Composition-Dependent Dielectric Properties of Poly(vinylidene

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Composition-Dependent Dielectric Properties of Poly(vinylidene fluoride-trifluoroethylene)s Near the Morphotropic Phase Boundary Yang Liu, Zhubing Han, Wenhan Xu, Aziguli Haibibu, and Qing Wang* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 United States

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S Supporting Information *

ABSTRACT: The discovery of relaxor behavior in poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers plays a crucial role in driving the formation of a morphotropic phase boundary (MPB) near which large enhancements of dielectric and piezoelectric responses are achieved. Here we study the dielectric spectra of P(VDFTrFE)s near MPB and analyze the relaxor behavior using two different theoretical models. We observe the largest dielectric constant of ∼76 at 1 kHz in morphotropic composition P(VDFTrFE) 50/50 mol % at around 69 °C. We show that the compositional dependence of dielectric peak temperature changes dramatically in the vicinity of VDF = 49 mol %. Moreover, we find that the parameters (i.e., dielectric relaxation strength) deduced from the fit of the experimental data by different models also changes abruptly at VDF = 49 mol %. These findings clearly signify the destabilization of normal ferroelectric phase occurring at the critical VDF content of 49 mol % and explicitly mark one boundary of MPB region.

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INTRODUCTION

It is known that semicrystalline P(VDF-TrFE) crystallizes with at least three different phases subsequently from the nonpolar α phase (TGTG conformation, T: trans, G: gauche), the ferroelectric β phase (all-trans conformation) to a newly discovered relaxor phase (3/1 helix conformation), with the increase of TrFE content.15,21,22 The evolution from normal ferroelectric to relaxor is mediated by a transition region reminiscent of a MPB frequently observed in piezoelectric ceramics.17−20 Given that only a few studies were conducted on TrFE-rich region, the prior reviews on P(VDF-TrFE)s differ pronouncedly on this subtle region from “antiferroelectric”-like phase,22 “cooled” phase (tilting of long trans segments)23 to a mixed ferroelectric and nonferroelectric phase.24 Importantly, relaxor nature of TrFE-rich P(VDF-TrFE) has not been demonstrated previously.22−26 The evolution from a ferroelectric to a relaxor in P(VDF-TrFE) bears a surprising resemble to benchmark MPB piezoelectric ceramics Pb(Zn1/3Nb2/3)O3− PbTiO3 (PZN−PT)27,28 and Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN−PT),29 in which the growth of the relaxor phase out of the normal ferroelectric phase is observed with a decrease of PT concentration. The concomitant competition between local and long-range ferroelectric orders in these materials may play a crucial role in driving the formation of MPB and thus achieving striking properties, e.g., ultrahigh piezoelectric coefficients and large dielectric responses.28,29 The observation of very similar

Poly(vinylidene fluoride) based ferroelectric polymers hold great promise for a wide variety of energy applications such as capacitive energy storage, electromechanical device, organic electronic, and electrocaloric cooler.1−8 Of particular interest in this family is poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer due to its strongest piezoelectric activity among polymer materials.9 Despite tentative approaches targeting to improve piezoelectric performances by enhancing ferroelectric β phase (all-trans conformation) fraction and/or crystallinity,9,10 previous experimental and theoretical results exhibit a mixture of both positive and negative evidence10−14 and the intrinsic piezoelectric coefficient d33 remains restricted to ∼−30 pC/N (ref 9). This limit has been overcome very recently thanks to design of a morphotropic phase boundary (MPB) in P(VDF-TrFE) copolymers.15,16 MPB is a well-established physical concept that is widely utilized in piezoelectric ceramics17−20 but never realized in organic piezoelectrics. A combined experimental and theoretical results15 indicate that there exists a transition region (49 mol % ≤ VDF ≤ 55 mol %) at around VDF = 50 mol %, allowing the interconvention between competing ferroelectric phase and relaxor phase. In the proximity of this intermediate region, piezoelectric coefficient reaches the anomalous maximum of −63.5 pC/N at VDF = 50 mol % which is as twice as the previous results, i.e., ∼−30 pC/N (ref 9). The well-known ferroelectric instability disappears near MPB, indicating abrupt crystal structure changes. © XXXX American Chemical Society

