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Poly (vinylidene fluoride) dielectric composites with both ionic nanoclusters and well dispersed graphene oxide
Composites Science and Technology 138 (2017) 98e105
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Poly (vinylidene fluoride) dielectric composites with both ionic nanoclusters and well dispersed graphene oxide Jipeng Guan a, b, c, Chenyang Xing a, Yanyuan Wang a, Yongjin Li a, *, Jingye Li b, c a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People's Republic of China CAS Center for Excellence on TMSR Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2016 Received in revised form 14 November 2016 Accepted 14 November 2016 Available online 16 November 2016
Nanostructured polymeric dielectric composites, based on poly (vinylidene fluoride) (PVDF), graphene oxide (GO), and an ionic liquid (IL), 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]), denoted as Nano-PVDF/IL/GO, were fabricated through casting method, electron beam (EB) irradiation and microphase separation processes. Through the strategy, GO nanosheets were dispersed homogeneously in PVDF matrix and the organic nanoclusters formed by microphase separation of IL grafted PVDF (PVDFg-IL) chain segments tended to adhere onto the surface of GO nanosheets. The Nano-PVDF/IL/GO composites displayed a drastic reduction in dc conductivity compared with PVDF/GO and PVDF/IL/GO composites because of impeding the direct contact between GO nanosheets by the nanoclusters. Furthermore, the mircocapacitors between the adjacent GO nanosheets and nanoclusters contributed the increase of dielectric constant. At the same time, the dielectric loss from the current leakage of GO nanosheets was almost fully eliminated due to the strong interfacial bonding between polymer matrix and fillers which restrained the formation of conductive network. Therefore, we proposed an effective strategy for using the organic nanoclusters to modify the inorganic conductive fillers and impeding the formation of conductive network in the dielectric materials, which paves new possibility to prepare polymer nanocomposites with both high dielectric constant and low dielectric loss. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nanostructured dielectric composites Ionic liquid Graphene oxide nanosheet Nanoclusters Electron beam irradiation
1. Introduction Two-dimensional (2D) conductive materials with high dielectric constant such as graphite, graphene oxide (GO) and graphene are the desirable conductive fillers for fabricating the high-permittivity polymer matrix composites [1e7]. Compared with dielectric ceramic fillers and one-dimensional conductive fillers, only small amount of 2D fillers is necessary to form the percolation structure, so the polymer composites can maintain the nature of polymer matrices with outstanding mechanical performance, such as flexibility and processability [3,8,9]. However, in most of the polymer matrices, it is still a challenge to obtain the homogeneous dispersed fillers because of the incompatibility between the polymer matrix and 2D fillers. In order to resolve the problem, numerous research works have been carried out to improve the dispersion of 2D fillers
* Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (J. Li). http://dx.doi.org/10.1016/j.compscitech.2016.11.012 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
in polymer matrices, mainly by the chemical/physical surface modification of the 2D fillers [10e16]. Nandi and co-workers have successfully prepared the poly (methyl methacrylate) (PMMA)functionalized graphene (MG) through atom transfer radical polymerization (ATRP) and the GO was further reduced with hydrazine hydrate [12]. PMMA on the surface of MG has excellent compatibility with PVDF and the functionalized MG can be homogeneously dispersed in PVDF matrix. Jiang and co-workers have reported a novel composites through blending the PVDF and poly (vinyl pyrrolidone) anchored reduced graphene oxide (rGO @PVP) nanosheets which enhanced the dielectric properties significantly [13]. Although the dispersion of graphene or GO in polymer matrices can be improved through the functionalization or modification on the 2D fillers with covalent/noncovalent interaction, the complex experimental steps and limited available polymer matrix restrict the further application in preparing 2D filler based composites. Due to the cations-p interactions between ionic liquids (ILs) and carbon nanoparticles, it was reported that ILs can effectively enhance the dispersion of graphene and GO in polymer matrix
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[17e23]. Kim et al. employed Poly (ionic liquid) (PIL) to modify the reduced GO (RGO) and successfully improved the wettability between ILs and PIL-RGO which contributed to high capacitance in PIL-RGO based supercapacitor [19]. In addition, with utilizing the physical cations-p and/or p-p interactions between graphene and ionic liquid, Xu et al. also achieved the PVDF/graphene nanocomposites with nice filler dispersion [23]. In our previous study, we have successfully dispersed multi walled nanotubes in PVDF matrix by using ILs as the linker because ILs have specific interactions with both carbon nanotubes and >CF2 groups of PVDF [24,25]. However, in the dielectric materials, dissociative molecules such as ILs definitely lead to the new leakage current and increase the dielectric loss due to the ion migration in electric field. Therefore, it is of great importance to find a way to restrain the mobility of ions in ionic liquids which improve the compatibility between polymer and 2D conductive fillers, for the dielectric application. Very recently, we have succeeded in constraining the immigration of ions by immobilizing the cations of ILs onto the PVDF chains through electron beam irradiation (EBI) in PVDF/IL composites [26e28]. The double bonds of IL were chemically grafted onto the PVDF molecular chains (PVDF-g-IL) by the irradiation and the PVDF-g-IL subsequently self-assembled into the nanoclusters with the size of about 20 nm by the ionic interactions in the PVDF matrix, which significantly depressed the dielectric loss and enhanced the dielectric performance of composites. The chemical bonding of IL and the microphase separation of PVDF-g-IL open a new strategy that may not only improve the dispersion of 2D fillers in polymer matrices but also decrease the dielectric loss of the final composite materials. In this work, we have first used an unsaturated IL, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]), to modify GO by the mechanical grinding and then blended the IL modified-graphene oxide (GO@IL) with PVDF. GO@IL exhibits a good compatibility with polymer matrix. After EBI, IL was successfully grafted onto the PVDF chains and confined in the nanoclusters by the microphase separation. Furthermore, the nanoclusters likely adhered to the surface of graphene oxide and improved the interfacial bonding between polymer matrix and 2D fillers. It was found that the formation of nanoclusters not only effectively impeded the direct contact of adjacent GO nanosheets and nearly eliminated the dielectric loss from GO nanosheets, but also increased the dielectric constant. 2. Experimental 2.1. Materials The homopolymer PVDF (KF850, Kureha Chemicals, Japan)
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which has Mw ¼ 209, 000, Mw/Mn ¼ 2.0 and MFI ¼ 3.57 (g/10min) at 200 C, respectively, and an ionic liquid (IL), 1-vinyl-3ethylimidazolium tetrafluoroborate ([VEIM] [BF4]) (Aldrich), were used as received. Graphene oxide (GO) (SG-001, Plannano Energy Technologies Co. Ltd., Tianjin, China) was used without any further purification. Dimethylformamide (DMF) and acetonitrile (CH3CN) (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were commercially available and used without any further purification. 2.2. Preparation 2.2.1. Preparation of PVDF/GO and PVDF/IL/GO composites The fabrication strategy of the nanocomposites was shown in Scheme 1. To obtain homogeneous GO dispersion in PVDF matrix, the GO was first treated by grinding with the IL, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]) in mortar with GO/IL mass ratios of 1/10. The physical grinding induced IL to be coated onto the surface of GO and the IL-modified GO was termed to be GO@IL hybrid. 10 wt% of PVDF solution in DMF was first prepared at 80 C. Different amounts of GO@IL hybrid and equivalent amount of IL ([VEIM] [BF4]) were added into the PVDF solutions, i.e., the ratio of IL over PVDF was 10 wt %, and the GO was 0, 0.5, 1 and 2 wt%. Here the polymer composites were denoted by PVDF/IL/GO (X), where X was referred to the amount of GO in the composites. Then the mixture solutions were casted on the Polytetrafluoroethylene (PTFE) film, and then evaporated at 80 C in an air-circulating oven for 24 h. Finally, the PVDF/IL/GO film were prepared by hot-pressing casting film at 200 C for 3 min. The PVDF/GO (Y) composite films were also prepared in the same method, where the Y was referred to the mass percent of GO. 2.2.2. Preparation of nanostructure PVDF/IL/GO composites The PVDF/IL/GO (X) films were radiated with electron beam (EB) at 45 kGy, which was denoted by EB-PVDF/IL/GO (X). Then the extraction process was performed to remove the possible excess IL and/or poly (ionic liquid) in CH3CN for 48 h. The PVDF/IL/GO films with nanostructure were obtained by hot-pressing the extracted films at 200 C for 30 min, denoted as Nano-PVDF/IL/GO (X). 2.3. Characterization 2.3.1. Sample characterization TEM (Hitachi HT 7700) and SEM (Hitachi S-4800) measurements were used to characterize the morphologies of samples. The TEM measurements were performed at 120 kV on an ultramicrotomed section with 80e90 nm thickness. The SEM measurements were conducted at 3 kV on the gold coated fracture
Scheme 1. Schematic view for the preparation of Nano-PVDF/IL/GO composites: ① grinding GO with IL in mortar; ② blending of PVDF and GO@IL in DMF; ③casting on the PTFE film and hot-press at 200 C; ④ radiating the films in EB, and then extracting samples in CH3CN for 48 h; ⑤ hot-pressing at 200 C.
