Transactions on the Built Environment vol 22, 1996 WIT Press, ISSN

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1 Properties of ferroelectric polymers under high pressure and shock loading F. Bauer Institute Franco-Allemande de Recherches, 5, rue de General Cassagnou, Saint-Louis, France Abstract Ferroelectric polymers are the most recent class piezoelectric and pyroelectric materials developed. The most common piezoelectric polymers are PVDF, based on the monomer CH2-CF2 and copolymers PVDF with C2FgH The effects of frequency, temperature and hydrostatic pressure on the dielectric properties, molecular relaxations, and phase transitions of PVDF and a copolymer with 30% trifluoroethylene are recalled. Pressure causes large slowing down of the 3 molecular relaxations as well as large increases in the ferroelectric transition temperatures and melting points, but the magnitudes of the effects are different for the different "transitions". A unique application of these polymers as time-resolved dynamic stress gauges based on PVDF studies under very high pressure shock compression is presented. In particular piezoelectric response of shock compressed PVDF film prepared with attention to mechanical and electrical processing exhibits precise, well defined reproducible behavior to 30 GPa. Studies to pressures of 30 GPa are available which show that the piezoelectric behavior is linearly dependent on volumetric strain to a close approximation. The first record of detonation profile is presented. P(VDF-TrFE) copolymers exhibit unique piezoelectric properties over a wide range of temperature depending on the compositions. Under shock loading up to 15 GPa, unique piezoelectric response is observed. Experimental and theoretical results obtained in current mode are presented and discussed. 1 Introduction The polar polymer polyvinylidene fluoride (PVDF) and its copolymers with 20 mol % - 50 mol % trifluoroethylene (TrFE) posses a variety of scientifically interesting and technologically important properties which have made them

2 512 Structures Under Shock And Impact among the most widely investigated polymers [1, 2, 9]. They exhibit large dielectric, piezoelectric and pyroelectric constants and were the first known ferroelectric polymers. PVDF is a partially crystalline linear polymer with a carbon backbone in which each monomer [-CH2-CF2-] unit has two dipole moments, one associated with CF2 and the other with CH2 In the crystalline phase, PVDF exhibits a variety of molecular conformations and crystal structures depending on the method of preparation [1]. Melt cast films have the helical a form in which the molecular conformation is transgauche (TGTG), and the chains are packed in an antipolar unit cell. By stretching, the a phase in the film transforms to the p phase in which the molecular conformation in the all-trans (TT) planar zigzag with the dipole moments perpendicular to the chain axis. This (3 form exhibits reversible spontaneous polarization and is therefore the most useful ferroelectric and strongly piezoelectric phase. An unusual property of PVDF is that the forces responsible for ordering the dipoles in the ferroelectric phase are sufficiently strong that the polymer melts before it undergoes a ferroelectric paraelectric transition It is known, however, that copolymers of VDF and TrFE, [ CHF-CF2 ], with mol % TrFE favour the P phase, and these copolymers exhibit ferroelectric transitions below the melting point (T^). Poling of PVDF and the copolymers in the ferroelectric (FE) phase results in well ordered molecular conformation of the crystalline phase with a well defined remanent polarization [5]. In the present paper, the effect of frequency, temperature and hydrostatic pressure on the dielectric properties, molecular relaxation and phase transitions of PVDF and a copolymer with 30% trifluoroethylene will be recalled [7]. The piezoelectric behavior of PVDF under shock wave action is presented. The electrical response of 500 pm thick VDF/TrFE copolymers under shock loading is described. 2 Ferroelectric polymers. PVDF: An orientation of the film is activated by stretching it just below the polymer's softening point with subsequent stretching and annealing. This process changes the crystalline structure of the film to the desired p polar phase. Biaxially orientation leads to a more uniform thickness distribution (25 Hm) and no wrinking effect. Such prepared films are isotropic in mechanical properties as well as in piezoelectric properties VDF/TrFE copolymers: The structure, ferroelectric properties and melting temperature [1] of VDF/TrFE copolymers are known to depend on composition. We have chosen for the present work the P(VDFg 7 -TrFEg 3) composition because (i) it has the same crystalline p phase as PVDF at room temperature, (ii) it exhibits well defined and widely separated T^ and T^ and (iii) it has potential for applications. The samples, 1 to 25 jim thick, were deposited by spin coating of the VDF/TrFE copolymers under a liquid form on a glass substrate. Then the film was removed from the substrate and annealed. The resin was available from Piezotech (Solvay). The samples 25 ^m to 1 mm

