and P(VDF-TrFE-CTFE)

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Antiferroelectric Behavior of P(VDF-TrFE) and P(VDFTrFE-CTFE) Ferroelectric Domains for Energy Harvesting Mudassar Shehzad, and Tayyaba Malik ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Energy Materials

Antiferroelectric Behavior of P(VDF-TrFE) and P(VDF-TrFE-CTFE) Ferroelectric Domains for Energy Harvesting *Mudassar Shehzad, Tayyaba Malik School of Chemical and Materials Engineering (SCME) National University of Science and Technology H-12 Islamabad, 44000, Pakistan *[email protected], [email protected] KEYWORDS__ ferroelectric polymer, electrostriction, polarization, antiferroelectric, double hysteresis loop, electrocaloric effect, paraelectric phase, and energy harvesting ___________________________________________________________________________________________ ABSTRACT: Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], one of well-known ferroelectric polymers, has relatively high crystallinity and shows large polarization-electric field hysteresis loop (PE loop). On the other hand, poly(vinylidene fluoridetrifluoroethylene-clorotrifluoroethylene) [P(VDF-TrFE-CTFE)], one of relaxor ferroelectric polymers, in which the third monomer, CTFE, break coherent polarization domain into nanopolar regions, shows narrow PE loop. In this study, P(VDF-TrFE) copolymer and P(VDF-TrFE-CTFE) terpolymer were blended together, and their polarization behavior was investigated. The enhancement in the polarization as compare to neat P(VDF-TrFE-CTFE) is observed with the antiferroelectric like behavior of blends which give rise to the additional energy density. The result shows the evolution in properties of ferroelectric polymer blends from paraelectric to antiferroelectric behavior, especially for α=0.1 & α=0.2 blend system, shows fast charged and discharged energy density. _____________________________________________________________________________________________

1.

INTRODUCTION

Poly(vinylidene fluoride) PVDF is piezoelectric electroactive polymer (EAP) having excellent potential for use in medical and industrial applications. However their applications are limited due to low strain and low energy density. But, the major challenge is to obtain high energy density by adopting different techniques. For an example in 2011 a researcher conducted his research to get high electrostrictive performance of copolymers and terpolymers to result a high energy density in the PVDF family, however he adopted complex synthesis and polymerisation processes [1-10]. Recently poly (vinylidenefluoride trifluoroethylene) P(VDF– TrFE) copolymer is reported to exhibit high polarization and electromechanical response among all known polymers [2]. Recently, researchers are interested in PVDF-based copolymers and terpolymers due to their valuable ferroelectric properties. The copolymer P(VDF-TrFE) give rise to a noticeable electric-strain after high-energy electron irradiation of films [3]. Of that another method, irradiated films also show high stored energy density, which is attractive for many electromechanical applications [3-5]. Figure 1, represents the three conformations of PVDF based polymer molecular chain e.g. all ‘trans’ (TTT) ‘trans gauche trans gauche’ (TG+TG-) and TTTG+TTTG-. However in the polar region of PVDF the packing morphologies play a vital role to enhance its polarization. Meanwhile the TG+TG- conformation is commonly called as α phase [5]. In an alpha (amorphous) phase no polarity and polarization is found due to antiparallel packing of polymer chains. In a beta phase there are two all trans chains packed with their dipoles which are pointed in the same direction [27-28]. The beta (crystalline) phase has most interesting electrical properties due to noticeable dipole moment in the unit cell. The alternating TTTG+TTTG- conformations are attributed

to the polar gamma phase in which the dipole moment aligned in the same direction.

+

-

[TTT] + [TTTG TTTG ]

β(S.T)+

Γ

+

-

[TG TG ] + [TTT..]

α(F.E) + β(L.T)

Coexistance Figure 1.Three conformations of PVDF, TT, TG+TG-, TTTG+TTTG-. The coexistence of long trans (L.T) and short trans (S.T) is responsible for the ferroelectricity, where α(F.E) is Ferroelectric Alpha phase.

PVDF based polymers after irradiation can be converted into the relaxor ferroelectric having high polarization and dielectric constants [28]. The main phenomenon to convert a normal ferroelectric into relaxor ferroelectric is to create defects in their chain structure. If we substitute a third monomer CTFE into the chain of the polymer material we can also get relaxor ferroelectric

1

ACS Paragon Plus Environment


properties. It was found (in Table 1) that the crystalline domain size and chain conformation can be modified by electron irradiationor copolymerization with a third bulky monomer such as chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) [7]. Figure 2 shows the clear difference between the relaxor ferroelectric and normal ferroelectric polymers. Table 1. shows the different phases and their polarization responses based on literature review Phases Dipole Polarization Comments alignment Alpha (α) TG+, TGBeta (ß)

TTTT

Gamma (Γ)

TTTG+, TTTG-

Delta (δ)

Paraelectric Phase

10 2

D (µC/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normal ferroelectric P(VDF-TrFE)

Pr

5 0

high dielectric constants as compare to ceramic ferroelectrics. The frequency dependence of the dielectric constant gives us the beneficial information such as the electrical relaxation over the bands of different frequency and polarization of their dipoles. However the dielectric constant of copolymers is usually lesser than that of terpolymers. The relaxor ferroelectric polymers (RFEPs) usually has high dielectric constants as compare to normal ferroelectric polymers (NFEPs). The dielectric properties of normal ferroelectric polymers (NFEPs) are usually depending on temperature having high temperature delta phase [10]. The dielectric permittivity εr=ε/ε0 is called relative permittivity and is given in eq. 1. Here ε is the electric field dependent dielectric constant. So by definition the polarization is the function of an electric filed i.e. P=ε0E(εr-1)=ε0εrE-ε0E=D-ε0E, where P is polarization (C/m2) and D is displacement (C/m2) [25]. εr=P/ε0E+1=(D-ε0E)/ε0E+1=D/ε0E (eq. 1)

Pm

Pm Ec

Ec

Pr

Beta Domain

Gamma Domain

β(S.T)

-5

-10

Page 2 of 11

Γ Delta Phase

δ(P.E)

Relaxor Ferroelectric P(VDF-TrFE-CTFE)

-200 -100 0 100 200 E(MV/m) Figure 2. shows the conversion of P(VDF-TrFE) to the P(VDF-TrFECTFE) after successful substitution of the third monomer CTFE into the bulky group of P(VDF-TrFE).