Received: July 7, 2019 Revised: August 14, 2019

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DOI: 10.1021/acs.macromol.9b01403 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Temperature dependence of the dielectric constants of P(VDF-TrFE)s with various VDF contents upon heating: (a) 55, (b) 53, (c) 50, (d) 49, (e) 48, (f) 47, and (g) 45 mol %. polymer was washed by vacuum filtration with both distilled water and methanol and then dried at 90 °C for 24 h to yield about 10 g of powder of P(VDF-TrFE). Film Preparation. P(VDF-TrFE) was dissolved in N,N-dimethylformamide (DMF; Sigma-Aldrich) at a concentration of 40 mg/mL. The solution was stirred at a rate of 700 r.p.m. overnight and subsequently cast onto glass plates and dried at 70 °C for 16 h in a vacuum oven. Subsequently, the films were peeled off from the glass plates and annealed at 130 °C for 12 h. The typical film thickness was about 10 μm. Characterization. Polymer compositions were determined by nuclear magnetic resonance (NMR). NMR spectra were acquired by a Bruker CDPX-300 spectrometer (300 MHz). Samples were dissolved in acetonitrile-d3 (VWR) and scanned 128 times to reduce the noise in the spectra. The 1H NMR spectra were used to determine the molar ratio of VDF and TrFE in the copolymers. Gold electrodes of a typical thickness of 60 nm and a diameter of 4 mm were sputtered (Denton Vacuum, Desk IV) on both sides of the polymer films for the dielectric measurements. Dielectric spectra were acquired over a broad temperature range using a dielectric E4980A precision LCR meter (Keysight) in conjunction with a Delta Design oven (model 9023). The data were recorded at a heating or cooling rate of 1.5 °C/min from 25 to 100 °C or 100 to 25 °C in the frequency range 100 Hz-1 MHz.

behaviors in P(VDF-TrFE)s as in the well-known MPB piezoelectric ceramics offers further confirmation of the existence of MPB in ferroelectric polymers. The observation of relaxor behavior in P(VDF-TrFE) copolymers is also of importance to reveal that relaxor property is intrinsic to P(VDF-TrFE) rather than induced extrinsically by irradiation3 or incorporation of bulky defects30−32 like CFE or CTFE (CFE: chlorofluoroethylene; CTFE: chlorotrifluoroethylene). In this work, in order to further shed light on the understanding of relaxor property discovered in P(VDF-TrFE)s, we focus on the investigation of the dielectric properties of P(VDF-TrFE)s especially near MPB at which relaxor property emerges. We study the evolution of relaxor property in P(VDFTrFE) as a function of VDF content by using two different models. By analyzing the deduced parameters versus VDF content, we find that relaxor phase in P(VDF-TrFE) develops and becomes stronger with decreasing the VDF content. We thus resolve an explicit boundary locating at a critical VDF content of 49 mol %, below which only a local ferroelectric instability occurs. The strong competition between long-range and local ferroelectric instabilities as revealed based on our findings not only help understanding of MPB formation in P(VDF-TrFE) copolymers but also may offer opportunities to rapidly search possible phase boundaries in piezoelectric polymers.

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RESULTS AND DISCUSSION Dielectric Properties near the MPB Region. Figure 1 summarizes the dielectric spectra of P(VDF-TrFE) copolymers for VDF ≤ 55 mol %. Indeed, dielectric responses of P(VDFTrFE) were already reported in a few previous studies.4,33−36 However, relaxor behavior was not unveiled in these works. Typically, differing from normal ferroelectrics, the signature of relaxor ferroelectric in dielectric spectra is that the dielectric peak temperature Tmax shifts toward higher temperatures with increasing the frequency of the applied electric field, which can be justified according to the temperature dependence of dielectric response under different frequencies (Figure 1). However, most of the previous works presented the dielectric data in terms of dielectric constant versus frequency under