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surface of samples fractured in liquid nitrogen. The differential scanning calorimeter (DSC, TA, Q2000) was used to evaluate the thermal behaviors of samples. The heating/cooling rate was 10 C/ min, and the temperature range was 20 C to 200 C. The crystallization behavior was measured with wide-angle X-ray diffraction (WAXD, Bruker D8) in the range of 2q from 5 to 45 with scanning speed of 1 /min. The small-angle X-ray scattering (SAXS, Shanghai Synchrotron Radiation Facility, China) patterns were obtained using a monochromatic X-ray beam with a wavelength of 1.24 Å. The sample-detector distance is 1820 mm and the exposed time of sample were 200 s. X-ray scattering intensity patterns were obtained by using a two-dimensional array with 2048 2048 pixels. Raman spectra of composites were obtained by using a Raman system (Bruker, Senterra, R200) with a laser wavelength of 785 nm to excite the samples. Ultrahigh-resistivity was used to measure electrical properties (Rs: surface resistivity) of samples with a ring electrode probe. The direct-current voltage between the 2 m was 10 V. An average value was chosen for Rs of each sample when at least three locations on each sample were measured. The novo-control Alpha-N high-resolution dielectric analyzer (GmbH Concept 40) was employed to evaluate the dielectric properties of samples. Measurements were carried out in the frequency range 100107 Hz at an ac of 1.0 V at room temperature. The both surfaces of each sample were evaporated by a layer of gold with a disk shape to serve as electrodes before measurement. 3. Results and discussion 3.1. Morphology study In previous work, we have reported that PVDF-g-IL can be microphase separated from the PVDF matrix and PVDF-g-IL aggregated into the nanoclusters due to the ionic interactions [26e28]. As shown in Fig. 1A and A1, numerous nanoclusters were observed after the IL grafting and microphase separation (Fig. 1A1),
while it is miscible one phase for the physically mixed PVDF/IL binary blends (Fig. 1A). It is well known that GO exhibits poor compatibility with PVDF matrix in the binary PVDF/GO composites. Without any modification, GO tends to aggregate together as shown in Fig. 1BeD. The size of GO agglomerations becomes larger with increasing the loading content of GO. In order to improve the dispersion of GO, the ionic liquid ([VEIM] [BF4]) was introduced to modify GO and the modified GO was mixed with PVDF. The NanoPVDF/IL/GO composites was obtained by following EB irradiation and the microphase separation in the melt state. As shown in Fig. 1B1eD1, in Nano-PVDF/IL/GO composites, GO is homogeneously distributed in PVDF matrix and the size of agglomerations is smaller than those in PVDF/GO composites. In other words, the addition of IL enhances the interfacial interaction of GO and PVDF and improves the dispersion of GO in PVDF matrix. Fig. 1B2eD2 shows the TEM images of Fig. 1B1eD1 with larger magnification. Numerous nanoclusters were observed in the PVDF matrix in addition to the well dispersed graphene oxide. Therefore, we have successfully prepared a novel nanocomposite with both organic nanoclusters and GO nanofillers. Fig. 2 compares the detail morphologies of the PVDF/GO (2) and the Nano-PVDF/IL/GO (2). Note that the magnifications for the two samples are different in Fig. 2. It is clear that GO in Nano-PVDF/IL/ GO (2) is well exfoliated because the GO layer thickness is much smaller than that in the PVDF/GO sample. On the other hand, it is also found that the surface of GO is smooth in PVDF/GO (2) composites. In contrast, the layer consisting of nanoclusters was observed on the both sides of GO sheets to form a sandwich structure in Nano-PVDF/IL/GO (2) composites. Such “nanoclustersGO-nanoclusters” sandwich structure was obtained by the selfassembly of the grafted IL in the melt, as depicted in Scheme 1. The “nanoclusters-GO-nanoclusters” sandwich structure enhances the interfacial strength and stabilizes the dispersion of GO in PVDF matrix. Fig. 3 shows the Raman spectra of the samples at each step
Fig. 1. (A) TEM images of PVDF/IL (100/10) sample without GO; PVDF/GO samples with different amounts (wt %) of GO: (B) 0.5; (C) 1.0; (D) 2.0; and Nano-PVDF/IL/GO samples with different amounts (wt %) of GO: (A1) and (A2) 0; (B1) and (B2) 0.5; (C1) and (C2) 1.0; (D1) and (D2) 2.0.
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Fig. 2. TEM images of (A) PVDF/GO (2) and (B) Nano-PVDF/IL/GO (2) composites.
Intensity(a.u.)
D band
ID/IG
G band
b
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c
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d 1.29
e
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a 1.07
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Fig. 3. Raman spectra of D and G bands of graphene oxide in the composites: (a) Graphene oxide; (b) GO @ IL composites; (c) PVDF/IL/GO composites before irradiation of EB; (d) EB-PVDF/IL/GO (2) composites; (e) Nano-PVDF/IL/GO composites.
during fabricating the Nano-PVDF/IL/GO composites, as detailed in the experimental section. After grinding GO with IL, the cation-p and/or p-p interaction induces IL coating onto the GO surface. This leads to the D-band of GO shifting to lower wavenumber. The Dband shifted from 1332.8 cm1 for the raw GO to 1259.9 cm1 for IL-modified GO [29e31]. In addition, the value of ID/IG also increases from 1.07 to 1.26 after the modification, where ID and IG correspond to the intensities of the peaks of D-band and G-band respectively. Simply solution mixing the modified GO with PVDF does not result in any Raman shift changing. As shown in Fig. 3 (c), PVDF/IL/GO composite before EB has almost same Raman profile with the GO@IL composites. This means that IL molecules are strongly coated on the GO surface and the solution mixing does not induce any separation of IL from the GO surface. The same results can also
be observed for the samples with the EB irradiation at room temperature. Both the peak position and the ID/IG value keep almost the same as the GO@IL composites. The results are rational because the solution mixing and the irradiation at room temperature does not induce the change of the interactions between the cations of IL and the conjugate structure of GO surface. However, the PVDF-g-IL aggregates to form nanoclusters in the melt state after the IL grafting onto the PVDF molecular chain. This means that the original ILs on the surface of GO may separate from the GO and aggregate together by the ionic interactions. Therefore, in Fig. 3e, the D-band shift from 1328 cm1 for the sample without microphase separation to 1320 cm1 for the Nano-PVDF/IL/GO sample. This means that some grafted ILs are peeled out from the surface of GO. At the same time, ID/IG value also decreases from 1.29 to 1.13 after the microphase separation. However, it should be noted that the nanoclusters by the microphase separation are still largely adhered to the GO surface to form a “nanocluster-GO-nanocluster” structure, as shown in Fig. 2. Therefore, in Nano-PVDF/IL/GO nanocomposites, the D-band appears at 1320 cm1, which is still lower than 1332 cm1 for the raw GO and the value of ID/IG is 1.13, compared 1.07 for the raw GO. This indicates that the microphase separated nanoclusters on the GO surface still have the interactions with the surface of GO. Such interactions not only enhance the interface between the nanoclusters and GO but also restrain the contact between the neighboring GO nanosheets.
3.2. Crystallization behaviors The melting and crystallization behaviors of the PVDF/IL/GO composites have been investigated by DSC (Fig. 4). For the PVDF/GO (2) composites, the melt-crystallization temperature (Tc) of PVDF is 145.9 C, which is higher than that of neat PVDF at 140.9 C in Fig. 4A. The obvious increase in Tc of PVDF was attributed to the
Fig. 4. Differential scanning calorimeter (DSC) curves of samples of each step for fabricating Nano-PVDF/IL/GO composites (the cooling curve corresponds to the first cooling process and the heating curve corresponds to the following heating process).
