Flexible hdC-G reinforced polyimide composites with high dielectric permittivity

Accepted Manuscript Flexible hdC-G reinforced polyimide composites with high dielectric permittivity Xiaojian Liao, Wan Ye, Linlin Chen, Shaohua Jiang, Guan Wang, Lin Zhang, Haoqing Hou PII: DOI: Reference:

S1359-835X(17)30234-8 http://dx.doi.org/10.1016/j.compositesa.2017.06.011 JCOMA 4698

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

22 March 2017 7 June 2017 7 June 2017

Please cite this article as: Liao, X., Ye, W., Chen, L., Jiang, S., Wang, G., Zhang, L., Hou, H., Flexible hdC-G reinforced polyimide composites with high dielectric permittivity, Composites: Part A (2017), doi: http://dx.doi.org/ 10.1016/j.compositesa.2017.06.011

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Flexible hdC-G reinforced polyimide composites with high dielectric permittivity Xiaojian Liao,ae Wan Ye,a Linlin Chen,a Shaohua Jiang,abe* Guan Wang, c Lin Zhang, d and Haoqing Houa* a

Department of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang,

330022, China. b

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037,

China. c

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and

International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China. d

Department of Nano Engineering, University of California, San Diego, La Jolla, California 92093,

USA e

Present address: Macromolecular Chemistry II, University of Bayreuth.

ABSTRACT Carbon nanotubes (CNTs) reinforced composites with high dielectric permittivity empower miscellaneous applications in flexible electronics but are hindered by the large addition and agglomeration of fillers and weak mechanical performance. Here, we prepare homogeneous dispersion of CNTs and graphene oxide (hdC-G) via solvent-exchange, and fabricate hdC-G/polyimide (PI) composite films by in situ polymerization and thermal imidization. The achieved hdC-G can construct a 3D network and keep a long term stability. The hdC-G/PI composites show high dielectric permittivity of 124.9 at 100 Hz, 4000% higher than that of pure PI. The hdC-G/PI composites also exhibit enhanced thermal stability and improved tensile strength 1

without sacrificing the flexibility. This solvent-exchange approach can greatly enrich the applications of synergistic uses of CNTs and GO in composites and the hdC-G/PI composites with simultaneously high dielectric permittivity, low content of fillers, good mechanical and thermal performances can be good candidates for flexible electronics. Keywords: A. Hybrid; B. Mechanical properties; Dielectric properties; Solvent-exchange;

1. Introduction High dielectric permittivity polymer-based composites have rapidly developed as advanced functional materials for various applications such as capacitors for energy storage, artificial muscles and optoelectronics [1-4]. Two types of polymer-based dielectric composites have been widely investigated to achieve excellent dielectric properties: dielectric-polymer composite with dielectric fillers [5-9], and conductor-polymer composite with conducting fillers [10, 11]. Recently, conducting fillers have been favorably used as reinforced fillers in polymeric composites, because of a dramatic growing in dielectric permittivity at a low concentration in comparison to dielectric fillers (i.e. ferroelectric ceramics) [12-16]. The performance and application of these composites are directly influenced by three main factors: dielectric properties, percolation threshold value (fc), and mechanical properties [1, 17]. Metal particles [18, 19], CNTs [20, 21], conductive polymer [14, 22], graphene oxide and graphene [23-25] are commonly used as conducting fillers. Within this research area, CNTs stand out from these fillers due to their mass production and low-cost, ultrahigh tensile strength (50~200 GPa), large and tunable aspect ratios (more than 1000:1), as well as superior pristine conductivity (105~108 S/m) [26, 27]. At the percolation threshold, an interconnected network 2