EXPERIMENTAL SECTION

Polymer Synthesis. Synthesis of P(VDF-TrFE) copolymers was accomplished via suspension polymerization using a 300 mL stainless steel Parr reaction vessel. Then, 100 mL of deionized water and 0.15 g of potassium peroxydisulfate initiator were added to the vessel which was subsequently sealed and degassed via vacuum pump and cooled using a liquid-nitrogen bath. Gaseous VDF and TrFE were pumped into the reaction vessel separately. The amount of monomer was controlled by varying the time each monomer was allowed to flow into the reaction vessel. After charging the vessel with monomer, the vessel was sealed. The vessel was then heated to 90 °C for 12 h. Once the reaction was complete, the vessel was cooled to room temperature and opened. The B

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Figure 2. (a) Dielectric constant as a function of temperature measured at 1 kHz upon heating. (b) Dielectric peak temperature Tmax as a function of VDF content. The lines in panel b are guides for the eyes.

Figure 3. Dielectric loss as a function of temperature measured at 1 kHz upon heating (a) and cooling (b). The lines are guides for the eyes.

different temperatures,33−35 which may be one of the reasons why relaxor nature of the copolymers is overlooked. The dielectric relaxation near Tmax was attributed to the molecular motions in the crystalline regions independent of the amorphous regions.34,35 In addition, in some studies,4,36 the temperature dependence of dielectric response under different frequencies was reported while the frequency dependence of dielectric peaks was not recognized. Interestingly, we find in Figure 1 that the shape of dielectric peak remains nearly unchanged as the VDF content decreases from 55 to 50 mol % (Figure 1a−c). The peak becomes increasingly broader (Figures 1d−g) with further decrease of the VDF content. The rapid increase in dielectric response at 100 Hz and near 100 °C shown in Figure 1d,f is extrinsically attributed to ionic conductivity. Now we focus on the evolution of dielectric peak of P(VDFTrFE) copolymers in terms of the magnitude of dielectric response and corresponding temperature Tmax. To clearly show the evolution of these parameters as a function of VDF concentration, we select dielectric data measured at a specific frequency of 1 kHz, which is shown in Figure 2a. It is known that the presence of MPB enables improved dielectric responses of ferroelectric materials. Such enhancement of dielectric constant was reported in our previous work and P(VDF-TrFE) 50/50 mol % exhibits the highest dielectric constant (∼18) at room temperature.15 Here we show that dielectric peak near Tmax is also largely enhanced for MPB compositions (Figure 2a). We observe the largest dielectric constant of ∼76 in morphotropic composition P(VDF-TrFE) 50/50 mol % near 69 °C. The dielectric peak drops significantly and smears out remarkably as the VDF content decreases below 49 mol %. Moreover, Figure 2b shows that dielectric peak temperature Tmax first decreases gradually with decreasing VDF content from 55 mol % to 49 mol % as a result of the reduction in the fraction of all-trans conformation related to the ferroelectric instability. Tmax reaches

the minimum at VDF = 49 mol %, below which it increases (with the decrease of the VDF content) toward the dielectric peak temperature (∼75 °C) in poly(trifluoroethylene) (PTrFE).37 Distinct composition dependence of dielectric peak temperature Tmax between the MPB region (49 mol % ≤ VDF ≤ 55 mol %) and relaxor region (VDF < 49 mol %) indicates that long-range ferroelectric distortion disappears at a critical VDF content of 49 mol %. In perovskite relaxors, the dielectric behavior above Tmax is usually not strongly frequency dependent. However, Figure 1 shows that the dispersion of dielectric constant is frequency dependent above Tmax. These relaxor-like dielectric spectra were frequently observed in polymer materials.30 We attribute it to the ionic contribution, which is active and strongly frequency dependent especially at high temperatures for polymer materials. Our argument is supported by suppression of the frequency dependent behavior above Tmax in a terpolymer relaxor with a significantly reduced Tmax ≈ 0 °C.38,39 Given that the ionic contribution only plays a major role at high temperatures, the frequency dependence of Tmax near 60 °C can be described by Vogel-Folcher (V-F) equation. In addition to classic V-F relation, there are various models available to understand dielectric behavior near the Tmax.40−48 Here we mainly select two typical models (see below) to analyze our dielectric data as a function of VDF content because the main interest of this work is to provide an explicit picture of the compositional evolution of phase transition and critical behavior in P(VDF-TrFE) copolymer. In relaxor, the frequency dependent dielectric permittivity (real part) reaches a maximum at Tmax and Tmax depends on the frequency of the applied electric field. V-F relation is a well-recognized method to testify if a dielectric material can be a relaxor system.3,38,39,49−53 In our case, we find that the dielectric data are well fitted by V−F relation and the other relation (see below). These results necessarily indicate C

