Achieving High Energy Density in PVDF-Based Polymer Blends

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Achieving high energy density in PVDF-based polymer blends: suppression of early polarization saturation and enhancement of breakdown strength Xin Zhang, Yang Shen, Zhonghui Shen, Jianyong Jiang, Long-Qing Chen, and Cewen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10016 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 1, 2016

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ACS Applied Materials & Interfaces

Achieving High Energy Density in PVDF-Based Polymer Blends: Suppression of Early Polarization Saturation and Enhancement of Breakdown Strength Xin Zhang1, Yang Shen1,*, Zhonghui Shen1, Jianyong Jiang1, Longqing Chen2, Ce-Wen Nan1,* 1 School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine Processing, Tsinghua University Beijing, 100084, China. 2 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA E-mail: [email protected]; [email protected] KEYWORDS polymer blends, dielectric, energy density, ferroelectricity, electric displacement.

ABSTRACT

Polymers with high dielectric strength and favorable flexibility have been considered promising materials for dielectrics and energy storage applications, while the achievable energy density （Ue） of polymer is rather limited by the intrinsic low dielectric constant and ferroelectric hysteresis. Polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE)) with ultrahigh εr of >50 is considered promising in achieving high Ue of polymer dielectrics. However, P(VDF-TrFE-CFE) only exhibits moderate Ue due to the early saturation of electrical polarization at low electric field. In this contribution, we show that, by blending P(VDF-TrFE-

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CFE) with polyvinylidene fluoride (PVDF), the early saturation of P(VDF-TrFE-CFE) is substantially suppressed, giving rise to concomitant enhancement of dielectric permittivity and breakdown strength. An ultrahigh energy density of 19.6 J/cm3 is thus achieved at ~ 640 kV/mm, which is 1600% greater than Ue of the bench-mark biaxially-oriented polypropylene (BOPP, 1.2 J/cm3 at 640 kV/mm). Results of phase field simulations reveal that the interfaces between PVDF and P(VDF-TrFE-CFE) play a critical role by not only suppressing early saturation of electrical polarization in P(VDF-TrFE-CFE) but also inducing additional interfacial polarization. Binary phase diagram of P(VDF-TrFE-CFE)/PVDF blends is also systematically explored with their dielectric and energy storage behavior studied.

Introduction As basic passive element and energy storage device, dielectric capacitors have been playing important roles in advanced electronics and electrical power systems such as stationary power grids, electronic weapons and hybrid electric vehicles.1,2 Polymer dielectric materials have been considered promising candidate for high energy density capacitors applications due to their favorable

dielectric

properties,

especially,

an

usually

ultrahigh

breakdown

strength(>500MV/m),3 and their superiority compared with the counter-parted ceramic capacitors in mechanical flexibility and the capability of being molded into various configurations for power electronic devices with reduced volume and weight.4 Generally, the energy density of dielectric materials can be illustrated as U e = ∫

0

Dmax

E dD , in which E is the

applied electric field and D is the dielectric displacement or charge density, for linear dielectrics, Ue=1/2εrε0EB2, where εr and EB are the dielectric permittivity and breakdown strength of the dielectrics respectively.5 According to this, developing polymer materials with high εr (high

2


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displacement), high breakdown strength, and optimal D-E curves with favorable discharged efficiency is quite essential to obtain high energy density. However, most polymers are currently still suffering from intrinsic low dielectric permittivity hence small electrical displacement (D), which severely limit the achievable energy density. For example, one of the commonly used commercial polymer capacitor films is the biaxially oriented polypropylene( BOPP ), PP is a non-polar polymer with an ultrahigh breakdown strength over 700 MV/m and low dielectric loss less than 0.02%, in spite of which, the energy density of the BOPP films is still rather limited to 1~2 J/cm3, owning to its over-low dielectric constant (εr =2.2 ).6,7 Some other polymers with polar groups of strong dipole moments in their molecular chains usually exhibit higher dielectric permittivity compared to the non-polar ones. Poly(vinylidene difluoride) (PVDF) is a typical polar polymer with a relatively high εr of 10. With the four different polymorphs, i.e., α (in trans-gauche conformation TGTG), β (in all trans planar zigzag conformation TTTT), γ (in a conformation of three trans linked to agauche TTTG) and δ (a polar version of α phase ),8 PVDF of β phase exibits the strongest polarity as well as ferroelectric hysteresis, while PVDF of α or γ phases usually lead to less polared phases as well as weaker hysteresis. For example, a high electrical polarzation of over 9 µC/cm2 ,can be obtained in wellpolarized β-phase PVDF.9 Recent years, PVDF was chemically modified by introducing momers including trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE), chlorofluoroethylene (CFE) and hexafluoropropylene (HFP) into the (VDF) chains to make PVDF-based copolymers and terpolymers, these polymers exhibit excellent dielectric and electric properties and have attracted much attention.10-12 Among which, terpolymer P(VDF-TrFE-CFE) is a relaxor ferroelectric with ultrahigh dielectric constant of over 50,13 in which CFE and TrFE monomers act as molecular defects to effectively break the large ferroelectric domains into nanosize, and consequently lead

