Reduced polyaniline decorated reduced graphene ...

Composites Part A 109 (2018) 578–584

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Reduced polyaniline decorated reduced graphene oxide/polyimide nanocomposite films with enhanced dielectric properties and thermostability

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Hao Fenga, Xinliang Fanga, Xiaoyun Liua, Qibing Peib, Zhong-Kai Cuic, Shifeng Denga, , ⁎⁎ Jinlou Gua, Qixin Zhuanga, a Key Laboratory of Advanced Polymer Materials of Shanghai, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China b Soft Materials Research Laboratory, Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA 90095, United States c Department of Chemistry, Université de Montréal, C.P. 6128, Succ. Centre Ville, Montréal, Québec H3C 3J7, Canada

A R T I C LE I N FO

A B S T R A C T

Keywords: A. Graphene A. Polymer-matrix composites (PMCs) B. Electrical properties B. High-temperature properties

This study reports the synthesis and characterization of reduced polyaniline decorated reduced graphene oxide/ polyimide (RGO@R-PANI/PI) nanocomposite films with enhanced dielectric properties and thermostability. The steric effects of R-PANI decorated on the surface of RGO not only prevented the aggregation of RGO but also improved the dispersion of RGO@R-PANI nanosheets in the PI matrix, leading to higher dielectric constant (ε) and lower dielectric loss (tanδ) of the RGO@R-PANI/PI nanocomposite films than those of the RGO/PI nanocomposite films. The highest dielectric constant (25.84) was observed in the 20 wt% RGO@R-PANI/PI nanocomposite film (the mass fractions of RGO was 3.44%) with a low dielectric loss of 0.11 at 1 kHz. Furthermore, the 5 wt% weight loss temperature of the 20 wt% RGO@R-PANI/PI nanocomposite films was 480 °C, indicating the nanocomposite films are good candidates for dielectric materials under extreme temperature.

1. Introduction Recently, with the fast development of renewable power generation technologies, tremendous effort has been devoted to the fabrication of power storage capacitors [1–5]. In comparison with the traditional perovskite ceramic capacitors, polymer-based dielectric materials with many excellent properties (higher flexibility, lower fabrication temperature, easy processing, and low cost) have drawn more attention in recent years [6–9]. Polyimide (PI) is a typical kind of high performance engineering polymer materials with excellent thermal stability and superior mechanical properties [10–13]. Despite numerous advantages, PI has a low dielectric constant (∼3). Therefore, improving the dielectric constant of PI-based dielectrics while maintaining the original excellent performance is one of the main challenges facing researchers. So far two approaches have been proven efficient for increasing the dielectric constant. One is to introduce high-k ceramic fillers (e.g. TiO2, Al2O3, CaCu3Ti4O12, and BaTiO3) into the polyimide matrix [14–18]. However, the polymer matrix loses the mechanical and processing properties with the increasing content of the ceramic fillers. The other ⁎

Corresponding author. Corresponding author. E-mail addresses: [email protected] (S. Deng), [email protected] (Q. Zhuang).

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https://doi.org/10.1016/j.compositesa.2018.03.035 Received 3 January 2018; Received in revised form 29 March 2018; Accepted 31 March 2018 Available online 02 April 2018 1359-835X/ © 2018 Elsevier Ltd. All rights reserved.

approach is to fabricate percolated composites based on the percolation theory [19,20] by using conductive fillers such as graphene [21,22], carbon nanotubes [23], and conductive polymers [24]. Graphene with great electrical conductivity has great potential to enhance the dielectric constant [25,26]. Due to the π−π interactions and high cohesive van der Waals potential energy, graphene tends to form irreversible agglomerations, which leads to significantly high dielectric loss near the percolation threshold. In order to improve the dispersion and prevent the aggregation of graphene in the polymer matrix, previous efforts on achieving high dielectric constant mainly focused on covalent functionalization of graphene. Chen et al. modified the surface of GO by chemical grafting of N-(2-hydroxyphenyl)methacrylamide, which increased the dielectric constant from 4.92 to 8.35 [27]. Fang et al. synthesized PPD-CFGO by amination and carboxylation of GO, and the dielectric constant of the 4 wt% (PPD-CFGO)/PI nanocomposite films rose to 36.9 at 1 kHz, which was 12.5 times higher than that of PI [28]. However, the covalent functionalization method deteriorates the electrical conductivity of graphene by destroying the ππ conjugated structure of graphene. In order to overcome the disadvantage of covalent


