Piezoelectric responses in poly(vinylidene fluoride/hexafluoropropylene) copolymers Bret Neese, Yong Wang, Baojin Chu, Kailiang Ren, Sheng Liu, Q. M. Zhang, Cheng Huang, and James West Citation: Applied Physics Letters 90, 242917 (2007); doi: 10.1063/1.2748076 View online: http://dx.doi.org/10.1063/1.2748076 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large magnetoelectric coupling coefficient in poly(vinylidene fluoride-hexafluoropropylene)/Metglas laminates J. Appl. Phys. 110, 104103 (2011); 10.1063/1.3660767 Electromechanical properties of poly(vinylidene-fluoride-chlorotrifluoroethylene) copolymer Appl. Phys. Lett. 88, 062904 (2006); 10.1063/1.2170425 Phase transition and properties of a ferroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer J. Appl. Phys. 97, 084101 (2005); 10.1063/1.1862323 Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films J. Appl. Phys. 92, 7442 (2002); 10.1063/1.1524313 Effect of electron irradiation on poly(vinylidene fluoride-trifluoroethylene) copolymers Appl. Phys. Lett. 77, 1713 (2000); 10.1063/1.1290266
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APPLIED PHYSICS LETTERS 90, 242917 共2007兲
Piezoelectric responses in poly„vinylidene fluoride/hexafluoropropylene… copolymers Bret Neese, Yong Wang, Baojin Chu, Kailiang Ren, Sheng Liu, and Q. M. Zhanga兲 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 and Electrical Engineering Department, The Pennsylvania State University, University Park, Pennsylvania 16802
Cheng Huang and James West Johns Hopkins University, Baltimore, Maryland 21218
共Received 14 January 2007; accepted 17 May 2007; published online 15 June 2007兲 The authors show that a high transverse piezoelectric response with both high piezoelectric d31 共d31 = 43.1 pm/ V兲 and electromechanical coupling k31 coefficients 共k31 = 0.187兲, much higher than those in the piezoelectric poly共vinylidene fluoride兲 and poly共vinylidene fluoride-trifluoroethylene兲 copolymers, can be obtained in poly共vinylidene fluoride-hexafluoropropylene兲 关P共VDF-HFP兲兴 10 wt % copolymers under quasistatic condition. Furthermore, the copolymers also display a higher d31 coefficient compared to the d33 coefficient, which seems to be unusual compared with most other piezopolymers. The experimental data suggest that the origin of the unusual piezoelectric response in these P共VDF-HFP兲 copolymers originates from a reversible change between a poled ␣-like structure and -like structure. The phase change nature also results in a large frequency dispersion of the piezoelectric response and a smaller d31 共=20.5 pm/ V兲 at 50 kHz. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2748076兴 Due to their light weight, flexibility, and ability to be formed into intricate shapes, polymers with high electromechanical responses are attractive for a broad range of applications such as actuators, transducers, microelectromechanical systems, and robotics, as well as bioengineering and medical applications.1–3 Among the known polymers, poly共vinylidene fluoride兲 共PVDF兲 and its copolymers with trifluoroethylene 共TrFE兲 are the best known piezopolymers and exhibit the highest piezoelectric responses.4–6 A few years ago, it was demonstrated that, by defect modifications, P共VDF-TrFE兲 copolymers of certain compositions can be converted into electrostrictive materials with giant electrostriction.7,8 It was further shown that the giant electrostriction originates from the molecular conformation change between the nonpolar and polar forms.9 In this letter, we investigate defect modifications to the PVDF homopolymer. Compared with P共VDF-TrFE兲 copolymers, PVDF homopolymer shows much richer phase structures and hence may offer opportunities for exploring different electromechanical phenomena.4–6 The experimental results from a defect modified PVDF polymer, i.e., copolymers of P共VDFHFP兲 共HFP: hexafluoropropylene兲, to be presented suggest that a molecular conformation change or phase change can also be employed to improve the piezoelectric responses of PVDF-based piezopolymers. Recently, several experimental investigations have revealed that a relatively high piezoelectric response, comparable to that in PVDF and P共VDF-TrFE兲 piezopolymers, can be obtained in this class of copolymers.10–13 Additionally, a high electrostrictive strain was also reported for P共VDF-HFP兲.14 P共VDF-HFP兲 copolymers of 10 and 12 wt % compositions 共Solef 11010/1001 and 21216/1001兲, which were supplied by Solvay, were examined in this study. It was found that the piezoelectric coefficient d31 and the electroa兲