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to much reduced remnant polarization.14 Despite of their high dielectric constant and suppressed remnant displacement, terpolymers only exhibit limited energy density of ~ 10 J/cm3. This is attributed to the early saturation of polarization at elevated electric field, which results in very limited discharge energy density of terpolymers.15 For further enhancing the energy density of PVDF-based materisals, one promising approach is to embed inorganic component, either conductive particles16-17 or creamic particles with high dielectric constant (on the order of hundreds or even thousands ) into the polymer matrix to fabricate polymer nanocomposites.18-22 Tremendous efforts have been made, by using core-shell strcutured ceramic fillers17-19 or nanofibers of large asepct ratios20-22 or adopting novel sandwich structure of polymer nanocomposites 23 , to achieve concomitant enhancement of dielectric constants and breakdown strength. However, enhanced dielectric constant is usually obtained at the cost of increased dielecttic loss, compromised breakdown strength and flexibility of the polymer nanocomposites, which is caused by structural defects such as voids or pores at the interfaces between polymers and inorganic fillers, especially when a large number of inorganic particles are introduced to induce high dielectric constants.21 Although recent synergistic approaches23 have raised the energy density of composite dielectrics up to over 20 J/cm3 which is enhanced by over 10 times that of BOPP，the fabrication of the composite dielectrics still remain a critical issue for the mass application of these composite dielectrics. In parallel to the composite approach, polymer blends consisted of polymers, espcially fluorine-containing polymers, with different polar groups on their side chains have also been explored for their dielectric and dielectric behaviors.

24-26

Within these polymer blends, the intermolecular actions between polar groups have been proved rather effective in enhancing the dielectric permittivity and breakdown strength, hence giving rise to much enhanced energy density.

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In this paper, PVDF with non-polar γ phases and P(VDF-TrFE-CFE) were blended together and casted into films. Enhanced dielectric constant of over ~56 was obtained in blend with composition of 20/80 vol%(PVDF/Terpolymer). Besides, electric breakdown strength is improved in blends compared with neat polymers and an ultrahigh energy density of 19.6 J/cm3 is achieved at the composition of 60/40 vol%(PVDF/Terpolymer), which is much higher than the neat PVDF (~13.1 J/cm3) and neat terpolymer (~11.2 J/cm3). And the results of phase field simulation indicate that the dielectric properties of blends are closely related to the role exchange between continuous phase and dispersed phase for the two polymers. Experimental section Fabrication of PVDF/P(VDF-TrFE-CFE) blends films. All the chemicals were purchased from China National Chemicals Corporation Ltd. if not otherwise specified. PVDF and P(VDFTrFE-CFE) (Arkema) were successively dissolved into N,N-dimethylformamide (DMF), then the mixture was stirred for 24h to make it stable and homogenous. The volume fractions of PVDF in the PVDF/P(VDF-TrFE-CFE) blend films increase from 0% to 100%. Polymer films were cast from solutions on glass substrates with a handy and facile scraper, after being dried at 40oC in vacuum, the films were heated at 200oC for 5 min and then quenched in ice water immediately. To improve the crystallinity of both PVDF and P(VDF-TrFE-CFE), the films were further annealed at 110 oC for 5h before naturally cooled to room temperature. After these thermal treatments, free-standing films of ~ 10µm in thickness were peeled from the glass substrate and characterized with a scanning electron microscopy (SEM) (Hitachi S-4500). The surface roughness is characterized with atomic force microscopy(AFM)(Infinity Oxford).

