Significantly Enhanced Dielectric Performance of Poly(vinylidene

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Significantly enhanced dielectric performance of Poly(vinylidene #uoride-co-hexa#uoropylene)-based composites filled with hierarchical flower-like TiO2 particles Nuoxin Xu, Liang Hu, Qilong Zhang, Xingrong Xiao, Hui Yang, and Enjie Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08987 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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Significantly enhanced dielectric performance of Poly(vinylidene fluoride-co-hexafluoropylene)-based composites filled with hierarchical flower-like TiO2 particles Nuoxin Xu, Liang Hu, Qilong Zhang*, Xingrong Xiao, Hui Yang and Enjie Yu

School of Materials Science and Engineering, State Key Lab Silicon Mat, Zhejiang University, Hangzhou 310027, PR China

ABSTRACT

In this study, we report a feasible strategy for fabricating high-dielectric-constant polymer composites for applications in energy storage devices and embedded capacitors. Hierarchical flower-like TiO2 particles were prepared via a facile solvothermal process and incorporated into the P(VDF-HFP) matrix. The temperature and frequency dependent dielectric properties of flower-like TiO2/P(VDF-HFP) composites as well as commercial TiO2/P(VDF-HFP) composites were investigated. The results reveal that the flower-like TiO2 particles are more effective in increasing the dielectric constant of P(VDF-HFP) when compared with commercial TiO2. Typically, the dielectric constant of the P(VDF-HFP) composite filled with 20 vol% flower-like TiO2 reaches 83.1 at 100 Hz, in contrast to 43.4 for the composite filled with 20 vol% 1 ACS Paragon Plus Environment

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commercial TiO2 and 11.3 for pristine P(VDF-HFP). Also, the flower-like TiO2-filled composites exhibit similar characteristic breakdown strengths to their commercial TiO2-filled counterparts. The significant improvement in the dielectric constant could be attributed to the enhancement of Maxwell-Wagner-Sillars polarization, which originates from the sophisticated morphology of flower-like TiO2 particles.

KEYWORDS: Poly(vinylidene fluoride-co-hexafluoropylene), polymer-based composite, flower-like particles, dielectric constant, interface polarization

INTRODUCTION Dielectric polymers have brought about considerable attention for applications in energy harvesting/storage devices, integrated capacitors, actuators, and power cable termination due to their inherent advantages of high breakdown strength, mechanical flexibility, facile processability and economical efficiency.1-7 In order to meet the miniaturization and low cost requirements for electronic devices, dielectric materials with large dielectric constants (εr) and high breakdown strengths are strongly desired.8,9 Nevertheless, the low dielectric constants of polymers have impeded the realization of their full potential.10 For instance, biaxially oriented polypropylene (BOPP), which is the benchmark polymer dielectrics in current commercial applications, possesses a low dielectric constant of 2.2.11 In the past decade, composites containing high-dielectric-permittivity ceramic particles embedded in organic matrices have emerged as an appealing strategy for the enhancement in dielectric constants of polymer materials. 2 ACS Paragon Plus Environment


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In general, ferroelectric or relaxor ferroelectric ceramic particles, such as BaTiO3(BT), Pb(Zr,Ti)O3(PZT) and Pb(Mg,Nb)O3-PbTiO3 (PMN-PT), have been the most commonly used fillers in polymer composites because of their high dielectric constants. 12-15 However, the large remnant polarizations and high coercive electric fields of ferroelectrics could adversely affect the energy storage efficiency of final composites. Furthermore, the huge contrast in dielectric permittivity between the filler and the matrix always leads to inhomogeneity of the electric fields, which could deteriorate the bulk dielectric properties.16,17 Therefore, the addition of moderate-dielectric-constant nonferroelectric fillers, such as TiO2 and ZrO2, have been intensively

investigated

recently.16,18-23

Nevertheless,

the

dielectric

constants

of

nonferroelectric-filled composites were usually lower compared with their ferroelectric-filled counterparts. Besides, a high volume fraction of the ceramic fillers is normally required to achieve a high dielectric permittivity, which will inevitably worsen the flexibility and processability of the composite film. Recent studies have demonstrated that the morphology of fillers also plays a vital part in the dielectric properties of the composites. Tang et al. reported that functionalized TiO2 nanowires with high aspect ratios could be more effective in enhancing the dielectric constants of composites compared with TiO2 nanoparticles. 24 Wu et al. utilized three-dimensional zinc oxide (3D ZnO) superstructures (i.e. flower-like and walnut-like) as fillers in the fabrication of inorganic-polymer composites.25 The results revealed that the 3D ZnO particles could