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mechanical coupling factor k31 of P共VDF-HFP兲 10 wt % 共4.5 mol % 兲 are significantly higher than those of PVDF and P共VDF-TrFE兲 copolymers. Moreover, the P共VDF-HFP兲 piezopolymers exhibit an unusual piezoelectric response, i.e., 兩d31 / d33 兩 ⬎ 1, which is unusual compared with other piezopolymers. The experimental data to be presented suggest that the large and unusual piezoelectric response for the P共VDF-HFP兲 copolymers originates from a reversible change between a poled ␣-like and -like structure, which also results in strong frequency dispersion. To achieve a high transverse piezoelectric response, copolymer films were uniaxially stretched. The P共VDF-HFP兲 polymer powders were first melt pressed to films of 60– 80 m thick at 230 ° C and then rapidly quenched in ice water to keep the crystallinity low so that the films can be stretched. A special zone drawing machine was designed and fabricated, which provides a very narrow heating zone with the drawing ratio to be controlled precisely by the differential speed of the two motors at the two ends of the thin film to be stretched.15 This machine was used to uniaxially stretch the P共VDF-HFP兲 films to four to five times of their original length with the temperature of the narrow heating zone at 100 ° C. Uniaxially stretched 共five times兲 PVDF films were purchased from K-Tech Co. and characterized here as a comparison. The piezoelectric state of these polymers was established by corona poling of the films, which were gold sputtered on one side. Corona poling of the samples was conducted with 30 kV at room temperature for 5 min.16 After poling, gold electrodes were sputtered on the opposite side of the films for electromechanical and dielectric evaluations. All the piezoelectric measurements were carried out after aging the piezopolymers for several days after the poling. The thickness strain 共corresponding to d33兲 and the transverse strain 共corresponding to d31兲 along the film stretching direction were measured using two specially designed dilatometers at a frequency of 1 Hz 共at quasistatic condition兲
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TABLE I. Comparison of the quasistatic piezoelectric coefficients and electromechanical coupling factors. Polymer PVDF P共VDF-TrFE兲25 mol % P共VDF-HFP兲10 wt % P共VDF-HFP兲12 wt %
d31 共pm/V兲 22.4 10.7 43.1 39.3
d33 共pm/V兲 −25.8 −33.5 −32.0 −26.9
Y 共GPa兲 3.0 3.0 2.0 0.45
k31 0.12 0.07 0.187 0.081
and at room temperature.17,18 Electromechanical resonance measurement was carried out by using a HP4294A impedFIG. 1. XRD taken from both unstretched and uniaxially stretched P共VDFance analyzer. Due to a low mechanical quality factor of HFP兲 10 wt % copolymers. The peak positions are labeled in reference to the ␣ and ␥ phases. The stretched films exhibit an x-ray pattern different polymer thin films, a computer curve fitting method based on from that of the  phase of PVDF. The diffraction peak of Si is used as a a modified Butterworth–Van Dyke 共BVD兲 circuit model was reference for calibration. employed to extract the electromechanical properties of the piezopolymer films from the conductance curve.19,20 Dielecing as well as corona poling 共also for direct contact poling兲 tric properties were characterized at different frequencies usdisplay an x-ray pattern which is very much different from ing a HP 共4284A兲 inductor, capacitor, resistor 共LCR兲 meter. the  phase as observed in the stretched PVDF films. This Differential scanning calorimetry 共DSC兲 was performed on a indicates that the bulkier HFP units in the PVDF chains act TA Q100 instrument at a heating rate of 10 ° C / min. X-ray as defects to destabilize the ferroelectric  phase. This is diffraction 共XRD兲 was conducted with a Scintag Cu K␣ difsomewhat analogous to that in P共VDF-TrFE兲 copolymers fractometer with an x-ray wavelength of 1.54 Å. Dynamic where adding bulkier monomer units such as chlorofluoroetmechanical analysis 共TA DMA 2980兲 was carried out to achylene, chlorotrifluoroethylene 共CTFE兲 and HFP converts the quire the elastic modulus of the polymer films at near static normal ferroelectric into a relaxor ferroelectric.9,22–24 condition. On the other hand, the introduction of HFP in P共VDFThe quasistatic piezoelectric responses measured for HFP兲 copolymers does not convert the ferroelectric  phase P共VDF-HFP兲 copolymers are presented in Table I, along 共for stretched PVDF films兲 into a paraelectric phase, as obwith the piezocoefficients of PVDF characterized here and served in the case of HFP and CTFE in the P共VDF-TrFE兲 P共VDF-TrFE兲 75/ 25 mol % copolymer.21 The data reveal based terpolymers. Instead, the x-ray data of the stretched two interesting features: 共i兲 The P共VDF-HFP兲 10 wt % coP共VDF-HFP兲 films still exhibit ␣-like structure, with a small polymers display a high d31 coefficient, which is 43.1 pm/ V, amount of a -phase component similar to that observed in nearly double that of the PVDF homopolymer. The P共VDFP共VDF-CTFE兲 copolymers.25,26 This difference in crystalline HFP兲 12 wt % samples show similar results with a lower phase structure might be responsible for the different electrovalue of d31. 