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nucleation effect of GO [24e26]. When IL was blended with PVDF matrix, the Tc of PVDF decreased from 140.9 to 133.6 C due to the miscibility of IL and PVDF in the PVDF/IL/GO (0) composites. With the addition of GO in the PVDF/IL/GO (2) composites, the Tc of PVDF locates at 134.2 C, which is lower than that of neat PVDF, but higher than that of the PVDF/IL/GO (0) composites. After EB irradiation and microphase separation processes, two Tc peaks of PVDF matrix at 144.1 and 150.2 C were observed in the Nano-PVDF/IL/ GO composites. The higher Tc of PVDF can be ascribed to the nucleation effect of nanoclusters [26,27]. The lower Tc, adjacent to the Tc of PVDF in the PVDF/GO (2) composites, was attributed to the nucleation effect of raw GO nanosheets. The melting behaviors of samples in each experimental step were also studied as shown in Fig. 4B. For the PVDF/GO (2) composites, the melting temperature (Tm) of PVDF is the same as that of neat PVDF at 173.2 C. The melting peak becomes narrower than neat PVDF for the nucleation effect of GO. Interestingly, in the PVDF/IL/GO (0) composites, the Tm of PVDF is lower than neat PVDF at 171.9 C and the melting peak becomes wider than neat PVDF. It can be ascribed to the miscibility of IL and PVDF which suppresses the crystallization of PVDF. The same melting behavior was observed in the ternary PVDF/IL/GO (2) composites. Compared to the binary PVDF/GO composites, the nucleation effect of GO was suppressed by IL coating on the GO surface in the PVDF/IL/GO (2) composites. After EB irradiation and the melt microphase separation processes, the nanoclusters were formed in the Nano-PVDF/IL/GO composites (Fig. 1) which exhibits the nucleating effect for PVDF matrix and increases the Tc during a cooling process (Fig. 4A), in agreement with our previous works [26e28]. In the melting process, two Tm peaks was also observed. It was considered that the higher Tm would come from the melting of the crystals with thicker lamellae nucleated by the nanoclusters, while the lower Tm corresponded to the normal a crystals of neat PVDF [26,28]. PVDF is a polymorphism polymer and it exhibits many crystal phases, such as a-phase, b-phase and g-phase. Fig. 5 shows the XRD patterns of PVDF/GO, PVDF/IL/GO and Nano-PVDF/IL/GO composites. In neat PVDF and the binary PVDF/GO composites, four characteristic diffraction peaks were observed at 17.7, 18.4, 19.9 and
26.6 , which are assigned to the (100), (020), (110) and (021) reflections of nonpolar a crystal phase (Fig. 5A). The results indicated that GO in PVDF matrix did not influence the crystal form due to the poor compatibility between GO and PVDF. In PVDF/IL/GO (0) composites, the diffraction peak of polar b crystal phase at 20.6 was observed except for the nonpolar a crystal phase. This means that the addition of IL induces the formation of polar crystal phase, as reported in our previous work [25e27]. In the PVDF/IL/GO composites, when the IL-modified GO was added into PVDF/IL composites, the interaction between > CF2 of PVDF and the planar cationic imidazolium ring coated on the GO surface induces the zigzag conformation of PVDF (Fig. 5B). Thus, the polar crystal phases were achieved and the proportion of polar b crystal phase increases gradually with increasing the content of modified GO, as shown in Fig. 5B. However, after EBI and the melting microphase separation processes, the diffraction peaks of polar b crystal phase at 20.6 disappears and the diffraction peaks of the nonpolar crystal phase appears again (Fig. 5C). In other words, in the ternary Nano-PVDF/IL/ GO composites, the nonpolar a crystal phase is prominent. All the grafted ILs were confined in the nanodomains and the >CF2 groups of PVDF have no specific interactions with the cationic imidazolium ring, so PVDF crystallizes into the nonpolar alpha phase. In order to investigate the morphology and crystallization behaviors of the composites, the SAXS measurements have been carried out (Fig. 5D). Two scattering peaks at the q of 0.10 and 0.61 nm1 were observed in Nano-PVDF/IL/GO (0) without GO. The q of 0.10 nm1 originated from the structure of nanoclusters and the q of 0.61 nm1 corresponded with the structure of lamellae crystal as discussed before [26e28,32]. Furthermore, the values of q about the structure of lamellae crystal and nanoclusters were not changed by loading GO, which indicated that the GO did not destroy the PVDF crystal lamellar structure in the composites. 3.3. Electrical conductive properties In Fig. 6 A, the dc electric properties of samples at different stages were investigated. The surface resistivity (Rs) of the binary PVDF/GO composites decreases due to the formation of conductive
Fig. 5. XRD curves of (A) PVDF/GO composites; (B) PVDF/IL/GO composites; (C) Nano-PVDF/IL/GO composites; and (D) the SAXS patterns of the Nano-PVDF/IL/GO composites.
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Fig. 6. (A) Electrical resistivity; (B), (C) and (D) ac conductivity of PVDF/GO composites and PVDF/IL/GO composites before and after EB irradiation.
network with increasing the content of GO. The introduction of IL into PVDF matrix induced the reduction of Rs drastically with five orders of magnitude because of the high mobility of ionic liquid in the PVDF/IL binary blends. However, the Rs increases instead when a small amount of GO was added into the PVDF/IL blends, which may be caused by restraining the mobility of IL modified on the GO surface. Further increasing the GO loadings leads to the slightly decreasing Rs value because of the formation of GO conductive network. After EB irradiation at room temperature, the Rs increases with three orders of magnitude, compared to PVDF/IL/GO composites. The result is rational that the IL grafting process restrained the mobility of IL in PVDF matrix. With the melting microphase separation process, the values of Rs increases drastically, compared to EB-PVDF/IL/GO and PVDF/GO composites. On the one hand, in the microphase separation process, IL was confined into the nanoclusters which limited the move of IL further in the composites. On the other hand, the addition of IL improved the dispersion of GO in PVDF matrix and disturbed the formation of conductive network. As a result, the Nano-PVDF/IL/GO composites exhibited a lower dc conductivity compared to other samples. Fig. 6BeD shows the ac conductivity of composites. In Fig. 6B, the ac conductivity of PVDF had a clear improvement with the addition of GO in the binary PVDF/GO composites. With increasing GO content, the percolation network was formed and the value of ac conductivity remain essentially constant when GO content was above 1.0 wt%. When IL was added into PVDF matrix, a huge increase of ac conductivity with five orders of magnitude was achieved in PVDF/IL/GO (0) composites than neat PVDF in Fig. 6C. The high mobility of IL contributed to the improvement of ac conductivity. With increasing the content of GO in PVDF/IL/GO composites, the ac conductivity decreases which was consistent with the study above in Fig. 6A. After EB irradiation and melting microphase separation, the ac conductivity decreases with three orders of magnitude compared with the composites before EB irradiation.
composites, the dielectric constant increases with addition of GO. When the loading is 2 wt%, the value of dielectric constant is 33.4, as compared with 13.2 of neat PVDF at 100 Hz (Fig. 7A). According to the Maxwell-Wagner-Sillars (MWS) effect [33,34], charges can be accumulated at the interface when electric current flow across the PVDF/GO interfaces. This largely enhanced the average electric field and the dielectric permittivity of PVDF matrix, so high dielectric constant can be achieved with the addition of GO. However, the poor compatibility of GO and PVDF induced the larger agglomerations of GO, which increased the leakage current and dielectric loss as shown in Fig. 7A1. The dielectric loss of PVDF/GO (2) sample is 0.9, while that of neat PVDF is 0.085 at 100 Hz. The modification of GO by IL not only leads to the homogenous dispersion of GO, but also enhances the polarization of PVDF matrix due to the interaction between ILs and PVDF. Therefore, significant enhancement of the dielectric constant can be achieved. PVDF/IL/ GO (2) shows the dielectric constant of 5400 at 100 Hz. Unfortunately, due to the movements of ions under the ac electric fields, the incorporation of IL induces drastic increasing in the dielectric loss. The PVDF/IL/GO (2) composite shows the loss value of 4.2 at 100 Hz in Fig. 7B1. In order to decrease the loss of composites, EB irradiation and the hot-pressing process was executed. After these processes, IL was chemically grafted onto the PVDF chains and confined into the nanoclusters which successfully suppressed the ion movement and dipoles of IL as shown in Fig. 7C1. Furthermore, the nanoclusters adhering on the GO surface not only impeding the direct contact of GO nanosheets, but also enhanced the interfacial strength between GO and PVDF and stabilized the dispersion of GO in PVDF matrix which effectively decreased the dielectric loss of GO nanosheets. Moreover, it is clear that the dielectric constant increases gradually with increasing loading of GO in Nano-PVDF/IL/ GO composites. At the same time, the dielectric loss of the NanoPVDF/IL/GO composites keeps almost constant at different GO loadings, as shown in Fig. 7C1. The phenomena is critically important for the fabrication of dielectric materials.
3.4. Dielectric properties 3.5. Mechanical properties Fig. 7 shows the dielectric constant and loss of PVDF/GO, PVDF/ IL/GO and Nano-PVDF/IL/GO composites. In the binary PVDF/GO
Fig. 8 shows the strain-stress curves for the PVDF/GO and Nano-
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Fig. 7. Frequency dependency of dielectric constant (A), (B) and (C); and dielectric loss (A1), (B1) and (C1) of PVDF/GO and Nano-PVDF/IL/GO composites with the indicated GO contents.
homogeneously. Such “nanoclusters-GO-nanoclusters” sandwich structure enhanced the interfacial strength between GO and PVDF matrix and improved the dielectric properties of the final nanocomposites. In the Nano-PVDF/IL/GO (2) composites, the dielectric constant is three times higher than neat PVDF which can be ascribed to the existence of nanoclusters and microcapacitors between adjacent GO nanosheets. Furthermore, the dielectric constant of Nano-PVDF/IL/GO composites increases with the addition of GO, but the dielectric loss keeps almost constant. This paves new possibility to fabricate novel nanocomposites with high dielectric constant and low dielectric loss.
Stress/MPa
60 50 40 30 20 10 0 0.00
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Fig. 8. The strain-stress curves for the PVDF/GO and Nano-PVDF/IL/GO composites with 2 wt% GO.