structure can be formed by CNTs. The resultant CNTs/polymer nanocomposites turn into a conductor from insulator along with an increase in dielectric permittivity [28, 29]. However, due to the π-π stacking interaction and hydrophobic force among CNTs, the pristine CNTs tend to aggregate [30], which leads to the worse dielectric properties and the detriment to the mechanical performance of the composites. Therefore, it is still a great challenge to achieve high dielectric permittivity and excellent mechanical performance simultaneously with small addition of fillers in the composites. Although traditional strategies by surface modification [31] and in-situ polymerization [32, 33] have been utilized to improve the dispersion of CNTs in polymer matrix, the above problems are not solved sufficiently. Recently, constructing thin methoxypolyethylene glycol and octa-acrylate silsesquioxane layers in CNTs/polymer composites were reported to improve the dispersion of CNTs and reduce the dielectric loss at the same time [34, 35]. However, the relatively high amount addition of fillers hints the flexibility of the dielectric composites and this strategy sacrifices high conductivity of CNTs. Therefore, the main challenge is to tackle tough distribution of CNTs meanwhile keep high conductivity of pristine CNTs. It has been revealed that 3D homogeneous and stable CNTs-graphene oxide network can be constructed in aqueous, due to amphiphilic nature of GO functionality and the strong π-π stacking interaction and hydrophobic force between CNTs and GO [36-39]. And the hybrid CNTs-GO networks show an excellent distribution in H2O. However, insulator GO invariably triggers the reduction in electrical conductivity of CNTs network. In addition, although solvent-exchange method has been successfully applied to disperse single layer of GO in water [40], it is still a problem to achieve GO dispersion in organic solvents due to the agglomeration of GO in organic solvents, which greatly limit its application in the systems with organic solvents [41, 42]. 3

Polyimide (PI) is a well-known high performance polymer with excellent mechanical properties and thermal stabilities [43-46]. However, most of PIs possess low dielectric permittivity in the range of 2.5-3.5 [47-50], which significantly hinder its applications in dielectric fields. Many works have been studied to improve the dielectric permittivity by adding dielectric fillers and conductive fillers. High dielectric permittivity of PI composites can be obtained by adding more than 40 vol% dielectric fillers [51-55] and 10 wt% conductive fillers [48, 56-60]. However, such high content of fillers was added to the PI for high dielectric permittivity sacrifice the excellent mechanical properties of PI. In this work, to improve the dielectric properties of PI without lose its mechanical flexibility, small amount of CNTs and GO are synergistically added into PI matrix via steps of a modified solvent-exchange approach, in situ polymerization and thermal imidization. The modified solvent-exchange approach leads to a homogeneous dispersion of CNTs and GO in organic solvent (hdC-G). Only with very small addition of fillers (hdC-G), the hdC-G/PI composites could achieve high dielectric permittivity, enhanced tensile strength and thermal stabilities without sacrificing the flexibility of the materials.

2. Experimental 2.1. Materials 4,4'-oxydianiline (ODA) (99%, Shanghai Chemical Regents Co., Shanghai, China), p-phenylenediamine (PDA) (99%, Shanghai Chemical Regents Co., Shanghai, China), and 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA) (99%, provided by CIAC, Changchun, China) were purified by sublimation. CNTs (8–15 nm diameters, 15–30 µm lengths, and containing 1.2 wt% –COOH, Fig. S1a) was purchased from Chengdu Organic Chemicals Co., 4

Ltd., China. GO was purchased from XFNANO Materials Tech Co., Ltd., Nanjing, China. N-methylpyrrolidone (NMP, boiling point: 203 °C) was purchased from Sigma Aldrich and used as received. 2.2. Preparation of hdC-G/NMP by solvent-exchange The homogeneous dispersion of CNTs with GO in NMP was prepared by a solvent-exchange method (Fig. 1). Graphene oxide (0.1 g) was exfoliated in 100 mL deionized water via sonication for 1.0 h. Then, 100 mL of NMP was poured into the obtained GO/H2O media and sonicated for another 1 h. Then H2O was removed by distillation under vacuum until a fraction of NMP was distilled out, and centrifugation (10,000 rpm, 10 min) was used to remove the unexfoliated GO. The morphology of the exfoiliated GO was shown in Fig. S1b. The concentration of GO in the obtained homogeneous GO suspension was determined gravimetrically to be 0.46 mg/mL. Then pre-weighted CNTs with 1/1 mass ratio to GO were added to the obtained homogeneous GO suspension. After sonication for 0.5 h, the final suspension (hdC-G/NMP) was obtained. For comparison, two controlled dispersions were prepared by direct sonication of pristine CNTs and C-G hybrids in NMP solvent with the same content as hdC-G/NMP. The mass ratio of CNTs and GO is kept to 1/1 in hdC-G/NMP and C-G/NMP dispersions.