共ii兲 Contrary to what is normally observed with mechanical responses observed between the P共VDF-HFP兲 PVDF-based piezopolymers, the magnitude of d33 coefficopolymers here and the P共VDF-TrFE兲 based terpolymers. cients is less than d31 for P共VDF-HFP兲 copolymers examined Based on these observations, we suggest that the unusual here. piezoelectric response observed here originates from a reThe high value of d31, which is also larger than d33 in versible phase transformation between a poled ␣-like strucmagnitude observed in P共VDF-HFP兲 copolymers, is interestture and a more polar 共 like兲 structure. This transformation ing and distinctively different from that observed in the generates a piezoelectric rather than electrostrictive response, poled ferroelectric 共-phase兲 PVDF and P共VDF-TrFE兲 coas observed in the relaxor P共VDF-TrFE兲 based terpolymers. polymers. This suggests that the piezoelectric response in the Using the lattice constants of PVDF ␣ phase and  phase P共VDF-HFP兲 copolymer originates from a mechanism differand assuming a perfect crystal with the polar axis parallel to ent from that in the  phase of PVDF as well as P共VDFthe applied field, we can estimate the relative magnitude of TrFE兲. The polarization-hysteresis loop 共D-E loop兲 measured d31 vs d33, assuming a field induced reversible change beon stretched P共VDF-HFP兲 10 wt % films displays a much tween an ␣-like structure and a -like structure.4,5 For a smaller remnant electric displacement Dr, ⬃30 mC/ m2, as phase change from a poled ␣ to  phase, d31 ⬃ 共2c compared with the -phase PVDF, which has a larger Dr − c␣兲 / c␣ = 0.032 and d33 ⬃ 共b − a␣兲 / a␣ = −0.01, where c and 共⬃55 mC/ m2兲 even though the crystallinity of the two polyc␣ are the lattice constants along the polymer chain direction mers 共as determined by DSC兲 is very similar. The higher and b and a␣ are the lattice constants along the polarization piezoelectric coefficients and smaller Dr in P共VDF-HFP兲 direction for the  and ␣ phases, respectively. The compared with those of the -phase PVDF further suggest 兩d31 / d33兩 ⬃ 1.4 observed in the experiment is much smaller that the piezoelectric response in the P共VDF-HFP兲 originates than the estimated ratio 共⬃3.2兲 based on the lattice constants. from a mechanism different from that in the  phase of PVDF.6 This is presumably due to the random orientation of the crysXRD of P共VDF-HFP兲 10 wt % copolymers for both untallites in the direction perpendicular to the stretching direcstretched and stretched films are presented in Fig. 1. For the tion, which increases d33, as well as the imperfect alignment as prepared and quenched films, the x-ray peaks can be laof the polymer chains along the stretching direction, resultbeled as those observed in the ␣ phase of PVDF. However, ing in a smaller d31. Nevertheless, these results are consistent the relative peak intensity among them is quite different from with the hypothesis that the observed piezoelectric response that of the ␣ phase of unoriented samples, indicating another originates from a field induced change between a poled Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 124.16.153.105 On: Fri, 22 Apr 2016 structure rather than a pure ␣ phase.14 The films after stretch␣-like structure and -like structure, which can generate a 01:43:16
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here, the reduction of the piezoresponse from the quasistatic values to that at 50 kHz is much larger than the normal frequency dispersion observed in PVDF and P共VDF-TrFE兲. This could be caused by the phase change nature of the piezoelectric response in the P共VDF-HFP兲, which usually involves larger frequency dispersion. This work was supported by NASA under Grant No. NNA05CV52H and NIH under Grant No. R21EY01679902. One of the authors 共B.N.兲 wishes to thank the support of a NASA fellowship 共Grant No. NNA05CV52H兲 and Harry Partridge for his insight to this project. 1
FIG. 2. Electric conductance curve measured near the transverse resonance of a poled and stretched P共VDF-HFP兲 10 wt % copolymer film. The fitting of the experimental data with a modified BVD circuit model is also presented, from which the piezoelectric properties are obtained.