PVDF/IL/GO composites with 2 wt% GO. It is seen that the nanostructured composites exhibit improved modulus and tensile strength as compared with the PVDF/GO composites without nanodomains. On the other hand, both of the two composites have the elongation at break of about 7%, which is significantly higher than the inorganic dielectric materials, indicating the pretty good flexibility of the Nano-PVDF/IL/GO composites. 4. Conclusions In this study, novel Nano-PVDF/IL/GO composites with excellent dielectric properties have been fabricated. Ionic liquid was used as a modifier to increase the dispersion of GO in PVDF matrix and induced the polar b crystal phase of PVDF in the PVDF/IL/GO composites. Through solvent casting, EB irradiation and melting microphase separation processes, the nanoclusters consisting of PVDF-g-IL chain segments were formed. At the same time, GO nanosheets modified with numerous nanoclusters through cationp and/or p-p interaction were dispersed in PVDF matrix
Acknowledgments This work was primarily supported by the National Natural Science Foundation of China (21674033, 21374027) and Program for New Century Excellent Talents in University (NCET-13-0762). References [1] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible nanodielectric materials with high permittivity for power energy storage, Adv. Mater. 25 (2013) 6334e6365. [2] Z.M. Dang, J.K. Yuan, J.W. Zha, T. Zhou, S.T. Li, G.H. Hu, Fundamentals, processes and applications of high-permittivity polymerematrix composites, Prog. Mater. Sci. 57 (2012) 660e723. [3] Z.M. Dang, M.S. Zheng, J.W. Zha, 1D/2D carbon nanomaterial-polymer dielectric composites with high permittivity for power energy storage applications, Small 12 (2016) 1688e1701. [4] F. He, S. Lau, H.L. Chan, J.T. Fan, High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates, Adv. Mater. 21 (2009) 710e715. [5] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Compos. Sci. Technol. 117 (2015) 410e416. [6] R. Sengupta, M. Bhattacharya, S. Bandyopadhyay, A.K. Bhowmick, A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites, Prog. Polym. Sci. 36 (2011) 638e670. [7] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282e286. [8] X.Y. Huang, P.K. Jiang, Core-shell structured high-k polymer nanocomposites
J. Guan et al. / Composites Science and Technology 138 (2017) 98e105 for energy storage and dielectric applications, Adv. Mater. 27 (2015) 546e554. [9] Z. Li, L.A. Fredin, P. Tewari, S.A. Dibenedetto, M.T. Lanagan, M.A. Ratner, et al., In situ catalytic encapsulation of core-shell nanoparticles having variable shell thickness: dielectric and energy storage properties of high-permittivity metal oxide nanocomposites, Chem. Mater. 22 (2010) 5154e5164. [10] D.H. Lan, L. Chen, C.T. Au, S.F. Yin, One-pot synthesized multi-functional graphene oxide as a water-tolerant and efficient metal-free heterogeneous catalyst for cycloaddition reaction, Carbon 93 (2015) 22e31. [11] R.K. Layek, A.K. Das, M.J. Park, N.H. Kim, J.H. Lee, Enhancement of physical, mechanical, and gas barrier properties in noncovalently functionalized graphene oxide/poly(vinylidene fluoride) composites, Carbon 81 (2015) 329e338. [12] R.K. Layek, S. Samanta, D.P. Chatterjee, A.K. Nandi, Physical and mechanical properties of poly(methyl methacrylate) -functionalized graphene/poly(vinylidine fluoride) nanocomposites: piezoelectric b polymorph formation, Polymer 51 (2010) 5846e5856. [13] H. Li, Z. Chen, L. Liu, J. Chen, M. Jiang, C. Xiong, Poly(vinyl pyrrolidone)-coated graphene/poly(vinylidene fluoride) composite films with high dielectric permittivity and low loss, Compos. Sci. Technol. 121 (2015) 49e55. [14] Y.J. Wan, W.H. Yang, S.H. Yu, R. Sun, C.P. Wong, W.H. Liao, Covalent polymer functionalization of graphene for improved dielectric properties and thermal stability of epoxy composites, Compos. Sci. Technol. 122 (2016) 27e35. [15] J.Y. Wang, S.Y. Yang, Y.L. Huang, H.W. Tien, W.K. Chin, C.C.M. Ma, Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situ polymerization, J. Mater. Chem. 21 (2011) 13569. [16] J. Tong, H.X. Huang, M. Wu, Facile green fabrication of well dispersed poly(vinylidene fluoride)/graphene oxide nanocomposites with improved properties, Compos. Sci. Technol. 129 (2016) 183e190. [17] Q. Ji, I. Honma, S.M. Paek, M. Akada, J.P. Hill, A. Vinu, et al., Layer-by-layer films of graphene and ionic liquids for highly selective gas sensing, Angew. Chem. Int. Ed. 49 (2010) 9737e9739. [18] N. Karousis, S.P. Economopoulos, E. Sarantopoulou, N. Tagmatarchis, Porphyrin counter anion in imidazolium-modified graphene-oxide, Carbon 48 (2010) 854e860. [19] T.Y. Kim, H.W. Lee, M. Stoller, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, et al., High-performance supercapacitors based on poly(ionic liquid)-modified graphene electrodes, ACS Nano 5 (2010) 436e442. [20] J.S. Lee, T. Lee, H.K. Song, J. Cho, B.S. Kim, Ionic liquid modified graphene nanosheets anchoring manganese oxide nanoparticles as efficient electrocatalysts for Zneair batteries, Energy. Environ. Sci. 4 (2011) 4148e4154. [21] B.C. Lin, T.Y. Feng, F.Q. Chu, S. Zhang, N.Y. Yuan, G. Qiao, et al., Poly(ionic liquid)/ionic liquid/graphene oxide composite quasi solid-state electrolytes
105
for dye sensitized solar cells, RSC Adv. 5 (2015) 57216e57222. [22] N. Maity, A. Mandal, A.K. Nandi, Interface engineering of ionic liquid integrated graphene in poly(vinylidene fluoride) matrix yielding magnificent improvement in mechanical, electrical and dielectric properties, Polymer 65 (2015) 154e167. [23] P. Xu, H.G. Gui, X.X. Wang, Y.D. Hu, Y.S. Ding, Improved dielectric properties of nanocomposites based on polyvinylidene fluoride and ionic liquidfunctionalized graphene, Compos. Sci. Technol. 117 (2015) 282e288. [24] C.Y. Xing, L.P. Zhao, J.C. You, W.Y. Dong, X.J. Cao, Y.J. Li, Impact of ionic liquidmodified multiwalled carbon nanotubes on the crystallization behavior of poly(vinylidene fluoride), J. Phys. Chem. B 116 (2012) 8312e8320. [25] C.Y. Xing, M.M. Zhao, L.P. Zhao, J.C. You, X.J. Cao, Y.J. Li, Ionic liquid modified poly(vinylidene fluoride): crystalline structures, miscibility, and physical properties, Polym. Chem. 4 (2013) 5726e5734. [26] C.Y. Xing, Y.Y. Wang, X.Y. Huang, Y.J. Li, J.Y. Li, Poly(vinylidene fluoride) nanocomposites with simultaneous organic nanodomains and inorganic nanoparticles, Macromolecules 49 (2016) 1026e1035. [27] C.Y. Xing, Y.Y. Wang, C. Zhang, L.F. Li, Y.J. Li, J.Y. Li, Immobilization of ionic liquids onto the poly(vinylidene fluoride) by electron beam irradiation, Ind. Eng. Chem. Res. 54 (2015) 9351e9359. [28] C.Y. Xing, J.C. You, Y.J. Li, J.Y. Li, Nanostructured poly (vinylidene fluoride)/ ionic liquid composites: formation of organic conductive nanodomains in polymer matrix, J. Phys. Chem. C 119 (2015) 21155e21164. [29] L. Zhao, Y. Li, X. Cao, J. You, W. Dong, Multifunctional role of an ionic liquid in melt-blended poly (methyl methacrylate)/multi-walled carbon nanotube nanocomposites, Nanotechnology 23 (2012) 255702. [30] L. Zhao, Y. Li, J. Qiu, J. You, W. Dong, X. Cao, Reactive bonding mediated high mass loading of individualized single-walled carbon nanotubes in an elastomeric polymer, Nanoscale 4 (2012) 6613e6621. [31] L.P. Zhao, Y.J. Li, Z.F. Liu, H. Shimizu, Carbon nanotube-conducting polymer coreshell hybrid using an imidazolium-salt-based ionic liquid as a linker: designed as a potential platinum electrode alternative material for large-scale solution processing, Chem. Mater. 22 (2010) 5949e5956. [32] C. Xing, J. Li, C. Yang, Y. Li, Locally grafting of ionic liquid in poly(vinylidene fluoride) amorphous region and the subsequent microphase separation behavior in melt, Macromol. Rapid Commun. 37 (2016) 1559e1565. [33] R. Tamura, E. Lim, T. Manaka, M. Iwamoto, Analysis of pentacene field effect transistor as a Maxwell-Wagner effect element, J. Appl. Phys. 100 (2006) 114515. [34] J.K. Yuan, S.H. Yao, Z.M. Dang, M. Genestoux, J. Bai, Giant dielectric permittivity nanocomposites: realizing true potential of pristine carbon nanotubes in poly(vinylidene fluoride) through an enhanced interfacial interaction, J. Phys. Chem. C 115 (2011) 5515e5521.