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Figure 1. Schematic illustration of the fabrication of hdC-G/PI composite films by solvent-exchange. 2.3. Preparation of hdC-G/PI nanocomposite films The polyamic acid (PAA) was prepared in the hdC-G/NMP dispersion by in-situ polymerization. BPDA, ODA, and PDA with molar ratio of 2/1/1 were added into the hdC-G/NMP dispersion and reacted at -5 °C for 24 h. Then the hdC-G/PAA was casted into films, followed by heating to 350 °C (2 °C/min, N2) and annealing for 1 h for imidization. The content of the hdC-G fillers in the nanocomposite was controlled from 1.0 wt%, 2 wt%, 2.5 wt%, 3 wt% to 4 wt%. The other two controlled samples of CNTs/PI and C-G/PI composite films were prepared in the same conditions. The thickness of the samples was about 20 µm. 2.4. Characterization The morphology and microstructure of samples were investigated by scanning electron microscopy (TESCAN vega3) and transmission electron microscopy (JEM-2010). Dielectric properties were measured using a TH2819-A precision LCR meter at a frequency range from 100 Hz to 100 kHz (Tonghui Electronic Co., Ltd). Tensile test was performed by CMT-8102 electromechanical testing machine (Shenzhen, China) with a stretching rate of 5 mm/min. The chemical structures were studied by using Fourier transform infrared spectroscopy (FT-IR, Bruker tensor 27) and Raman (LabRAM HR-800). Thermal gravimetric analysis (TGA) was carried out on a thermo gravimetric analysis (PerkinElmer Pyris 1) with a heating rate of 10 °C/min in N 2. All the solutions were sonicated by ultrasonic cleaner (KQ-500DE, Kunshan UL ultrasonic unstruments Co., Ltd) at a frequency of 40 kHz and power of 500 W.

3. Results and discussion 6

3.1. Modified solvent-exchange approach To obtain the homogeneous dispersion of CNTs-based fillers in organic medium, we modified the solvent-exchange method to fabricate homogeneous dispersion of CNTs-GO in organic solvent. During the solvent-exchange process, the GO can be exfoliated into individual GO sheets after sonification in the aqueous medium [30]. Then, because of the compatibility to water, the organic solvents (such as NMP, DMSO, DMF, and DMAc) with high boiling point can facilely insert the interspace between the GO layers. Even though the H2O evaporates out from the dispersion by reduced pressure distillation, the individual GO sheets can still maintain homogeneity and stability in NMP (Fig. 2a). Owing to the π-π stacking interaction and hydrogen bonding with GO and amphiphilic nature of GO functionality, the CNTs can lay on the surface of GO sheet or bridge adjacent GO sheets (Fig. 2d). Therefore, the GO sheets not only contribute to improve the distribution of CNTs but also help to construct a 3D network with CNTs in the NMP medium, which leads to the homogeneous distribution of CNTs in NMP with the presence of GO. The obtained hdC-G/NMP dispersion also exhibits a long-term stability without any agglomerations and precipitations even after 15 days (Fig. 2a). In comparison, agglomerations were observed in CNTs/NMP and C-G/NMP dispersions after 2 and 12 h, respectively (Fig. 2b, c), due to the re-agglomeration of GO in the organic solvent after ceasing sonicate [41, 42]. The homogeneous hdC-G/NMP dispersion by solvent-exchange approach can open a great wide applications for the composites with synergistic addition of CNTs and GO.

