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Flexible FKM/mRGO nanocomposites with excellent thermal, mechanical and electrical properties
⁎Corresponding author at: Centre for Nanoscience and Technology, Department of Basic Sciences, Amal Jyothi College of Engineering, Kanjirappally, Kottayam, Kerala, India. soneygeo@gmail.com (Soney C. George)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Nanocomposites having multifunctional properties with simultaneous improvement in thermal, mechanical and dielectric properties along with improved ferroelectric signature are of considerable interest due to the rapidly growing need in the technological aspect. In the present work, microwave reduced graphene oxide (mRGO)/fluoroelastomer nanocomposites with enhanced dielectric performance and ferroelectric characteristics were developed. mRGO was synthesized from natural graphite via GO and was confirmed by analyzing the chemical changes using X-ray diffraction technique, Fourier transform infrared and Raman spectroscopy studies. The addition of the mRGO improved the mechanical, dielectric and ferroelectric characteristics of the nanocomposites by its better dispersion in the polymer matrix as confirmed by TEM and AFM analysis. The enhanced polymer filler interaction was also confirmed by analyzing the Kraus Plot. Nanocomposite with 0.75 phr of filler loading showed optimum mechanical properties with increased Tg of about −12.25 °C. This is an evidence for the enhancement in properties due to the improved filler-polymer electrostatic interaction. Nanocomposites showed several-fold increase in the dielectric constant value compared to the gum sample. Moreover, the nanocomposites have minimum dielectric loss at 1 MHz frequency. The incorporation of mRGO created a conductive pathway by the microcapacitor effect and the nanocomposites showed enhanced AC conductivity. Ferroelectric studies revealed that the coercive field (Ec) and the remnant polarization (Pr) of the nanocomposites were increased with filler loading that signifies the improvement in ferroelectric signature of the nanocomposites. The samples show excellent dielectric and ferroelectric properties along with better thermal and mechanical stability, and hence, will be promising candidate for ultra-modern storage devices.
Keywords
Nanocomposites
Fluoroelastomer
Reduced graphene oxide
Dielectric
Ferroelectric
1 Introduction
Fluoroelastomers (FKM) have emerged as a great promise in industrial areas as they exhibit excellent thermo-mechanical properties when processed with different reinforcing agents. The outstanding heat stability and oil resistance of these materials are due to the high ratio of fluorine to hydrogen, the strength of the carbon-fluorine bond and the absence of unsaturation. These characteristic properties transformed FKM to achieve a greater attraction in the fields of aircraft, automotive, chemical, petroleum and energy industries (Ameduri, 2009; Ameduri et al., 2001; Teng, 2012). FKM nanocomposites with different filler materials having good capabilities were developed by several researchers (Gao and Guo, 2017; Heidarian and Hassan, 2014; Dubeya et al., 2015; Maiti and Bhowmick, 2009; Kader et al., 2006; Rooj et al., 2011). Among various fillers graphene and its derivatives such as graphite nanoplatelets, exfoliated graphite, graphene oxide, reduced graphene oxide etc. are of great interest and extensive research is ongoing in this field. Due to the multifunctional properties of graphene and its derivatives (Hu et al., 2014; Chee et al., 2015); graphene based polymer nanocomposites are widely used for various applications in different dimensions such as sensors (Turcheniuk et al., 2015; Tian et al., 2018), energy harvesters (Mahmood et al., 2014; Ren et al., 2017); photocatalysts (Divya et al., 2014; Ma et al., 2017) etc. Allyl functionalized graphene oxide/FKM nanocomposites with enhanced thermal properties were developed by Wei and Qiu (2014). In another work, exfoliated nano graphite reinforced flouroelastomer composites were prepared and the dielectric and dynamic mechanical properties were investigated (Sridhar et al., 2009). Graphene oxide incorporated FKM was solution-processed by Wei et al. to augment the mechanical and liquid barrier properties of FKM (Wei et al., 2014). Xing et al. fabricated rGO incorporated FKM nanocomposites by solvothermal process and they observed 3-fold increase in dielectric permittivity values for 5 phr rGO loaded sample as compared with neat FKM (Xing et al., 2014). Cure characteristics and polymer-filler interactions in nitrogen dopped graphene nanoribons incorporated FKM matrix were also reported (Khajehpour et al., 2014).
