ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november

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1 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november Special Issue Papers History and Recent Progress in Piezoelectric Polymers Abstract Electrets of carnauba wax and resin have exhibited good stability of trapped charges for nearly 50 years. Dipolar orientation and trapped charge are two mechanisms contributing to the pyro-, piezo-, and ferroelectricity of polymers. Since the 1950s, shear piezoelectricity was investigated in polymers of biological origin (such as cellulose and collagen) as well as synthetic optically active polymers (such as polyamides and polylactic acids). Since the discovery of piezoelectricity in poled polyvinylidene fluoride (PVDF) in 1969, the pyro-, piezo-, and ferroelectricity were widely investigated in a number of polar polymers, such as copolymers of vinylidene fluoride and trifluoroethylene, copolymers of vinylcyanide and vinylacetate, and nylons. Recent studies involve submicron films of aromatic and aliphatic polyureas prepared by vapor deposition polymerization in vacuum and the piezoelectricity of polyurethane produced by the coupling of electrostriction and bias electric fields. Gramophone pickups using a piece of bone or tendon were demonstrated in Microphones using a stretched film of polymethyl glutamate were reported in Ultrasonic transducers using elongated and poled films of PVDF were demonstrated in Headphones and tweeters using PVDF were marketed in Hydrophones and various electromechanical devices utilizing PVDF and its copolymers have been developed during the past 30 years. This paper briefly reviews the history and recent progress in piezoelectric polymers. I. Introduction We will review in this paper the development of studies on electroactive properties of polymers for the last 50 years. The studies date back to the electrets of carnauba wax and resin prepared by Eguchi in 1925 [1]. When a high electric field is applied across insulators at elevated temperatures, two kinds of charge are generally generated. The first is an injected charge from the electrodes, which is called homocharge and the sign is the same as that of the injected electrode. The second is polarization caused by the field-induced orientation of dipolar atomic groups, which is called heterocharge and the sign is opposite to that of homocharge. Heterocharge also is produced by the movement of impurity ions followed by trapping. In ferroelectric polymers, residual polarization due to the orientation of dipoles is stabilized and contributes to the pyro- and piezoelectric activities. Manuscript received July 5, 1999; accepted January 7, The author is with the Kobayasi Institute of Physical Research, Kokubunji, Tokyo, Japan ( eiichifukada@msn.com). Eiichi Fukada (Invited Paper) /$10.00 c 2000 IEEE Early in the1950s, Fukada found piezoelectricity in various kinds of biopolymers. Only shear piezoelectricity was observed in the uniaxially oriented systems of crystallites of cellulose and collagen. Shear piezoelectricity also was observed in a number of uniaxially elongated films of optically active synthetic polymers as well as biological polymers. The tensile piezoelectricity in stretched and poled films of polyvinylidene fluoride (PVDF) was first demonstrated by Kawai in This discovery triggered widely spread investigations on the pyro-, piezo-, and ferroelectricity of PVDF, its copolymers, nylons, and other polymers for subsequent years. Recently, submicron pyro- and piezoelectric films of polyurea were prepared by the method of vapor deposition polymerization. High piezoelectric activities also were found in polyurethane films, which are caused by the coupling of electrostriction and DC bias fields. II. Electrets The original electrets, permanently charged dielectrics, were prepared by Eguchi in 1924 using a mixture of carnauba wax and resin [1]. An electric field of about 1.5 MV/m was applied on a molten mixture at about 130 C. A disk of electret made of carnauba wax and resin, 20-cm in diameter and 1-cm thick, is preserved at the Science Museum in Tokyo. Its surface charges remain 45 years after preparation and were observed to be approximately one-seventh the original charges [2]. Sumoto prepared a number of carnauba wax electrets around He wrapped them with metal foils and stored them in desiccators [3]. Takamatsu carried out measurements of their surface charges at 22, 27, and 35 years after preparation. The results are shown in Fig. 1. The measured charges are plotted against the poling temperature. Just after poling of the electrets, the surface charges were negative, indicating heterocharge. However, after more than 22 years, positive charges were observed, indicating homocharge. The amount of homocharge increases with increasing poling temperature. The results indicate the good stability of injected charge (homocharge) in carnauba wax. Another example of excellent stability of injected homocharge is shown in FEP teflon films, which are installed in electret microphones in millions of portable telephones.