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Poly (vinylidene fluoride) dielectric composites with both ionic nanoclusters and well dispersed graphene oxide Jipeng Guan a, b, c, Chenyang Xing a, Yanyuan Wang a, Yongjin Li a, *, Jingye Li b, c a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People's Republic of China CAS Center for Excellence on TMSR Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 September 2016 Received in revised form 14 November 2016 Accepted 14 November 2016 Available online 16 November 2016
Nanostructured polymeric dielectric composites, based on poly (vinylidene fluoride) (PVDF), graphene oxide (GO), and an ionic liquid (IL), 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]), denoted as Nano-PVDF/IL/GO, were fabricated through casting method, electron beam (EB) irradiation and microphase separation processes. Through the strategy, GO nanosheets were dispersed homogeneously in PVDF matrix and the organic nanoclusters formed by microphase separation of IL grafted PVDF (PVDFg-IL) chain segments tended to adhere onto the surface of GO nanosheets. The Nano-PVDF/IL/GO composites displayed a drastic reduction in dc conductivity compared with PVDF/GO and PVDF/IL/GO composites because of impeding the direct contact between GO nanosheets by the nanoclusters. Furthermore, the mircocapacitors between the adjacent GO nanosheets and nanoclusters contributed the increase of dielectric constant. At the same time, the dielectric loss from the current leakage of GO nanosheets was almost fully eliminated due to the strong interfacial bonding between polymer matrix and fillers which restrained the formation of conductive network. Therefore, we proposed an effective strategy for using the organic nanoclusters to modify the inorganic conductive fillers and impeding the formation of conductive network in the dielectric materials, which paves new possibility to prepare polymer nanocomposites with both high dielectric constant and low dielectric loss. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nanostructured dielectric composites Ionic liquid Graphene oxide nanosheet Nanoclusters Electron beam irradiation
1. Introduction Two-dimensional (2D) conductive materials with high dielectric constant such as graphite, graphene oxide (GO) and graphene are the desirable conductive fillers for fabricating the high-permittivity polymer matrix composites [1e7]. Compared with dielectric ceramic fillers and one-dimensional conductive fillers, only small amount of 2D fillers is necessary to form the percolation structure, so the polymer composites can maintain the nature of polymer matrices with outstanding mechanical performance, such as flexibility and processability [3,8,9]. However, in most of the polymer matrices, it is still a challenge to obtain the homogeneous dispersed fillers because of the incompatibility between the polymer matrix and 2D fillers. In order to resolve the problem, numerous research works have been carried out to improve the dispersion of 2D fillers
* Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (J. Li). http://dx.doi.org/10.1016/j.compscitech.2016.11.012 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
in polymer matrices, mainly by the chemical/physical surface modification of the 2D fillers [10e16]. Nandi and co-workers have successfully prepared the poly (methyl methacrylate) (PMMA)functionalized graphene (MG) through atom transfer radical polymerization (ATRP) and the GO was further reduced with hydrazine hydrate [12]. PMMA on the surface of MG has excellent compatibility with PVDF and the functionalized MG can be homogeneously dispersed in PVDF matrix. Jiang and co-workers have reported a novel composites through blending the PVDF and poly (vinyl pyrrolidone) anchored reduced graphene oxide (rGO @PVP) nanosheets which enhanced the dielectric properties significantly [13]. Although the dispersion of graphene or GO in polymer matrices can be improved through the functionalization or modification on the 2D fillers with covalent/noncovalent interaction, the complex experimental steps and limited available polymer matrix restrict the further application in preparing 2D filler based composites. Due to the cations-p interactions between ionic liquids (ILs) and carbon nanoparticles, it was reported that ILs can effectively enhance the dispersion of graphene and GO in polymer matrix
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[17e23]. Kim et al. employed Poly (ionic liquid) (PIL) to modify the reduced GO (RGO) and successfully improved the wettability between ILs and PIL-RGO which contributed to high capacitance in PIL-RGO based supercapacitor [19]. In addition, with utilizing the physical cations-p and/or p-p interactions between graphene and ionic liquid, Xu et al. also achieved the PVDF/graphene nanocomposites with nice filler dispersion [23]. In our previous study, we have successfully dispersed multi walled nanotubes in PVDF matrix by using ILs as the linker because ILs have specific interactions with both carbon nanotubes and >CF2 groups of PVDF [24,25]. However, in the dielectric materials, dissociative molecules such as ILs definitely lead to the new leakage current and increase the dielectric loss due to the ion migration in electric field. Therefore, it is of great importance to find a way to restrain the mobility of ions in ionic liquids which improve the compatibility between polymer and 2D conductive fillers, for the dielectric application. Very recently, we have succeeded in constraining the immigration of ions by immobilizing the cations of ILs onto the PVDF chains through electron beam irradiation (EBI) in PVDF/IL composites [26e28]. The double bonds of IL were chemically grafted onto the PVDF molecular chains (PVDF-g-IL) by the irradiation and the PVDF-g-IL subsequently self-assembled into the nanoclusters with the size of about 20 nm by the ionic interactions in the PVDF matrix, which significantly depressed the dielectric loss and enhanced the dielectric performance of composites. The chemical bonding of IL and the microphase separation of PVDF-g-IL open a new strategy that may not only improve the dispersion of 2D fillers in polymer matrices but also decrease the dielectric loss of the final composite materials. In this work, we have first used an unsaturated IL, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]), to modify GO by the mechanical grinding and then blended the IL modified-graphene oxide (GO@IL) with PVDF. GO@IL exhibits a good compatibility with polymer matrix. After EBI, IL was successfully grafted onto the PVDF chains and confined in the nanoclusters by the microphase separation. Furthermore, the nanoclusters likely adhered to the surface of graphene oxide and improved the interfacial bonding between polymer matrix and 2D fillers. It was found that the formation of nanoclusters not only effectively impeded the direct contact of adjacent GO nanosheets and nearly eliminated the dielectric loss from GO nanosheets, but also increased the dielectric constant. 2. Experimental 2.1. Materials The homopolymer PVDF (KF850, Kureha Chemicals, Japan)
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which has Mw ¼ 209, 000, Mw/Mn ¼ 2.0 and MFI ¼ 3.57 (g/10min) at 200 C, respectively, and an ionic liquid (IL), 1-vinyl-3ethylimidazolium tetrafluoroborate ([VEIM] [BF4]) (Aldrich), were used as received. Graphene oxide (GO) (SG-001, Plannano Energy Technologies Co. Ltd., Tianjin, China) was used without any further purification. Dimethylformamide (DMF) and acetonitrile (CH3CN) (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were commercially available and used without any further purification. 2.2. Preparation 2.2.1. Preparation of PVDF/GO and PVDF/IL/GO composites The fabrication strategy of the nanocomposites was shown in Scheme 1. To obtain homogeneous GO dispersion in PVDF matrix, the GO was first treated by grinding with the IL, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM] [BF4]) in mortar with GO/IL mass ratios of 1/10. The physical grinding induced IL to be coated onto the surface of GO and the IL-modified GO was termed to be GO@IL hybrid. 10 wt% of PVDF solution in DMF was first prepared at 80 C. Different amounts of GO@IL hybrid and equivalent amount of IL ([VEIM] [BF4]) were added into the PVDF solutions, i.e., the ratio of IL over PVDF was 10 wt %, and the GO was 0, 0.5, 1 and 2 wt%. Here the polymer composites were denoted by PVDF/IL/GO (X), where X was referred to the amount of GO in the composites. Then the mixture solutions were casted on the Polytetrafluoroethylene (PTFE) film, and then evaporated at 80 C in an air-circulating oven for 24 h. Finally, the PVDF/IL/GO film were prepared by hot-pressing casting film at 200 C for 3 min. The PVDF/GO (Y) composite films were also prepared in the same method, where the Y was referred to the mass percent of GO. 2.2.2. Preparation of nanostructure PVDF/IL/GO composites The PVDF/IL/GO (X) films were radiated with electron beam (EB) at 45 kGy, which was denoted by EB-PVDF/IL/GO (X). Then the extraction process was performed to remove the possible excess IL and/or poly (ionic liquid) in CH3CN for 48 h. The PVDF/IL/GO films with nanostructure were obtained by hot-pressing the extracted films at 200 C for 30 min, denoted as Nano-PVDF/IL/GO (X). 2.3. Characterization 2.3.1. Sample characterization TEM (Hitachi HT 7700) and SEM (Hitachi S-4800) measurements were used to characterize the morphologies of samples. The TEM measurements were performed at 120 kV on an ultramicrotomed section with 80e90 nm thickness. The SEM measurements were conducted at 3 kV on the gold coated fracture
Scheme 1. Schematic view for the preparation of Nano-PVDF/IL/GO composites: ① grinding GO with IL in mortar; ② blending of PVDF and GO@IL in DMF; ③casting on the PTFE film and hot-press at 200 C; ④ radiating the films in EB, and then extracting samples in CH3CN for 48 h; ⑤ hot-pressing at 200 C.