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Fig. 1. Synthesis of the RGO@R-PANI/PI nanocomposite films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1.88 mmol) was poured into the mixture followed by stirring for another 12 h. The sample of GO@PANI nanosheets was obtained after washing and drying at 60 °C under vacuum for 24 h. PANI was prepared with a similar process without GO.

functionalization, non-covalently functionalized graphene with polymers via π−π stacking was reported [29–32]. Herein, insulating reduced polyaniline (R-PANI) was decorated on the surface of RGO by a strong π−π stacking to introduce steric effects, effectively preventing the irreversible agglomerations of RGO. As shown in Fig. 1, PANI was firstly wrapped on the GO via in situ polymerization to obtain decorated graphene oxide (GO@PANI). Then, reduced polyaniline decorated reduced graphene oxide (RGO@R-PANI) was prepared through the reduction of GO@PANI by hydrazine. Thereafter, not only R-PANI converted from conductor (PANI) to insulator (R-PANI), but also the electrical conductivity was remarkably improved from GO to RGO. Finally, the “insulator-conductor-insulator” structured RGO@R-PANI nanosheets were obtained with uniform distribution in the PI matrix. As a result, RGO nanosheets are completely isolated by R-PANI, which is conducive to the formation of microcapacitor structure and preventing the current leakage. Moreover, R-PANI decorated on the surface of RGO improved the compatibility between RGO and the PI matrix. Compared to the RGO/PI nanocomposite films, the RGO@R-PANI/PI nanocomposite films have vast potential applications in the microelectronic industries because of its enhanced dielectric properties.

2.3. Preparation of RGO@R-PANI nanosheets In a typical process, the as prepared GO@PANI and excessive N2H4·H2O (5 mL) were added in 80 mL deionized water and stirred at 95 °C for 5 h. The mixture was washed with deionized water by filtration and dried at 60 °C under vacuum for 24 h to obtain reduced polyaniline decorated reduced graphene oxide (RGO@R-PANI) nanosheets. Following the same procedure, RGO and R-PANI were synthesized by substituting GO and PANI for GO@PANI. 2.4. Preparation of RGO@R-PANI/PI and RGO/PI nanocomposite films ODA (1 g, 5 mmol) and predetermined amount of RGO@R-PANI were dissolved in 20 mL NMP with mechanical stirring at room temperature with the gradual addition of PMDA (1.09 g, 5 mmol) during 4 h. After the complete dissolution of PMDA in the NMP, the solution was stirred for another 8 h to obtain poly(amic acid) (PAA). Then the solution was poured into a glass dish and dried in air atmosphere at 80 °C for 48 h, followed by heat treatment in nitrogen atmosphere at 100, 200 and 300 °C for 1 h, respectively. Finally, the RGO@R-PANI/PI nanocomposite films containing 3, 5, 7, 10, 15, 20 and 30 wt% of RGO@R-PANI were prepared. The neat PI film and 0.52, 0.86, 1.21, 1.72, 2.58, 3.44 and 5.16 wt% RGO/PI nanocomposite films were prepared as controls following the same process for comparison.

2. Experimental 2.1. Materials Graphite was obtained from Alfa Aesar (Massachusetts, USA). GO was prepared according to the process reported by Hummers and Offeman [33]. Hydrazine hydrate (80%) (N2H4·H2O), pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA) were purchased from Aladdin (Shanghai, China). Concentrated hydrochloric acid (HCl), aniline (ANI), N-methylpyrrolidone (NMP), Ammonium persulfate (APS) and ethanol were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).