2 1278 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 TABLE I Shear Piezoelectricity of Biopolymers Polysaccharides Cellulose wood 0.1 ramie 0.2 Chitin crab shell 0.2 lobster apodeme (demineralized) 1.5 Amylose starch 2.0 d 14 (pc/n) Fig. 1. The observation of surface charge on carnauba wax electrets at 22, 27, and 35 years after preparation. The magnitude of charge is plotted against the poling temperature Tp. The poling electric field was 4 MV/m [2]. Proteins Collagen bone 0.2 tendon 2.0 skin 0.2 Keratin wool 0.1 horn 1.8 Fibrin elongated films of fibrinogen-thrombin clot 0.2 Deoxyribonucleic acids salmon DNA (at 100 C) 0.07 III. Shear Piezoelectricity of Biopolymers Quartz Crystal d 11 =2.2pC/N The observation of piezoelectric effects in wood was first reported by Bazhenov in 1950 [4], [5]. The experimental verification of both direct and inverse piezoelectric effects and the determination of the piezoelectric matrix were carried out by Fukada in 1955 [6]. Only the shear piezoelectric constants d 14 =d 25 are finite and other components are zero, according to a symmetry D ( 2) for the uniaxially oriented system of cellulose crystallites. The symmetry of cellulose crystal C 2 2 indicates that there are 8 components in the piezoelectric matrix. When the z axis of each crystal aligns in the same direction with antiparallel form and with the x and y axes being uniformly distributed, only two components, d 14 =d 25, remain to be finite without cancellation. Due to this symmetry relation, most biopolymers with fibrous configurations may manifest the shear piezoelectric effect. In 1953, Yasuda et al. [7] discovered that bone produces electricity by bending deformation, that the compressed regions are negatively polarized and the elongated regions are positively polarized, and that callus is formed at the negatively polarized regions when bone is in the in vivo state. The quantitative verification of the shear piezoelectric effects in bone and tendon were first demonstrated by Fukada and Yasuda in 1957 and 1964 [8], [9]. The shear piezoelectric constant for dry bone d 14 = 0.2 pc/nwas about twice that of dry wood, but that for dry tendon d 14 = 2.0 pc/n was 10 times that of dry bone and was comparable with that of quartz crystal d 11 =2.2 pc/n. Most biological polymers including polysaccharides, proteins, and polynucleotides have been found to exhibit shear piezoelectricity as shown in Table I. Most biopolymers are in the fibrous form, in which the uniaxial symmetry exists. The physiological significance of piezoelectricity in biopolymers was discussed by many authors, but no obvious conclusions were obtained. In connection with piezoelectricity in bone, great interest was aroused in the field of orthopedics. For the origin of stress-generated potential in bone, the steaming potential due to fluid flow in the structure of bone was found to be more important than the stress-induced piezoelectric polarization [10]. The electrical stimulation of fracture and growth of bone has been very actively investigated for the past several decades. Different types of clinical devices, including DC current, AC current, pulsing electromagnetic field, and ultrasonics, were developed [11]. IV. Piezoelectricity of Optically Active Polymers The sign of the shear piezoelectric constant depends upon the chirality of asymmetric carbon atoms. Shear piezoelectricity is originated from the internal rotation of polar atomic groups associated with an asymmetric carbon atom. The magnitude of the piezoelectric constant is proportional to the degree of orientation and the degree of crystallinity. Table II shows a comparison of the piezoelectric constant d 25 for three optically active polymers [12] [14].