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Measurements of dielectric properties. For dielectric measurements, copper electrodes (4 mm in diameter and 50 nm in thickness) were deposited on both sides of the blends films. Dielectric permittivity and dielectric loss were measured with a HP 4294A precision impedance analyzer (Agilent) at room temperature within a broad frequency range from 102 to 107 Hz at 1V rms. Electric displacements-electric ﬁeld (D-E) loops were measured at 100 Hz in silicon oil with a commercial high voltage integrated ferroelectric test system (Radiant Technologies, Inc.). Electric breakdown tests were carried out with Dielectric Withstand Voltage Test System (Beijing Electro-Mechanical Research Institute Super-Voltage Technique) at a ramping rate of 200 Vs-1 and a limit current of 5mA. Result and discussion Morphology and crystal structure of blends films. PVDF/P(VDF-TrFE-CFE) blend films with thickness of ~10 µm were prepared through a carefully treated solution-casting method, the asprepared blend films were then heated to 200oC and followed by quenching into ice water. Such quenching processes induces PVDF of γ phase, as indicated by the XRD patterns of the blends films (Figure 1a). For neat PVDF, the two major peaks at 18.5°, 20.1° could be assigned to (020), (110) directions of γ phase.27 For PVDF of γ phase, the conformation of three trans linked to a gauche (TTTG) leads to small polarizability and hence suppressed remnant displacement (Dr). The neat terpolymer only exhibits one peak at 18.4o. With increasing contents of terpolymer in the blends the (110) peak of PVDF gradually decreases and finally disappeares in the 90/10 blends, indicating that the PVDF chains have not interpenetrate with the terpolymer chains and form co-crystals to change the basic crystallographic structure.26 The surface SEM image, AFM mapping image and sample photo image (Figure 1b, c, d) show that the final super-flexible

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polymer films are of high qualities with smooth surface (surface roughness is 13.8 nm according to the AFM result) and few defects, such as voids or pores. Dielectric performances of blend films. The dependences of dielectric constants on the frequency are shown in Figure 2a, dielectric constants of all blend films tend to decrease with increasing frequency. At lower frequency (< 104 Hz), the addition of P(VDF-TrFE-CFE) into the blends can obviously enhance the dielectric constant, while at higher frequency (>106 Hz), the improvement is quite mild. Increasing amount of terpolymer also results in larger dielectric loss, but unlike the trends of dielectric constants, the increase of dielectric loss at low frequency is rather limited, as shown in Figure 2b. The variations of dielectric constant and loss with the concentration of terpolymer in the blends could be better distinguished in Figure 2c. At 100 Hz, P(VDF-TrFE-CFE) has much larger dielectric constants (~53) than that of PVDF (~12), owning to the response of dipoles in the randomly distributed polar nano-region in terpolymer.28 For the blends, dielectric constants continuously increases with increasing amount of P(VDF-TrFECFE). High dielectric constants of ~ 56 is obtained in polymer blend with composition of 90/10 vol% (Terpolymer/PVDF), which is even higher than that of the neat terpolymer. Similar enhancement of dielectric constants or displacement also have been observed in other blend systems, such enhancement is usually restricted within a small interval of composition, which can be mainly ascribed to interfacial displacement between PVDF and terpolymer as well as structural reconfiguration or improved crystallinity.26,29 Along with the much enhanced dielectric constants, the dielectric loss are maintained at low level. The blends exhibit even lower dielectric loss than both neat PVDF and terpolymer. High-field dielectric behavior of polymer blends. Dielectric displacement - Electric field loops (D-E loops) of neat PVDF, neat P(VDF-TrFE-CFE) and polymer blends with different volume

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ratios are shown in Figure S1. The quenching process induces PVDF of γ-phase with slim D-E loops which tend to be more like paraelectric instead of ferroelectric.30 Yet, PVDF still exhibits much stronger hysteresis compared with terpolymer which shows strong relaxor ferroelectricity. Interestingly, small loading of terpolymer into PVDF induces even slimmer D-E loops with enhanced maximum displacement ( Dmax ) and suppressed remnant displacement ( Dr ). With continuous increase in the contents of terpolymer up to over 60 vol%, obvious trends of early saturation can be observed from the D-E loops of blends, which is typical symbol of relaxor ferroelectric terpolymer. The variations of Dmax and Dr with the content of terpolymer in blends are shown in Figure 3a. At 400 kV/mm, the Dmax of PVDF is ~5 µC/cm2 with a high Dr of 1.8 µC/cm2. With increasing amount of terpolymer in the blends, the Dmax increases continuously up to ~10.45 µC/cm2 at the composition of 90/10 vol.% (Terpolymer/PVDF), which is even higher than that of neat terpolymer (9.8 µC/cm2 ), while Dr of blends remains at a favorable low level. An interesting feature can be observed that, taking the composition of 50/50 vol. % as a demarcation point, the enhancement of displacement for blends with terpolymer less than 50 vol. % is obviously milder than the later part, which might be due to the role exchange between continuous phase and dispersed phase (here we consider the volumetric dominant phase more likely to be the continuous phase) for the two polymers. For better understanding of this trend, a phase field simulation method was employed to investigate the displacement behaviors of blend under high electric field. To obtain the electric displacement, we employ the Spectral Iterative Perturbation Method (SIPM) 31 to solve the electrostatic equilibrium equations for the blends with different compositions. The two polymers are defined as two phases, e.g., PVDF as phase a (use a as the order parameter to define the PVDF phase, a =1 represents the PVDF phase and