3 Structures Under Shock And Impact 513 thick, were prepared by compression molding of the melt followed by quench into either water or air. The samples were annealed at 413 K. Poling process: In the preparation of piezoelectric polymers and copolymers it is in general necessary to apply a high poling electric field to an essentially insulating material. The poling process is that established for PVDF [5, 6], and provides a means for achieving a homogeneous polarization at a / * ,6 (kv) VDF/TrFE Figure 1: Hysteresis curves versus voltage : PVDF 25um thick P(VDF-TrFE) (70/30) 8pm thick. predetermined level Histories of hysteresis loops on 25 jam thick biaxially stretched PVDF films as well as on 8 pm spin coated P(VDF-TrFE) (70/30) film are given Figure 1. 3 Hydrostatic pressure studies of polyvinylidenefluoride (PVDF) and for a VF2/VF3 copolymer [7J. The pressure dependences of T^ (melting temperature) of PVDF and T^ (normal ferroelectric transition) and T^ of a VF2/VF] (70/30) copolymer were determined from complex dielectric constant and differential thermal analysis (DTA) measurements. The complex dielectric constant e = e' - ie" was measured as a function of frequency (10%_ 10 6 Hz ), temperature ( K) and hydrostatic pressure (0-20 kbar). Homopolymer PVDF: The dielectric constant of PVDF exhibits strong pressure and temperature dependences and frequency dispersion in the frequency range investigated 1(K - 10& Hz [8]. Above 320 K, with increasing T, e' exhibits a broad maximum and then decreases fairly rapidly up to the melting point (% 440 K) above which temperature itrisesvery sharply [8]. The pressure dependence of T^ is shown in Figure 2 which also shows that the frequency dispersion of T^ increases with pressure. The slope dt^/dp is frequency dependent and is of the order of 30 K/kbar.

4 514 Structures Under Shock And Impact PRESSURE (kbar) Figure 2: Pressure dependences of both T^ and copolymer and of T of PVDF. of VF2/VF] 70/30 Copolymer VF2/VF$ 70/30: Figure 3 shows the pressure dependences of both 1^ and Tm of VF^/VFg 70/30 copolymer The slope dt<vdp is 30 K/kbar below 2 kbar. The initial slope is dt^/dp = 53 K/kbar at 10^ Hz. ^-Relaxation Process: The p-relaxation process strongly influences the electrical and mechanical responses of theses polymers. The dielectric signature of the relaxation for the present polymers is the classic one observed for other polymers and can be described by the methods used to treat relaxationnal phenomena in polymers, liquids and glasses [7, 8]. With increasing frequency the e' (T) and e"(t) curves shift to higher temperatures, and the magnitude of e "(T) peak increases With increasing pressure there is a large displacement of the e"(t) and e'(t) responses to higher temperatures. This effect is seen explicitly in Figure 3 where the shift with pressure of the e"(t) peak, T^^ (= Tg), at 100 khz is plotted. The T^ax (P) dependence is somewhat nonlinear, the average slope between 0 and 5 kbar being dt^^x/dp = 10.0 ±0.5 K/kbar. This result is qualitatively and quantitatively very similar to that for PVDF [8] demonstrating that the p-relaxation process is essentially independent of composition. The analysis done in reference [7] has produced thefirstevaluation of pressure dependences of TQ (T0 is a reference temperature where all relaxation times diverge: TQ can thus be viewed as the "static" dipolar freezing temperature for the relaxation process). All three parameters increase with pressure, Figure 3. The results can be qualitatively understood. The pressure, by reducing the sample volume and available free volume, causes closer packing of PVDF chains and chains segments and hinders the motion of the dipolar groups responsible for the P-relaxation. Furthermore, it has been experimentally demonstrated that the adiabatic piezoelectric response under hydrostatic

5 Structures Under Shock Ami Impact (3 100 Figure 3: Shifts with pressure of TO, Tp, TC and T^of P( VDFo 7o-TrFEo 30) compression pressure of both PVDF and P(VDF-TrFE) copolymers is reversible until and above 0.76 GPa [6]. 4 Piezoelectric response of PVDF under shock Ioading.[3, 5, 6, 10, 12, 13]. Studies of shock compression response of PVDF [3, 6] in a "standard" configuration, Figure 4, have been especially a continuing effort of ISL, Sandia Figure 4: PVDF 'standard' Gauge. National Laboratories to develop high quality, reproducible sensors material and to determine their physical characteristics under high pressure shock compression In fact, shock-compression gauges cannot be "calibrated".