3 fukada: progress in piezoelectric polymers 1279 TABLE II Comparison of the Piezoelectric Constant for Three Optically Active Polymers Polypropylene oxide CH 3 H T g = 60 C At 100 C 1969, Furukawa and Fukada [12] d = 0.1 pc/n O C C e =0.2 mc/m 2 ε =1.9 ε 0 H H n c =2GN/m 2 Poly-β-hydroxybutyrate CH 3 H T g =20 C At 0 C 1984, Ando and Fukada [13] d =1.3 pc/n O C C C H H O n e =4mC/m 2 ε =3ε 0 c =4.5 GN/m 2 Poly-lactic acid CH 3 T g =85 C At 50 C 1991, Fukada [14] d = 10pC/N O C C e =18mC/m 2 ε =3.5 ε 0 H O c =2GN/m 2 n Fig. 2. The frequency characteristics of trial pickups using baleen, bone, and tendon as electromechanical transducers [15], [16]. V. Demonstrations of Electromechanical Devices Using Shear Piezoelectricity of Biopolymers The first demonstrations of electromechanical devices using shear piezoelectricity of biopolymers were conducted by Fukada in Gramophone pickups using tendon, bone, and baleen were produced to demonstrate piezoelectric effects in biopolymers. Rochelle salt elements used in commercial pickups were substituted by small rectangular bimorph plates in which collagen fibers are oriented 45 degrees aslantly to the direction of length. Oscillation of a needle causes bending oscillation of the bimorph plates, which result in the piezoelectric output voltage. The frequency characteristic of output voltage for such pickups is illustrated in Fig. 2 [15], [16]. The sensitivity increases in the order of baleen, bone, and tendon suggests the increase of content and alignment of collagen crystallites in their structures. These primitive demonstrations were effective to persuade people to believe the shear piezoelectricity of biopolymers. Following the discovery of shear piezoelectricity in biopolymers, the shear piezoelectricity of various kinds of synthetic polypeptides was actively investigated. An example is polymethyl-l-glutamate (PMG), which shows d 14 = 2 pc/n. In 1968, a microphone using PMG as an electromechanical transducer was demonstrated [17], [18]. Fig. 3 shows the design of the microphone. A thin strip of PMG film, which is elongated twice the original length and cut 45 degrees aslantly to the draw direction, is set between the center of a paper cone and edges of the microphone. The sound vibrates the cone, which then strains the PMG strip longitudinally. The electric output from the strip is led to a cathode follower and main amplifier. Fig. 3 also shows the frequency characteristic for such a trial microphone. VI. Tensile Piezoelectricity of Poled Polymers Kawai [19] discovered large piezoelectricity in elongated and poled films of polyvinylidene fluoride (PVDF, PVF 2 ) in The chemical structure of PVDF molecules is given by (CH 2 -CF 2 ) n.cf 2 dipoles are aligned normal to the surface of the film after poling and form a residual polarization. The symmetry of poled films is represented by C 2v (2 mm), if the z axis is assigned in the poling direction, normal to the surface of films, the x axis in the direction of elongation, and the y axis is perpendicular to both the x and z axes. Five components of the piezoelectric matrix are finite, and their observed values are d 31 =20pC/N, d 32 =1.5 pc/n,d 33 = 32 pc/n, d 15 = 27pC/N, and d 24 = 23pC/N, respectively [20].

4 1280 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 A. The Dimensional Effect If residual polarization caused by oriented dipoles or trapped charges is not changed under applied stress, the lateral stretching reduces the thickness of the film, which then increases the induced charge in electrodes. The piezoelectric constant e 31 (polarization P 3 /strain S 1 )isgiven by the product of Poisson s ratio, ν 31, times the residual polarization, Pr. B. The Intrinsic Effect Residual polarization in the crystalline phases is changed by applied stress, which includes polarization due to oriented dipoles even in the noncrystalline phases. This intrinsic effect is determined by the product of the electrostriction constant times residual polarization Pr. Fig. 3. An audio frequency microphone using a piezoelectric strip of polymethyl L-glutamate (PMG) [17], [18]. VII. The Origin of Piezoelectricity in Poled Polymers Poled PVDF films show tensile, thickness, and shear piezoelectricity similar to piezoelectric ceramics. Residual polarization produced by poling consists of both oriented dipoles and trapped charges. Dipoles are oriented in the crystalline phases inside lamellae and in the interfaces between the crystalline and noncrystalline phases in which molecules are aligned in parallel. The ions and electrons either injected from electrodes or originally