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significantly increase the dielectric constants of their polymer composites, whereas the sphere-ZnO-filled composites exhibited moderately enhanced dielectric constants. In the present study, we chose Poly(vinylidene fluoride-co-hexafluoropylene) [P(VDF-HFP)] as the composite polymer matrix because of its relatively high dielectric constant at approximate 10. Hierarchical flower-like TiO2 (F-TiO2) particles were prepared via a solvothermal process and incorporated into the polymer matrix. A detailed study concerning the influence of flower-like TiO2 particles on the crystalline structure, dielectric properties and breakdown strengths of P(VDF-HFP) composites is presented. Compared with the commercial TiO2-filled composites, the flower-like TiO2/P(VDF-HFP) composites showed much more enhanced dielectric constants and similar breakdown strengths. EXPERIMENTAL DETAILS Chemicals and materials. Tetrabutyl titanate (TBT), N, N-dimethyl formamide (DMF) and Titanium oxide(anatase) were all analytical grade and purchased from Aladdin Industrial Corporation, China. Glycerol and ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd., China. Poly(vinylidene fluoride-co-hexafluoropylene) [P(VDF-HFP)] pellets were provided by Sigma-Aldrich. All chemicals were used as received without any further purification . Synthesis of flower-like TiO2 particles. The hierarchical flower-like TiO2 particles were prepared via a template-free solvothermal approach. In a typical synthetic procedure, 5.85mmol TBT was added into a clear solution containing 30 mL ethanol and 10 mL glycerol. After stirring for 5 min at room temperature, the mixture was transferred into a Teflon-lined stainless steel



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autoclave, sealed and maintained at 180℃ for 24 h. After natural cooling down, the resultant white precipitates were collected by centrifugation at 7000rpm for 5 min, washed thoroughly with ethanol and dried at 60℃ for 12h. Finally, the products were calcined in air at 450℃ for 2 h and harvested for subsequent preparation. Preparation of TiO2-P(VDF-HFP) composites. The typical procedures for the preparation of TiO2-P(VDF-HFP) composites were carried out as follows: P(VDF-HFP) was first dissolved in DMF and stirred for 4 h at room temperature. Then a required amount of TiO2 was dispersed in the solution by sonication for 30 min. After vigorously stirring at room temperature for 24 h, the uniform suspension was cast onto a clean glass plate and dried at 60℃ for 12 h to remove the residual solvent. Finally, the obtained films were hot-pressed at 180℃ under a pressure of 2500 psi. The composites containing different volume fractions (5%, 10%, 15%, and 20%) of TiO2 particles were prepared. The composite films were about 70-80µm in thickness. For comparison, pure P(VDF-HFP) film was also prepared. Characterization. The crystalline phases of as-prepared powders and composites were identified by X-ray diffraction (XRD, EMPYREAN, PANalytical Co., Netherlands) analysis with Cu Kα radiation. A field emission scanning electron microscope (FESEM, S-4800, Hitachi Ltd., Japan) was used to observe the morphology of the particles. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250XI X-ray photoelectron spectrometer (Thermo Fisher Scientific, USA). The cross-section morphology of the films was observed using a field emission scanning electron microscope (FESEM, SU-70, Hitachi Ltd.,


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Japan) with all the samples freeze-fractured in liquid-nitrogen before testing. Differential scanning calorimetry (DSC) was conducted using a Perkin-Elmer DSC-7 analyzer between 80℃ and 200℃ with a heating/cooling rate of 10℃min−1 under a nitrogen atmosphere. Prior to electrical measurements, copper electrodes of 150 nm in thickness were evaporated on both surfaces of the films using a mask with 20 mm diameter eyelets. The dielectric properties of the samples were measured using a Novocontrol Alpha-N high resolution Dielectric Analyzer (GmbH Concept 40) in the frequency range of 10-1 to 106 Hz at several temperatures between -25 ℃ and 125℃. The DC breakdown test was carried out using a breakdown voltage instrument (CS2674AX, Nanjing Changsheng Instrument Co. Ltd, China) at room temperature. Ten breakdown tests were performed on each film. RESULTS AND DISCUSSION Figure 1(a) shows the morphology of the solvothermal products. The as-prepared particles exhibits a flower-like morphology and they are uniform in shape and size with an average diameter of ∼2.2µm. Detailed observations reveal that the entire architecture is actually composed of numerous nanopetals with thorn-like tip ends. The outwards radiating nanopetals interconnect with each other and organize into quasi-spheres. The flower-like structures wouldn’t collapse into scattered nanopetals even after prolonged sonication or grinding, implying that they are not random aggregations but ordered self-assemblies.26 The XRD pattern of the as-synthesized precursor is shown in Figure 1(b). Although the pattern couldn’t be matched to any known one in the crystallographic databases, it is similar to that of previously-reported