Another approach to get the good properties in between the ferroelectric relaxor and normal polymers is to blend normal ferroelectric (NFE) and relaxor ferroelectric (RFE) with each other. After blending copolymer with terpolymer some properties of both sides should be inherited into the blend following the expected rule of mixture. There are many blend systems reported in literature, e.g. it was reported that if we blend PMMA with P(VDF-TrFE-CFE) we can get high polarization, dielectric constant and high mechanical strength [5]. Secondly, a recent study has investigated the blends of P(VDF-TrFE) and P(VDFTrFE-CFE) with very good properties which give rise to the additional electrocaloric effect (ECE) and enhancement in the polarization of these blends [9,10]. The study conducted in 2012 showed that if we graft PEMA with P(VDF-TrFE) we can get an enhanced discharged energy density with minimal energy loss [14]. The antiferroelectric like double hysteresis loop is also observed which give rise to the high dielectric and electroactive properties suitable for contemporary industrial and medical use. As PVDF based copolymers and terpolymers are also called dielectrics and they have reasonable dielectric constants. However the polymer materials possess high energy density and

Figure 3. Shows the reaction between beta and gamma domains that results delta phase where ß(S.T) is Beta short trans and σ(P.E) is paraelectric delta phase. Table 2. The suggested characterization of transmission spectra of P(VDF) depending on copolymers and terpolymers based on literature review. Experimental Wavenumber

Group

Vibration

Comments

3430

N-H

Symmetric Stretching

--

3016

CH2

Symmetric Stretching

--

2971

C-H

Asymmetric Stretching

--

1453

CH2

In-plane bending or scissoring

--

1335

CH2

In-plane bending or (wagging or twisting)

--

840

CH2CF2

CH2 Rocking and CF2 Asymmetric Stretching

Beta phase

763

--

In-plane or Rocking

bending

Alpha phase

745

--

In-plane or Rocking

bending

Beta Phase

615

CF2,

CF2 bending and CCC

Alpha Phase

2


CCC

skeletal vibration

500-1000

C-F

Bending

510

CF2

Bending wagging

490

CF2

--

HH TT config and

Beta Phase Alpha Phase

The antiferroelectric state in which ions are spontaneously polarized, but neighboring ions polarized in antiparallel directions, so that the spontaneous macroscopic polarization of the crystal as a whole is zero [12]. Therefore, an anti-ferroelectric polymer dielectric material exhibit high maximum polarization (Pm), low remnant polarization (Pr), and high electrostriction (Es). The double hysteresis loop (DHL) only appeared in the first several cycles and eventually transformed into a single hysteresis loop on repeated cycling, consisted lower field dipole depolarization step and a polarization reversal step at higher fields. Therefore, the defective crystalline P(VDF-TrFE) crystals cooled from the melt shows the antiferroelectric like double hysteresis loop [1-3]. This antiferroelectric like behavior of cooled crystals is due to the coexistence of both FE (ferroelectric /or α and ß) and PE (paraelectric /or Γ) phases [13]. The more desirable is to achieve high Pm (maximum polarization) under a high electric field along with low Pr (remnant polarization) for high energy storage applications [14]. This rectangular-shaped D-E loop is used for the applications such as non-volatile energy storage capacitors and transistor-type ferroelectric memories (FeRAMs) [15]. However R. Gregorio (2006), correlates the characteristic reflections (as shown in Table 2) of FTIR spectroscopy with the antiferroelectric like double hysteresis loop behavior [16]. The coexistence of long trans and short trans indicates the relaxor ferroelectric (RFE) has disordered crystalline structure [17]. However, this disordered structure dictates the electric-displacement electric field (DE) loop behavior and net dipole moment become equal to zero. On the further investigation of antiferroelcetric like behavior by R. Su et al. (2012), it was attributed to the nonpolar “antiferroelectric-like phase” in P(VDF-TrFE) [18]. It shows that the antiferroelectriclike behavior is a universal phenomenon for semicrystalline ferroelectric polymers [19] in which the small amount of crystalline material is blended into the amorphous polymer material. The dipole moment is restricted by the parallel and antiparrallel dipoles due to the coexistence of amorphous and crystalline materials which give rise to the coexistence of ferroelectric (ß) and paraelectric (Γ) phases together. The reaction between beta and gamma domain results the delta phase which further leads to the antiferroelectric properties of polymer blends as shown in Figure [Fig 3]. The antiferreoelctric (AFE) like behavior is distinguished by the double hysteresis loops (DHL) where it bulges at the centre. The propeller shaped double hysteresis loop is the distinct property of AFE polymers which give rise to the high energy density and high dielectric constant. The beta phase is the most important phase where we can get the good crystalline properties and also desirable for AFE like double hysteresis loop behavior [1-5, 28]. The normal ferreoelctric material is that which has less energy density and shows the broader polarization while the relaxor ferroelectric polymer materials usually shows the higher