present in the polymer are trapped in various places in the polymer. Many theories and interpretations were put forward to explain the origin of piezoelectricity in poled polymers [21] [23]. Very simple explanations are given by two mechanisms as follows. VIII. Interfacial Piezoelectricity Piezoelectric properties of metal-dielectric interfaces were investigated by Lewis and co-workers [24] [26]. Fig. 4 illustrates the experimental cell [25]. Annular ring electrodes are mounted on an insulating base. The liquid dielectric covers the electrodes to a depth of 1 mm. A thin mirror floats on the surface above the electrodes. The displacement of the mirror is precisely measured by an optical interferometer. Cyclic motion of the mirror normal to the liquid surface is found to occur when alternating voltages are applied to the electrodes. As the liquid dielectric, water, glycol, glycerol, and silicone oil were investigated. Normalized displacements, δ, of the mirror at 205 Hz were 90, 2.0, 0.5, and 0.1 pm/v, respectively. For insulating liquids as glycol and silicone, when a bias voltage V is superimposed, δ increases with V and its phase changes 180 degrees at V = 0. The phenomena is similar to electrostriction. For gels such as gelatin and agarose, large values of δ (>200 pm/v) were found. In these experiments electric double layers are present on the surface of the electrodes. When an alternating voltage is applied, the mobile counter ions will oscillate along field lines between the electrodes. A shear wave is thus generated in the liquid and propagated to the floating mirror. The charged double layer can be the origin of piezoelectricity. The idea can be extended to the interfaces between crystalline phases and amorphous phases in poled polymers such as PVDF. Polarization in polar domains is compensated by trapped charges at amorphous boundaries. It is suggested that charged double layers located in interfaces can generate piezoelectric phenomena [27]. IX. Dielectric Hysteresis Since the finding of large piezoelectricity, the presence of ferroelectric properties was anticipated for polyvinylidene fluoride. The observation of the hysteresis loop between the electric displacement and the electric field was successfully performed in 1974 [28]. However, above room temperature, the contribution of ionic conduction to hysteresis cannot be excluded. Furukawa et al. [29] under-

5 fukada: progress in piezoelectric polymers 1281 Fig. 5. Hysteresis loops between the electrical displacement D and the applied electric field E observed for PVDF at low temperatures below the glass transition temperature Tg = 50 C [29]. Fig. 4. An experimental set up to show interfacial piezoelectricity [25]. The mm thick coplanar electrodes, consisting of a central 11-mm diameter disk surrounded at a distance of 3.5 mm by a 2-mm wide annulus, are mounted on an insulating base. A 0.25-mm thick, 10-mm diameter glass mirror floats on the liquid surface [24], [25]. took measurements of the hysteresis loop at a low temperature range below the glass temperature of 50 C, at which ionic conductivity is insignificant, by applying high electric fields up to 240 MV/m. As shown in Fig. 5, even at 100 C, hysteresis loops were clearly observed with a residual polarization of about 60 mc/m 2, which does not change much with varying temperature. Only the coercive field increases with decreasing temperature. Other evidence of the field-induced orientation of dipoles was obtained by the observation of infrared absorptions under varying electric field [30], [31]. The moment of the bending vibration of the CF 2 group at a wave number 512 cm 1 directs in the same direction as that of the CF 2 dipole. If dipoles orient in the direction of the applied electric field, the transmission of ir ray should increase in magnitude. Under the cyclic variation of applied electric field, the transmission showed a butterfly curve corresponding to dielectric hysteresis loop (Fig. 6.). X. Curie Temperature In 1979, at the spring meeting of the Japan Society of Applied Physics, Yagi et al. (Daikin), Tamura et al. (Pioneer), and Kitayama et al. (NTT) reported at the same time that copolymers of vinylidene fluoride and trifluoroethylene (50/50 mol %) exhibit a peculiar transition at about 50 C below the melting temperature of about 160 C. Both the dielectric constant and the piezoelectric constant showed a peak at about 50 C (Fig. 7.). Above the peak, the reciprocal of the dielectric constant plotted against temperature obeyed the Curie-Weiss law (Fig. 8) [32]. Yagi et al. [33] showed that the critical temperature