An exceptionally high electrostrictive response ( approximately 4 percent) was observed in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. The material exhibits typical relaxor ferroelectric behavior, suggesting that the electron irradiation breaks up the coherent polarization domain (all-trans chains) in normal ferroelectric P(VDF-TrFE) copolymer into nanopolar regions (nanometer-size, all-trans chains interrupted by trans and gauche bonds) that transform the material into a relaxor ferroelectric. The expanding and contracting of these polar regions under external fields, coupled with a large difference in the lattice strain between the polar and nonpolar phases, generate an ultrahigh strain response.
A ceramic-powder polymer composite, making use of a relaxor ferroelectric polymer that has a high room-temperature dielectric constant as the matrix, is developed. The experimental data show that the dielectric constant of the composites with Pb(Mg1/3Nb2/3)O3–PbTiO3 powders can reach more than 250 with weak temperature dependence. In addition, the composites under a proper preparation procedure exhibit a high breakdown field strength (>120 MV/m), leading to a maximum energy storage density of more than 15 J/cm3. Experimental results also indicate that the high electron irradiation does not have much effect on the dielectric behavior of Pb(Mg1/3Nb2/3)O3–PbTiO3 powders, possibly due to the relaxor nature of the ceramic.
The effect of high-energy electron irradiation on structural and polarization properties of 50/50 mol % copolymer of poly(vinylidene fluoride-trifluoroethylene) was investigated for both mechanically stretched and unstretched films. Although stretching can significantly enhance the polarization and dielectric responses in unirradiated films, it was observed that this enhancement was not significant in irradiated films. In addition, the polarization in both types of films after irradiation can be described quite well by a logarithmic mixing law of composites, which consist of crystallites embedded in an amorphous matrix with nearly the same fitting parameters. On the other hand, the enhancement of the mechanical properties from stretching persists after the irradiation, and the elastic modulus along the stretching direction remains high after irradiation in comparison with unstretched films. It was found that the dielectric dispersion in both types of films after irradiation fits well to the Vogel–Fulcher law. It was also observed that the crystallinity decreases and the crosslinking coefficient increases continuously with dose. However, there was no direct one to one type relationship between the crystallinity and the crosslinking coefficient. Although stretching can reduce the rate of crosslinking, the reduction of crystallinity with dose for stretched and unstretched films does not show a marked difference.
Structural and molecular conformation changes of high-energy electron-irradiated poly-(vinylidene fluoride-trifluoroethylene) 50/50 copolymer have been investigated by means of FT-IR spectroscopy, X-ray diffraction, and cross-linking density measurement and are compared with the change of polarization hysteresis loops with dose. Although in general the irradiation reduces the macroscopic polar ordering, which leads to the eventual disappearance of the remanent polarization in the copolymer at room temperature, the change in the mesoscopic structure and molecular conformation with dose is not monotonic. In the intermediate dose range, there is a reversal of the change of local ordering with dose, as revealed by the decrease of the fraction of the TG conformation in the copolymer and contraction of the lattice in directions perpendicular to the polymer chain with dose, which could be caused by the high cross-linking density due to irradiation. In addition, for irradiated polymers at doses above 30 Mrad, no transition behavior with temperature is observed in the FT-IR spectra, consistent with the early experimental results that the ferroelectric-paraelectric transition has been eliminated by irradiation.
Poly(vinylidene fluoride) (PVDF) and its family of copolymers are arguably the best‐known examples of a class of high‐performance polymers noted for their remarkable piezoelectric and ferroelectric properties. After more than 30 years of study and development, the piezoelectricity and electromechanical properties of PVDF and its copolymers have been improved markedly. Today this class of polymer still possesses the highest electromechanical responses over a broad temperature range among known synthetic organic materials. Further, when considered along with their easy conformability, flexibility, robustness, and lightness, it is not surprising that electroactive polymers continue to be the focus of interest of the designers of high‐performance electromechanical devices. When PVDF is stretched and poled in a strong electric field, it exhibits piezoelectricity. In its piezoelectric form, PVDF finds use in transducer devices requiring the interconversion of mechanical and electrical energy. Piezoelectric PVDF can be fabricated and used in a variety of sensors and actuators such as artificial muscles and organs, medical imaging, blood‐flow monitors, microphones, smart skins, underwater acoustic transducers, seismic monitors, fluid pumps and valves, surface acoustic wave devices, robots, and tactile sensing devices. P(VDF‐TrFE) copolymers display similar and in some cases even superior properties.
Many organic substances in fact exhibit a key ferroelectric property that is called polarization hysteresis. The copolymer of PVDF with trifluoroethylene (TrFE) and tetrafluoroethylene (TFE), however, is the only polymeric system that shows both a well‐defined polarization hysteresis loop and a transition to a paraelectric phase with increased temperature.
The high-energy electron irradiated poly͑vinylidene fluoride-trifluoroethylene͒, P͑VDF-TrFE͒, copolymer exhibits many features resembling the relaxor ferroelectric behavior. In polymer systems, there are local dipolar motions at the monomer or unit cell scale, which manifest themselves as various relaxation processes. In this paper we investigate the relationship between the relaxor ferroelectric behavior, especially, Vogel-Fulcher ͑V-F͒ behavior and these local dipolar relaxation processes in irradiated P͑VDF-TrFE͒ 65/35-mol % copolymer. In order to cover the change in polarization dynamics of the copolymer system, the dielectric behavior of copolymer is measured over a broad frequency ͑0.01 Hz-10 MHz͒ and temperature ͑Ϫ40 to 80°C͒ range. The results indicate that there is an increased coupling among the local dipolar motions with reduced temperature in the crystalline region. On the other hand, the randomness introduced in the irradiation prevents the formation of a polar phase, on both the macroscale and the microscale, in the polymer. The observed relaxor behavior is a consequence of the competition of these two effects. The results further show that the V-F process of the irradiated copolymer system is different from the glass transition, which occurs in the amorphous phase of the copolymer.
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