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surface of samples fractured in liquid nitrogen. The differential scanning calorimeter (DSC, TA, Q2000) was used to evaluate the thermal behaviors of samples. The heating/cooling rate was 10 C/ min, and the temperature range was 20 C to 200 C. The crystallization behavior was measured with wide-angle X-ray diffraction (WAXD, Bruker D8) in the range of 2q from 5 to 45 with scanning speed of 1 /min. The small-angle X-ray scattering (SAXS, Shanghai Synchrotron Radiation Facility, China) patterns were obtained using a monochromatic X-ray beam with a wavelength of 1.24 Å. The sample-detector distance is 1820 mm and the exposed time of sample were 200 s. X-ray scattering intensity patterns were obtained by using a two-dimensional array with 2048 2048 pixels. Raman spectra of composites were obtained by using a Raman system (Bruker, Senterra, R200) with a laser wavelength of 785 nm to excite the samples. Ultrahigh-resistivity was used to measure electrical properties (Rs: surface resistivity) of samples with a ring electrode probe. The direct-current voltage between the 2 m was 10 V. An average value was chosen for Rs of each sample when at least three locations on each sample were measured. The novo-control Alpha-N high-resolution dielectric analyzer (GmbH Concept 40) was employed to evaluate the dielectric properties of samples. Measurements were carried out in the frequency range 100107 Hz at an ac of 1.0 V at room temperature. The both surfaces of each sample were evaporated by a layer of gold with a disk shape to serve as electrodes before measurement. 3. Results and discussion 3.1. Morphology study In previous work, we have reported that PVDF-g-IL can be microphase separated from the PVDF matrix and PVDF-g-IL aggregated into the nanoclusters due to the ionic interactions [26e28]. As shown in Fig. 1A and A1, numerous nanoclusters were observed after the IL grafting and microphase separation (Fig. 1A1),
while it is miscible one phase for the physically mixed PVDF/IL binary blends (Fig. 1A). It is well known that GO exhibits poor compatibility with PVDF matrix in the binary PVDF/GO composites. Without any modification, GO tends to aggregate together as shown in Fig. 1BeD. The size of GO agglomerations becomes larger with increasing the loading content of GO. In order to improve the dispersion of GO, the ionic liquid ([VEIM] [BF4]) was introduced to modify GO and the modified GO was mixed with PVDF. The NanoPVDF/IL/GO composites was obtained by following EB irradiation and the microphase separation in the melt state. As shown in Fig. 1B1eD1, in Nano-PVDF/IL/GO composites, GO is homogeneously distributed in PVDF matrix and the size of agglomerations is smaller than those in PVDF/GO composites. In other words, the addition of IL enhances the interfacial interaction of GO and PVDF and improves the dispersion of GO in PVDF matrix. Fig. 1B2eD2 shows the TEM images of Fig. 1B1eD1 with larger magnification. Numerous nanoclusters were observed in the PVDF matrix in addition to the well dispersed graphene oxide. Therefore, we have successfully prepared a novel nanocomposite with both organic nanoclusters and GO nanofillers. Fig. 2 compares the detail morphologies of the PVDF/GO (2) and the Nano-PVDF/IL/GO (2). Note that the magnifications for the two samples are different in Fig. 2. It is clear that GO in Nano-PVDF/IL/ GO (2) is well exfoliated because the GO layer thickness is much smaller than that in the PVDF/GO sample. On the other hand, it is also found that the surface of GO is smooth in PVDF/GO (2) composites. In contrast, the layer consisting of nanoclusters was observed on the both sides of GO sheets to form a sandwich structure in Nano-PVDF/IL/GO (2) composites. Such “nanoclustersGO-nanoclusters” sandwich structure was obtained by the selfassembly of the grafted IL in the melt, as depicted in Scheme 1. The “nanoclusters-GO-nanoclusters” sandwich structure enhances the interfacial strength and stabilizes the dispersion of GO in PVDF matrix. Fig. 3 shows the Raman spectra of the samples at each step
Fig. 1. (A) TEM images of PVDF/IL (100/10) sample without GO; PVDF/GO samples with different amounts (wt %) of GO: (B) 0.5; (C) 1.0; (D) 2.0; and Nano-PVDF/IL/GO samples with different amounts (wt %) of GO: (A1) and (A2) 0; (B1) and (B2) 0.5; (C1) and (C2) 1.0; (D1) and (D2) 2.0.
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Fig. 2. TEM images of (A) PVDF/GO (2) and (B) Nano-PVDF/IL/GO (2) composites.
Intensity(a.u.)
D band
ID/IG
G band
b
1.28
c
1.30
d 1.29
e
1.13
a 1.07
1100
1210
1320
1430
1540
1650
Raman Shift/cm
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-1
Fig. 3. Raman spectra of D and G bands of graphene oxide in the composites: (a) Graphene oxide; (b) GO @ IL composites; (c) PVDF/IL/GO composites before irradiation of EB; (d) EB-PVDF/IL/GO (2) composites; (e) Nano-PVDF/IL/GO composites.
during fabricating the Nano-PVDF/IL/GO composites, as detailed in the experimental section. After grinding GO with IL, the cation-p and/or p-p interaction induces IL coating onto the GO surface. This leads to the D-band of GO shifting to lower wavenumber. The Dband shifted from 1332.8 cm1 for the raw GO to 1259.9 cm1 for IL-modified GO [29e31]. In addition, the value of ID/IG also increases from 1.07 to 1.26 after the modification, where ID and IG correspond to the intensities of the peaks of D-band and G-band respectively. Simply solution mixing the modified GO with PVDF does not result in any Raman shift changing. As shown in Fig. 3 (c), PVDF/IL/GO composite before EB has almost same Raman profile with the GO@IL composites. This means that IL molecules are strongly coated on the GO surface and the solution mixing does not induce any separation of IL from the GO surface. The same results can also
be observed for the samples with the EB irradiation at room temperature. Both the peak position and the ID/IG value keep almost the same as the GO@IL composites. The results are rational because the solution mixing and the irradiation at room temperature does not induce the change of the interactions between the cations of IL and the conjugate structure of GO surface. However, the PVDF-g-IL aggregates to form nanoclusters in the melt state after the IL grafting onto the PVDF molecular chain. This means that the original ILs on the surface of GO may separate from the GO and aggregate together by the ionic interactions. Therefore, in Fig. 3e, the D-band shift from 1328 cm1 for the sample without microphase separation to 1320 cm1 for the Nano-PVDF/IL/GO sample. This means that some grafted ILs are peeled out from the surface of GO. At the same time, ID/IG value also decreases from 1.29 to 1.13 after the microphase separation. However, it should be noted that the nanoclusters by the microphase separation are still largely adhered to the GO surface to form a “nanocluster-GO-nanocluster” structure, as shown in Fig. 2. Therefore, in Nano-PVDF/IL/GO nanocomposites, the D-band appears at 1320 cm1, which is still lower than 1332 cm1 for the raw GO and the value of ID/IG is 1.13, compared 1.07 for the raw GO. This indicates that the microphase separated nanoclusters on the GO surface still have the interactions with the surface of GO. Such interactions not only enhance the interface between the nanoclusters and GO but also restrain the contact between the neighboring GO nanosheets.
3.2. Crystallization behaviors The melting and crystallization behaviors of the PVDF/IL/GO composites have been investigated by DSC (Fig. 4). For the PVDF/GO (2) composites, the melt-crystallization temperature (Tc) of PVDF is 145.9 C, which is higher than that of neat PVDF at 140.9 C in Fig. 4A. The obvious increase in Tc of PVDF was attributed to the
Fig. 4. Differential scanning calorimeter (DSC) curves of samples of each step for fabricating Nano-PVDF/IL/GO composites (the cooling curve corresponds to the first cooling process and the heating curve corresponds to the following heating process).