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titanium glycerolate.27-29 After calcination, the flower-like morphology of the precursor is well preserved, as illustrated in Figure 1(c). All diffraction peaks of the calcinated samples could be fully indexed to anatase TiO2 (JCPDS No. 21-1272). No additional phases are detected, indicating that the titanium glycerolate precursor has been converted into pure anatase TiO2. In order to detect the existence of Ti3+ defects, the XPS spectra of flower-like and commercial TiO2 particles are presented. The binding energy data are calibrated with the C 1s signal at 284.8 eV. As shown in Figure S1, there are two peaks in the Ti 2p spectra, which could be identified as Ti4+ in the TiO2 environment.30 The symmetric peaks indicate low defect concentrations in the samples. No Ti3+ signals could be observed in the spectra.31


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Figure 1. (a) SEM image, and (b) XRD pattern of the solvothermal products; (c) SEM image, and (d) XRD pattern of the calcined particles.

Figure 2 presents the XRD patterns of flower-like TiO2 (F-TiO2) particles, pure P(VDF-HFP), and F-TiO2/P(VDF-HFP) composites with different volume fractions of F-TiO2. In the XRD pattern of the pure P(VDF-HFP), four distinguished diffraction peaks at 2θ = 17.7°, 18.3°, 19.9°, and 26.5° could be observed, corresponding to the diffractions in α-phase crystal planes (100), (020), (110), and (021), respectively.32, 33 In the XRD patterns of the composite films, both diffraction peaks from anatase TiO2 fillers and those from P(VDF-HFP) matrix can be observed. With the increase of TiO2 content, the total intensity of α-P(VDF-HFP) characteristic peaks decreases gradually. This phenomenon indicates that the incorporation of TiO2 might exert a dominant effect on the crystallization behavior of the P(VDF-HFP) matrix, which will be further discussed later in this paper. The freeze-fractured cross-sectional SEM images of pure P(VDF-HFP) and its composites are shown in Fig. S2. As illustrated in Figure S2(a) and Figure S2(b), commercial and flower-like TiO2 particles are dispersed homogenously in the matrix, and the films contain almost no voids between the fillers and the matrix. The result indicates that good interfacial adhesion is formed between TiO2 particles and the polymer matrix, which is one of the key factors in deciding the dielectric properties of the composites.



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Figure 2. XRD patterns of flower-like TiO2 (F-TiO2) particles, pure P(VDF-HFP), and F-TiO2/P(VDF-HFP) composites with different volume fractions of F-TiO2.

Figure 3. (a) Heating curves and (b) cooling curves of P(VDF-HFP) and F-TiO2/P(VDF-HFP) composites

DSC analysis was conducted to further investigate the influence of F-TiO2 particles on the crystallization behavior of the P(VDF-HFP) matrix. The heating and cooling curves of samples were shown in Figure 3(a) and (b), respectively. As presented in Figure 3(a), each sample exhibits only one single melting peak, whose position shifts towards lower temperatures with


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increasing TiO2 content. The crystallinity (χc) of P(VDF-HFP) in the F-TiO2 composites could be calculated according to the following formula: χc =

∆H m × 100% (1 − ω ) × ∆H m0

(1)

where ΔHm is the heat enthalpy of the sample obtained by the integration of the endothermic peak area, ∆Hm0 is the enthalpy of 100% crystalline α-P(VDF-HFP) (93.07 J/g), and ω is the weight percentage of TiO2 particles in the polymer matrix.34, 35 The melting peak temperatures and calculated crystallinities of samples are summarized in Table 1. The temperature measurement error is within ±0.1℃. It is manifest that the addition of F-TiO2 has a complex effect on the crystallization behavior of P(VDF-HFP) in the composites. The crystallinity is improved at a low loading level of TiO2, from 39.52% for the pristine P(VDF-HFP) to 40.33% for the composite with 5 vol% particles. However, the crystalline fraction decreases as the content of F-TiO2 particles further increases. This phenomenon can be interpreted by the two-side influence of particles on the crystallization of polymers.15, 36 On one hand, the existence of F-TiO2 fillers brings about more nucleation sites, which will reduce the nucleation energy and promote the crystallization of the P(VDF-HFP) matrix. On the other hand, the fillers act as physical obstacles, which could retard the movement of P(VDF-HFP) macromolecular chain segments. This retardation gets stronger with higher F-TiO2 loading. Thus, the degree of crystallinity increases with small amounts of particles, while it decreases with higher F-TiO2 volume fractions. The crystallization process of the pure P(VDF-HFP) and its composites was also investigated by DSC. As illustrated in Figure 3(b), the crystallization temperatures (Tc) first