maximum polarization (Pm) and lower remnant polarization (Pr). We can distinguish the polymers into broad categories of ferreoelctric (FE), paraelectric (PE) and antiferroelectric (AFE) polymers. In which the paraelectric and antiferroelectric polymer materials are called linear materials. The materials in which the polarization is changed by the application of an electric field is called the linear materials and their energy densities are proportional to their electric fields (U ∞ E2) [20]. The energy density is the energy stored or release by the ferroelectric and electroactive polymers (EAP). By modifying PVDF with bulky comonomers such as chlorotrifluoroethylene (CTFE) and hexafluoropropylene (HFP), P(VDF-CTFE) and P(VDF-HFP) random copolymers recently have achieved a discharged energy density as high as 17-25 J/cm3 at ~600 MV/m for millisecond discharge [20]. On the application of an electric field the dipoles will align on the direction of an applied electric field. Especially with the high energy density dielectric permanent dipoles, the orientation is changed and major part of an electric field become trapped or stored [28]. The only way to discharge the stored electric energy by using an antiferroelectric like polymers which have the net dipole moment is nearly equal to zero (shown in Figure 4), the dipole switching behavior is critical because when on the reversal the negative energy density would be stored. Therefore the antiferroelectric (AFE) like behavior of the DE loop is desired due to all energy will be released upon reversal of an electric field. The charged and discharged energy density is measured by the DE loop behavior by estimating the area under the shaded region in Figure 4. The unreleased (or undischargerd) energy density can be defined as Undischarged = 100*(1Ureleased/Ustore), where Ustore has same value as Uunrelease. [21]. This study is focused on normal and relaxor ferroelectric polymer blends, which changes the different phases upon addition of normal polymer into relaxor polymer, ranging from α=0.0 to α=1.0. We have investigated that the antiferroelectric behavior is responsible in getting high energy density of α=0.1, 0.2 and 0.3 polymer blends. The ongoing study is mainly focused on the paraelectric to antiferroelectric phase transition, which will help to satisfy our arguments. 3

Energy Density (J/cm )

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D (µC/cm )

Page 3 of 11

E(MV/m) Figure 4. shows the measurement of stored energy density by measuring the area under the shaded region.

3



content (α) in the blended polymers increases.

P(VDF-TrFE) copolymer (75/25 mol%), one of normal ferroelectric polymers, was purchased from Kureha Chemical Industry Co., Ltd. (Tokyo, Japan), and P(VDF-TrFE-CTFE) terpolymer (62.7/29.6/7.7 mol%), one of relaxor ferroelectric polymers, was synthesized by using a suspension polymerization process at Piezotech S. A. (Hésingue, France). Ten grams of P(VDF-TrFE) copolymer and P(VDF-TrFE-CTFE) terpolymer were separately dissolved in 90 g of methylisobutylketone (MIBK) and the solutions were filtered with a polytetrafluoroethylene (PTFE) filter with 1.0 µm-sized pores. The filtered copolymer and terpolymer solutions were then mixed together in the weight ratios of P(VDF-TrFE):P(VDF-TrEFCTFE) = α:α-1 (α = 0.0, 0.1, 0.2, …, 1.0). The blended solutions were kept at room temperature for 24 hours to remove trapped micro-bubbles. In order to make a free standing film, the blended solution was dispensed and applied onto a glass plate with a film applicator. The MIBK solvent was evaporated from the solution and thus a dry blended film forms on the glass plate. The blended film was debonded from the glass plate in deionized water, and annealed at 115 °C for 2 hours and at 135 °C for 6 hours to remove remaining solvent and increase crystallinity. For electrical measurements, bottom platinum electrode was sputtered on a Si wafer and then the blended solution was spin-coated on the Si wafer at 1300 rpm for 60 s and 3000 rpm for 30 s. The blended film was annealed at 115 °C for 2 hours and at 135 °C for 6 hours. Platinum was again sputtered on the blended film with shadow mask so that an array of circular platinum electrodes was formed. The films were then subjected for DSC (TA. Instruments' Q. SeriesTM, Heating rate 10°C/m), XRD (D8 ADVANCE, BRUKER, 30KV / 1.5mA, Cu K-α), FTIR (SHIMADZU, ATR spectra), and Sawyer Tower Model for dielectric measurement.

3. RESULTS AND DISCUSSION 3.1 DIFFERENTIAL SCANNING CALORIMETRY (DSC) MEASUREMENTS Figure 5 (a) and (b) show the DSC second heating and first cooling curves, respectively, for P(VDF-TrFE) copolymer, P(VDF-TrFE-CTFE) terpolymer, and their blends, after melting at 170 °C for 5 min to nullify previous thermal history. The normal ferroelectric polymer (α = 1.0) exhibits two peaks in the endothermic heat flow curves during the second heating of the DSC measurement (Figure 5. (a)), one melting peak at TmNF = 152.4 °C and the other ferroelectric-to-paraelectric phase transition peak around 118 °C [18]. On the other hand, the relaxor ferroelectric polymer (α = 0.0) has a melting peak at TmRF = 132.5 °C but does not exhibit any phase transition peak. As the copolymer content (α) in the blended polymers decreases, the ferroelectric-to-paraelectric phase transition peaks gradually decrease and broaden, and the phase transition peaks almost disappear for α ≤ 0.3. Among the blended polymers, only when P(VDFTrFE):P(VDF-TrFE-CTFE)=0.1:0.9 (α = 0.1), the endothermic heat flow curve exhibits dual melting peaks at 129.4 °C and 149.6 °C. Interestingly, these dual melting temperatures are slightly lower than TmRF = 132.5 °C (melting temperature of P(VDFTrFE-CTFE)) and TmNF 152.4 °C (melting temperature of P(VDFTrFE)). The blended polymers for α ≥ 0.2 have only one melting peak at TmNF = 152.4 °C, which becomes larger as the copolymer

NF

Heat Flow Endo Up

2. EXPERIMENTS

Heat Flow (Arbitrary Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 11

o

Tm =152.4 ( C) ferroelectric-to-paraelectric phase transition α=1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

NF

o

T =132.5 ( C) (a) 0 40 80 120 160 o Temperature ( C) m

RF o FE-PE Tm = 107 ( C) NF o Tm = 136 ( C) phase transition

α=1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

(b) 0 40 80 120o 160 Temperature ( C)

Figure 5. (a) DSC ‘2nd heating curve’ for P(VDF-TrFE) and P(VDF-TrFE-CTFE) and its blends. (b) first cooling curve to estimate the cold crystallization properties of blends.