2.5. Characterization Fourier transformed infrared (FTIR) spectroscopy was performed using Nicolet Magna-IR 550. X-ray diffraction (XRD) patterns were obtained using a D/MAX 2550 VB/PC rotating anode X-ray multicrystal diffraction spectrometer with Ni-filtered Cu Kα radiation and operated at 60 mA and 40 kV. Raman spectra were acquired with a backscattering configuration at 514 nm on a Renishaw inVia Reflex Raman spectrometer. The morphologies of GO, RGO and RGO@R-PANI were characterized by the field-emission scanning electron microscopy (FESEM, Hitachi S-4800) and the high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). FESEM (Hitachi S-4800) was

2.2. Preparation of GO@PANI nanosheets The polyaniline decorated graphene oxide (GO@PANI) nanosheets were prepared by the polymerization of aniline with GO. Typically, aniline (0.7 g, 7.53 mmol) was dispersed in 1 mol/L HCl (80 mL), and then GO (0.1 g) was added to the resulting solution, using tip sonication (0.5 h). Next, in an ice-water bath, the dispersion was stirred for 0.5 h and then 20 mL of 1 mol/L HCl solution containing APS (0.429 g, 579


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Fig. 2. (a) FTIR spectra, (b) Raman spectra and (c) X-ray diffraction patterns of GO@PANI, RGO@R-PANI, PI and 20 wt% RGO@R-PANI/PI; (d) TGA curves of GO, RGO, PANI, R-PANI and RGO@R-PANI. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

witnesses stronger C]C stretching absorption (1495 cm−1), due to the introduction of RGO@R-PANI. Raman spectra of GO, RGO, PANI, R-PANI, GO@PANI, RGO@RPANI, PI, and RGO@R-PANI/PI are shown in Figs. 2b and S2 to further investigate the interactions between RGO@R-PANI and PI. The spectrum of PANI shows peaks at 1162, 1345, 1478 and 1597 cm−1 corresponding to CeH bending of the quinoid ring, CeN+ stretching of the bipolaron structure, NeH bending of the bipolaronic structure and CeC stretching of the benzenoid ring, respectively [36]. Due to the low content of GO in GO@PANI, the characteristic peaks (1350 and 1600 cm−1) of GO disappear in the spectrum of GO@PANI. In R-PANI and RGO@R-PANI, the relative intensities of the characteristic peaks at 1345 cm−1 decrease, owing to the reduction of the quinoid structural units to benzenoid structure units. The main characteristic peaks of PI are summarized as follows, CeNeC axial stretching, 1425 cm−1; aromatic imide ring vibration, 1615 cm−1; C]O symmetric stretching, 1796 cm−1. The Raman absorption peak at 1796 cm−1 shifted to 1793 cm−1 in the RGO@R-PANI/PI nanocomposites, due to the strong interactions between RGO@R-PANI and PI. The X-ray diffraction patterns of Graphite, GO, RGO, GO@PANI, RGO@R-PANI, PI, and RGO@R-PANI/PI are shown in Figs. 2c and S3. The XRD pattern of GO@PANI shows some specific diffraction peaks of PANI and no diffraction peak of GO, implying that GO were wrapping with PANI via in situ polymerization [37]. The XRD pattern of RGO@RPANI shows only a diffraction peak at 2θ = 19.6° (0.45 nm) while other diffraction peaks of PANI have disappeared, owing to the reduction by hydrazine. As for PI, an obvious broad peak centered at 18.5° indicates that PI is amorphous. The XRD pattern of the RGO@R-PANI/PI nanocomposites presents the same characteristic peaks of the PI, indicating that RGO@R-PANI nanosheets have excellent dispersing property in the

also used to characterize the fractured surface of the samples. Thermogravimetric analyses (TGA) were carried out in N2 atmosphere at a heating rate of 10 °C/min. The concept 40 Broadband dielectric analyzer (Novocontrol Technologies GmbH & Co. KG, Germany) was used to measure the dielectric properties at room temperature. The electronic conductivities of the samples were measured using a fourpoint probe resistivity measurement system (RTS-8, Four Probes Tech.), after the powder samples were pressed into disks at 20 MPa with a diameter of 10 mm and a thickness of 0.5 mm. The dielectric breakdown strength was measured using a CS2674AX ultrahigh hipot tester (Allwin Instrument Co., China) at room temperature.