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Figure 2. Digital pictures showing the dispersions of hdC-G/NMP (a), CNTs/NMP (b), and C-G/NMP (c), and SEM images of hdC-G hybrid fillers (d). The arrows indicate typical adjacent of CNTs and GO. 3.2. Morphology and microstructures of composites Remarkably, this 3D network of CNTs and GO can be applied here as fillers to fabricate polymer-based dielectric composites. Through in situ polymerization, the PI precursor, polyamic acid, can fill the void space of this 3D network. After high temperature imidization, not only the polyamic acid thermally converts into polyimide but the hdC-G filler can resist the reduction of electrical conductivity due to the insulation from GO [61, 62]. As shown in the Supporting Information Fig. S2, S3, both FT-IR spectra and Raman spectra show the characteristic signals of CNTs and GO in the hdC-G complexes and hdC-G/PI composites, which confirmed the co-existence of CNTs and GO in the hdC-G complexes and hdC-G/PI composites. Furthermore, Fig. 3a, b and c present the lamellar structures in the cross-section of hdC-G/PI film, and no obvious agglomeration of hdC-G is observed. In comparison, the C-G fillers show an inhomogeneous distribution in the PI film (Fig. 3d, e and f). The agglomerates of CNTs and GO (C-G) were observed in the PI matrix. These comparisons demonstrate that the solvent-exchange method in this work can facilitate the uniform distribution of CNTs with GO in the PI matrix, which would lead to the improvement in dielectric, mechanical and 8

thermal properties.

Figure 3. SEM images of the cross section of hdC-G/PI composites (a, b and c) and C-G/PI composites (d, e and f), respectively. The yellow dashed circles in (e) and (f) indicates the agglomerates of C-G in matrix. 3.3. Dielectric properties Generally, in conductor-dielectric composites, the polymer-based composites can transform from dielectric to conductor with increasing the content of conductive fillers around percolation threshold, where the conductive fillers form a network of conductive paths in the polymer matrix [63, 64]. Meanwhile, the dielectric permittivity can obtain a huge raise yet unfortunately the dielectric loss also shows a high value. The frequency dependence of dielectric permittivity (εr), dielectric loss (tanδ), and AC electrical conductivity (σ) of hdC-G/PI composites at room temperature are shown in Fig. 4a-c and summarized in Table 1. The σ is calculated by dielectric permittivity and loss at the same angular frequency ( ):  ( )   0     r( )   0     r ( )  tan  ( ) (1)

Where εr" is the imaginary part of the dielectric permittivity and 0 (8.85×10-12 F/m) is the dielectric 9

permittivity of free space.

Figure 4. Frequency dependence of dielectric permittivity (a), dielectric loss (b), and AC conductivity (c) of hdC-G/PI composites with different amount of fillers. Table 1. Dielectric, mechanical and thermal properties of hdC-G/PI composites. hdC-G (wt%) 0 1 2 2.5 3 4

Dielectric permittivity 100 Hz 1 kHz 3.1 53.8 87.5 124.9 100.0 92.5

3.1 27.6 81.6 116.8 93.8 81.0

Dielectric loss 100 Hz 0.004 1.65 1.93 1.97 5.87 6.60

1 kHz 0.01 0.63 0.88 0.90 1.05 1.12

Conductivity (S/m) 100 Hz -10

1.0×10 4.89×10-7 2.74×10-5 2.79×10-5 5.85×10-5 1.01×10-4

1 kHz 1.06×10-9 1.36×10-6 2.83×10-5 2.89×10-5 5.66×10-5 1.03×10-4

Strength (MPa)

Toughness (J/g)

T5% (°C )

193±5.3 296±3.4 232±4.8 213±5.4 196±6.2 190±3.2

85±1.4 81±0.7 65±1.1 36±1.5 16±1.8 8±0.6

568 570 572 573 575 576

The dielectric permittivity of pure PI is 3.12 at 100 Hz which is almost independence of frequency