increases with increasing mol % of VDF and approaches the melting temperature of PVDF. The presence of Curie temperature was discovered for the copolymers of vinylidene fluoride and trifluoroethylene. It was proved that the crystalline phase at the low temperature range is a ferroelectric state with molecules of all-trans configuration and that at the high temperature range is the paraelectric state with molecules of random mixture of trans and gauche configurations. The crystalline structures and their phase changes have been thoroughly investigated by many authors [34] [35]. XI. Amorphous Ferroelectric Polymers A theoretical model for electrets due to frozen-in dipolar orientation was presented to explain the piezoelectricity and pyroelectricity in amorphous polymers such as polyvinyl chloride [36]. In 1980, Miyata et al. [37] discovered the piezoelectricity in a copolymer of vinylidene cyanide (VDCN) and vinyl acetate (VAc), which is a completely amorphous polymer. A film of the copolymer was poled at 150 C near its glass transition temperature of 170 C and cooled to room temperature under the field. Then a piezoelectric constant d 31 of about 5 pc/n was observed. The dielectric strength measured at the glass transition temperature was as large as ε/ε o = 120. A large

6 1282 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 Fig. 6. Hysteresis loops between the infrared transmission at a wave number of 512 cm 1 and the applied electric field. The bending vibration of CF 2 dipoles takes place at 512 cm 1. If the moment of CF 2 orients in the field direction, the infrared transmission is increased [30]. Fig. 8. Plot of reciprocal dielectric constant 1/ε against temperature. C 1 and C 2 indicate the Curie constants above and below the transition temperature [32]. dielectric polarization is frozen in at room temperature as residual polarization, which causes piezoelectricity [38]. XII. Ferroelectricity of Nylon Fig. 7. Temperature dependence of the dielectric constant ε = ε iε in the frequency range of 1 Hz 100 khz for a VDF-TrFE (55:45) copolymer [32]. Scheinbeim et al. first reported hysteresis effects in the piezoelectric constants of the odd-numbered nylons in 1984 [39], and clearly demonstrated ferroelectric polarization switching and D-E hysteresis loops in 1991 [40]. Fig. 9 shows D-E hysteresis loops observed for nylon-7, nylon-11, and PVDF at 1 mhz at room temperature [41]. The samples were pretreated by applying a static electric field of MV/m for a few hours for the purpose of field sweeping out most of the mobile ionic species. The residual polarization between 50 and 100 mc/m 2 is observed. Fig. 10 shows the temperature dependence of the piezoelectric constant d 31 for nylon-11, nylon-7, and PVDF measured at 104 Hz. Higher values of d 31 at the higher temperature range for nylons may be explained by the increase of Poisson s ratio above the glass transition temperature of about 70 C. Odd-numbered nylons have a crystal structure consisting of closely packed hydrogen-bonded sheets [42]. X-ray

7 fukada: progress in piezoelectric polymers 1283 Fig dipole switching of chains in the hydrogen-bonded sheet structure [43]. XIII. Evaporated Thin Films of Piezoelectric Polymers Fig. 9. D-E hysteresis loops observed for nylon-7, nylon-11, and PVDF at 1 mhz at room temperature [41]. Fig. 10. Temperature dependence of the piezoelectric constant d 31 for nylon-11, nylon-7, and PVDF at 104 Hz [41]. studies reveal that the sheets initially lie parallel to the surface of nylon films [43]. When a poling field is applied to films, 90 rotations of NH and CO dipoles take place in the field direction, so that the hydrogen-bonded sheets are perpendicular to the plane of poled films. During subsequent hysteresis measurements with electric field reversals, 180 switching of dipoles takes place in the hydrogen-bonded sheet structure (Fig. 11.). A new technique called vapor deposition polymerization was invented to prepare submicron thin films of polyurea. The thickness of films can be varied from about 500 nm to about 10 µm. The polymer films deposited on substrates of any shape exhibit pyro- and piezoelectricity after poling treatments. The dipole moment of a urea bond NH-CO-NH is 4.9debye and is larger than 3.4 debye of a peptide bond NH-CO. In 1989, Takahashi et al. [44] first reported on pyroelectricity in poled films of aromatic polyurea. The field-induced orientation of urea dipoles yields large polarization similar to odd-numbered nylons [45]. A diamine monomer H 2 N-A 1 -NH 2 and a diisocyanate monomer OCN-A 2 -NCO are evaporated concurrently