3. Results and discussion 3.1. Structural characterization of RGO@R-PANI/PI nanocomposites GO, RGO, PANI, R-PANI, GO@PANI, RGO@R-PANI, PI, and RGO@ R-PANI/PI were characterized via FTIR (Figs. 2a and S1). Compared to GO, the absorption peaks at 1564 cm−1, 1481 cm−1, 1294 cm−1, 795 cm−1 in GO@PANI represent the stretching vibration of C]C in the quinonoid ring and benzenoid ring, CeN in the secondary aromatic amine and deformation vibration of the aromatic CeH out-of-plane, respectively [34]. After the reduction of quinoid structure in GO@PANI by hydrazine, the relative intensities of the absorption peaks at 1589 cm−1 and 1148 cm−1 of quinoid structure decrease in comparison with that of the absorption peak at 1495 cm−1 from the benzenoid structure in RGO@R-PANI. The absorption peaks at 1776 cm−1, 1726 cm−1 and 1379 cm−1 are associated with symmetrical and asymmetrical C]O stretching vibrations of imide group and CeN stretching modes in PI [35]. Compared to PI, RGO@R-PANI/PI 580


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3.3. Dielectric properties of RGO@R-PANI/PI and RGO/PI nanocomposite films

polymer matrix. In order to calculate the mass fractions of RGO in RGO@R-PANI/PI nanocomposites, the thermal properties of GO, RGO, PANI, R-PANI, RGO@R-PANI were studied with the thermogravimetric analyses and the curves were shown in Fig. 2d. Due to the thermal decomposition of oxygen groups, the second weight loss of GO is near 200 °C [38]. The weight loss of RGO is 17.7%, implying the removal of the oxygen containing groups after reduction. Meanwhile, there is no mass loss for R-PANI below 400 °C. The degradation of R-PANI backbone begins at about 450 °C, indicating that R-PANI possesses good thermal stability [39]. The TGA curve of RGO@R-PANI is the superposition of RGO and R-PANI. The weight ratio of RGO in RGO@R-PANI is calculated to be 17.2 wt% by Eq. (1):

wt %(RGO) =

ΔW2−ΔW1 × 100% ΔW2−ΔW3

Fig. 5a, b display the dielectric constant and the dielectric loss of the RGO@R-PANI/PI nanocomposite films as a function of frequency. For comparative analysis, the dielectric constant of the RGO/PI nanocomposite films as a function of frequency was shown in Fig. S4. As summarized in Fig. 5c, d, the 1 kHz dielectric constant and dielectric loss of the 20 wt% RGO@R-PANI/PI nanocomposite films are 25.84 and 0.11, which is considered as a combination of exceptionally high dielectric constant and reasonably low dielectric loss among state-of-theart graphene based nanocomposites with high temperature resistance [42,43]. The corresponding RGO/PI nanocomposite films (3.44 wt%) exhibits low dielectric constant of 8.23 and high dielectric loss of 56.4. The improvement of the dielectric constant of the RGO@R-PANI/PI nanocomposite films mainly ascribes to the formation of microcapacitor network [44]. This phenomenon can be explained by the percolation theory, a typical theory in polymer nanocomposites with conductive fillers. Since the mass is proportional to the volume, in order to directly exploit the experimental data, the volume fraction is replaced by mass fraction. Therefore, the percolation threshold is achieved by Eq. (2) [45]

(1)

where the ΔW1, ΔW2 and ΔW3 is the weight loss of RGO@R-PANI, RPANI and RGO from 100 to 700 °C. According to the TGA data, the mass fractions of RGO in 3, 5, 7, 10, 15, 20 and 30 wt% RGO@R-PANI/PI nanocomposites are 0.52, 0.86, 1.21, 1.72, 2.58, 3.44 and 5.16 wt%, respectively. The morphology of GO, RGO, PANI, RGO@R-PANI nanosheets was characterized with SEM and TEM. As shown in Fig. 3e, the GO@PANI nanosheets become coarser and more wrinkled than GO (Fig. 3a) and RGO (Fig. 3c) [40]. Compared to GO (Fig. 3b) and RGO (Fig. 3d), RGO@R-PANI (Fig. 3f) surface presents a lot of circular granule with a mean diameter of ∼20 nm, which is PANI loaded on the surface of graphene. Meanwhile, no obvious exfoliated PANI or naked RGO nanosheets appear in the TEM, owing to the strong π-π stacking interactions between the R-PANI and RGO [37,41].