The need of materials for energy storage applications is increasing day by day due to the rapid advancement in technological areas (Wang and Zhu, 2011; Romasanta et al., 2015). Even though ceramic materials can be used for such application (Damjanovic, 1998) major drawbacks of these materials include their reduced breakdown strength in spite of its large dielectric constant and the chance of agglomeration when incorporated into the polymer matrix (Luo et al., 2015). Elastomeric nanocomposites or polymer nanocomposites find their advantage in this loop-hole. Polymeric systems have high breakdown strength, low leakage currents and comparative dielectric constants which make them an alternative to the ceramic materials. These features of polymer nanocomposites enable them to use as thin layers in such applications without the loss of its functionalities (Barbar et al., 2009; Dang et al., 2012; Nan et al., 2010; Shankar et al., 2007). Nowadays fluoro polymers with enhanced breakdown strength and better dielectric and ferroelectric properties are used in this respect (Li and Wang, 2016; Prateek Thakur and Gupta, 2016). Shaohui et al. developed poly(vinylidene fluoride) nanocomposites from surface modified BaTiO3 nanofibers and achieved dielectric constant of 24 with a dielectric loss of 0.018 by the incorporation of 7.5 vol% filler at 1 kHz and enhanced energy storage capability (Liu et al., 2014). Sawane et al. fabricated ferroelectric PVDF-HFP insulator with highly tunable contact angle values and dielectric constant around 8 (Sawane et al., 2016).
Even though there are some reports available on the study of FKM/graphene nanocomposites, to the best of our knowledge the detailed study in the morphology, interaction between FKM/mRGO and the properties with respect to the dielectric and ferroelectric behaviour have not been published so far. In the present work we are trying to combine the higher breakdown strength and ferroelectric properties of FKM and the enhanced dielectric performance of mRGO prepared via microwave reduction of GO which is an ecofriendly method. The nanocomposites were prepared by the simple two-roll mixing technique. Synthesized mRGO was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman analysis. Morphological analysis of the samples was carried out by using TEM and AFM imaging studies. The interaction between the polymer and the filler was confirmed by atomic force microscopy (AFM), differential scanning calorimetry (DSC) and Kraus plot. By the support of results related to the polymer-filler interactions the properties of the nanocomposites were analyzed in terms of mechanical and dielectric properties and ferroelectric PE-loop characteristics.
2 Materials and methods
The polymer matrix, Tecnoflon FOR 532 fluoroelastomer is a copolymer of vinylidene fluoride and hexafluoropropylene (VDF-co-HFP) containing bisphenol curatives with specific gravity of 1.81 g/cm3 and mooney viscosity 45 MU at 121 °C was procured from Synthetic and Chemicals Ltd, Bareilly, UP, India. Graphite which is used for the synthesis of GO was procured from Sigma Aldrich, Bangalore, India. The chemicals such as Ca(OH)2 and MgO used for compounding were purchased from Nice Chemicals Pvt. Ltd. Mumbai, India and all other reagents used were of analytical grade and used without further purification.
2.1 Preparation of mRGO via GO synthesis
The synthesis of mRGO was carried out by the microwave reduction of GO which is prepared by Tours Method (Marcano et al., 2010). In short, by Tours Method a 9:1 mixture of 96% H2SO4 and 85% H3PO4 was added to a 6:1 mixture of graphite and KMnO4, which produces a slight exotherm to 35–40 °C. The reaction mixture was heated to 50 °C, stirred for 12 h, then cooled to room temperature and poured onto ice made from deionised water. The reaction was terminated by adding 30% H2O2 and the resultant filtrate was washed with distilled water until the slurry reached neutral pH. Obtained graphene oxide was dried in an oven at 40 °C. The reduction of GO was carried out in a conventional microwave oven at 800 W for 1.5 min to get mRGO.
2.2 Preparation of the FM nanocomposites
Compounding of the fluoroelastomer nanocomposites was carried out in a two-roll mill and the ingredients for the preparation of the nanocomposites in terms of phr (parts per hundred rubber) are displayed in Table 1.
| Ingredients (Phr) | FM 0 | FM 0.25 | FM 0.5 | FM 0.75 | FM 1.25 | FM 2 |
|---|---|---|---|---|---|---|
| FKM | 100 | 100 | 100 | 100 | 100 | 100 |
| MgO | 3 | 3 | 3 | 3 | 3 | 3 |
| Ca(OH)2 | 6 | 6 | 6 | 6 | 6 | 6 |
| mRGO | 0 | 0.25 | 0.5 | 0.75 | 1.25 | 2 |
| Paraffin wax | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 | 2.5 |
Cure time of the rubber vulcanisates was determined according to ASTM D-1646 using oscillating disc type rheometer. The rubber samples were then press-cured for the respective cure time at 177 °C and post cured at 230 °C for 8 h in an air oven. The Schematic representation for the preparation of the nanocomposites is given in Fig. 1.