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from 100 Hz to 100 kHz. With addition of a small amount (1.0 wt%) of conducting fillers, the dielectric permittivity of composites increases to 53.8 (100 Hz) and 27.6 (1 kHz) respectively, which indicates that the introduction of fillers has strong enhancement on dielectric properties of polymer matrix. The dielectric permittivity of the composites increases with increasing fillers content from 0 wt% to 2.5 wt% then decreases. Notably, the hdC-G/ PI composites show a high dielectric permittivity at the low weight content of fillers. The dielectric permittivity of hdC-G/PI containing 2.5 wt% of hdC-G is up to 124.9 at 100 Hz, which is 40 times than the value of pure PI. When the filler content is around 1.0 to 2.5 wt%, the dielectric permittivity and loss of these composites show a similar frequency dependence and there is a new relaxation process arising at low frequency range from 100 Hz to 1 kHz, exhibiting a rapid increase in the loss and  at the low frequency. Furthermore, the dielectric loss of hdC-G/PI composites do not show a significantly grow and dependency on the frequency when the content of the fillers around 1.0 to 2.5 wt%. For the conductivity part, it is well known that the measured AC conductivity can originate from the real conductivity and the dielectric relaxation. When fillers content increases to 3.0 and 4.0 wt%, the dielectric permittivity decreases and the loss increases to a high value (>5). The dramatically decreased loss with frequency and the almost independent conductivity to frequency indicate that the loss or the  observed of composites with 3.0 and 4.0 wt% of fillers are mainly determined by electrical conduction. Based on the curve shown in Fig. 4a, it is clearly observed that the dielectric permittivity is also strongly dependent on the content of fillers. Especially, the dielectric permittivity of composite with 2.5 wt% of fillers shows a stronger frequency dependence, which indicates that the dielectric behavior is dominated by the percolation phenomenon. It is known that for the conductor-dielectric 11

composite with a composition close to fc, the frequency dependence of the dielectric permittivity can be expressed as:  eff    1 (2)

where the  (0 <  < 1) is a constant. For a random binary system, the values of “” is 0.75±0.05 [63, 65]. If Eq. (2) is used to analyze the dielectric permittivity of composite with 2.5 wt%, it is found that Eq. (2) can be used to fit the experimental result, but there are two frequency regimes as shown in Fig. 5a. The fitting constant 1 is close to 1, which is much higher than the value at high frequency regime. The 2 at high frequency regime is 0.72 which is very close to the universal value (I.e. 0.75±0.05) obtained using numerical simulation. The two fitting constants mean two different dielectric relaxation processes in the composite, which indicates the fillers content of composites with 2.5 wt% is very close to percolation threshold.

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Figure 5. Dielectric permittivity versus frequency of composite with 2.5 wt% and fitting curve (fitted by Eq. (2)) (a); dependence of dielectric permittivity of the three kinds of PI based composite films on the weight percent of fillers (b); and dielectric permittivity (1 kHz) of PI composites in this work and other previous studies with different fillers (c). To further study the composition dependence, the dielectric permittivity at 1 kHz is plotted against the fillers content as shown in Fig. 5b. A typical convex-shaped curve exhibits the effect of the weight fraction of fillers on the dielectric properties of hdC-G/PI composite films. In contrast, the CNTs/PI and C-G/PI composites both show little increase in dielectric permittivity at such a low concentration of fillers. Compared with these two systems, the high dielectric permittivity obtained in hdC-G/PI composites is due to the homogeneous dispersion of hdC-G fabricated via the solvent-exchange method. Fig. 5c presents the Ashby plot of 13