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Figure 4. (a) Logarithm frequency as a function of dielectric peak temperature (heating) of P(VDF-TrFE)s with different VDF contents, where solid lines represent the fits by the V-F relation. (b−d) The parameters logarithm f 0, Tf and log10(e)Ea/kB deduced from the V-F fit of dielectric data as a function of VDF content upon both heating and cooling. The lines in panels b−d are guides for the eyes.

are attributed to trans−gauche rotation to stabilize a disordered 3/1-helix conformation. Therefore, the change in the dielectric constant near 70 °C is strongly related to the dielectric relaxation effect in the crystalline domain of P(VDF-TrFE). For morphotropic composition (VDF from 49 mol % to 55 mol %), the existence of phase transition would lead to either an enhancement or a reduction of dielectric peak values as the VDF content decreases, which cannot be used to explain the maximum dielectric peak achieved at VDF = 50 mol %. This result rules out the dominant role of phase transition in determining the dielectric constant near 70 °C. For the pure relaxor compositions (VDF < 49 mol %), no phase transition and therefore the density change occurs, which therefore rules out the density fluctuations.59 Evolution of Relaxor Property As a Function of VDF Content. Different from normal ferroelectric, relaxor ferroelectric exhibits a broad dielectric peak which strongly depends on the frequency of the applied electric field (Figure 1). Such characteristic can be described by the V−F law ln f = ln f 0 − Ea/ kB(Tmax − Tf), where f is the frequency, f 0 is the attempt frequency, Ea is the activation energy, kB is the Boltzmann constant, Tmax is the dielectric peak temperature, and Tf is the freezing temperature. In addition to the V-F law, a new relation42 is used here to fit the experimental data such that ln f = ln f 0 − (T0/Tmax)p, where T0 is the equivalent temperature of the activation energy and p is a common parameter characterizing quantitatively the dielectric relaxation strength (DRS) of relaxor. As p decreases from infinity to 1/2, the relation can describe dielectric response of dielectrics from normal ferroelectric to glass. For instance, p corresponds to infinity in the case of normal ferroelectrics, in which dielectric peak temperature (the Curie temperature) is independent of measurement frequency.

that copolymers near MPB regions show relaxor behavior; the ionic contribution, leading to a frequency dependence of dielectric constant above Tmax, plays a minor role near Tmax. The frequency dependent dispersion above Tmax cannot be used to rule out the fact that copolymers are relaxors as similar behaviors have also been observed in many ceramic counterparts.54−58 In addition to ionic conduction, the temperature range may be not high enough to reach the regime with no dispersion, which may also lead to the intermediate range with a frequency dependent dispersion. To evaluate the evolution of dielectric property especially the relaxation processes, the analysis on the dielectric loss as a function of temperature and frequency is relevant. Indeed, these analyses in terms of fitting to the relaxation times by different models has been intensively analyzed in the previous works.33−35 These studies attributed the dielectric peaks shown in Figure 1 to the dielectric relaxation in the crystalline domain of P(VDF-TrFE) copolymers, which is also in line with recent analysis on the conformational evolution.15 Since the crystalline part of P(VDF-TrFE) is responsible for the ferroelectric instability, the appearance of the short-range ferroelectric order should also occur in crystalline part. Moreover, in Figure 3 we present dielectric loss a function of temperature measured at 1 kHz for different VDF content. It is shown that the temperature corresponding to dielectric loss peak gradually decreases as the VDF content decreases, which was also found in previous data.34,35 Moreover, we find that the magnitude of dielectric loss reaches the maximum at the critical VDF content of 49 mol % regardless of heating or cooling, which indicates the strongest molecular motions occurring at this composition. Since the intramolecular conformation transition occurs at VDF = 49 mol %,15 the significant molecular motions D