2.3 Characterization techniques
The characterization of mRGO was carried out using the Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis. FTIR spectrum was obtained from the Perkin Elmer analyzer at a spectrum range of 400–4000 cm−1. XRD studies were performed using Bruker AXS D8 advance X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm) as the X-ray source. Transmission electron microscopic (TEM) analysis was carried out on JEM-2100 HRTEM. Cryocut specimens for the TEM analysis were prepared by ultramicrotome (Leica, Ultracut UCT) placed on 300 mesh Cu grids with 35 mm diameter. The instrument used for AFM analysis was a WITec alpha300RA AFM, SNOM & RAMAN combined system with image resolution of 1024 × 127 pixels. The mechanical properties of the nanocomposites were analyzed on a pneumatic universal testing machine, INSTRON-4411 with the velocity of the moving cross head of about 500 mm/min. DSC measurements were performed using a Mettler Toledo DSC 822e with an operating temperature varying from −50 °C to 200 °C. Dielectric studies were carried out using Agilent E4980A precision LCR meter with frequency ranging from 100 Hz to 2 MHz and the ferroelectric loop were measured using P-E loop tracer (Radiant technologies).
The reinforcement effect is explained on the basis of Kraus Plot and the data for analysis was procured from the solvent-diffusion studies. For diffusion studies xylene was used as the solvent and is done using the given procedure. Circular samples were punched out from the compression molded samples using a sharp-edged steel die. The sample thickness was measured by a micrometer screw gauge. Weighed samples were soaked in 25 ml of the solvent taken in sorption bottles kept at room temperature. The samples were taken out from the diffusion bottles in regular intervals and the excess solvent adhered to the sample surface was wiped off gently. The samples were then weighed in a precise electronic balance and placed back into the test bottles such that the weighing process took a total of 30 s in order to avoid the discrepancy caused. The weighing was continued to equilibrium and from the data diffusion parameters were calculated.
3 Results and discussion
3.1 Characterization of microwave reduced graphene oxide
The formation of mRGO by the chemical transformation of graphite was explained using the FTIR and XRD spectra. The functional groups present in the compounds were identified by FTIR Spectroscopy. Fig. 2(a) shows the FTIR spectrum of graphite, GO and mRGO. In graphite there was only sp2 hybridized carbons linked together, therefore it does not show any characteristic peak in the FTIR spectrum. When graphite was oxidized to GO, oxygen functionalities were added to the carbon network structure and corresponding peaks were identified in the spectrum. The peaks at 3220, 1736, 1625, 1364, 1217, 1045 cm−1 correspond to the —OH stretching, C⚌O stretching, aromatic C⚌C, carboxy C—O, epoxy C—O and alkoxy C—O respectively, which confirmed the oxidation of graphite to GO. When GO was reduced some of the oxygen functionalities were removed from the structure. It was confirmed by the reduction of peak corresponding to —OH stretching and by the increase in intensity of —CH stretching vibrational peaks (Marcano et al., 2010).
Fig. 2(b) shows the XRD pattern of graphite, GO and mRGO. The XRD pattern of graphite shows a diffraction peak at 26.5° with d-spacing value 3.36 Å. When it was converted into GO the peak was shifted to 9.8° with a basal spacing of 8.98 Å (Marcano et al., 2010). The intense intercalation of graphite by oxidation resulted in increasing the d-spacing value from 3.36 to 8.98. When GO was reduced all the XRD peaks disappeared as a result of exfoliation and also due to the loss of long-term ordering of graphitic structure (Zhu et al., 2010). Hence the reduction of GO to mRGO was established by its structural changes as revealed in XRD studies.