6 516 Structures Under Shock And Impact Even in the direct shock experiment, the controlled shock-compression experiment serves as a shock-calibration only if the reproducibility of the piezoelectric polymer is evaluated quantitatively and a persistent reproducible material is available. At Sandia National Laboratories the principal experimental tool used to study the shock response is the compressed-gas gun which subjects the piezoelectric samples to precise, controlled shock loading with impactors and targets of well defined standard materials to control the stress input. Impact velocities are measured to accuracies and precisions of 0.1%. At the Institute of Saint-Louis, the experimental measures of electrical response of shock compressed piezoelectric polymer films are carried out on new impact-loading facility which is a powder gun 20 mm in diameter. In both laboratories, low loss coaxial cables and 1 GHz digitizers provide the highfrequency recording capability required to properly interpret the sensor responses. The responses are measured in the "current mode" to provide both a simple circuit and the most revealing electrical behavior. The symmetrical impact of an impactor and target Z cut quartz, Z cut sapphire or copper provides the loading. The insulated PVDF gauge element is placed on the impact surface of the target material. In this configuration, Figure 5, the initial stress wave produced by impact is that typical of the impact of the copper impactor on the PVDF gauge. This wave then, reverberates between the impactor and target until stress is reached equal to that for the standard impactor and target [6]. The electrical signal from the shock-compressed PVDF gauge is recorded in the current mode : the gauge provides a signal dependent on the stress rate called here "multiple shock". Upon integration of the current pulse of current pulse, the electrical charge-versus-time is obtained, Figure 5. PVDF gauge ,4#r =qr Y o 100 a Time (nanoseconds) Ctargt (uc/w cm) y i.» o_ i r '" Wf^* r\ ^ 200 Time (nanoseconds) Figure 5: Experimental studies of PVDF under shock loading. PVDF results in a behavior in which the observed polarization with volume compression is approximately linear with pressure [3, 12].

7 Structures Under Shock And Impact 517 It is observed that PVDF stress rate gauges in a standard configuration as well as low inductance gauges poled via a new poling procedure [6] show continuous response to pressures approaching 30 GPa, Figure 6. The experimental results as well as the computed response done by Moulard [12] are plotted on the same Figure II 241, L bl.ll 1.11 Ml b.4l 8.11 chirgi Figure 6: Piezoelectric polarization data for PVDF gauges : pressure versus electrical charge. Detonation profile measurement: We have tried to use these new gauges in low inductance configuration and with an «ad hoc» shielding in an explosive single shock experiment. The explosive is a mixture of hexogen and wax 70/30 in weight percent The Figure 7, gives the theoretical and experimental pressure in the vicinity of the explosive. ISL-PVDF ID computation Figure 7: Pressure profile in the vicinity of an explosive in a detonation regime.

8 518 Structures Under Shock And Impact 5 Electrical response of thick P(VDF-TrFE) copolymer under shock loading. It should be recalled here that the poling process is the same as that established for PVDF [5, 10], and provides a mean for achieving a homogeneous polarization at a predetermined level. High voltage up to 60 kv are used to pole the thicker samples. The P(VDFQ 77 -TrFEQ 23) in mole % gauges with low crystallinity show a remanent polarization value close to 4 uc/cnf. All P(VDFo 70 -TrFEo 30) in mole % gauges, with high crystallinity show a remanent polarization value close to 7.2 uc/cm*. The spatial distribution of the polarization is homogenous. The configuration of the sputtered electrodes on the copolymer gauge is based on that established by Graham [3] called «Sandia guard ring configuration», as shown in Figure 8. active area 4 mm* thickness 500 urn electroded outside part length 8 mm width 8 mm Figure 8: Electrical configuration of a copolymer sample. Experimentation: Impact loading is produced by controlled impact in a powder gun (caliber 20 mm) at ISL [6]. The symmetrical impact of an impactor and target copper provides the loading. The short duration of the loading creates conditions in which no heat is exchanged. For 500 jam thick copolymers, the gauge element is placed behind a 3 mm thick target material. In the latter case, an induced shock of 10 GPa can be achieved but is completely dependent on the visco-plastic behavior of the copper. Nevertheless, the shock induced can be computed. Results: The level of the shock wave induced into the copolymer was chosen in the range of 1 to 4 GPa. One example of current record is given Figure 9, at a stress level of 3.4 GPa The record obtained shows deviations from the expected signal which, theoretically, corresponds to a current jump followed by a current slightly increasing and coming back to zero when the wave is passed. As we can see on Figure 9, the current is jumping and begins to slightly increase in time and is drastically distorded. The control of the homogeneity of the polarization in the copolymer does not reveal any anomalies. After many non destructive controls of the preparation process of the samples, as well as of the annealing process, electron microscope