higher piezoelectric response than those within a single phase and yield a larger d31 response compared with d33. The low frequency dielectric constant of P共VDF-HFP兲 10 wt % stretched films measured at room temperature is 16 before poling and 12 after. The dielectric loss after corona poling remains the same at about .03. The elastic modulus measured along the drawing direction for P共VDF-HFP兲 10 and 12 wt % is 2 and 0.45 GPa, respectively. Based on the piezoelectric d31 coefficient, the dielectric constant K, as well as the elastic modulus 共=1 / s11兲, the electromechanical coupling factor k31 can be determined as 2 = k31
2 d31 E K0s11
,
共1兲
where 0 is the vacuum permittivity. k31 for 10 wt % copolymer is 0.187, which is larger than that from PVDF and P共VDF-TrFE兲 piezopolymers.4,21 A high piezoelectric coefficient alone is not adequate for most electromechanical applications; a high electromechanical coupling factor is also very important.27 k31 is much smaller for the P共VDF-HFP兲 copolymer with 12 wt % due to the much lower modulus, which could be caused by a higher amount of HFP in the copolymer leading to less crystallinity. The high frequency electromechanical properties of P共VDF-HFP兲 10 wt % were characterized using the resonance measurement and the data are shown in Fig. 2. The fitting to the conductance curve yields k31 = 0.11 and d31 = 20.5 pm/ V at ⬃50 kHz. The reduction of piezoelectric and dielectric constants with frequency is common to piezoelectric polymers. However, for the P共VDF-HFP兲 copolymer
T. T. Wang, J. M. Herbert, and A. M. Glass, The Applications of Ferroelectric Polymers, 1st ed. 共Blackie, New York, 1988兲, pp. 118-379. 2 Y. Bar-Cohen, Electroactive Polymer (EAP) Actuators as Artificial Muscles, 2nd ed. 共SPIE, Washington, 2004兲, pp. 89-138. 3 F. Xia, S. Tadigadapa, and Q. M. Zhang, Sens. Actuators, A 125, 346 共2006兲. 4 H. S. Nalwa, Ferroelectric Polymers 共Dekker, New York, 1995兲, pp. 183232. 5 A. Lovinger, Science 220, 1115 共1983兲. 6 T. Furukawa, Phase Transitions 18, 143 共1989兲. 7 Q. M. Zhang, V. Bharti, and X. Zhao, Science 280, 2101 共1998兲. 8 H. Xu, Z. Y. Cheng, D. Olson, T. Mai, Q. M. Zhang, and G. Kavarnos, Appl. Phys. Lett. 78, 2360 共2001兲. 9 Q. M. Zhang, C. Huang, F. Xia, and J. Su, Electroactive Polymer (EAP) Actuators as Artificial Muscles 共SPIE, Washington, 2004兲, Chap. 4, pp. 97-99. 10 W. Kunstler, M. Wegener, M. Seib, and R. Gerhard-Multhaupt, Appl. Phys. A: Mater. Sci. Process. 73, 641 共2001兲. 11 X. He, K. Yao, and B. Gan, J. Appl. Phys. 97, 084101 共2005兲. 12 M. Wegener, W. Kunstler, K. Richter, and R. Gerhard-Multhaupt, J. Appl. Phys. 92, 7442 共2002兲. 13 Y. Huan, Y. Liu, Y. Yang, and Y. Wu, J. Appl. Polym. Sci. 104, 858 共2007兲. 14 X. Ku, A. Schirokauer, and J. Scheinbeim, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47, 1291 共2000兲. 15 K. Yamada, M. Oie, and M. Takayanagi, J. Polym. Sci., Polym. Phys. Ed. 21, 1063 共1983兲. 16 P. Pillai, N. Shroff, and A. Tripathi, J. Electrost. 17, 269 共1985兲. 17 J. Su, P. Moses, and Q. M. Zhang, Rev. Sci. Instrum. 69, 2480 共1998兲. 18 Z.-Y. Cheng, T.-B. Xu, V. Bharti, S. Wang, and Q. M. Zhang, Appl. Phys. Lett. 74, 1901 共1999兲. 19 IEEE Standard on Piezoelectricity, ANSI/IEEE Standard No. 176-1987 共1996兲 20 Q. C. Xu, A. R. Ramachandran, and R. E. Newnham, J. Wave-Mater. Interact. 12, 105 共1987兲. 21 H. Wang, Q. M. Zhang, L. E. Cross, and A. O. Sykes, J. Appl. Phys. 74, 3394 共1993兲. 22 F. Xia, Z. Cheng, H. Xu, H. Li, Q. M. Zhang, G. J. Kavarnos, R. Y. Ting, G. Abdel-Sadek, and K. D. Belfield, Adv. Mater. 共Weinheim, Ger.兲 14, 1574 共2002兲. 23 T. C. Chung and A. Petchsuk, Macromolecules 35, 7678 共2002兲. 24 Y. Tajitsu, A. Hirooka, A. Yamagish, and M. Date, Jpn. J. Appl. Phys., Part 1 36, 6114 共1997兲. 25 B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer, and Q. M. Zhang, Science 313, 334 共2006兲. 26 Z. Li, Y. Wang, and Z.-Y. Cheng, Appl. Phys. Lett. 88, 062904 共2006兲. 27 G. Kino, Acoustic Waves: Devices, Imaging, and Analog Signal Processing 共Prentice-Hall, Englewood Cliffs, NJ, 1994兲, pp. 17-71.
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