The main aim of Raman analysis is to find the structural variations in the carbon backbone of carbonaceous fillers such as CNT, graphene, etc. The transformation from sp2 to sp3 hybridization and the degree of disorder in the carbon network was confirmed by Raman analysis. The Raman spectra of graphite, GO and mRGO are given in Fig. 2(c). The Raman spectra consist of D-band, G-band and 2D-band that represent the disorder band, graphitic band and the breathing mode band respectively. In graphite, the D-band and G-band were present at 1353 and 1581 cm−1 respectively. But when it was converted into GO, the peaks shifted to 1373 and 1609 cm−1 respectively due to the addition of oxygen functionalities in the carbon structure. Due to the presence of ordered structure in graphite the 2D band centered at 2710 cm−1 was also present. In graphite the intensity of G-band is higher compared to D-band. When it was converted into GO the intensity of the G-band decreased at the same time the intensity of D-band increased due to the increase in disorder (Marcano et al., 2010). It was also clear from the ID/IG ratio of both graphite and GO. For graphite ID/IG ratio is 0.1381 and it increases to 0.7852 when it was converted into GO. It confirms that the introduction of oxygen functionalities increased the disorder of the graphitic structure and hence the ID/IG value increases. mRGO was also characterized by Raman analysis and the intensity of D-band increased with the reduction process, because of the use of high power microwave radiations for the reduction process. The ID/IG ratio of mRGO was 1.029 which represents the increased disorder in the graphitic structure (Liu et al., 2012). Hence the reduction of GO to mRGO was confirmed.
3.2 Characteristic properties of FM nanocomposites
3.2.1 Cure characteristics of FM nanocomposites
The vulcanization kinetics of the FM nanocomposites was analyzed from the cure graph. The parameters such as scorch time (tS2) which represents the beginning of curing, the optimum cure time (t90), the maximum torque value (MH), the minimum torque value (ML), delta torque value (ΔS = MH − ML) and cure rate index (CRI) were determined from the curing curve and are depicted in Table 2.
| Samples | ML | MH | ΔS | tS2 | t90 | CRI |
|---|---|---|---|---|---|---|
| (dNm) | (dNm) | (dNm) | (min) | (min) | (min−1) | |
| FM 0 | 1.07 | 11.49 | 10.42 | 0.92 | 1.8 | 113.63 |
| FM 0.25 | 1.13 | 12.3 | 11.17 | 1.03 | 1.84 | 123.45 |
| FM 0.5 | 1.18 | 12.81 | 11.63 | 1.16 | 2.16 | 100.00 |
| FM 0.75 | 1.1 | 12.87 | 11.77 | 1.22 | 2.27 | 95.23 |
| FM 1.25 | 1.23 | 13.36 | 12.13 | 1.28 | 2.51 | 81.30 |
| FM 2 | 1.23 | 13.87 | 12.64 | 1.57 | 3.22 | 60.60 |
The extent of crosslinking and the polymer-filler interaction in the composites were determined from the MH values. The MH value increases considerably with the filler concentration. This gives an evidence for the enhanced filler-polymer interaction and increased crosslinking with the FKM matrix. The ML value also increased with filler loading from FM 0 (1.07 dNm) to FM 2 (1.23 dNm). Thus the incorporation of mRGO has influenced the sliding movement of the elastomeric chains that affects the properties of the nanocomposites. ΔS values represent the density of vulcanizates and it gives an idea about the quality of interaction at the polymer-filler interface. The value of ΔS also increased with filler loading. This might be due to the influence of rGO on the crosslink density of the rubber vulcanization or due to some chemical interaction between the interfaces. ts2, the scorch time which represents the induction period and t90, the optimum cure time also increased and correspondingly the CRI decreased with filler loading. The presence of accelerator with medium scorch safety was the reason for increased ts2. The increase in t90 value with filler loading might be due to the presence of mRGO with high surface area which absorbed the curing agents that resulted in a prolonged vulcanization process of the nanocomposites (Wang and Chen, 2013; Chang et al., 2002).
3.2.2 Reinforcement mechanism; The Kraus plot
Interaction between the polymer matrix and the filler material is an important factor which affects the characteristic properties of the nanocomposites. Hence the degree of reinforcement which gives an idea about the interaction between mRGO and FKM was assessed by analyzing the Kraus plot. According to Kraus equation (Kraus, 1963)
The reinforcing ability of the nanocomposites increased with filler loading mainly due to the enhanced polymer filler interaction. In the case of FKM/mRGO nanocomposites there is an electrostatic interaction between the fluorine atoms present in the FKM matrix and the hydrogen atoms attached to the functional groups present in mRGO. The schematic representation showing the interaction between FKM and mRGO is given in Fig. 4.