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nucleation effect of GO [24e26]. When IL was blended with PVDF matrix, the Tc of PVDF decreased from 140.9 to 133.6 C due to the miscibility of IL and PVDF in the PVDF/IL/GO (0) composites. With the addition of GO in the PVDF/IL/GO (2) composites, the Tc of PVDF locates at 134.2 C, which is lower than that of neat PVDF, but higher than that of the PVDF/IL/GO (0) composites. After EB irradiation and microphase separation processes, two Tc peaks of PVDF matrix at 144.1 and 150.2 C were observed in the Nano-PVDF/IL/ GO composites. The higher Tc of PVDF can be ascribed to the nucleation effect of nanoclusters [26,27]. The lower Tc, adjacent to the Tc of PVDF in the PVDF/GO (2) composites, was attributed to the nucleation effect of raw GO nanosheets. The melting behaviors of samples in each experimental step were also studied as shown in Fig. 4B. For the PVDF/GO (2) composites, the melting temperature (Tm) of PVDF is the same as that of neat PVDF at 173.2 C. The melting peak becomes narrower than neat PVDF for the nucleation effect of GO. Interestingly, in the PVDF/IL/GO (0) composites, the Tm of PVDF is lower than neat PVDF at 171.9 C and the melting peak becomes wider than neat PVDF. It can be ascribed to the miscibility of IL and PVDF which suppresses the crystallization of PVDF. The same melting behavior was observed in the ternary PVDF/IL/GO (2) composites. Compared to the binary PVDF/GO composites, the nucleation effect of GO was suppressed by IL coating on the GO surface in the PVDF/IL/GO (2) composites. After EB irradiation and the melt microphase separation processes, the nanoclusters were formed in the Nano-PVDF/IL/GO composites (Fig. 1) which exhibits the nucleating effect for PVDF matrix and increases the Tc during a cooling process (Fig. 4A), in agreement with our previous works [26e28]. In the melting process, two Tm peaks was also observed. It was considered that the higher Tm would come from the melting of the crystals with thicker lamellae nucleated by the nanoclusters, while the lower Tm corresponded to the normal a crystals of neat PVDF [26,28]. PVDF is a polymorphism polymer and it exhibits many crystal phases, such as a-phase, b-phase and g-phase. Fig. 5 shows the XRD patterns of PVDF/GO, PVDF/IL/GO and Nano-PVDF/IL/GO composites. In neat PVDF and the binary PVDF/GO composites, four characteristic diffraction peaks were observed at 17.7, 18.4, 19.9 and
26.6 , which are assigned to the (100), (020), (110) and (021) reflections of nonpolar a crystal phase (Fig. 5A). The results indicated that GO in PVDF matrix did not influence the crystal form due to the poor compatibility between GO and PVDF. In PVDF/IL/GO (0) composites, the diffraction peak of polar b crystal phase at 20.6 was observed except for the nonpolar a crystal phase. This means that the addition of IL induces the formation of polar crystal phase, as reported in our previous work [25e27]. In the PVDF/IL/GO composites, when the IL-modified GO was added into PVDF/IL composites, the interaction between > CF2 of PVDF and the planar cationic imidazolium ring coated on the GO surface induces the zigzag conformation of PVDF (Fig. 5B). Thus, the polar crystal phases were achieved and the proportion of polar b crystal phase increases gradually with increasing the content of modified GO, as shown in Fig. 5B. However, after EBI and the melting microphase separation processes, the diffraction peaks of polar b crystal phase at 20.6 disappears and the diffraction peaks of the nonpolar crystal phase appears again (Fig. 5C). In other words, in the ternary Nano-PVDF/IL/ GO composites, the nonpolar a crystal phase is prominent. All the grafted ILs were confined in the nanodomains and the >CF2 groups of PVDF have no specific interactions with the cationic imidazolium ring, so PVDF crystallizes into the nonpolar alpha phase. In order to investigate the morphology and crystallization behaviors of the composites, the SAXS measurements have been carried out (Fig. 5D). Two scattering peaks at the q of 0.10 and 0.61 nm1 were observed in Nano-PVDF/IL/GO (0) without GO. The q of 0.10 nm1 originated from the structure of nanoclusters and the q of 0.61 nm1 corresponded with the structure of lamellae crystal as discussed before [26e28,32]. Furthermore, the values of q about the structure of lamellae crystal and nanoclusters were not changed by loading GO, which indicated that the GO did not destroy the PVDF crystal lamellar structure in the composites. 3.3. Electrical conductive properties In Fig. 6 A, the dc electric properties of samples at different stages were investigated. The surface resistivity (Rs) of the binary PVDF/GO composites decreases due to the formation of conductive
Fig. 5. XRD curves of (A) PVDF/GO composites; (B) PVDF/IL/GO composites; (C) Nano-PVDF/IL/GO composites; and (D) the SAXS patterns of the Nano-PVDF/IL/GO composites.
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Fig. 6. (A) Electrical resistivity; (B), (C) and (D) ac conductivity of PVDF/GO composites and PVDF/IL/GO composites before and after EB irradiation.
network with increasing the content of GO. The introduction of IL into PVDF matrix induced the reduction of Rs drastically with five orders of magnitude because of the high mobility of ionic liquid in the PVDF/IL binary blends. However, the Rs increases instead when a small amount of GO was added into the PVDF/IL blends, which may be caused by restraining the mobility of IL modified on the GO surface. Further increasing the GO loadings leads to the slightly decreasing Rs value because of the formation of GO conductive network. After EB irradiation at room temperature, the Rs increases with three orders of magnitude, compared to PVDF/IL/GO composites. The result is rational that the IL grafting process restrained the mobility of IL in PVDF matrix. With the melting microphase separation process, the values of Rs increases drastically, compared to EB-PVDF/IL/GO and PVDF/GO composites. On the one hand, in the microphase separation process, IL was confined into the nanoclusters which limited the move of IL further in the composites. On the other hand, the addition of IL improved the dispersion of GO in PVDF matrix and disturbed the formation of conductive network. As a result, the Nano-PVDF/IL/GO composites exhibited a lower dc conductivity compared to other samples. Fig. 6BeD shows the ac conductivity of composites. In Fig. 6B, the ac conductivity of PVDF had a clear improvement with the addition of GO in the binary PVDF/GO composites. With increasing GO content, the percolation network was formed and the value of ac conductivity remain essentially constant when GO content was above 1.0 wt%. When IL was added into PVDF matrix, a huge increase of ac conductivity with five orders of magnitude was achieved in PVDF/IL/GO (0) composites than neat PVDF in Fig. 6C. The high mobility of IL contributed to the improvement of ac conductivity. With increasing the content of GO in PVDF/IL/GO composites, the ac conductivity decreases which was consistent with the study above in Fig. 6A. After EB irradiation and melting microphase separation, the ac conductivity decreases with three orders of magnitude compared with the composites before EB irradiation.
composites, the dielectric constant increases with addition of GO. When the loading is 2 wt%, the value of dielectric constant is 33.4, as compared with 13.2 of neat PVDF at 100 Hz (Fig. 7A). According to the Maxwell-Wagner-Sillars (MWS) effect [33,34], charges can be accumulated at the interface when electric current flow across the PVDF/GO interfaces. This largely enhanced the average electric field and the dielectric permittivity of PVDF matrix, so high dielectric constant can be achieved with the addition of GO. However, the poor compatibility of GO and PVDF induced the larger agglomerations of GO, which increased the leakage current and dielectric loss as shown in Fig. 7A1. The dielectric loss of PVDF/GO (2) sample is 0.9, while that of neat PVDF is 0.085 at 100 Hz. The modification of GO by IL not only leads to the homogenous dispersion of GO, but also enhances the polarization of PVDF matrix due to the interaction between ILs and PVDF. Therefore, significant enhancement of the dielectric constant can be achieved. PVDF/IL/ GO (2) shows the dielectric constant of 5400 at 100 Hz. Unfortunately, due to the movements of ions under the ac electric fields, the incorporation of IL induces drastic increasing in the dielectric loss. The PVDF/IL/GO (2) composite shows the loss value of 4.2 at 100 Hz in Fig. 7B1. In order to decrease the loss of composites, EB irradiation and the hot-pressing process was executed. After these processes, IL was chemically grafted onto the PVDF chains and confined into the nanoclusters which successfully suppressed the ion movement and dipoles of IL as shown in Fig. 7C1. Furthermore, the nanoclusters adhering on the GO surface not only impeding the direct contact of GO nanosheets, but also enhanced the interfacial strength between GO and PVDF and stabilized the dispersion of GO in PVDF matrix which effectively decreased the dielectric loss of GO nanosheets. Moreover, it is clear that the dielectric constant increases gradually with increasing loading of GO in Nano-PVDF/IL/ GO composites. At the same time, the dielectric loss of the NanoPVDF/IL/GO composites keeps almost constant at different GO loadings, as shown in Fig. 7C1. The phenomena is critically important for the fabrication of dielectric materials.
3.4. Dielectric properties 3.5. Mechanical properties Fig. 7 shows the dielectric constant and loss of PVDF/GO, PVDF/ IL/GO and Nano-PVDF/IL/GO composites. In the binary PVDF/GO
Fig. 8 shows the strain-stress curves for the PVDF/GO and Nano-
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Fig. 7. Frequency dependency of dielectric constant (A), (B) and (C); and dielectric loss (A1), (B1) and (C1) of PVDF/GO and Nano-PVDF/IL/GO composites with the indicated GO contents.
homogeneously. Such “nanoclusters-GO-nanoclusters” sandwich structure enhanced the interfacial strength between GO and PVDF matrix and improved the dielectric properties of the final nanocomposites. In the Nano-PVDF/IL/GO (2) composites, the dielectric constant is three times higher than neat PVDF which can be ascribed to the existence of nanoclusters and microcapacitors between adjacent GO nanosheets. Furthermore, the dielectric constant of Nano-PVDF/IL/GO composites increases with the addition of GO, but the dielectric loss keeps almost constant. This paves new possibility to fabricate novel nanocomposites with high dielectric constant and low dielectric loss.
Stress/MPa
60 50 40 30 20 10 0 0.00
PVDF/GO (2.0) Nano-PVDF/IL/GO (2.0)
0.02
0.04
0.06
Strain(mm/mm)
0.08
0.10
Fig. 8. The strain-stress curves for the PVDF/GO and Nano-PVDF/IL/GO composites with 2 wt% GO.