The phenomenon showing only one melting peak in the blended polymers for α ≥ 0.2 is contrary to the previous observation reported in literature [10]. They reported dual melting peaks for 0.5 ≥ α ≥ 0.1 and stated that the copolymers and terpolymers do not co-crystallize. However, our results imply that the blended polymers with α ≥ 0.2 do not partially melt at TmRF = 132.5 °C but completely melt at TmNF = 152.4 °C. Therefore, the P(VDF-TrFE) and P(VDF-TrFE-CTFE) blends with α ≥ 0.2 fabricated in this study are considered to have molecularly well-mixed microstructures. The exothermic heat flow curves during the first cooling of the DSC measurement (Fig. 5(b)) show a trend similar to the endothermic heat flow curves (Fig. 5(a)) The normal ferroelectric P(VDF-TrFE) copolymer (α = 1.0) exhibits two peaks, one crystallization peak at 136 °C and the other paraelectric-to-ferroelectric phase transition peak around 70 °C. On the other hand, the relaxor ferroelectric P(VDF-TrFE-CTFE) terpolymer (α = 0.0) has a crystallization peak at 107 °C but does not exhibit any phase transition peak. As the copolymer content (α) in the blended polymers decreases, the paraelectric-to-

4


Page 5 of 11

ferroelectric phase transition peaks gradually decrease and almost disappear for α ≤ 0.2. Also, it is worth noting that as the copolymer content (α) decrease, the crystallization temperature and crystallization enthalpy (peak area) gradually decrease, which can also be considered as an evidence that the P(VDF-TrFE) and P(VDF-TrFE-CTFE) blends with α ≥ 0.2 fabricated in this study have molecularly well-mixed microstructures.

3.2 X-RAY DIFFRACTION (XRD) ANALYSIS

Intensity (Arbitrary Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P(VDF-TrFE-CTFE) o 2θ=18.50 d = 4.795 Angstrom

P(VDF-TrFE) o 2θ=20.0 d = 4.439 Angstrom α=1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

14 16 18 20 22 24 26 2θ (degree) .

Figure 6. XRD Spectra for P(VDF-TrFE) and P(VDF-TrFE-CTFE) and its blends.

The blended polymers were also studied with X-ray diffraction (XRD) patterns as shown in Fig. 6. The normal ferroelectric P(VDF-TrFE) copolymer (α = 1.0) shows a diffraction peak at 2θ= 20.0°, which corresponds to an interchain lattice spacing of 4.439 Å (calculated from Bragg’s equation 2dsinθ =nλ) from the (110, 200) reflection in ferroelectric crystalline phases. On the other hand, the relaxor ferroelectric P(VDF-TrFE-CTFE) terpolymer (α = 0.0) displays a diffraction peak at 2θ= 18.50°, corresponding to the interchain lattice spacing of 4.795 Å [24]. The position of the diffraction peak, 2θ = 18.50°, was obtained by curve-fitting the X-ray diffraction curve with Gaussian function to reduce the effect of noise. The third monomer, CTFE, in P(VDF-TrFE-CTFE) terpolymers is known to act as random defects in P(VDF-TrFE) copolymers so that the interchain lattice spacing of P(VDF-TrFE-CTFE) terpolymer is larger than that of P(VDF-TrFE) copolymer, and thus, polar phases in P(VDF-TrFE) copolymers transform into non-polar nanophases in P(VDFTrFE-CTFE) terpolymer. For blends with low copolymer contents (0.1 < α < 0.3), two X-ray diffraction peaks were observed as shown in Figure 6. As the content of copolymer in the blends increases, the intensity of the first peak (non-polar reflection) gradually decreases, but the intensity of the second peak (polar reflection) gradually increases. Furthermore, the positions of the first peaks were calculated by curve-fitting with Gaussian function to be 18.50°, 18.47°, 18.42°, and 18.42°, for α = 0.0, 0.1, 0.2, and 0.3, respectively. This shift of the first peak originates from the slight increase of interchain lattice spacing of non-polar phases. For α > 0.4, only one X-ray diffraction peak corresponding to polar phases was observed around 2θ = 20.0°. This result is contrary to those reported in literature in which nonpolar peak was also observed for α= 0.4 and 0.5 [10].

3.3 FOURIER SPECTROSCOPY

TRANSFORM

INFRARED

The Fourier transform inferred spectroscopy is conducted at room temperature and various characteristics of reflections are investigated. However some researchers correlate the characteristic reflections of Fourier transform infrared spectroscopy with the antiferroelectric like double hysteresis loop behavior [8, 21, 26]. The coexistence of long trans and short trans is one of the evidence which indicate that the relaxor ferroelectric (RFE) has disordered crystalline structure. However, this disordered structure dictates the electric-displacement electric field (DE) loop behavior and net dipole moment become equal to zero. At room temperature the characteristic of reflections of PVDF based crystals is showing smooth trend which is interpreted in section (3.4). From Figure 7. (a) of FTIR for blends containing (α=0.0, 0.1, 0.2… 1.0) where α is the copolymer content. Various phases are investigated within the blends name as short trans (T3G), all trans (TTT or T3), Tm>3 and trans and gauche (TG) at 844, 1173, 1287, and 1400 cm-1. The Tm>3 phase is gradually increasing at (1287cm-1) and TGTGTG` at (1400cm-1) decreases with the characteristic reflections. The phase at 1173cm-1 is all trans which is increased with in the vicinity of the other peaks as the copolymer content α is increasing. The Figure 7b is plotted by the formula where three major characteristic reflections are chosen and by using simple formula.