3.2.3 Morphological analysis of FM nanocomposites
AFM is one of the most important tools employed for the analysis of surface morphologies with the aid of 3D images in nanometer scale. Surface roughness values evaluated over the complete 3D surface are represented in terms of Sq (root mean square roughness) and Sa (average roughness) which gives an overall idea about the texture of the surface (Maiti and Bhowmick, 2006). The 3D and 2D topographical images of the fluoroelastomer nanocomposites are shown in Fig. 5. The addition of mRGO has modified the surface of FM nanocomposites and the raised topographies are indicative of the presence of mRGO in the polymer matrix. From the careful analysis of the topographies it is clear that with the addition of 0.75 phr mRGO, the surface of FKM matrix becomes homogeneous without much heaps and leaps. But on further addition of mRGO, agglomeration of the filler occurred and the homogeneity is disturbed. This is also clear from the surface roughness values as given in Table 3.
| Sample | Sq (Root Mean Square) nm | Sa (Average Surface Roughness) nm | Sq-Sa (nm) |
|---|---|---|---|
| FM 0 | 129.86 | 101.89 | 27.97 |
| FM 0.25 | 72.83 | 55.20 | 17.63 |
| FM 0.75 | 41.62 | 25.84 | 15.77 |
| FM 2 | 51.24 | 39.74 | 11.50 |
The Sq and Sa surface roughness values are found to be decreasing with the addition of filler material until the loading is 0.75 phr, but with further filler loading roughness value increases. This is because as the filler loading increases the size of aggregates increases due to agglomeration. Hence in the case of nanocomposite with 0.75 phr of filler loading an even distribution of mRGO in the FKM matrix can be observed. A similar observation was also reported in the case of SBR and PDMS polymer nanocomposites (Abraham et al., 2016; Irani et al., 2013).
The values of Sq and Sa are also analyzed in a different manner and if the values of Sq and Sa are similar there is no layer deviation from the mean surface. Therefore the difference, Sq-Sa gives an overview about the presence of filler particles on the surfaces (Chang et al., 2002). In the present study the Sq-Sa values for the nanocomposites are found to be greater than zero. Hence the incorporation of filler materials in the nanocomposites is confirmed.
The extent of interaction of the filler with the matrix material can be analyzed from the TEM micrographs. The TEM image of nanocomposite with 0.75 phr of filler loading is given in Fig. 6. In the figure the bright portions represent the polymer matrix and the dark portions represent mRGO. From TEM images (FM 0.75) the well dispersion of filler material in the FKM matrix can be visualized. AFM topographies also support the morphological analysis from the TEM images.
3.2.4 Mechanical properties of FM nanocomposite
Tensile and tear properties of the fluoroelastomer nanocomposites are displayed in Table 4. Tensile strength is found to be increasing with filler loading and the value is maximum for the nanocomposite with 0.75 phr of mRGO content. It is also clear from the stress-strain plot given in the supplementary data as Fig. S1. FM 0.75 shows a tensile strength of about 6.718 MPa which is about 37% more than the gum sample. Correspondingly the elongation at break also increases. With further filler loading, the tensile strength of the nanocomposites decreases. Tear strength is also found to be increasing with filler loading and is maximum for the nanocomposite with 2.0 phr of filler loading. For the gum sample the value is 18.63 N/mm which increases with filler loading to a value of 27.48 N/mm for 2.0 phr mRGO. The increase in mechanical properties of FM nanocomposite up to 0.75 phr filler loading is due to the proper dispersion of mRGO in the FKM matrix which facilitates the transfer of stress uniformly through the matrix material. The improvement in mechanical properties with filler loading is due to the better reinforcing effect of mRGO in the FKM matrix as revealed from the Kraus plot. From TEM images and AFM analysis it is evident that mRGO is evenly dispersed in the polymer matrix at 0.75 phr loading. Hence FM 0.75 shows the optimum tensile properties.