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shifted towards higher temperatures then back towards lower ones with increasing F-TiO2 concentration. This result can also be explained by the competition between the heterogeneous nucleation effect and the hinderance of the polymer chain movement.37 Table 1. Evolution of the melting temperature Tm and crystallinity χc of P(VDF-HFP) and F-TiO2/P(VDF-HFP) composites

Sample

P(VDF-HFP)

5vol% F-TiO2

10vol% F-TiO2

15vol% F-TiO2

20vol% F-TiO2

Tm/℃

153.85

152.85

152.18

151.68

151.35

χc/%

39.52±0.003

40.33±0.003

38.35±0.004

35.93±0.004

31.11±0.006

Figure 4 presents the variations of the dielectric constant (εr), dielectric loss tangent (tanδ) and electrical conductivity (σ) of P(VDF-HFP) and flower-like TiO2/P(VDF-HFP) composites as a function of frequency with varied F-TiO2 volume fractions at room temperature. For comparison, the frequency dependent dielectric constant of the composite filled with 20 vol% commercial TiO2 (C-TiO2) is also plotted. As demonstrated in Figure 4(a), the dielectric constant of the composite increases with increasing F-TiO2 content over the whole frequency range. For each sample, the dielectric constant declines exponentially with increasing frequency in the low-frequency region (102~105Hz), and the frequency dependence becomes stronger with the increase of the loading of TiO2 in the composites. When the frequency is over 105 Hz, the variation trend of εr becomes stable. The frequency dependence behavior shows the typical characteristic of interface polarization.38 It is worth noticing that the dielectric constants of flower-like TiO2-incorporated composite samples are significantly higher than those of 11 ACS Paragon Plus Environment

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commercial TiO2-filled ones at the same filler volume fraction. Typically, the dielectric constant of the P(VDF-HFP) composite with 20 vol% F-TiO2 reaches 83.1 at 100 Hz, in contrast to 43.4 for the composite with 20 vol% C-TiO2 and 11.3 for pristine P(VDF-HFP). The differences in dielectric loss tangent and conductivity between the P(VDF-HFP) and its composites also show strong dependency on the TiO2 content. As shown in Figure 4(b) and Figure 4(c), the composite with larger loading levels of TiO2 shows not only higher dielectric loss, but also higher conductivity, and the dielectric loss tangent exhibits a decreasing trend at low frequencies (102~105Hz). This phenomenon could be explained by the existence of extra sources of space charge carriers in the system which are induced by the inorganic fillers.39 However, the high-frequency dielectric loss tangent of the composite is slightly lower than that of the pure P(VDF-HFP) , and further decreases with increasing TiO2 concentration. This result is similar to that reported for other inorganic/polar polymer composites in previous studies.40, 41 One possible assumption is that at high frequencies the space charge conduction in the composites is significantly reduced, and the dipole orientation makes the dominant contribution to the dielectric loss. The inorganic fillers confine the motion of polymer chains and reduce dipole orientation, thus resulting in low dielectric loss at high frequencies. It is worthy to be mentioned that the conductivities of composites are less than 10-8 S/cm at frequencies below 100Hz, implying that the composites could still serve as dielectric materials. And the electrical conductivity of each sample shows strong dependence on frequency owing to the insulating nature.



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Figure 4. Frequency dependence of (a) dielectric constant, (b) dielectric loss, and (c) conductivity of P(VDF-HFP) and TiO2/P(VDF-HFP) composites with different volume ratios of TiO2. In order to obtain further explanation about the differences in dielectric properties between samples, Figure 5 illustrates the frequency dependence of the dielectric constant of P(VDF-HFP) and TiO2/P(VDF-HFP) composites at various temperatures. As can be seen from it, the shape of the spectra of the composite filled with 5 vol% F-TiO2 is similar to that of P(VDF-HFP). That's to say, each of their dielectric constants exhibits not only an increase with increasing temperature or decreasing frequency, but also a strong temperature-dependent dielectric response particularly 13 ACS Paragon Plus Environment