Fi =

Ai A1 + A2 + A3

(eq. 2) Where Ai is the area of the reflection of curve under observation and A1, A2, and A3 are the characteristic reflections of Tm>3 (at 1287cm-1), Tm>4 (at 1400 cm-1) and T3G (at 845cm-1). By analyzing these three major reflections (Tm>3,TG , and T3G), we see that, it is very difficult to grasp the changes in reflection because of the relaxor ferroelectric (RFE) phase already contains a distorted structure with short and long trans sequence. The DSC curve also confirms that the copolymer and terpolymer cocrystallize at melting point. The copolymer content in terpolymer matrix acted like pinning (embedded) defects. At melt crystallization most of the pinning or embedded defects are removed. Resultantly, ferroelectric domains increases in size and ferroelectricity is reduced. It can be seen that at low copolymer content (α3 and Tm>4 conformation is quite subtle, which means the copolymer adopts a mixed structure consisting of three conformations just as terpolymer. As copolymer content becomes higher than α>0.2, a quick increase and/or decrease is observed, which indicates the influence of copolymer becomes larger. The T3 conformation only increases in the whole copolymer content range. In the DE hysteresis loop, at 0.1 to 100 Hz poling frequency the large hysteresis is observed which is attributed to the large domain size in the relaxor ferroelectric (RFE) phase (future work). At 25°C, a significant amount of relaxor ferroelectric (RFE) phase should transform into the paraelectric delta (δ) phase. Due to the (RFE/PE) FE phase transition upon poling and FE (RFE/PE) phase transition upon removing the field the remnant polarization reduces nearly to zero and typical double hysteresis loop DHL is observed. The characteristic reflections of FTIR spectroscopy show the influence of copolymer content in terpolymer matrix. As copolymer content (α) increases from

5



2

2

(a)

D(µC/cm )

blends. Energy density (which is measured by calculation of area under the curve) is proportional to E2 is measured in section (3.5). It is noted that energy density for α=0.1 and 0.2 blends is higher than rest of the other polymer blends.

D(µC/cm )

α=0.2, the clear transition is observed in a whole spectra. The Tm>3 phase at 1287 cm-1 is gradually increases with increasing the copolymer content (α>0.2). While for TG (or Tm>4) conformation at 1400cm-1 is gradually decreases and reaches up to the certain value for α=1.0. The characteristic reflection of 1173 cm-1 is regarded as long trans (TTT or all trans) is increased little bit for the whole spectra. We speculate that the presence of Tm>3 phase at (1287cm-1), which on the broader sense could be regarded as a δ phase, is due to the coexistence of short and long trans which finally resulted the antiferroelectric like DHL behavior.

α=0.0

(b) α=0.1

(b) -1

1400cm -1 845cm -1 1287 cm

2

α=0.4

(g) α=0.6

2

2

D(µC/cm )

3.4 ELECTRICAL DISPLACEMENT-ELECTRICAL FIELD (DE) HYSTERESIS LOOP Figure (8) shows the DE loop for P(VDF-TrFE) copolymer and P(VDF-TrFE-CTFE) terpolymer and their blends. The DE loops are measured at various electric fields and frequency. The normal ferroelectric shows the higher remnant polarization (Pr), maximum polarization (Pm) and coercive field (Ec) than relaxor ferroelectric polymer. Enhancement in polarization is observed by addition of copolymer into terpolymer matrix. Therefore, due to small addition of copolymer content in α=0.1 blend enhanced the polarization behavior to higher level than α=0.0, 0.2 and 0.3

(i) α=0.8

E(MV/m) 2

(f) α=0.5

E(MV/m)

E(MV/m)

D(µC/cm )

Figure 7. (a) FTIR spectra for terpolymer, copolymer and their blends from 0.0 to 1.0 blend. (b) Variation of infrared absorption intensities for polymer chains of (T3G or Tm>3) (at 845cm-1), (Tm>3) (1287cm-1), and (TG orTm>4) (1400cm-1) conformation with respect to copolymer content.

E(MV/m) D(µC/cm )

(e)

E(MV/m)

0.0 0.2 0.4 0.6 0.8 1.0 Copolymer Content (α)

D(µC/cm )

2

E(MV/m)

2

800 1000 1200 1400-11600 Wavenumber (cm )

α=0.3

(h)

D(µC/cm )

α=0.2 α=0.1 α=0.0

α=0.2

(d)

2

α=0.4 α=0.3

(c)

2

α=0.6 α=0.5

E(MV/m)

D(µC/cm )

α=0.8 α=0.7

D(µC/cm )

α=0.9

2

(a) α=1.0

D(µC/cm )

T3 Τm>3 TG

T3G

D(µC/cm )

Intensity (arbitrary unit)

E(MV/m)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 11

α=0.7

E(MV/m) (j) α=0.9

E(MV/m)

(k) α=1.0

E(MV/m) Figure 8. DE loop for the blends (α=0.0, 0.1, 0.2, …… 1.0) at same frequency, where α represents the copolymer content in terpolymer, from (a) to (k).

The antiferroelectric like double hysteresis loop for α=0.1, 0.2 and 0.3 blends is also observed which is attributed to the addition of highly crystalline material into highly amorphous matrix material.

6


2

D (µC/cm )

12 Copolymer (a) Content ( α) 8 0.0 0.1 4 0.2 Copolymer 0.3 0 Content (α) 0.4 0.6 0.5 -4 0.7 0.8 -8 0.9 1.0 -12 -200 -100 0 100 200 E(MV/m)

0.2 Hz to 200 Hz

3 (b)

0 -3 -6

α=0.1

-150-100-50 0 50 100 150

E(MV/m)

8

(c)

α=0.1

2

4

-8

2

D(µC/cm )

0

-4

280

(d)

245 210 175

Co/Ter Breakdown Slope Content(%) (MV/m) 50% 228 2.64 60% 206 3.98 70% 211 2.86 80% 226 3.15 90% 194 3.11 100% 180 7.21

Co/Ter Breakdown Slope Content(%) (MV/m) 0% 240 3.31 10% 270 2.24 20% 243 2.84 30% 226 3.45 40% 219 2.90

0.0 0.2 0.4 0.6 0.8 1.0 Copolymer Content (α)

Figure 9. (a) DE loop comparison of blends at same frequency and electric field from 0.0 to 1.0 blend. (b) The frequency dependent antiferroelectric like behavior (c) The antiferroelectric like DHLs of α=0.1 blend at various electric field (d) dielectric breakdown strength for α=0.0 to 1.0 blends.