PVDF/IL/GO composites with 2 wt% GO. It is seen that the nanostructured composites exhibit improved modulus and tensile strength as compared with the PVDF/GO composites without nanodomains. On the other hand, both of the two composites have the elongation at break of about 7%, which is significantly higher than the inorganic dielectric materials, indicating the pretty good flexibility of the Nano-PVDF/IL/GO composites. 4. Conclusions In this study, novel Nano-PVDF/IL/GO composites with excellent dielectric properties have been fabricated. Ionic liquid was used as a modifier to increase the dispersion of GO in PVDF matrix and induced the polar b crystal phase of PVDF in the PVDF/IL/GO composites. Through solvent casting, EB irradiation and melting microphase separation processes, the nanoclusters consisting of PVDF-g-IL chain segments were formed. At the same time, GO nanosheets modified with numerous nanoclusters through cationp and/or p-p interaction were dispersed in PVDF matrix
Acknowledgments This work was primarily supported by the National Natural Science Foundation of China (21674033, 21374027) and Program for New Century Excellent Talents in University (NCET-13-0762). References [1] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible nanodielectric materials with high permittivity for power energy storage, Adv. Mater. 25 (2013) 6334e6365. [2] Z.M. Dang, J.K. Yuan, J.W. Zha, T. Zhou, S.T. Li, G.H. Hu, Fundamentals, processes and applications of high-permittivity polymerematrix composites, Prog. Mater. Sci. 57 (2012) 660e723. [3] Z.M. Dang, M.S. Zheng, J.W. Zha, 1D/2D carbon nanomaterial-polymer dielectric composites with high permittivity for power energy storage applications, Small 12 (2016) 1688e1701. [4] F. He, S. Lau, H.L. Chan, J.T. Fan, High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates, Adv. Mater. 21 (2009) 710e715. [5] Z. Wang, M. Fang, H. Li, Y. Wen, C. Wang, Y. Pu, Enhanced dielectric properties in poly(vinylidene fluoride) composites by nanosized Ba(Fe0.5Nb0.5)O3 powders, Compos. Sci. Technol. 117 (2015) 410e416. [6] R. Sengupta, M. Bhattacharya, S. Bandyopadhyay, A.K. Bhowmick, A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites, Prog. Polym. Sci. 36 (2011) 638e670. [7] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282e286. [8] X.Y. Huang, P.K. Jiang, Core-shell structured high-k polymer nanocomposites
J. Guan et al. / Composites Science and Technology 138 (2017) 98e105 for energy storage and dielectric applications, Adv. Mater. 27 (2015) 546e554. [9] Z. Li, L.A. Fredin, P. Tewari, S.A. Dibenedetto, M.T. Lanagan, M.A. Ratner, et al., In situ catalytic encapsulation of core-shell nanoparticles having variable shell thickness: dielectric and energy storage properties of high-permittivity metal oxide nanocomposites, Chem. Mater. 22 (2010) 5154e5164. [10] D.H. Lan, L. Chen, C.T. Au, S.F. Yin, One-pot synthesized multi-functional graphene oxide as a water-tolerant and efficient metal-free heterogeneous catalyst for cycloaddition reaction, Carbon 93 (2015) 22e31. [11] R.K. Layek, A.K. Das, M.J. Park, N.H. Kim, J.H. Lee, Enhancement of physical, mechanical, and gas barrier properties in noncovalently functionalized graphene oxide/poly(vinylidene fluoride) composites, Carbon 81 (2015) 329e338. [12] R.K. Layek, S. Samanta, D.P. Chatterjee, A.K. Nandi, Physical and mechanical properties of poly(methyl methacrylate) -functionalized graphene/poly(vinylidine fluoride) nanocomposites: piezoelectric b polymorph formation, Polymer 51 (2010) 5846e5856. [13] H. Li, Z. Chen, L. Liu, J. Chen, M. Jiang, C. Xiong, Poly(vinyl pyrrolidone)-coated graphene/poly(vinylidene fluoride) composite films with high dielectric permittivity and low loss, Compos. Sci. Technol. 121 (2015) 49e55. [14] Y.J. Wan, W.H. Yang, S.H. Yu, R. Sun, C.P. Wong, W.H. Liao, Covalent polymer functionalization of graphene for improved dielectric properties and thermal stability of epoxy composites, Compos. Sci. Technol. 122 (2016) 27e35. [15] J.Y. Wang, S.Y. Yang, Y.L. Huang, H.W. Tien, W.K. Chin, C.C.M. Ma, Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situ polymerization, J. Mater. Chem. 21 (2011) 13569. [16] J. Tong, H.X. Huang, M. Wu, Facile green fabrication of well dispersed poly(vinylidene fluoride)/graphene oxide nanocomposites with improved properties, Compos. Sci. Technol. 129 (2016) 183e190. [17] Q. Ji, I. Honma, S.M. Paek, M. Akada, J.P. Hill, A. Vinu, et al., Layer-by-layer films of graphene and ionic liquids for highly selective gas sensing, Angew. Chem. Int. Ed. 49 (2010) 9737e9739. [18] N. Karousis, S.P. Economopoulos, E. Sarantopoulou, N. Tagmatarchis, Porphyrin counter anion in imidazolium-modified graphene-oxide, Carbon 48 (2010) 854e860. [19] T.Y. Kim, H.W. Lee, M. Stoller, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, et al., High-performance supercapacitors based on poly(ionic liquid)-modified graphene electrodes, ACS Nano 5 (2010) 436e442. [20] J.S. Lee, T. Lee, H.K. Song, J. Cho, B.S. Kim, Ionic liquid modified graphene nanosheets anchoring manganese oxide nanoparticles as efficient electrocatalysts for Zneair batteries, Energy. Environ. Sci. 4 (2011) 4148e4154. [21] B.C. Lin, T.Y. Feng, F.Q. Chu, S. Zhang, N.Y. Yuan, G. Qiao, et al., Poly(ionic liquid)/ionic liquid/graphene oxide composite quasi solid-state electrolytes
105
for dye sensitized solar cells, RSC Adv. 5 (2015) 57216e57222. [22] N. Maity, A. Mandal, A.K. Nandi, Interface engineering of ionic liquid integrated graphene in poly(vinylidene fluoride) matrix yielding magnificent improvement in mechanical, electrical and dielectric properties, Polymer 65 (2015) 154e167. [23] P. Xu, H.G. Gui, X.X. Wang, Y.D. Hu, Y.S. Ding, Improved dielectric properties of nanocomposites based on polyvinylidene fluoride and ionic liquidfunctionalized graphene, Compos. Sci. Technol. 117 (2015) 282e288. [24] C.Y. Xing, L.P. Zhao, J.C. You, W.Y. Dong, X.J. Cao, Y.J. Li, Impact of ionic liquidmodified multiwalled carbon nanotubes on the crystallization behavior of poly(vinylidene fluoride), J. Phys. Chem. B 116 (2012) 8312e8320. [25] C.Y. Xing, M.M. Zhao, L.P. Zhao, J.C. You, X.J. Cao, Y.J. Li, Ionic liquid modified poly(vinylidene fluoride): crystalline structures, miscibility, and physical properties, Polym. Chem. 4 (2013) 5726e5734. [26] C.Y. Xing, Y.Y. Wang, X.Y. Huang, Y.J. Li, J.Y. Li, Poly(vinylidene fluoride) nanocomposites with simultaneous organic nanodomains and inorganic nanoparticles, Macromolecules 49 (2016) 1026e1035. [27] C.Y. Xing, Y.Y. Wang, C. Zhang, L.F. Li, Y.J. Li, J.Y. Li, Immobilization of ionic liquids onto the poly(vinylidene fluoride) by electron beam irradiation, Ind. Eng. Chem. Res. 54 (2015) 9351e9359. [28] C.Y. Xing, J.C. You, Y.J. Li, J.Y. Li, Nanostructured poly (vinylidene fluoride)/ ionic liquid composites: formation of organic conductive nanodomains in polymer matrix, J. Phys. Chem. C 119 (2015) 21155e21164. [29] L. Zhao, Y. Li, X. Cao, J. You, W. Dong, Multifunctional role of an ionic liquid in melt-blended poly (methyl methacrylate)/multi-walled carbon nanotube nanocomposites, Nanotechnology 23 (2012) 255702. [30] L. Zhao, Y. Li, J. Qiu, J. You, W. Dong, X. Cao, Reactive bonding mediated high mass loading of individualized single-walled carbon nanotubes in an elastomeric polymer, Nanoscale 4 (2012) 6613e6621. [31] L.P. Zhao, Y.J. Li, Z.F. Liu, H. Shimizu, Carbon nanotube-conducting polymer coreshell hybrid using an imidazolium-salt-based ionic liquid as a linker: designed as a potential platinum electrode alternative material for large-scale solution processing, Chem. Mater. 22 (2010) 5949e5956. [32] C. Xing, J. Li, C. Yang, Y. Li, Locally grafting of ionic liquid in poly(vinylidene fluoride) amorphous region and the subsequent microphase separation behavior in melt, Macromol. Rapid Commun. 37 (2016) 1559e1565. [33] R. Tamura, E. Lim, T. Manaka, M. Iwamoto, Analysis of pentacene field effect transistor as a Maxwell-Wagner effect element, J. Appl. Phys. 100 (2006) 114515. [34] J.K. Yuan, S.H. Yao, Z.M. Dang, M. Genestoux, J. Bai, Giant dielectric permittivity nanocomposites: realizing true potential of pristine carbon nanotubes in poly(vinylidene fluoride) through an enhanced interfacial interaction, J. Phys. Chem. C 115 (2011) 5515e5521.