ε ∝ (mc −mRGO @ R − PANI )−S for mRGO @ R − PANI < mc

(2)

where ε is the dielectric constant of the RGO@R-PANI/PI nanocomposite films, mc is the critical mass fraction at the percolation threshold, mRGO@R-PANI is the mass fraction of RGO@R-PANI and s is the critical exponent. The dielectric constant of the experiment fits Eq. (2) with mc = 0.212 and s = 0.73 in Fig. 5c. When the RGO@R-PANI content increases to 30 wt%, the destruction of the percolation networks and the formation of conductive networks lead to the decrease of dielectric constant and increase of the dielectric loss [46]. As shown in Fig. S5, the dielectric constant of the 20 wt% RGO@R-PANI/PI nanocomposite film increases from 25.84 to 41.35 when the temperature is raised from room temperature to 200 °C, owing to the promoted polarization with increasing temperature [47]. In order to further explain the high dielectric constant and low dielectric loss of the RGO@R-PANI/PI nanocomposite films, the electronic conductivities of GO, RGO, PANI, R-PANI, and RGO@R-PANI were measured. The electrical conductivity of RGO in this study is observed to be 23.6 S/cm, four orders of magnitude greater than that of GO (6.78 × 10−4 S/cm). PANI has the high conductivity of 4.6 S/cm,

3.2. Dispersion of RGO@R-PANI in PI matrix The morphology of RGO@R-PANI/PI and RGO/PI was investigated using SEM techniques. As opposed to the smooth fracture surface of PI (Fig. 4a), the fracture of the RGO@R-PANI/PI nanocomposite is rough and ridged (Fig. 4b, c). The RGO@R-PANI nanosheets are uniformly distributed throughout the PI matrix, and no apparent aggregation (pointed by red arrows) in Fig. 4b, c. On the contrary, the RGO/PI nanocomposites display large RGO aggregation (pointed by red arrows) in Fig. 4d, which mainly attribute to the incompatibility between RGO and PI and the strong interactions between RGO.

Fig. 3. SEM/TEM images of GO (a/b), RGO (c/d); SEM image of GO@PANI (e); TEM image of RGO@R-PANI (f). 581


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Fig. 4. SEM images of the fractured cross section of the neat PI (a), the 10 wt% RGO@R-PANI/PI nanocomposite films (b), the 20 wt% RGO@R-PANI/PI nanocomposite films (c), and the 3.44 wt% RGO/PI nanocomposite films (d). (The inserts are 10X-magnified images.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 5. The frequency dependent dielectric constant (a) and dielectric loss (b) of the neat PI and the RGO@R-PANI/PI nanocomposite films; 1 kHz dielectric constant and dielectric loss of the RGO@R-PANI/PI nanocomposite films (c) and RGO/PI nanocomposite films (d) at room temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In comparison, the current leakage caused by the agglomeration of RGO leads to the increased dielectric loss in the RGO/PI nanocomposite films (Fig. 6b). Furthermore, the breakdown strength of neat PI and RGO@RPANI/PI nanocomposite films as a function of RGO@R-PANI content

while the electrical conductivity of R-PANI is under the detection limit (10−6 S/cm). RGO@R-PANI has very low conductivity (5.9 × 10−4 S/ cm) because of the insulating R-PANI decorated on the surface of RGO, which could block the flow of electric charges in the PI matrix (Fig. 6a). 582