and deposited for polymerization on the surface of a substrate in vacuum as depicted in Fig. 12. A 1 and A 2 can be various aromatic or aliphatic atomic groups. An example of aromatic polyurea P(MDI/MDA) is shown in Fig. 13. The physical properties of polymerized films depend on the composition ratio of the two monomers MDI and MDA. The component ratio is varied by changing the evaporation temperatures of the monomers. If the evaporation temperature is high, the vapor pressure and thus the deposited amount of the monomer increase [46]. Fig. 14 shows the dependence on the evaporation temperature of MDI of the piezoelectric constant e 31 and pyroelectric constant p 3 for P(MDA/MDI). The maximum values of both e 31 and p 3 are found at a balanced state, at which equal amounts of the two monomers are deposited. Fig. 15 shows the temperature dependence of e 31 and tan δ =e 31/e 31. The piezoelectric constant is almost constant up to 200 C. Multilayer films are prepared by evaporating aluminum electrode layers and polyurea layers alternately. The apparent piezoelectric constant e 31 observed for such multilayered films increased in proportion to the number of layers [45]. An example of aliphatic polyurea, polyurea-5, is shown in Fig. 16 [47]. Dielectric hystetesis loops observed for polyurea-5 at 30 C are shown in Fig. 17. The frequency of applied electric field was varied from 0.02 Hz to 10 Hz. At low frequencies the hysteresis loops are rounded, suggesting the contribution of ionic DC conduction. At higher

8 1284 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 Fig. 12. Schematic diagram of the apparatus for the vapor deposition polymerization of polyurea from diamine and diisocyanate monomers [45]. Fig. 14. Dependence of the piezoelectric constant e 31 and the pyroelectric constant p 3 on the evaporation temperature of MDI, which changes the deposited monomer ratio MDA/MDI. The evaporation temperature of MDA is kept at 100 C [46]. Fig. 13. An example of aromatic polyurea, P(MDI/MDA) [45]. temperatures the hysteresis loops are more rounded. Residual polarization, Pv at E=0, is the sum of true residual polarization Po and the charge due to direct current, which increases linearly with increasing period T of the applied field (Pv = Po +at). Fig. 18 shows plots of Pv against T. From the linear relationship we obtain Po. The values of Po were about 25 mc/m 2 independent of temperature as shown in Fig. 19[48]. Polarization switching was observed for a 0.4-µm thick film of polyurea-5 [49]. Fig. 20 shows the results when a step function field of 240 MV/m was applied on a poled film at 20 C in forward and reverse directions with regard Fig. 15. Temperature dependence of the piezoelectric constant e 31 and loss tan δ =e 31 /e 31 for P(MDI/MDA) [47].

9 fukada: progress in piezoelectric polymers 1285 Fig. 16. An example of aliphatic polyurea, polyurea-5 [47]. Fig. 17. Hysteresis loops observed for polyurea-5 at Hz at room temperature [48]. to the poling direction. In the forward direction, the electric displacement increases with time due to DC conduction. In the reverse direction, the switching of polarization takes place with DC conduction. The difference between two curves gives the amount of reversed polarization 2Pr. The value Pr = 20 mc/m 2 was independent of the applied field. Switching time was about τ s =0.3 second at 20 C, and decreased with increasing field. XIV. Single Crystalline Films of P(VDF/TrFE) In 1995, Ohigashi et al. [50] succeeded in preparing single crystalline films of ferroelectric copolymers of vinylidene fluoride and trifluoroethylene P(VDF/TrFE) (75/25 molar ratio) with the thickness of µm. The method of preparation is as follows. First, a solution cast film is drawn 5 times the original length. Second, the film is an- Fig. 18. Plots of the apparent residual polarization against the period of the applied electric field [48]. nealed and crystallized at 138 C for 2 hours. The temperature range of the paraelectric (hexagonal) phase is between 123 C and 150 C. Third, the film is poled by applying a 0.05 Hz AC field at room temperature. During annealing, both ends of the film are clamped at a constant length, and its surfaces should not contact any solid material. If the surface of the film is contacted with a solid material, the stresses from the solid induce the nucleation of lamellar crystals. Single crystalline films, a few centimeters long, exhibit beautiful colors due to birefringence. A very sharp D-E hysteresis loop was observed with a remanent polarization of 100 mc/m 2 and a coercive field Ec = 38 MV/m at 20 C.