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whereas not a =0) and P(VDF-TrFE-CFE) as phase b, hence the phase dependent dielectric constants can be written as

()

()

()

a b κ r = κ a r +κ b r

(1)

The electric displacement D(r) in each blends can be obtained by

D( r )=ε 0κ( r )E( r )+PS( r ) ,

(2)

Where ε 0 is the dielectric permittivity of the vacuum, κ(r) the phase dependent relative dielectric constants, κ(r) the total electric field distribution, and PS(r) the phase dependent spontaneous displacement of the blend. Using the SIPM method to solve the electrostatic equilibrium equation

∇ D ( r )= 0 ,

(3)

we can obtain the spatially total electric field distribution E(r). The total displacement distribution is given by

P( r )=ε 0 ( κ( r )−δ )⋅E( r )+PS( r )

(4)

With δ the Dirac delta function. 32 In the simulation, we artificially set the role of continuous or dispersed phase for two polymers separately, and displacement of blends with content of terpolymer ranging from 10 vol.% to 90 vol.% is simulated and given in Figure 3b. As seen, with the same composition, the displacement of blends that terpolymer serving as continuous phase is consistently higher than the blends when PVDF serving as continuous phase. Then the simulated data are plotted by using composition of 50/50 vol. % as the demarcation point and compared with experimentally

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measured displacement values. As shown in Figure 3a, simulated result fits well with the experimental data, and an obvious difference in the displacement changing tendency can be observed when different polymers serving as continuous phase. Both experimental and simulated results indicate that different roles of continuous phase or dispersed phase in the blends induces different dielectric properties. Since displacement D ∝ q d , here q represents the diploes and d means the dipole mobile distance,33 the continuous phase tend to hold larger d hence produces higher displacement, and this advantage is more notable when the continuous phase holds higher dielectric constant. Energy storage properties of polymer blends. Discharged energy density is directly derived from the D-E loops by integration of dielectric displacement to electric field. As can be seen from Figure 4a, compared with neat PVDF, incorporation of terpolymer can consistently improve the discharged energy density of the blends, in spite of which, the energy density of blends is still lower than the neat terpolymer when the applied electric field is fixed at 300kV/mm. While at higher electric field (500kV/mm), the change of energy density is no longer keeping increasing, instead, two peaks appear when increasing the content of terpolymer. The first peak energy density (~12.2 J/cm3 ) is obtained at the composition of 40/60 vol.% (terpolymer/PVDF),

the formation of this peak value can be ascribed to the continuously

enhanced Dmax and simultaneously improved efficiency induced by suppressed Dr (Figure 3b & Figure 4b). Afterwards, the energy efficiency turns to drop down with further increasing the content of terpolymer to over 40 vol.%, in spite of which, the energy density is still improved to the second peak value of 12.55 J/cm3 with terpolymer concentration of 80 vol.% owning to notably enhanced Dmax in blends by interfacial effects. Of particular note is that, unlike trends at lower electric fields, both two peak energy density of bends are larger than that of neat

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terpolymer. Which indicate that blends hold obvious advantage over neat terpolymer in enhancing energy density at higher electric fields. This can be explained by the early saturation behaviors at low electric field (~100 kV/mm) of P(VDF-TrFE-CFE).

