پیزوالکتریسیته در کریستال هایPVDFفاز β: مطالعه ای با شبیه سازی مولکولی

Based on the energy-minimization method in molecular simulation, the origin of piezoelectricity in β-phase PVDF crystals was investigated. Simulation results indicated that under the action of a constant applied voltage, PVDF crystal would be stretched or compressed, dependent on the orientation of dipoles and the polarity of the applied voltage. Our simulation confirmed that piezoelectricity in polymers would be mostly attributed to the dimensional effect. Piezoelectric coefficient calculated from our model was well consistent with experimental data, which showed the credibility of this model.

In 1969, Kawai [1] discovered strong piezoelectricity in poly(vinylidene fluoride) (PVDF, as abbreviated); in 1971, Bergman [2] and Wada [3] found its pyroelectricity; and soon typical ferroelectric hysteresis loops were also found from the direct polarization measurements using the standard Sawyer-Tower circuit [4]. All these discoveries in PVDF family have been regarded as the milestone in organic transducers [5]. Before the discovery of organic ferroelectrics, it is usually believed that piezoelectricity and ferroelectricity are characteristics of crystals and ceramics, and the related mechanism has been intensively studied. However, polymers are mostly semi-crystalline with coexistence of crystalline phase and amorphous phase, so ferroelectricity and piezoelectricity of polymers are inevitably different from those of inorganic materials. Studies on these polymers have elicited great interest because of their potential applications to transducers, sensors, actuators, and potential ultra-high-density data storage. A widely accepted mechanism, called a dimensional effect, has been put forward to explain the piezoelectricity and pyroelectricity in polymers, and this mechanism assumes that piezoelectric and pyroelectric activities arise from macroscopic dimensional changes when a sample is deformed, while the moment of constituent molecular dipoles is kept constant. Theoretical considerations by Broadhurst et al. [6] and Wada and Hayakawa [7] have shown that macroscopic dimensional changes played an important role in the piezoelectric activity of drawn and poled PVDF. Furukawa et al. [8] had examined the contribution of the dimensional effect to the piezoelectricity in VDF/TrFE copolymers by experimental measurements and theoretical calculations. In our previous work [9], we introduced the molecular simulation method to the study of ferroelectricity in PVDF and illuminated the effect of trapped charges on ferroelectric switching process in polymers. In this paper by the molecular simulation method we attempted to study the origin of piezoelectricity in β-phase PVDF crystals.

Based on the energy-minimization method in molecular simulation we studied the origin of piezoelectricity in β-PVDF crystal. Simulation results confirmed that organic piezoelectricity would be attributable to the dimensional effect, which was very different from inorganic piezoelectrics. Piezoelectric coefficient obtained from our work was well consistent with experimental data, and our model also explained the origin of the negative piezoelectric effect from microscopic scale.