9 Structures Under Shock And Impact -LW I,?b ti*# in (istc ti#* in ftstc (A) (B) Figure 9: Anomalous current response of a 450 urn thick copolymer (A), Experimental current records at different shock levels (B). analysis revealed defects corresponding to microscopic cracks in the bulk of the copolymer during the annealing process. In the meantime, because of the lack of resin of P(VDFQ 79 -TrFEgjo) in mole %, we have decided to study and to optimize the annealing process on the available resin i.e. P(VDFg 77 - TrFE0 23) '" mole % whose composition is close to thefirstcomposition studied. 500 pm samples were prepared by compression molding of the melts and were quenched into water. The crystallinity was lowered in this case and after poling at a level of 4 uc/cnf, no crack was detectable We have repeated the shock experiments with the same set-up Typical current records are shown on Figure 9(B). One should contrast this record with that shown in Figure 9(A) to appreciate the distortion obtained with the high crystallized copolymer The current from the P(VDFg 77 -TrFEQ 23) in mole % is observed to increase as the wave propagates through the 500 urn sample. The extent of the effect depends largely on the stress amplitudes and on the high deformation as we can see on the record obtained at 9.4 GPa. We also can note a rise time of a few nanoseconds for thefirstcurrent jump. The nonlinear behavior of the observed records is noticeable, and has to be analyzed with the same approach used for PVDF as described in reference [12]. 6 Conclusion In this paper we have recalled investigation of the effects on hydrostatic pressure on the p relaxation process in the copolymer P(VDF0 7-TrFEg 3) and obtained results on the ferroelectric (TJ and melting (T^) transitions as well as in PVDF. Pressure causes large increases in T^, T^, and Tp, the dynamic dipolar freezing temperature associated with the P relaxation, and the magnitude of the change increases with increasing transition temperature. Pressure has a strong influence on both the dynamics and energetics of the P relaxation

10 520 Structures Under Shock And Impact Some high pressure applications of ferroelectric polymers has been presented. PVDF stress rate gauges show continuous response with high reproducibility to pressure approaching 30 GPa. After ten years studies of PVDF gauge shock response we were able to record the detonation pressure profile of a specific explosive. The first experimental study of the piezoelectric response of thick piezoelectric copolymers was realized. First use of such copolymer for measuring short stress pulses (2.5 ns) induced by laser in a aluminum target was initiated by St. Couturier, T. de Resseguier et al. in Poitiers [11]. We have defined the process of preparation as well as the poling process of a copolymer which can respond to shock level of 10 GPa in a single shock compression. References 1. Lovinger, A.J. JapJ. Appl. Phys., 1985, 24, Supplement 24-2, Ferroelectrics 32 Nos 1-4 (1981) and 115 N 4 (1991). 3. Graham, R.A., ch 5.2, Solids under High Pressure Shock Compression, pp , Springer Verlag, New York, Scheinbeim, J, New Ferroelectric and Piezoelectric Polymers pp , Proceedings ISAF'92, Bauer, F, Ferroelectrics, 49, 231 (1983), 115, (1991) 6. Bauer, F, Graham, R.A., Anderson, M.U., Lefebvre, H, Lee, L.M. and Reed, R.P., Piezoelectric Response of Precisely Poled PVDF to Shock Compression greater than 10 GPa, pp , Proceedings of the ISAF 92, 1992, 7. Samara, G, Graham, R.A., Bauer, F in Proceeding ISE8, 1994, pp Samara, G, Bauer, F, Ferroelectrics vol. 135, De Reggi A., Transduction Phenomena in Ferroelectric Polymers and their Role in Pressure Transducers, Ferroelectrics, 1983, Vol.50, Bauer, F, Graham, R.A., Lee, L.M., Properties of VF2/ VF] Ferroelectric Copolymers; Electrical Response under High Pressure Shock Loading, pp \proceedings of the ISAF'90, Couturier, St., Boustie, M, de Resseguier, T, Hallouin, M, Romain, J. P., Laser Driven Shock Measurements by VF]/VF3 and PVDF Gauges of Pulses of 2.5 ns up to 10^ W/cnf, to be published in Proceedings APS Shock Wave Meeting^ Seattle, 1996, American Institute of Physics AIP 800 Woodbury NY Moulard, H, Bauer, F, Lagrangian Analysis of the PVDF Shock Sensor, Proceedings APS Shock Wave Meeting, Colorado Spring in American Institute of Physics AIP 800 Woodbury NY , Graham, R.A., Anderson, M.U., Bauer, F, Setchell, R.E., Piezoelectric Polarization of the Ferroelectric Polymer PVDF from 10 MPa to 10 GPa: Studies of Loading - Path Dependence, pp , Shock Waves in Condensed Matter-1991, 1992