16.3 12.3 7.5 7.4 7.5

0.80 0.10 0.07 0.08 0.07

39.3 41.4 25.1 8.7 7.5

86.7 0.91 0.31 0.16 0.09

33.7 12.3 7.5 9.9 8.7

199 61.7 3.24 0.14 0.11

13.9 36.3 38.3 16.5 10.4

5918 178 216 1.32 0.22

The standard deviation is 5% of the reported values.

Figure 8. Schematics showing the capacitive and dissipative features of binary CNT/polymer, binary GNR/polymer, and hybrid CNT/GNR/PVDF nanocomposites.

remained unchanged (around 7.5) over the measured frequency range. Figure 7b explicates that the lowest real permittivity was observed for the GNR binary nanocomposite, due to poor ability of GNRs to neighbor each other, as confirmed by AC conductivity. In fact, GNRs did not have the capability to form effective nanocapacitors, leading to their low real permittivity. Moreover, it was captivatingly observed that all the hybrid nanocomposites presented higher real permittivity than the binary nanocomposites over the whole frequency range. For instance, at 1000 Hz, the real permittivity of the MWCNT binary nanocomposite was 13.9, whereas hybrids 3:1, 1:1, and 1:3 showed real permittivity equal to 36.3, 38.3, and 16.5, respectively. Lower imaginary permittivity and higher real permittivity of the hybrid nanocomposites compared to MWCNT binary nanocomposites prove their loftier performance. This is attributed to the role of GNRs as additional nanoelectrodes and also reduced thickness of nanodielectrics. In order to scrutinize the performance of the hybrid nanocomposites further, real permittivity and dissipation factor, tan δ (ε″/ε′) of the binary and hybrid nanocomposites with various nanofiller contents at a frequency of 1000 Hz are tabulated in Table 1. As shown in Table 1, the nanocomposites far below the percolation threshold had a real permittivity around 7.5, matching the real permittivity of pure PVDF, and a dissipation factor around 0.07. However, both real permittivity and dissipation factor enhanced as the nanocomposites approached the percolation threshold and above. It is also vivid that the real permittivity and dissipation factor of the

higher innate conductivity of MWCNT. It was also observed that the imaginary permittivity of the hybrid nanocomposites was between the binary nanocomposites. Hence, it can be claimed that there was no synergy between MWCNT and GNR in terms of forming a conductive network. This is contrary to the results of a study performed by Zhang et al.,32 who developed a hybrid nanocomposite of expanded graphite/ MWCNT/cyanate ester and showed that, at some concentrations, expanded graphite can act as a bridge between two adjacent MWCNTs. It is worth noting that the descending trend of imaginary permittivity with frequency is a natural behavior of conductive CPNs.9,90 Figure 7b illustrates the real permittivity of the binary and hybrid nanocomposites with 2.7 wt % nanofiller content. In a CPN, all constituents, i.e., polymer matrix, conductive nanofiller, and interface, can be polarized, and their contribution to overall real permittivity depends on the frequency range.29,97,98 In general, real permittivity in CPNs originates from different sources, interfacial polarization, dipolar and electronic polarizations of polymer matrix, and space charge polarization within nanofillers.7,30,31 Even though all the polarization mechanisms are present at the lower frequencies (

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