10 1286 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 Fig. 19. Temperature dependence of the residual polarization Po [48]. Thick multilayered samples also were prepared by cementing thin, single crystalline films. All the matrix components of the elastic, dielectric, and piezoelectric constants were determined with an impedance analyzer. Fig. 21 depicts the temperature dependence of the electromechanical coupling factors k ij of various modes. The coupling factor k 33 related to thickness vibration mode is as large as at room temperature and is almost independent of temperature up to Curie temperature Tc = 125 C. The coupling factor k 24 related to shear mode in the plane of films is very large and almost independent of temperature [51]. XV. Electrostriction of Polymers One of the nonlinear electromechanical properties is electrostriction. The strain S can be expressed as a function of the electric field E, omitting the higher terms, S=d o E+RE 2. (1) The first term is linear piezoelectricity; the second term is electrostriction. If the first term is zero, the derivative of S by E gives the piezoelectric constant, d= S/ E =2RE. (2) Fig. 22 illustrates the principle of electrostrictive relation between S and E. If a large DC field is given as a bias field E b and the small AC field E ac is applied, the AC response of strain S can be generated. The piezoelectric constant d = 2RE b is observed under a DC bias field E b. In 1994, Zhenyi et al. [52] first investigated the piezoelectric effect due to electrostriction in polyurethane films Fig. 20. Plots of D and its derivative D/ log t against log t after the application of a stepwise electric field of 240 MV/m on a polyurea-5 film at 20 C in the reverse and forward directions [49]. and observed the piezoelectric constant in the thickness direction d as a function of bias DC field as shown in Fig. 23. The maximum value of the piezoelectric constant d is found to be about 600 pc/n, which is higher than that of PZT ceramics. Su et al. [53] and Zhang et al. [54] investigated in more detail electrostriction in segmented polyurethane. Fig. 24 shows the electrostrictive response for 0.1 mm and 2 mm thick films of polyurethane. Electrostriction is enhanced for the thinner films. Under high electric fields, charges injected from the electrodes, as well as impurity ions in the sample, bring about the nonuniform distribution of trapped space charges. Local fields due to trapped charges exist particularly at interfaces between electrodes and the sample. The effects become relatively smaller when the

11 fukada: progress in piezoelectric polymers 1287 Fig. 23. The thickness piezoelectric constant of dry polyurethane film plotted against the bias DC field [52]. Fig. 21. Temperature dependence of the electromechanical coupling factors k 33,k 24,k 15,k 32,andk 31 for a single crystalline film of P(VDF/TrFE) [51]. Fig. 24. The electrostriction observed for polyurethane films with the thickness of 0.1 mm and 2 mm [54]. thickness of the film is large. Local fields also exist at interfaces between hard segments and soft segments of polyurethane. The nonuniform distribution of local fields is cancelled out for linear effects like piezoelectricity, because E(x)dx = 0, but is not cancelled out for nonlinear effects like electrostriction, because E 2 (x) dx 0. Zhang et al. [55] recently discovered large electrostriction in electron irradiated copolymers of vinylidene fluoride and trifluoroethylene (50/50). All-trans chains are interrupted by trans and gauche bonds and polarization domains are reduced to nanometer-size. A relaxor ferroelectric polymer is suggested. Fig. 22. The principle of electrostriction and the piezoelectric response with a bias electric field [52]. XVI. Industrial Applications of Poled Polymers A number of industrial applications have been developed for PVDF and its copolymers. Ultrasonic transducers in the MHz range, utilizing the piezoelectric constant d 33, were first produced by Ohigashi and Shige-

12 1288 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 47, no. 6, november 2000 Fig. 25. The principle of conversion between tensile strain of a poled PVDF film and sound pressure [59]. Fig. 27. A model of tweeter utilizing the piezoelectric constant d 31 of PVDF films and its frequency characteristic [59]. Fig. 26. A model of microphone utilizing the piezoelectric constant d 31 of PVDF films and its frequency characteristic [59]. nari in 1972 [56] and by Sussner et al. in 1973 [57]. Various types of hydrophone have been constructed and commercialized [58]. In 1975, electroacoustic transducers for audio frequencies such as microphones, tweeters, and headphones utilizing the piezoelectric constant d 31,were first produced and commercialized [59]. Fig. 25 depicts the principle to convert sound pressure to transverse strain of a poled PVDF film and vice versa. Electromechanical transduction is performed using the piezoelectric constant d 31. Fig. 26 shows a model of microphone with a bent PVDF diaphragm and its frequency characteristic. Fig. 27 depicts a model of high frequency tweeter using a round PVDF film and its frequency characteristic. Applications of pyroelectricity of poled polymers also have been developed [60]. Some significant overviews of practical applications of piezoelectric polymers are given in [61] [63]. XVII. Conclusions A historical survey for the development of piezoelectric polymers is presented. Piezoelectricity