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PANI/PI nanocomposite films with enhanced dielectric properties and thermostability. Due to the steric effects of R-PANI decorated on the surface of RGO, RGO@R-PANI with an “insulator-conductor-insulator” structure was well dispersed in the PI matrix, leading to enhanced dielectric properties. For instance, the dielectric constant of the RGO@RPANI/PI nanocomposite films (25.84) is about 8 times higher than that of PI (∼3) at 1 kHz, while the dielectric loss is 0.11. As controls, the RGO/PI nanocomposite films were synthesized as well. The dielectric constant and dielectric loss of the RGO/PI nanocomposite films are 8.23 and 56.4, respectively, which are worse than those of the RGO@RPANI/PI nanocomposite films. Furthermore, the RGO@R-PANI/PI nanocomposite films presented satisfactory thermal stability, namely, remaining stable below 300 °C. With the improved dielectric properties and heat-resistance, the RGO@R-PANI/PI nanocomposite films have a wide application as high-temperature dielectric materials. Acknowledgements Fig. 6. (a) The model for the insulating R-PANI blocking the flow of electric charge in the RGO@R-PANI/PI nanocomposite films; (b) The formation of conductive network in the RGO/PI nanocomposite films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

This work was financially supported by the National Natural Science Foundation of China (51573045) and the International Collaboration Research Program of Science and Technology Commission of Shanghai (16520722000). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.compositesa.2018.03. 035. References [1] Li Q, Chen L, Gadinski MR, Zhang S, Zhang G, Li U, et al. Flexible high-temperature dielectric materials from polymer nanocomposites. Nature 2015;523(7562):576–9. [2] Zhang D, Liu W, Tang L, Zhou K, Luo H. High performance capacitors via aligned TiO2 nanowire array. Appl Phys Lett 2017;110(13):133902. [3] Zhang X, Shen Y, Zhang Q, Gu L, Hu Y, Du J, et al. Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering. Adv Mater 2015;27(5):819–24. [4] Luo S, Shen Y, Yu S, Wan Y, Liao W-H, Sun R, et al. Construction of a 3D-BaTiO3 network leading to significantly enhanced dielectric permittivity and energy storage density of polymer composites. Energy Environ Sci 2017;10(1):137–44. [5] Chen C, Gu Y, Wang S, Zhang Z, Li M, Zhang Z. Fabrication and characterization of structural/dielectric three-phase composite: continuous basalt fiber reinforced epoxy resin modified with graphene nanoplates. Compos A 2017;94:199–208. [6] He D, Wang Y, Chen X, Deng Y. Core–shell structured BaTiO3@Al2O3 nanoparticles in polymer composites for dielectric loss suppression and breakdown strength enhancement. Compos A 2017;93:137–43. [7] Luo H, Ma C, Zhou X, Chen S, Zhang D. Interfacial design in dielectric nanocomposite using liquid-crystalline polymers. Macromolecules 2017;50(13):5132–7. [8] Dang ZM, Zheng MS, Zha JW. 1D/2D carbon nanomaterial-polymer dielectric composites with high permittivity for power energy storage applications. Small 2016;12(13):1688–701. [9] Li YL, Jiang T, Lin SL, Lin JP, Cai CH, Zhu XY. Hierarchical nanostructures selfassembled from a mixture system containing rod-coil block copolymers and rigid homopolymers. Sci Rep 2015;5:10137. [10] Chen L, Ding Y, Yang T, Wan C, Hou H. Synthesis and properties of a high dielectric constant copolymer of a copper phthalocyanine oligomer grafted to amino-capped polyimide. J Mater Chem C 2017;5(33):8371–5. [11] Xu Z, Zhuang X, Yang C, Cao J, Yao Z, Tang Y, et al. Nitrogen-doped porous carbon superstructures derived from hierarchical assembly of polyimide nanosheets. Adv Mater 2016;28(10):1981–7. [12] Liao X, Ye W, Chen L, Jiang S, Wang G, Zhang L, et al. Flexible hdC-G reinforced polyimide composites with high dielectric permittivity. Compos A 2017;101:50–8. [13] Lin D, Zhuo D, Liu Y, Cui Y. All-integrated bifunctional separator for Li dendrite detection via novel solution synthesis of a thermostable polyimide separator. J Am Chem Soc 2016;138(34):11044–50. [14] Hui S, Lizhu L, Ling W, Weiwei C, Xingsong Z. Preparation and characterization of Polyimide/Al2O3 nanocomposite film with good corona resistance. Polym Compos 2016;37(3):763–70. [15] Chi QG, Dong JF, Zhang CH, Wong CP, Wang X, Lei QQ. Nano iron oxide-deposited calcium copper titanate/polyimide hybrid films induced by an external magnetic field: toward a high dielectric constant and suppressed loss. J Mater Chem C 2016;4(35):8179–88. [16] Liu L, Zhang Y, Lv F, Tong W, Ding L, Chu PK, et al. Polyimide composites composed of covalently bonded BaTiO3@GO hybrids with high dielectric constant and low dielectric loss. RSC Adv 2016;6(90):86817–23.