2

D(µC/cm )

6

D(µC/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Breakdown Potential (MV/m)

Page 7 of 11

α=0.1 E=MV/m

-200 -100 0 100 200 E(MV/m)

Below 150MV/m (or even at slightly higher) due to parallel and antiparallel dipolar orientation which on reversal process gives antiferroelectric-like double hysteresis loop (DHL) is observed, Fig 9. (c). The Figure 9. (b) illustrates the antiferroelectric like behavior of copolymer and terpolymer blends [21]. The copolymer is highly crystalline and terpolymer is highly amorphous polymer. In terpolymer, when the addition of copolymer is induced, due to foreign addition of highly crystalline polymer the dipolar orientation is disturbed [28]. When crystallization process begins, higher compatibility (miscibility) of the copolymer into terpolymer matrix disturbs the crystallinity and crystalline size. The disturbance of the crystalline size and crystallinity causes the enhancement of polarization and increases the antiferoelectric like DHL behavior. It is noted the double hysteresis loop (DHL) behavior is only observed in α=0.1, 0.2, and 0.3 prominently and at lower electric field (0.2) is gradually decreased from the neat polymer and finally it reaches to the level of α=1.0 blend [5].

3.5 STORED AND DISCHARGED ENERGY DENSITY The stored energy density is obtained for α=0.0, 0.1, 0.2… 1.0 blends. It is noted that the stored energy density for α=0.1 and 0.2 blends is higher than the rest of the other blends. However α=0.0 and 1.0 blends conventionally giving less energy densities than 7



3


9 6

α=0.3 α=0.1 α=0.2

3

(a) α =0.0

0

3

(a)

100 Unrelease (%)

α= 0.1 and 0.2 blends either. Also α=0.1 and 0.2 blends shown the higher value of the energy density than the other blends. The increase in the energy density is due to the antiferroelectric like behavior which gives rise to the double hysteresis loop [28]. Consequently, the double hysteresis loop is also having relatively energy densities and their reasonable dielectric constants. Figure 10. shows the comparison between the different energy densities for α=(0.0, 0.1, 0.2…1.0). Apparently, the increase in the stored energy density (UE) is due to the applied electric field. Besides that, slightly reduction of the Pm due to the content of polymer (which causes the switching in the DE loop) exhibit slightly negative UE due to reduction of Pr. The energy density of the antiferroelectric like behavior is increased and then decreased by the increment of frequency from 0.01 to 100Hz (Figure 10. (c)). From the Figure (Fig 10. (c)) the discharged energy density for α=0.0, 0.1, 0.2… 1.0 is almost linearly increased by the increasing the copolymer content (α).


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 11

% Unrelease

(C)

50 0 -50

-100

Negative stored energy 1 10 Frequency (Hz)

100

Figure 10. (a) The discharged energy density curve of blends from 0.0 to 1.0 blend.(b) Stored and Discharged energy density comparison for α=0.1 blend at various frequency. (c) The percentage of unreleased energy density for α=0.1 blend.

The discharged energy density for blends is enhanced due to the AFE behavior in contrary to the studies conducted in 2010 and 2012 separately [14, 20]. The percentage of unreleased energy density is reported only 12% at 3.33 Hz for α=0.1 blend where in the previous researchers reported upto 30% to 40% unreleased energy density which is undesirable to use in fast charge and discharge equipments [14].

3.6 DIELECTRIC CONSTANT

0

50 100 150 E(MV/m)

200

Stored 4.0 Discharged 3.5 3.0 Antiferroelectric 2.5 2.0 1.5 Paraelectric 1.0 (b) 1 10 Frequency (Hz)

100

For the electrical characterization the complex dielectric constant is given as ε*=ε’+iε”, where ε’ is real and ε” is imaginary part of complex dielectric constant [23, 25]. The pure copolymer P(VFDTrFE) is a low dielectric polymer and terpolymer P(VFD-TrFECTFE) is highly dielectric polymer. The dielectric constant of copolymer is 25 at 1 KHz at room temperature, where the dielectric constant for terpolymer at 1 KHz is 48. It is reported that the higher value of dielectric constant of terpolymer at lower frequency and room temperature is due to the response of dipoles in the randomly distributed nano-polar region. In Figure (Fig 11. (a)) the ε’ of α=0.0, 0.1, 0.2… 1.0 blends are measured as function of frequency at lower electric filed. It is observed that the ε’ of α=0.1 to 0.3 blends is increased gradually and when the copolymer content α reaches at 0.4 the ε’ is reduced. We speculate that the increase in the imaginary part of dielectric constant (ε’) of α (0.1 ~ 0.3) blends is due to the randomly distributed nano-polar region caused by the addition of small amount of copolymer into terpolymer. Interestingly, the increase in the dielectric relative permittivity (εr) is obtained by the addition of small amount of lower dielectric constant polymer [Fig. 11 (d)] which is also reported in literature [10]. However the dielectric constant of blends is lower than pure terpolymer, but the enhancement in the dielectric constant is observed due to antiferroelectric like behavior. The relaxor ferroelectric having antiferroelectric like behavior is known by its high energy density characteristics.