34

As a result of early

saturation at low electric field, further increasing the electric field leads to quite small increment of electric displacement, which hindered the further enhancement of energy density in terpolymer.35 The electric displacement growth rate can be obtained by calculating the derivative of displacement to electric field, which can act as the parameter to analyze the displacement changing behaviors along with the electric fields. Thus the displacement growth rates are calculated and plotted with electric field for neat PVDF, neat terpolymer and blends with different compositions and shown in Figure 5. Neat terpolymer has the largest starting growth rate, a strong peak is appeared around 100 kV/mm, which refers to a sharp increase of displacement, then the growth rate turns to a sharp drop and maintains at fairly low level at higher electric fields. Comparatively, the change of growth rate for neat PVDF is quite mild and no peak is observed. After incorporation of PVDF into terpolymer, the strength of the peak turns to be continuously weaker and obvious left shift of the peak can be observed, finally almost no peak can be find when the content of PVDF reached 80 vol% in the blends. The displacement growth rate changing behaviors indicate that the displacement saturation has be weaken in the blends compared with terpolymer, which consequently gives rise to higher displacement growth rate as well as energy density growth rate at higher electric fields. This trend is clearly revealed in the variation of energy density with electric fields for neat PVDF, terpolymer and polymer blends with different compositions as shown in Figure S2, according to which, the energy density of blends grow much faster the neat terpolymer after electric field of 400kV/mm.

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The synergistic effects of interface enhanced electric displacement and eliminated displacement saturation behaviors give rise to much improved energy density in the blends compared with the neat polymers. Since the energy density is quadratically dependent on breakdown strength, the electric breakdown behaviors of blends are of significance to be studied. Thus the characteristic electric breakdown strength of the neat polymer and blends are analyzed with a two-parameter Weibull distribution function: P(E)=1-exp(-(E/EB) β), where P(E) is the cumulative probability of electric failure, E is experimental breakdown strength, EB is a scale parameter refers to the breakdown strength at the cumulative failure probability of 63.2% which is also regarded as the characteristic breakdown strength, and β is the Weibull modulus associated with the linear regressive fit of the distribution.36 As shown in Figure 6a, PVDF has much higher electric breakdown strength(~ 607.5 kV/mm) than that of terpolymer(~ 493.7 kV/mm), and improved electric breakdown strength can be observed in blends compared with neat polymers, specifically, ~ 665.1 kV/mm for blends with 40/60 vol. % (Terpolymer/PVDF) and ~512.1 kV/mm for blends with 80/20 vol.%. The enhancement of breakdown strength may firstly be ascribed to the increased modulus of elasticity in the blends, for polymers with a higher modulus of elasticity usually have higher breakdown strength37; Most importantly, the enhanced breakdown strength could be attributed to the intermolecular interactions, or polar interactions, in the polymer blends. Results of an atomic simulation by Yao et.al.,38 indicate that there is a greater decrease in the cohesive energy level via formation of a crystalline structure in the blend compositions as compared to the constituent pure polymers. Formation of a crystalline phase in the blend is therefore more favored compared to pure PVDF or PVDF-TrFE-CFE. This greater crystallinity could be the result of polar interactions between electronegative fluorine in the CF3 groups in the terpolymer and the electropositive hydrogens in PVDF. The polar interaction is the

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major force constraining the chains to be parallel with one another and changes the local structural configuration and improves the crystallinity of the polymer blend. The improved configuration and crystallinity in the blends is responsible for the observed enhancement of the elastic modulus and hence the dielectric breakdown strength. The intermolecular interactions could also reduce the mobility of the polymer chains, resulting in reduced probability of charge carriers transferring through the loose polymer chains. This can be further evidenced by the high β value of blends (~ 17.6 for blends of 80/20 vol. % and ~16.9 for blends of 40/60 vol. %), which are higher than neat polymers. Since β quantifies the scattering in the experimental data and a higher value of β represents less scattering, the higher β value of the blends indicates that the blending process make the films rather tight with very few defects. Owning to much enhanced electric

breakdown

strength,

polymer

blends

with

composition

of

40/60

vol.%(PVDF/Terpolymer) delivers the highest discharged energy density of 19.6 J/cm3 (See Figure 6b), which is enhanced by almost 60% over that of the neat PVDF (~ 12.5 J/cm3 at 610 kV/mm), over 70% over that of the neat terpolymer(~ 11.3 J/cm3 at 490 kV/mm), and is 1600% greater than the energy density of commercial BOPP (1.2 J/cm3 at 640 kV/mm).6 Conclusion In summary, P(VDF-TrFE-CFE)/PVDF blends films are prepared with a solution-casting method and followed with carefully treated quenching and annealing process, much enhanced dielectric constant as well as electric displacement at high electric fields are achieved in the blends owning to interfacial coupling effects and compounding effects compared with neat polymers. Both simulated and experimental results indicate that the electric displacement of blends with terpolymer serving as continuous phase is consistently higher than the blends with PVDF serving as continuous phase. High fields displacement behaviors show that early