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at low frequencies. However, the shape of the spectra alters when we further raise the TiO2 loading. The temperature dependent dielectric constant of the composite incorporated with a higher TiO2 content only shows similar tendency to that of P(VDF-HFP) at higher frequencies. In the low-frequency region, the dielectric constant varies with elevating temperature in an N-shaped pattern. The magnitude of the spectra also varies among samples. The composites exhibit much greater dielectric constants except at high temperatures and extremely low frequencies. Fig 5(e) and Fig 5(f) further imply that the flower-like TiO2 particles are more effective in increasing the dielectric constant of P(VDF-HFP) over the whole temperature and frequency range when compared with commercial TiO2. These phenomena could be interpreted by the combined effect of different polarization mechanisms. For composite systems, the dielectric constant originates from the contributions of dipole orientation and interfacial polarization. The interfacial polarization is also known as Maxwell-Wagner-Sillars(MWS) polarization, which occurs when there is an accumulation of charge carriers at the interfaces of heterogeneous systems.42 The MWS polarization in the TiO2/P(VDF-HFP) composites include MWSP(VDF-HFP) and MWSTiO2-P(VDF-HFP). The MWSP(VDF-HFP) polarization originates from blocked charges at the boundaries between the lamellar crystals and interlamellar amorphous regions. The entrapment of charge carriers at the interfaces between TiO2 fillers and the P(VDF-HFP) matrix results in MWSTiO2-P(VDF-HFP) polarization. The mobility of charge carriers increases with elevating temperature (-25≤T≤50 ℃ ), resulting in the enhancement of MWSTiO2-P(VDF-HFP) polarization. When the temperature is further increased, the high mobility of charge carriers



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begins to inhibit the accumulation at the TiO2/P(VDF-HFP) interfaces, leading to the decline of the dielectric constant. At higher temperatures, as observed both in the pure P(VDF-HFP) and its composites, the dielectric constant increases sharply. This implies that the MWSP(VDF-HFP) polarization begins to exert the dominant effect on the dielectric constant. The high dielectric constants at low frequencies and high temperatures might also be associated with sample-electrode non-ohmic effect, which is caused by the mismatch of the Fermi energy level between the sample and metal electrodes.43, 44 The dielectric response becomes stronger for the composite with a higher TiO2 content since there are more TiO2/P(VDF-HFP) interfaces in it. And compared with commercial TiO2 particles, hierarchical flower-like TiO2 superstructures make the TiO2/P(VDF-HFP) interfaces more sophisticated. In other words, the incorporation of flower-like TiO2 brings about more interface area, creates more entrapped charge carriers, and thus gives rise to the greater dielectric constant. The MWS polarization only makes contributions at low-frequency range due to its long relaxation time, therefore the variation of εr becomes stable at higher frequencies38.


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Figure 5. Frequency dependence of the dielectric constant at various temperatures of (a) pure P(VDF-HFP), (b) P(VDF-HFP) -5 vol% F-TiO2, (c) P(VDF-HFP) -10 vol% F-TiO2, (d) P(VDF-HFP) -15 vol% F-TiO2, (e) P(VDF-HFP) -20 vol% F-TiO2, and (f) P(VDF-HFP) -20 vol% C-TiO2. In order to obtain more information about the MWS polarization, we utilize electric modulus formalism to analyze the dielectric relaxation behaviors of the composites. The electric modulus formalism can minimize the parasitic effect of electrode polarization and reflect the relaxation existing in different energy environments. 45, 46 The complex electric modulus formalism M* is defined by the following equation46: 16 ACS Paragon Plus Environment


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M *=

1

ε

*

=M ′+iM ′′=

ε′ ε ′′ +i 2 2 ε ′ +ε ′′ ε ′ +ε ′′2 2

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(2)

where M′ and M″ are the real and the imaginary part of electric modulus, respectively, and ε′ and ε″ are the real and the imaginary part of dielectric constant. The imaginary part of the electric modulus(M″), which takes the form of loss curves, is usually adopted to interpret the bulk relaxation properties.47 Figure 6 displays the frequency dependence of M″ at various temperatures. Two relaxation processes can be observed in the M″ curves. The relaxation peaks at the high frequency side exhibit the αa relaxation, which is associated with the segmental motions in the amorphous region of P(VDF-HFP).48 The peaks appearing at lower frequencies and higher temperatures (above 50℃) could be attributed to the MWSP(VDF-HFP) polarization. The MWSP(VDF-HFP) peak shifts to higher frequencies with increasing temperature, which is due to the reduction of the relaxation time caused by the enhancement of mobility of charge carriers at high temperatures. For TiO2/P(VDF-HFP) composites, the relaxation intensity of MWSP(VDF-HFP) polarization subsides with the increasing loading of TiO2, indicating that the introduction of TiO2 particles suppresses the charge aggregation at the crystal/amorphous boundaries in the P(VDF-HFP) matrix. Besides, new relaxation peaks appear at low frequencies in the temperature range of -25~50℃. This phenomenon is consistent with the dielectric anomaly observed in Figure 5, which could be ascribed to the MWSTiO2-P(VDF-HFP) polarization.