8


ε' (Real Part)

60

ε''(Imaginary Part)

Copolymer content, α 0.0 0.1 0.4 0.5 0.8 0.9

0.2 0.6 1.0

160

0.3 0.7

120

40 20 0

(a) 3

4

5

10 10 Frequency (Hz)

10

30 Copolymer content, α 0.0 0.1 0.2 0.3 25 0.4 0.5 0.6 0.7 0.8 0.9 1.0 20 15 10 5 0 (b) -5 3 4 5 10 10 10 Frequency(Hz) 0.6

tan(δ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Copolymer Content (α) α=0.0 0.1 0.2 0.4 0.5 0.6 0.9 1.0

Dielectric Permittivity (εr)

Page 9 of 11

Copolymer Content (α) 0.0 0.3 0.6 0.1 0.4 0.7 0.2 0.5 0.8

(d) 0.9 1.0

80 40 0

50 100 150 Electric Field (MV/m)

Figure 11. (a) Real part of dielectric constant (ε’) of blends from 0.0 to 1.0 blend. (b) Imaginary part of dielectric constant (ε”) of blends from 0.0 to 1.0 blend. (c) tanδ for blends and (d) relative permittivity from 0.0 to 1.0 blend.

If the antiferrolectric relexor polymer additionally has higher dielectric constant then it is strikingly enough to address the problem to reduce the technology gap. The correlation of real and imaginary dielectric constant is quite interesting. The trend of real part and imaginary part is quite opposite upon the increase in the frequency (Hz). It is noted that at the higher frequency the trend of dielectric constant (ε’) is decreased while the dielectric loss is increased. But as a whole the dielectric loss of blend system is very less. We investigate that the dielectric loss is quite visible [Fig 11 c] while the relaxor ferroelectric gradually converted to ferroelectric phase due to overwhelming addition of copolymer content (α).

CONCLUSION 0.3 0.7

The enhancement in the polarization is shown with response of copolymer content. The antiferroelectric like behavior is observed for lower copolymer content which gives rise to the tunable dielectric constant. The energy density is investigated at various fields and frequency in order to address the AFE behavior. The stored and discharged energy density of α=0.1 and 0.2 blends are enhanced and the negative energy stored is eliminated by estimating the useful frequency range for antiferroelectric-like polymer blends.

0.8

0.3

0.0 (c)

ACKNOWLEDGMENT 3

4

10 10 Frequency (Hz)

5

10

Mudassar Shehzad wants to thanks, in particular, invaluable support received from Dr. Seung Tae Choi, Dr. Mohammad Ali Mohammad, Dr Malik Adeel Umar, Dr Ahmed Umar Munawar and Dr. Harris Masood Ansari.

AUTHOR INFORMATION Corresponding Author Mudassar Shehzad was born in the Islamic Republic of Pakistan in 1986. He received B.S degree in Metallurgy and Materials Engineering in 2010 from Punjab University Lahore, Pakistan. After

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

his graduation he served as a Lab Engineer in Ghulam Ishaq Khan Institute (GIKI), Pakistan (from 2010 to 2012). He completed his Masters in Mechanical Engineering from University of Ulsan, South Korea (Republic of Korea). Recently he is working as a Lecturer at the National University of Sciences and Technology (NUST) in the School of Chemical and Materials Engineering. His research interests include ferroelectric polymers actuators and sensors, characterization of polymer materials, and electrocaloric effect (ECE) measurement for ferroelectric polymers.

Author Contributions Tayyaba Malik was born in the Islamic Republic of Pakistan in 1994. She completed her B.S degree in Electronics Engineering in 2015 International Islamic University Islamabad, Pakistan. Right after her graduation she worked as Graduate Trainee in Quality Testing Lab at National Institute of Electronics (NIE), Pakistan. At the same time, she enrolled herself in Masters in Nanoscience and Engineering at National University of Sciences and Technology (NUST) in the School of Chemical and Materials Engineering. Besides her studies she engage herself in research work, her areas of interest include ferroelectric polymers, actuators, flexible electronic devices, strain sensors and flexible supercapacitor, characterization and fabrication of PVDF and PDMS based devices.

REFERENCES [1]. An, N.; Liu, H.; Ding, Y.; Zhang, M.; Tang, Y. Preparation and Electroactive Properties of a PVDF/nano-TiO2 Composite Film Applied Surface Science 2011, 257, 3831–3835. [2]. Wang, T. T.; Herbert, J. M.; Glass. A. M. The Applications of Piezoelectric Polymers Blackie London 1988. [3]. Zhang, Q. M.; Bharti, V.; Zhao, X. Giant Electrostriction and Relaxor Ferroelectric Behavior in Electron-Irradiated Poly(vinylidene fluoridetrifluoroethylene) Copolymer Science 1998, 280, 2101- 2104. [4]. Xia, F.; Cheng, Z. Y.; Xu, H. S.; Li, H. F.; Zhang, Q. M.; Kavarnos, G. J.; Ting, R. Y.; Abdel-Sadek, G.; Belfield, K. D. High Electromechanical Responses in a Poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) Terpolymer Adv. Mater, 2002, 21, 1574 [5]. Chu, B.; Zhou, X.; Ren, K.; Neese, B.; Lin, M.; Wang, Q.; Bauer, F.; Zhang, Q. M. A Dielectric Polymer With High Electric Energy Density And Fast Discharge Speed Science 2006, 313, 1887–1887. [6]. Li, J. Nanocomposites Based On Ferroelectric Polymers For Electrical Energy Storage Ms thesis 2009. [7]. Xu, H.; Cheng, Z.-Y.; Olson, D.; Mai, T.; Zhang, Q. M.; Kavarnos, G. Ferroelectric and Electromechanical Properties of Poly(vinylidenefluoride-trifluoroethylene-chlorotrifluoroethylene) Terpolymer Appl. Phys.Lett. 2001, 78, 2360. [8]. Zhang, Q.; Zhang, S.; Cheng, A.; Liu, S.; Zellers, B.; Anderson, D., Relaxor Ferroelectric EAPs And Their Applications Nature 2011, 419, 2002-2010. [9]. Bharti, V.; Xu, H.; Cheng, Z. Y.; Mai, T.; Zhang, Q. M. Quantitative Analysis of Structural, Relaxational And Electrostrictive Properties of PVDF-TRFE/PMMA Films Irradiated With High-Energy Electrons IEEE Trans. Dielectr. Electr. Insul. 2001, 8, 718. [10]. Chen, X. Z.; Li, X., Qian, X. S.; Wu, S.; Lu, S. G.; Gu, H. M.; Lin, M.; Shen, Q. D.; Zhang, Q.M. A Polymer Blend Approach To Tailor The Ferroelectric Responses in P(VDF-TrFE) Based Copolymers, Polymer 2013, 54, 2373-2381.