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Figure 5. (a) Comparison of fitting results by the V−F law and a new relation (solid lines) for VDF = 50 mol % (heating). (b−d) p, T0 and logarithm f 0 versus the VDF content deduced from the fit of dielectric data by a new relation upon both heating and cooling. The lines in panels b−d are guides for the eyes.

The value of p decreases with increasing DRS of relaxor (p > 1). The value of p for Debye medium and glass is 1 and 1/2, respectively. As a result, this new relation can help to depict the evolution from normal ferroelectric to relaxor in P(VDF-TrFE) copolymers near MPB in terms of the change in DRS (p). Figure 4 shows the fitting results by the V-F relation. It can be seen in Figure 4a that logarithm frequency versus peak temperature can be reasonably described by the V-F relation, indicating a typical relaxor behavior occurring in P(VDF-TrFE) copolymers. Figure 4b−4d present the deduced parameters as a function of the VDF content. Interestingly, we find that all three parameters change very sharply at a critical VDF content of 49 mol % regardless of heating or cooling of P(VDF-TrFE)s (Figure 4b−Figure 4d). Two distinct regions are clearly seen and the remarkable changes in the deduced parameters occurring at VDF = 49 mol % are attributed to the disappearance of ferroelectric instability, which is also consistent with the analysis based on Figure 2b. Figure 5 summarizes the fitting results by the new relation. We compare the results with the V-F equation in P(VDF-TrFE) 50/ 50 mol %, which is shown in Figure 5a. A remarkably good fit to the new relation further supports relaxor nature of P(VDFTrFE) copolymers (VDF ≤ 55 mol %). Similar to the results by the V-F relation, we find that the parameters (Figure 5c,d) deduced from the new relation also change abruptly at VDF = 49 mol %. Specifically, we show that DRS (p) exhibits a dramatic drop at VDF = 49 mol %, indicating that dielectric relaxation of relaxor P(VDF-TrFE) becomes significantly stronger with decreasing the VDF content and the local ferroelectric order is dominant when VDF < 49 mol %. These results provide clear evidence supporting the strong competition between normal

ferroelectric and relaxor phases which can be induced simply by changing the VDF concentration in P(VDF-TrFE) copolymers.

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CONCLUSIONS

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ASSOCIATED CONTENT

This work reports the detailed analysis of dielectric responses of P(VDF-TrFE) copolymers near MPB to determine phase evolution and the phase boundary in P(VDF-TrFE)s. Specially, it demonstrates that a local ferroelectric instability develops and becomes stronger as the VDF content decreases. As a result, relaxor phase is dominant below a critical VDF content of 49 mol %, which is supported by the abrupt changes in dielectric properties at this composition in terms of the compositional dependence of dielectric peak temperature and several parameters deduced from two different models. In addition, the temperature dependence of dielectric properties shows the largest dielectric constant of ∼76 at 1 kHz occurring in P(VDFTrFE) 50/50 mol % near 69 °C. These results may offer a unique route to search novel MPB piezoelectric polymers for applications in flexible, wearable and biocompatible devices.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01403. (Dielectric data in P(VDF-TrFE) copolymer upon cooling and theoretical fits of experimental data by using different models PDF) E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yang Liu: 0000-0002-3086-418X Wenhan Xu: 0000-0002-4347-2601 Qing Wang: 0000-0002-5968-3235 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors would like to acknowledge the support from Air Force Office of Scientific Research through MURI FA9550-191-0008.

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DOI: 10.1021/acs.macromol.9b01403 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b01403 Macromolecules XXXX, XXX, XXX−XXX