| Sample | Tensile strength | Elongation at break | Tear strength | Modulus at 100% |
|---|---|---|---|---|
| (MPa) | (%) | (N/mm) | (MPa) | |
| FM 0 | 4.221 ± 0.13 | 314 ± 2 | 18.632 ± 1 | 1.181 ± 0.1 |
| FM 0.25 | 4.632 ± 0.2 | 326 ± 5 | 16.561 ± 2 | 1.170 ± 0.14 |
| FM 0.50 | 5.197 ± 0.12 | 320 ± 3 | 17.186 ± 2 | 1.433 ± 0.1 |
| FM 0.75 | 6.713 ± 0.01 | 373 ± 1 | 18.563 ± 1 | 1.301 ± 0.03 |
| FM 1.25 | 5.534 ± 0.1 | 274 ± 1 | 23.631 ± 1 | 1.365 ± 0.15 |
| FM 2 | 3.096 ± 0.12 | 197 ± 2 | 27.484 ± 1.5 | 1.512 ± 0.13 |
3.2.5 Thermal analysis of FM nanocomposites
The weight changes of the sample with respect to temperature are determined from TGA analysis and hence the thermal stability of the sample can be predicted. TGA plot of weight% of the sample versus temperature is shown in Fig. 7(a) and the corresponding derivative plot is given in Fig. 7(b). From the graph it is clear that with increase in the amount of mRGO the thermal stability of the nanocomposites has increased to some extent.
It is also evident from the plot that higher thermal stability is attained for the nanocomposite with 2 phr of filler loading. Also the residue at Tmax is higher for FM 2, which again accounts for its improved thermal stability. A detailed description of the data extracted from the TGA plot is included in the supplementary data as Table S1. The improved thermal stability of the nanocomposites is attributed to the presence of mRGO with better heat insulation efficiency that prevents the volatile degradation of the material.
3.2.6 Differential scanning calorimetric analysis of FM nanocomposites
DSC measures the amount of heat energy absorbed or released by a sample when it undergoes a physical or chemical change. DSC analysis of polymer nanocomposites gives an idea about the extent of intercalation or exfoliation of nanoparticles in the matrix. The interaction of the intercalated or exfoliated polymer chains with the nanofillers affects the segmental mobility of the polymer matrix thereby enhancing the glass transition temperature (Tg) of the polymer (Corcione and Frigione, 2012). In the present work the change in Tg of the samples with filler loading is analyzed. The Tg values have been taken from the DSC plot by considering the onset, mid-point and endset of Cp. Here in the present case Tg (midpoint) is taken as the glass transition temperature. (DSC plot of the FM nanocomposites is displayed in Fig. S2 of supplementary data.) Tg of the nanocomposites was analyzed with respect to filler loading and the extracted values from the DSC plot are depicted in Table 5.
| Sample | Tg (onset) | Tg (midpoint) | Tg (endset) |
|---|---|---|---|
| FM 0 | −21.70 | −18.22 | −14.74 |
| FM 0.25 | −21.00 | −17.14 | −13.28 |
| FM 0.5 | −15.38 | −12.75 | −10.12 |
| FM 0.75 | −15.11 | −12.25 | −9.39 |
| FM 1.25 | −15.23 | −12.56 | −9.89 |
| FM 2 | −18.35 | −13.67 | −8.99 |
The value of the Tg increased with increase in filler loading. Increase in Tg can be attributed to the fact that the incorporation of mRGO into the polymer matrix restricted the movement of the matrix material by its better dispersion in the polymer matrix. mRGO which is used as the filler material is evenly distributed in the matrix material as evident from the TEM images, and hence the Tg increases. At lower filler loading there is no significant effect of mRGO in the matrix material, so the value of Tg is near to the Tg of pristine polymer. But with the increase in mRGO loading up to 0.75 phr, Tg increases rapidly from −17.14 to −12.25 and with further increase in filler loading the value of Tg decreases. This might be due to the aggregate formation in nanocomposite as evident from the increase of surface roughness value emanating from AFM studies.
3.2.7 Dielectric properties and the ferroelectric PE-Loop characteristics of FM nanocomposites
The effect of filler loading on the electrical properties of the nanocomposites was analyzed. The variation of AC Conductivity with frequency of the nanocomposites is given in Fig. 8(a). The addition of mRGO increased the AC conductivity of the nanocomposites. It is due to the incorporation of the conducting filler, mRGO to the matrix component and interfacial polarization that forms a continuous conducting network (Tantis et al., 2012; Devi and Kumar, 2018; Yousefi et al., 2014). From the figure it is evident that AC conductivity is increasing up to 0.75 phr of filler loading which gives the optimum σAC conductivity value. This is because of the presence of conductive mRGO sheets isolated by FKM matrix that forms a lot of microcapacitor structures to accumulate the charge carriers at the interphase. Due to the presence of the microcapacitors, with increase in filler loading more conductive pathways are formed which open a way for increased AC conductivity values. Conductivity decreases with further filler loading due to the agglomeration of the mRGO in the FKM matrix.
