Piezoelectric effect polymers

Polymer Ferroelectrics. In 1969, it was found that strong piezoelectric effects could be induced in the polymer poly(vinyhdene fluoride) (known as PVD2 or PVDF) by apphcation of an electric field (103). Pyroelectricity, with pyroelectric figures of merit comparable to crystalline pyroelectric detectors (104,105) of PVF2 films polarized this way, was discovered two year later (106.)... 

The compressibility of polymers is strongly nonlinear at pressures of a few GPa. In order to consider the nonlinearity of the piezoelectric effect at shock pressure, it is of interest to consider the piezoelectric polarization in terms of the volume compression as shown in Fig. 5.9. The pressure-versus-volume relation for PVDF is not accurately known, but the available data certainly provide a relative measure of changes in compressibility. When considered versus volume, the piezoelectric polarization is found to to be remarkably linear. Thus, large volume compression does not appear to introduce large nonlinearities. Such a behavior will need to be considered when the theory of piezoelectricity for the heterogeneous piezoelectric polymer is developed. 

The piezoelectricity of polymer films and its inverse effect, strain induced by applied voltage, have been observed for as-cast films, oriented films, and films which has been polarized under a static field. It is at present believed that all kinds of polymer films exhibit more or less piezoelectricity. 

The anisotropy of piezoelectricity in oriented polymer films is quite different among polymers. The piezoelectric effect in oriented polypeptide films is greatest when elongated along a direction at 45° to the draw-axis. On the contrary, the effect is most remarkable for elongation along the draw-axis for roll-drawn poly(vinylidene fluoride) film. 

As will be shown in the theory, the electrostriction effect plays an important role in the piezoelectric effect of polymer films. Moreover, a knowledge of the complex electrostriction constant as a function of frequency reveals a new aspect of the relaxational behavior of polymers. In this review a new method for measuring complex electrostriction constant with varying frequency will be presented with some results for poly(vinylidene fluoride). 

In the usual experiment where E0 = 0, the term kSE in Eq. (9) does not make any contribution as far as the electrical response with the same frequency as the mechanical excitation is concerned. However, as will be described in 2.2 and 2.4, the piezoelectric constant of a polymer film is sometimes a function of the electrostriction constant which plays an important role in the anisotropy and relaxational behavior of the piezoelectric effect. 

Fig. 7. Sketch of the apparatus for measuring inverse piezoelectric effect in polymer films. B vibrating cantilever beam, E electromagnetic exciter, P electromagnetic pick-up, S specimen film, M weight, C oscillator, Vt, V2 and V3 voltmeters, Sw switch, V d.c. bias source for measuring electrostriction effect. Drawn after Kawai (1) (1969) by permission of the Japan Society of Applied Physics...

As stated in the Introduction, as-cast polymer films in general show a weak piezoelectric effect in both elongation and bending. The results by Furukawa, Uematsu, Asakawa and Wada (1968) for five kinds of polymer films are illustrated in Figs. 18 and 19. The effect is ascribed to space charges embedded in the film. 

Piezoelectricity in polymer films is greatly increased by polarizing the film under a high d.c. field at high temperatures. This effect, the piezoelectricity of a polymer electret, has attracted the attention of many investigators from the scientific as well as industrial point of view, since the early work by Kocharyan and Pachadzhyan (1963,1966) on PMMA and PVC. 

Konaga,T., Fukada,E. Piezoelectric effect in oriented films of poly-y-benzyl L-glutamate. Rep. Progr. Polymer Phys. Japan 13, 379 (1970). 

As pointed out above, the occurrence of a pronounced piezoelectric effect in synthetic polymers can be mainly attributed to the presence of a non-eentro-symmetric unit cell and a net polarization in the material. To achieve this in polymeric materials one requires [28] (a) a large dipole moment in the chemical repeat unit (b) feasibility of crystallizing in a non-centrosymmetric unit cell (c) alignment of molecular dipoles. A dipole moment arises from a... 

However, in addition to their thermoplasticity, representatives of PHAs have optical activity, increase induction period of oxidation, exhibit the piezoelectric effect and, what is most important, they are characterized as being biodegradable and biocompatible. At the same time, the PHAs have disadvantages (high cost, brittleness), which can be partially or completely compensated by using composite materials based on blends with other polymers, with dispersed fillers or plasticizers. Taking into account all the above, we have suggested to create a mixed polymer composite based on poly-3-hydroxybutyrate (PHB) and polyisobutylene (PIB). 

A major advance was made in 1969 when a strong piezoelectric effect was discovered in poly(vinylidene fluoride) (PVDF). The effect is much greater than for other polymers. In 1971, the pyroelectric properties of PVDF were also first reported, and as a consequence, considerable research and development has continued during the last two decades. 

There are mainly two kinds of piezoelectric polymer materials as mentioned before. First, the polymer materials intrinsically have the piezoelectric effect. This kind of polymer materials mainly are PVDF and its copolymer of trifluoroethylene (PVDF-TrFE) (Furukawa, 1989), nylon-11 (Newman et al., 1980), and polyuria (Hattori et al., 1996). However, most polymer-based piezoelectric generators are fabricated from PVDF and its copolymers. The other polymer materials might endow the generator with thermo-resisting properties, while it has not been verified yet. 

The piezoelectric effect stems from hydrogen and fluorine atoms in the VDF, which are positioned perpendicularly to the polymer backbone. Fig. 5.7A shows a typical molecular structure of PVDF with different crystalline phases (Chang et al., 2012). The piezoelectric performance of PVDF is dependent on the nature of the crystalline phase (Crossley et al., 2014). Typically, PVDF has three crystalline phases, namely a, p, and y, and it is the a-phase that typically forms in most situations. While it is polar p-phase that shows the strongest piezoelectric behavior so this material needs to be electrically poled using an electric field with the order of 100 MV m or mechanically stretched. A higher P-phase crystalline can lead to a higher piezoelectric coefficient Note that the copolymer of P(VDF-TrFE) [(CH2-CF2) -(CHF-CF2)ml crystallizes more easily into the P-phase due to steric factors (Furukawa, 1989). So, the most applied material in piezoelectric generator is P(VDF-TrFE). 

Thus, the greater the dielectric constant, the greater is the piezoelectric effect. This is true only when comparing polymers with similar symmetry. There are certainly high dielectric constant polymers that are not piezoelectric. The electric displacement, D, (equation 5.6) for the combined effects of stress and temperature is given by the linear relation... 

PVDF is known to exhibit a strong piezoelectric effect [136] with the Pha.se I ( p form) being the most effective crystalline form for piezoelectric activity. Since molecular relaxation modes also contribute to overall piezoelectricity, high energy irradiatirm will affect the piezoelectric activity. This is due mainly to the effect of crosslinking which will increase the mechanical strength and change the molecular mobility of the polymer chains. A restriction in chain mobility will reduce reorientation of the molecular electric... 

Piezoelectric materials have, in their structure, dipoles that can be aligned when an electric field is applied in response, dimensional changes occur. The best performance has been obtained with poly(vinylidene fluoride) (PVDF) and derivative. Fluorine on the backbones makes the polymer highly polar and allows reversible conformational change. For PVDF only the beta phase is polar and useful to produce the piezoelectric effect. 

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