Fig. 7. TGA curves of the neat PI and the RGO@R-PANI/PI nanocomposite films with various contents of RGO@R-PANI. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

was investigated in Fig. S6. The breakdown strength of RGO@R-PANI/ PI nanocomposite films decreases with the addition of RGO@R-PANI content, but the breakdown strength of 20 wt% RGO@R-PANI nanocomposite films still remains at a high level (81 ± 7.5 kV mm−1). 3.4. Thermal properties of RGO@R-PANI/PI nanocomposite films As high-performance engineering plastics, PI-based nanocomposites possess excellent thermal stability. From Fig. 7, it is observed that the 5 wt% loss temperatures of the neat PI, 3, 5, 7, 10, 15, 20 and 30 wt% RGO@R-PANI/PI nanocomposites are 593, 568, 551, 536, 515, 501, 480 and 465 °C, respectively. The introduction of RGO@R-PANI lower the thermal stability of the RGO@R-PANI/PI nanocomposites because the degradation of the R-PANI backbone begins at about 450 °C. Nevertheless, the RGO@R-PANI/PI nanocomposites remain stable below 300 °C. 4. Conclusions We have demonstrated a novel strategy of preparing the RGO@R583


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[17] Luo H, Roscow J, Zhou X, Chen S, Han X, Zhou K, et al. Ultra-high discharged energy density capacitor using high aspect ratio Na0.5Bi0.5TiO3 nanofibers. J Mater Chem A 2017; 5(15): 7091–102. [18] Zhang D, Liu W, Guo R, Zhou K, Luo H. High discharge energy density at low electric field using an aligned titanium dioxide/lead zirconate titanate nanowire array. Adv Sci 2017;1700512. [19] Dang ZM, Lin YH, Nan CW. Novel ferroelectric polymer composites with high dielectric constants. Adv Mater 2003;15(19):1625–9. [20] Zhu J, Ji X, Yin M, Guo S, Shen J. Poly (vinylidene fluoride) based percolative dielectrics with tunable coating of polydopamine on carbon nanotubes: toward high permittivity and low dielectric loss. Compos Sci Technol 2017;144:79–88. [21] Feng H, Ma W, Cui Z-K, Liu X, Gu J, Lin S, et al. Core/shell-structured hyperbranched aromatic polyamide functionalized graphene nanosheets-poly(p-phenylene benzobisoxazole) nanocomposite films with improved dielectric properties and thermostability. J Mater Chem A 2017;5(18):8705–13. [22] Liu L, Lv F, Zhang Y, Li P, Tong W, Ding L, et al. Enhanced dielectric performance of polyimide composites with modified sandwich-like SiO2@GO hybrids. Compos A 2017;99:41–7. [23] Ren J, Yu D, Feng L, Wang G, Lv G. Nanocable-structured polymer/carbon nanotube composite with low dielectric loss and high impedance. Compos A 2017;98:66–75. [24] Lv P, Zhao Y, Liu F, Li G, Dai X, Ji X, et al. Fabrication of polyaniline/polyimide composite fibers with electrically conductive properties. Appl Surf Sci 2016;367:335–41. [25] Li Y, Shi Y, Cai F, Xue J, Chen F, Fu Q. Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent. Compos A 2015;78:318–26. [26] Wu Y, Lin X, Shen X, Sun X, Liu X, Wang Z, et al. Exceptional dielectric properties of chlorine-doped graphene oxide/poly (vinylidene fluoride) nanocomposites. Carbon 2015;89:102–12. [27] Chen Y, Zhang S, Liu X, Pei Q, Qian J, Zhuang Q, et al. Preparation of solutionprocessable reduced graphene oxide/polybenzoxazole nanocomposites with improved dielectric properties. Macromolecules 2015;48(2):365–72. [28] Fang X, Liu X, Cui Z-K, Qian J, Pan J, Li X, et al. Preparation and properties of thermostable well-functionalized graphene oxide/polyimide composite films with high dielectric constant, low dielectric loss and high strength via in situ polymerization. J Mater Chem A 2015;3(18):10005–12. [29] Lin P, Cong Y, Sun C, Zhang B. Non-covalent modification of reduced graphene oxide by a chiral liquid crystalline surfactant. Nanoscale 2016;8(4):2403–11. [30] Wang Y, Yang C, Mai Y-W, Zhang Y. Effect of non-covalent functionalisation on thermal and mechanical properties of graphene-polymer nanocomposites. Carbon 2016;102:311–8. [31] Wang Z, Han NM, Wu Y, Liu X, Shen X, Zheng Q, et al. Ultrahigh dielectric constant and low loss of highly-aligned graphene aerogel/poly(vinyl alcohol) composites with insulating barriers. Carbon 2017;123:385–94. [32] He Z-z YuX, J-h Yang, Zhang N, Huang T, Wang Y, et al. Largely enhanced dielectric