The value of dielectric permittivity indicates the ability of a material to store energy from the electric field. Variation of the dielectric permittivity (ε′) value as a function of frequency for different nanocomposites is shown in Fig. 8(b). For FM 0 the ε′ value remains constant in the whole frequency range. But when the filler component, mRGO is added to FKM, the ε′ value is found to be dependent on frequency. The nanocomposites achieved the percolation threshold at lower filler loading (0.25 phr) and showed a sudden increase in dielectric constant at low frequencies. This is due to the better dispersion and high aspect ratio of mRGO which acts as a microcapacitor throughout the FKM Matrix. The higher dielectric permittivity value of 32.4 was achieved with 2 phr of filler loading, which is about 30 times greater than the gum sample. Hence with increase in filler loading the energy storage capacity of the nanocomposites has increased. The extent of dissipation of energy due to charge migration or conversion to thermal energy was represented by dielectric loss. The variation of dielectric loss of the nanocomposites with frequency is given in Fig. 8(c). Here in the case of FM nanocomposites the dissipation of energy is found to be very small i.e. 0.005 unit at 1 MHz frequency. Hence FM nanocomposites with increased breakdown strength, decreased dielectric loss and increased dielectric permittivity have better dielectric capabilities. This is mainly due to the better adhesion between the filler material and the FKM matrix.
Fluoroelastomers are specialty polymers which are mechanically durable and can withstand both high and low temperatures. The incorporation of the filler material mRGO improves both the dielectric and ferroelectric signature of the nanocomposites to a greater extent. The PE-Hysteresis loop of the nanocomposites with 0.25 and 2 phr of filler loading are given in Fig. 9. The concave region in the graph is indicative of the ferroelectric properties of the fluoropolymer. From the P-E loop it can be seen that the remnant polarization (Pr) and coercive field (Ec) values improved with filler loading, which shows that the addition of mRGO enhances the ferroelectric property of the composite system. Ec of the hysteresis loop increases with filler loading. The value of Ec for FM 0.25 is 98 which is increased with filler loading and is found to be higher for FM 2 with a value of 146. The presence of mRGO with large interfacial area promotes the exchange coupling effect through the dipolar interface layers and results in a higher polarization levels and ferroelectric responses. Thus the PE-loop reflects the enhanced properties associated with the addition of mRGO to the fluoroelastomer matrix.
4 Conclusion
FM nanocomposites with enhanced mechanical, thermal, dielectric and ferroelectric PE- Loop characteristics were prepared by two-roll mixing. The negative slope of the Kraus plot indicated the enhanced interaction between FKM and mRGO, which was a reason for the improvement in the characteristic properties of the nanocomposites. DSC analysis showed an increase in Tg value compared to FM 0 and was found to be higher for the nanocomposite with 0.75 phr of filler loading. This showed the efficient incorporation of mRGO into the FKM matrix as confirmed from the TEM and AFM analysis. Mechanical properties of the nanocomposites were enhanced to a greater extent with the addition of the filler material mRGO. Incorporation of mRGO significantly influenced the dielectric properties of the nanocomposites. Nanocomposite with 0.75 phr of filler loading showed improved dielectric performance along with better thermal and mechanical properties. The superior dielectric performance and improved ferroelectric signature of the nanocomposite with increase in filler loading was due to the electrostatic interaction between mRGO and fluoroelastomer. From the above studies it is evident that with increase in filler loading, the effect of mRGO is significant such that it enhances the dielectric and PE-Loop characteristics of the nanocomposites. Thus the increase in dielectric permittivity and exceptionally large piezoelectric compliances make the FM nanocomposite suitable for nanotechnology related applications of ultra modern energy storage devices.
Acknowledgment
Grace Moni is thankful to the University Grants Commission (UGC), Government of India, for the financial assistance (JRF).
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Appendix A
Supplementary material
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.03.015.
Appendix A
Supplementary material
Supplementary data 1
Supplementary data 1