usually has been investigated in crystalline materials such as quartz and PZT ceramics. The early studies for piezoelectricity in cellulose and collagen stimulated interests for the semicrystalline materials with a low elastic stiffness. Chiral polymers may show shear piezoelectricity in the uniaxially oriented states of molecules and crystallites. The magnitudes of the piezoelectric constant d 14 of biopolymers and synthetic polypeptides are in the same level as that of quartz. However, the value of d 14 recently found for oriented films of poly-l-lactic acid is 10 pc/n [14]. The appearance of piezoelectric polymers with ferroelectric properties was looked for until 1969, when large piezoelectricity was discovered for elongated and poled films of PVDF [19]. The dielectric hysteresis and Curie temperature were confirmed in PVDF and other polar polymers. The large values of piezoelectric constants for PVDF, d 31 = 20 pc/n and d 33 = 30 pc/n, stimulated

13 fukada: progress in piezoelectric polymers 1289 practical applications such as pressure sensors and ultrasonic transducers. The studies have been extended to various polymers as copolymers of vinylidene fluoride and trifluoroethylene, copolymers of vinylidene cyanide and vinylacetate, nylons, polyureas, and polyurethanes. It is likely that more new polymers will be found to be piezoelectric or ferroelectric in the future. In the study of single crystalline films of P(VDF/TrFE), the highest values of electromechanical coupling factors are k ij =0.3 [51]. The higher values of k ij are not likely to be found in poled polymers. For practical applications, piezoelectric polymers are compared with piezoelectric ceramics with higher coupling factors such as k ij =0.6. The feature of piezoelectric polymers is the low dielectric constant and the low elastic stiffness, which result in the high voltage sensitivity and the low acoustic impedance. These merits have been well utilized in hydrophones. Availability of large area and processability would be other features of polymers. Piezoelectric polymers should possess their own established area for technical applications. Practical applications of piezoelectric chiral polymers are rather limited. One example is that elongated rods of poly-l-lactic acid (PLLA) implanted in bone accelerates the growth of bone [64]. The piezoelectric polarization in PLLA caused by mechanical strain induces electric current in bone, which stimulates the biological activity of osteocytes. The concept of the experiment comes from the original idea that the piezoelectric polarization in bone caused by mechanical strain stimulates the biologic activity of osteocytes. PLLA is biodegradable and can be formed to pins and jigs used for orthopedic operations. Most protein molecules assume an α-helical conformation, which exhibits shear piezoelectricity. Electromechanical transduction in hearing sense is carried out by ionchannel protein molecules located in the membrane of hair cells [65]. It is suspected that the applied electrical or mechanical stress changes the conformation of proteins, which induces the transport of ions through pores formed by protein molecules. Because the shear piezoelectricity is caused by internal rotation of peptide dipoles, it should result in the change of conformation of helical molecules. The piezoelectric polarization also should change the interaction between protein molecules constituting ion-channels. The molecular piezoelectricity in proteins and other optically active polymers appears to be an unexplored field remaining for future research. References [1] Eiichi FukadaEiichi Fukada and M. Eguchi, On the permanent electret, Phil. Mag., vol. 49, pp , [2] T. Takamatsu and I. Sumoto, Life time of carnauba wax electrets, Riken Hokoku, vol. 45, pp , 1969 (in Japanese). [3],, 1986, unpublished. [4] V. 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Glasgow, Scotland: Blackie, [62] P. M. Galetti, D. DeRossi, and A. S. DeReggi, Eds. Medical Applications of Piezoelectric Polymers. New York: Gordon and Breach, [63] L. F. Brown, Ferroelectric polymers: Current and future ultrasound applications, in Proc. IEEE Ultrason. Symp., 1993, pp [64] Y. Ikada, Y. Shikinami, Y. Harada, M. Tagawa, and E. Fukada, Enhancement of bone formation by drawn poly(l-lactide), J. Biomed. Mater. Res., vol. 30, pp , [65] L. Bolis, R. D. Keynes, and S. H. P. Maddrell, Eds. Comparative Physiology of Sensory System. Cambridge: Univ. Press, Eiichi Fukada was born March 28, 1922, at Kokura, Japan. He earned a B.Sc. degree in 1944 and a D.Sc. degree in 1960, both at the Department of Physics, University of Tokyo, Tokyo. He was a research member at Kobayasi Institute of Physical Research from 1944 to 1963; Research Director of Biopolymer Physics Laboratory, the Institute of Physical and Chemical Research (RIKEN) at Tokyo from 1963 to 1980; and Executive Director at RIKEN from 1980 to He was a research advisor at the Institute for Super Materials, ULVAC, Tsukuba from 1987 to He has been Executive Director, Kobayasi Institute of Physical Research since His current interests are piezoelectricity and ferroelectricity in polymers and their applications. He belongs to the Japanese Society of Applied Physics, the Society of Polymer Science, Japan. He received Scientific Achievement Awards from the Society of Polymer Science, Japan, and the Society of Rheology and Poiseuille Gold Medal Award from the International Society of Biorheology.