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Figure 6. Frequency dependence of the imaginary part of electric modulus at various temperatures of (a) pure P(VDF-HFP), (b) P(VDF-HFP) -5 vol% F-TiO2, (c) P(VDF-HFP) -10 vol% F-TiO2, (d) P(VDF-HFP) -15 vol% F-TiO2, (e) P(VDF-HFP) -20 vol% F-TiO2, and (f) P(VDF-HFP) -20 vol% C-TiO2. 18 ACS Paragon Plus Environment


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Figure 7. The Weibull plots of the breakdown strength for (a) F-TiO2/ P(VDF-HFP) composites, and (b) C-TiO2/ P(VDF-HFP) composites; (c) characteristic breakdown strength of the composites.

Electric breakdown strength(BDS) is also an important factor in determining the operation electric field and maximum energy densities of the composites. The characteristic breakdown strength of each sample could be analyzed with a two-parameter Weibull distribution function49:   E β  P = 1-exp  −      E0  

(3)

Where P is the cumulative probability of electric failure, β is the shape parameter and is positive, E is the measured breakdown strength, E0 represents the breakdown strength at the 19 ACS Paragon Plus Environment

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cumulative failure probability of 0.632 (i.e. 1-1/e where e is the exponential constant) which is also regarded as the characteristic breakdown strength. The BDS Weibull distribution and characteristic breakdown strength of F-TiO2/ P(VDF-HFP) and C-TiO2/ P(VDF-HFP) composites are illustrated in Figure 7. All of the F-TiO2/P(VDF-HFP) composites could withstand a high electric field over 50 MV/m. It’s worth noting that the F-TiO2-filled composite shows similar characteristic breakdown strength to that of the C-TiO2-filled one with the same filler volume fraction. This result is similar to that reported for other inorganic/polymer composites in literature25. Aforementioned results indicate that F-TiO2/ P(VDF-HFP) composites show much greater dielectric constants. Therefore, the flower-like-TiO2-filled composites exhibit the superiority in fabricating energy storage devices over their commercial-TiO2-filled counterparts. CONCLUSIONS In summary, hierarchical flower-like TiO2 (F-TiO2) particles have been successfully prepared via a facile solvothermal process. A detailed study concerning the temperature and frequency dependent dielectric properties of P(VDF-HFP)-based composites incorporated with flower-like and commercial TiO2 particles has been conducted. The experimental results reveal that flower-like TiO2-filled composites show not only much greater dielectric constants, but also similar characteristic breakdown strengths in comparison with their commercial TiO2-filled counterparts. For instance, the dielectric constant of the P(VDF-HFP) composite filled with 20 vol% F-TiO2 reaches 83.1 at 100 Hz, in contrast to 43.4 for the composite filled with 20 vol%



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C-TiO2 and 11.3 for pure P(VDF-HFP). Meanwhile, the F-TiO2/P(VDF-HFP) composites can withstand a high electric field over 50 MV/m. The improvement in the dielectric constant could be attributed to the enhancement of Maxwell-Wagner-Sillars polarization, which originates from the large amount of charge carriers accumulated at the sophisticated interfaces between flower-like TiO2 particles and P(VDF-HFP) matrix. Our findings could provide a feasible approach to the fabrication of high-dielectric-constant polymer composites for applications in energy storage and integrated capacitors. SUPPORTING INFORMATION XPS spectra of TiO2 particles. Freeze-fractured cross-sectional SEM images of pure P(VDF-HFP) and its composites.

AUTHOR INFORMATION Corresponding Author

* Email: [email protected] (Qilong Zhang).

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National High Technology Research and Development Program of China (863 Program) (No. 2013AA030701) and the Fundamental Research Funds for the Central Universities (No. 2015QNA4007).

REFERENCES


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Significantly Enhanced Dielectric Performance of Poly(vinylidene

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