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[11]. Kasap, S. O. Dielectric Materials and Insulation. Principles of Electronic Materials and Devices 2nd ed Tata McGraw-Hill New Delhi 2005, 2000, 507-575. [12]. Kittel, C. Theory of Antiferroelectric Crystals Phys. Rev. 1951, 82, 729 – 732. [13]. Murata, N.; Tsunashima, H. Polarization Reversal and Double Hysteresis Loop in Copolymers of Vinylidene Fluoride And Trifluoroethylene IEEE Transactions on Electrical Insulation 1986, 21, 543-548. [14]. Li, J.; Tan, S.; Ding, S.; Li, H.; Yang, L.; Zhang, Z. Observation of a New Particle in the Search For the Standard Model Higgs Boson With the ATLAS detector at the LHC physics letter B 2012, 716-720. [15]. Ducharme, S.; Reece, T. J.; Othon, C. M.; Rannow, R. K. Ferroelectric Polymer Langmuir-Blodgett Films For Nonvolatile Memory Applications IEEE Transactions on Device And Materials Reliability 2005, 5, 720-735. [16]. Gregorio, R. Determination of The Alpha, Beta, and Gamma Crystalline Phases of Poly(vinylidene fluoride) Films Prepared at Different Conditions J Appl Polym Sci. 2006, 100, 32723279. [17]. Yang, L.; Li, X.; Allahyarov, E.; Taylor, P. L.; Zhang, Q. M.; Zhu, L. Novel Polymer Ferroelectric Behavior via Crystal Isomorphism And The Nanoconfinement Effect Polymer 2013, 54, 1709-1728. [18]. Furukawa, T.; Takahashi, Y.; Nakajima, T. Recent Advances in Ferroelectric Polymer Thin Films for Memory Applications Current Applied Physics 2010, 10, 62-67. [19]. Chang, C.; Tran, V. H; Wang, J.; Fuh, Y. K.; Lin, L. DirectWrite Piezoelectric Polymeric Nanogenerator With High Energy Conversion Efficiency Nano Letters 2010, 10, 726-731. [20]. Guan, F.; Wang, J.; Yang, L.; Tseng, J. K.; Han, K.; Wang, Q.; Zhu, L. Confinement-Induced High-Field Antiferroelectric like Behavior in a Poly(vinylidene fluoride-co-trifluoroethylene cochlorotrifluoroethylene)-Graft-Polystyrene Graft Copolymer Macromolecules 2011, 44, 2190–2199. [21]. Su, R.; Tseng, J. K.; Lu, M. S.; Lin, M.; Fu, Q.; Zhu, L. Ferroelectric Behavior in The High Temperature Paraelectric Phase in a poly(vinylidene fluoride-co-trifluoroethylene) random copolymer Polymer 2012, 53, 728-739. [22]. Li. X,; Lu, S. G.; Chen, X. Z.; Gu, H. Qian, X.; Zhang. Q. M.; Pyroelectric and Electrocaloric Materials, J. Mater. Chem. C. 2013, 1, 23-27. [23]. Bobnar, V.; Levistik, A.; Huang, C.; Zahang, Q.M. Enhanced Dielectric Response in All-Organic Polyaniline-poly(vinyldenefluoridene-trifluoroethylene-chlorotrifluoroethylene) Composite Journal of Non-Crystalline Solids 2007, 353, 205-209. [24]. Zhao, X. L.; Wang , J. L.; Liu, B. L. Enhanced Piezoelectric Response in The Artificial Ferroelectric Polymer Multilayers Appl. Phys. Letter 2014, 105, 222907. [25]. Zhao, X. L.; Wang , J. L.; Liu, B. L.; Tian, B. B. Enhanced Dielectric And Ferroelectric Properties in The Artificial Polymer Multilayers Appl. Phys. Lett. 2014, 104, 082903. [26]. Liu., B. L.; Tian, B.B.; Geiger, S.;Hu, Z. H,; Zaho, X. L. The Intermediate Temperature T* Revealed in Relaxor Polymers Appl. Phys. Lett. 2014, 104, 222907. [27]. Zahang, Z.; Meng, Q. Mike-Chung,; T. C. Energy Storage Study of Ferroelectric Poly(vinylidene fluoride-trifluoroethylenechlorotrifluoroethylene) Terpolymers Polymer 2009, 50, 707715. [28]. Bharti. V.; Shanthi. G.; Xu. H.; Zhang. Q. M. Evolution of Transitional Behavior And Structure of Electron-Irradiated Poly (vinylidene fluoride–trifluoroethylene) Copolymer Films Materials Letters 2001, 47, 107–111.

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Page 11 of 11

Table of Content (TOC) Graphics

α(F.E + ) β(L.T)

β(S.T)

T3

TG

Intensity

β(S.T)++ Γ

Tm>3

Γ δ(P.E)

cm

-1

Displacement

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electric Field

Electric Field Electric Field Figure shows the reaction between ß(S.T) and Γ domains of ferroelectric polymer blends to obtain α and ß(L.T) which results Paraelelctric σ phase. The paraelectric σ phase is visible in FTIR (ATR) spectroscopy which shows the transition of Paraelectric domains to antierroelectric domains at high wavenumber (cm-1). Finally, the Double Hysteresis Loop (DHL) is responsible for fast charging and discharging of energy density.

11


and P(VDF-TrFE-CTFE)

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