dielectric permittivity and the filler content in PI composites with different fillers. Both areas C (GO [45, 66, 67]; carbon nanofiber (CNF) [68]) and D (Al2O3 [69]; Nano-Al [70]; CNTs [20, 71] ;BaTiO3 (BT) [51-55]; TiC [72]; TiO2 [73]; ZrO2 [74]; K0.5Na0.5NbO3 [75]; SiO2 [76]) cover the PI composites with relatively low dielectric permittivity (<40). In area B (BT@GO [77]; BT/CNTs [78]; Ag [48]; AgNWs [60]; CCTO [79, 80]; CCTO/Fe3O4 [80]; CNTs [56-59]; GO [81]), the high dielectric permittivity of PI composites are achieved based on the high filler addition (>5 wt%). But this high addition of fillers leads the sacrifice of mechanical properties especially toughness/flexibility, which limits their applications in flexible electronics. In comparison, area A covers an important position where both high dielectric permittivity and low filler addition could be achieved. It is obvious that the hdC-G/PI composite films in this work were filled in area A, suggesting the relatively high dielectric permittivity and the flexibility of the composites due to the small addition of fillers. To further investigate the dielectric behavior of the hdC-G/PI composites, micro-capacitor network model (Fig. 6) is proposed to interpret the crucial role of the hdC-G in tailoring the composites with high dielectric permittivity yet low content of fillers loading. This model can be briefly stated as followed: numerous micro-capacitors, formed by the electrodes of the neighbor hdC-G fillers and the dielectric layer of PI between them, constitute a large capacitance in the PI matrix. When the content of fillers increases to near fc, the neighbor fillers are very close to each other. Then the capacitance can have a correlated significant rise with the decrease of the channel between the adjacent two electrodes in the local electric field. According to the relationship between the capacitance and dielectric permittivity by the formula: C   r  0 S / d (3) 14

the dielectric permittivity has an uprush near fc. At the same time, in the case of certain content of fillers, the homogeneous dispersion of hdC-G fillers can form more micro-capacitors in the PI matrix and larger interface area with the PI matrix than the agglomeration of the fillers such as CNTs and C-G. Finally, the hdC-G/PI composites acquire higher dielectric permittivity than those of CNTs/PI and C-G/PI composites. On the other hand, compared to the nonuniform fillers embedded in the CNTs/PI and C-G/PI composite, the hdC-G/PI composites need less content of fillers to reach the fc based on the same principle.

Figure 6. Schematic illustration of the structure of hdC-G/PI film with a micro-capacitor network model. 3.4. Mechanical properties

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Figure 7. Mechanical properties of composites. Typical stress-strain curves of the C-G/PI (a) and hdC-G/PI (b) composite films with different weight content of fillers C-G and hdC-G. The tensile strength (c) and toughness (d) of two materials with different weight percent of fillers, respectively. Insert of (b) shows the flexible hdC-G/PI composite film (2.5 wt% hdC-G). Because of the inhomogeneous distribution of CNTs and GO, the C-G/PI composite films show limited reinforcement as shown in Fig. 7a, c. In comparison, the hdC-G/PI composite films exhibit significant improvement on mechanical properties (Fig. 7 and Table 1). The hdC-G/PI composite films with 1 wt% of hdC-G showed a tensile strength of 294 MPa, 52% higher than that of the pure PI films (193 MPa). As increasing the amount of hdC-G to 2.0, 2.5, 3.0 and 4.0 wt%, the tensile strength slightly decreased to 232, 213, 196 and 190 MPa, respectively, but still higher than or at the 16