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displacement saturation of terpolymer has been eliminated in the blends, consequently, higher electric displacement as well as energy density growth rate than neat polymers can be obtained at high electric fields. In addition, the dielectric breakdown strengths of blends are also improved owning to increased modulus of elasticity and reduced mobility of the polymer chains. As a result, a high energy density of 19.6 J/cm3 is achieved in blends with composition of 40/60 vol.%(PVDF/Terpolymer). The polymer blending approach provides chances of tuning and enhancing the dielectric and energy storage properties of polymers, and the intrinsic superior dielectric strength and flexibility are simultaneously reserved.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Dielectric displacement-electric field (D-E) loops of all prepared samples, simulated polarization result, variation of energy density and efficiency with electric fields of all samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected];(Y.S.); [email protected](C.W.N.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Grant No. 2015CB654603), the NSF of China (Grant No. 51572141 and 51532003), and Research fund of Science and Technology in Shenzhen (JSGG20150331155519130). REFERENCES [1] Chu, B.; Zhou, X.; Ren, K.; Neese, B.; Lin, M.; Wang, Q.; Zhang, Q. M. A Dielectric Polymer with High Electric Energy Density and Fast Discharge Speed. Science. 2006, 313, 334-336. [2] Wang, Q.; Zhu, L. Polymer Nanocomposites for Electrical Energy Storage. J. Polym. Sci., Part B.: Polym. Phys. 2011, 49, 1421-1429. [3] Wang, Y.; Zhou, X.; Chen, Q.; Chu, B.; Zhang, Q. Recent Development of High Energy Density Polymers for Dielectric Capacitors. IEEE Trans. Dielectr. Electr. Insul. 2010, 17, 1036-1042. [4] Dang, Z. M.; Yuan, J. K.; Yao, S. H.; Liao, R. J. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25, 6334-6365. [5] Zhang, X.; Shen, Y.; Zhang, Q.; Gu, L.; Hu, Y.; Du, J.; Nan, C. W. Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@ TiO2 Nanofibers by Atomic‐ Scale Interface Engineering. Adv. Mater. 2015, 27, 819-824. [6] Rabuffi, M.; Picci, G. Status Quo and Future Prospects for Metallized Polypropylene Energy Storage Capacitors. IEEE Trans. Plasma Sci. 2002, 30, 1939-1942. [7] Laihonen, S. J.; Gäfvert, U.; Schütte, T.; Gedde, U. W. DC. Breakdown Strength of Polypropylene Films: Area Dependence and Statistical Behavior. IEEE Trans. Dielectr. Electr. Insul. 2007, 14, 275-286. [8] Davis, G. T.; McKinney, J. E.; Broadhurst, M. G.; Roth, S. Electric-field-induced Phase

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Figure 1. (a) XRD patterns of neat polymers and blends with different compositions; (b) Surface SEM image, (c) AFM image and (d) sample photo image of polymer blends with composition of 40/60 vol.%(Terpolymer/PVDF)

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Figure 2. The dependences of (a) dielectric constant and (b) dielectric loss on frequency for neat polymers and blends with different compositions; (c) Variations of dielectric constant (square) and dielectric loss (circle) with the volume fraction of terpolymer in the blends.

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Figure 3. a) Variations of maximum electric displacement (red square), remnant displacement (blue circle) and simulated maximum electric displacement with volume fraction of terpolymer in the blends, and the blends with PVDF (shaded in pink) and terpolymer (shade in blue) serving as continuous phase are highlighted respectively; b) Dependency of simulated displacement on the volume faction of terpolymer for different polymers serving as continuous phase.

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Figure 4. (a) Discharged energy density and (b) efficiency of polymer blends as a function of the volume fraction of terpolymer, the electric fields are fixed at 300kV/mm, 400kV/mm and 500kV/mm respectively.

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Figure 5. The variation of displacement growth rate with electric fields for neat polymers and blends with different compositions.

Figure 6. (a) Failure probability of dielectric breakdown deduced from Weibull distribution for neat PVDF, terpolymer and blends with composition of 40/60 vol% and 80/20 vol.%(Terpolymer/PVDF); (b) Maximum discharged energy density of neat PVDF, terpolymer and blends with composition of 40/60 vol% and 80/20 vol. %(Terpolymer/PVDF) at their characteristic breakdown strength.

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Achieving High Energy Density in PVDF-Based Polymer Blends

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