[33] [34]

[35]

[36] [37]

[38]

[39]

[40]

[41] [42]

[43]

[44]

[45] [46]

[47]

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properties of poly(vinylidene fluoride) composites achieved by adding polypyrroledecorated graphene oxide. Compos A 2018;104:89–100. Humers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80(6):1339. Liu XY, Lv YL, Zhuang QX, Li YM, Zhang SD, Lan FX. Polybenzobisoxazoles-based nanocomposites with high microwave absorption performance and excellent thermal stability. Polymer 2016;99:605–13. Li Y, Pei X, Shen B, Zhai W, Zhang L, Zheng W. Polyimide/graphene composite foam sheets with ultrahigh thermostability for electromagnetic interference shielding. RSC Adv 2015;5(31):24342–51. Wang H, Hao Q, Yang X, Lu L, Wang X. A nanostructured graphene/polyaniline hybrid material for supercapacitors. Nanoscale 2010;2(10):2164–70. Li M, Huang X, Wu C, Xu H, Jiang P, Tanaka T. Fabrication of two-dimensional hybrid sheets by decorating insulating PANI on reduced graphene oxide for polymer nanocomposites with low dielectric loss and high dielectric constant. J Mater Chem A 2012;22(44):23477. McAllister MJ, Li JL, Adamson DH, Schniepp HC, Abdala AA, Liu J, et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem Mater 2007;19(18):4396–404. Shang J, Zhang Y, Yu L, Luan X, Shen B, Zhang Z, et al. Fabrication and enhanced dielectric properties of graphene–polyvinylidene fluoride functional hybrid films with a polyaniline interlayer. J Mater Chem A 2013;1(3):884–90. Xu JJ, Wang K, Zu SZ, Han BH, Wei ZX. Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage. ACS Nano 2010;4(9):5019–26. Loh KP, Bao QL, Ang PK, Yang JX. The chemistry of graphene. J Mater Chem A 2010;20(12):2277–89. Liu H, Xu P, Yao H, Chen W, Zhao J, Kang C, et al. Controllable reduction of graphene oxide and its application during the fabrication of high dielectric constant composites. Appl Surf Sci 2017;420:390–8. Liu X, Li Y, Guo W, Sun X, Feng Y, Sun D, et al. Dielectric and mechanical properties of polyimide composite films reinforced with graphene nanoribbon. Surf Coat Technol 2017;320:497–502. Dang ZM, Wang L, Yin Y, Zhang Q, Lei QQ. Giant dielectric permittivities in functionalized carbon-nanotube/electroactive-polymer nanocomposites. Adv mater 2007;19(6):852–7. Nan CW, Shen Y, Ma J. Physical properties of composites near percolation. Annu Rev Mater Res 2010;40(1):131–51. He F, Lau S, Chan HL, Fan JT. High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates. Adv mater 2009;21(6):710–5. Riad AS, Korayem MT, Abdel-Malik TG. AC conductivity and dielectric measurements of metal-free phthalocyanine thin films dispersed in polycarbonate. Phys B 1999;270(1–2):140–7.