same level as the pure PI films. In the hdC-G/PI composites, the stiff hdC-G fillers are wrapped by the soft PI polymer chains and their relationship can be act as the steel reinforced concrete structure. When the content of the fillers is very low, the fillers hdC-G can distribute homogeneously in the matrix and therefore lead to the enhancement of tensile strength. However, when the fillers amount increases, the fillers start to intertwine and form defect district in the films and lead to the loss of mechanical performance. Although increasing the amount of hdC-G fillers led to the decrease of elongation at break, the hdC-G/PI composites with 1.0, 2.0 and 2.5 wt% of hdC-G fillers can still keep an elongation at break of 48%, 47% and 28%, respectively, suggesting the flexibility of the composite films. To further quantitatively investigate the flexibility of hdC-G/PI composites, the toughness (area under the stress-strain curves divided by the density of the materials [82]) of the composite films are calculated (Fig. 7d, Table 1). Because of the more homogeneous distribution of fillers, the hdC-G/PI composite films show much higher toughness than that of the C-G/PI composites when adding the same amount of fillers. When the hdC-G amount is smaller than 2.5 wt%, the hdC-G/PI composite films possess toughness in the range of 36-81 J/g, which is much higher than that of the PVA cast films (2.5 J/g) and the CNTs reinforced PVA electrospun fibers (16 J/g) [83]. Such high toughness is enough to bear folding for many times (inserted photo in Fig. 7b, SI-video) and provides these composite films for further application in flexible electronic fields. 3.5.Thermal properties PI is a well-known polymer with excellent thermal stability. In this work, the pure PI film exhibits a T5% of 564 °C and char yield of 56% at 1000 °C in N2 atmosphere (Fig. 8, Table 1). After incorporation of hdC-G fillers, the T5% and char yield at 1000 °C gradually increased to 576 °C and 17

64%, respectively (Fig. 8, Table 1). These enhanced thermal properties of hdC-G/PI composite films can be attributed to the super excellent thermal stability of CNT and GO in N 2 atmosphere and the good compatibility between the fillers and the matrix. These hdC-G/PI composite films with outstanding thermal stability and good dielectric properties can be a promising candidate for the application in high temperature fields.

Figure 8. TGA curves of the PI and the hdC-G/PI composite films with different weight content of fillers at N2 atmosphere. 4. Conclusions In conclusion, high dielectric permittivity hdC-G/PI composite films with excellent mechanical and thermal properties have been successfully fabricated based on solvent-exchange process. The solvent-exchange process can lead to the homogeneous dispersion of CNTs and GO in organic solvents and therefore CNTs and GO can also uniformly distribute in PI matrix by an in situ polymerization. Compared with the CNTs/PI and C-G/PI composites, the hdC-G/PI composite films not only possess the high dielectric permittivity but also show a low content of fillers. More importantly, the hdC-G/PI composite films also possess higher tensile strength than the pure PI. The relatively high toughness of the hdC-G/PI composites guarantees the flexibility of the sample, which 18

suggests an application in wearable electronic materials. At the same time, the composites show outstanding thermal stability. It is believed that the outstanding properties of the hdC-G/PI composites can make it to be a good candidate applying to the flexible electronics even in high temperature atmosphere, and the homogeneous hdC-G dispersions in organic solvents by solvent-exchange approach provide promising opportunities for applications of the composites. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 21374044]; the Major Special Projects of Jiangxi Provincial Department of Science and Technology [grant number 20114ABF05100]; and the Technology Plan Landing Project of Jiangxi Provincial Department of Education [grant number GCJ2011-24]. References [1] Dang ZM, Yuan JK, Yao SH, Liao RJ. Flexible nanodielectric materials with high permittivity for power energy storage. Advanced Materials. 2013;25(44):6334-65. [2] Prateek, Thakur VK, Gupta RK. Recent Progress on Ferroelectric Polymer-Based Nanocomposites for High Energy Density Capacitors: Synthesis, Dielectric Properties, and Future Aspects. Chemical Reviews. 2016;116(7):4260-317. [3] Dang Z, Yuan J, Zha J, Zhou T, Li S, Hu G. Fundamentals, processes and applications of high-permittivity polymer–matrix composites. Progress in Materials Science. 2012;57(4):660-723. [4] Luo H, Zhang D, Wang L, Chen C, Zhou J, Zhou K. Highly enhanced dielectric strength and energy storage density in hydantoin@BaTiO3–P(VDF-HFP) composites with a sandwich-structure. RSC Advances. 2015;5(65):52809-16. [5] Zhang L, Xu Z, Cao L, Yao X. Synthesis of BF–PT perovskite powders by high-energy ball 19

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