Piezoelectric effects polymer electricity

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.)... 

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. 

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. 

Polymers that exhibit the piezoelectric effect include polyvinyl chloride, polyvinyl fluoride, and difluor polyethylene. These polymers acquire their properties through technological processing. The thin plastic foil samples are exposed to strong electric fields and then cooled to room temperature. This process results in a polarization of the material. 

PLLA is a crystalline polymer with an asymmetric carbon in the repeating unit of the main chain, PLLA obtains high piezoelectricity when the polymer chains are highly oriented by the drawing process. On the other hand, PDLLA that is amorphous does not exhibit piezoelectric effects under any conditions. Hence, the electric potential induced in PLLA by adding additional stress can promote pseudo-bone formation. 

When a small AC electric field is imposed on a DC electric bias field, an apparent piezoelectric effect can be obtained in an electrostrictive polymer, as shown in Figure 16.1. For the field along the 3-direction, the dominating AC strain response term (Ar) is obtained using Equation (16.9) as ... 

A piezoelectric sensor is a device that can convert mechanical stress into an electrical charge, and vice versa. An electric polarization occurs in a fixed direction when the piezoelectric crystal is deformed. The polarization causes an electrical potential difference over the crystal. Natural piezoelectric materials are quartz and tourmaline, and synthetic polymers such as polyvinyUdene fluoride (PVDF) exhibit piezoelectricity several times greater than quartz. Because the effect is reversible, which means that the electrical stimuli can lead to mechanical deformations, the piezoelectric effect is also useful to create some actuators in smart clothing. 

Mellinger A (2003) Dielectric resonance spectroscopy a versatile tool in the quest for better piezoelectric polymers. IEEE Trans Dielectr Electr Insul 10 842-861 Meyer RB (1969) Piezoelectric effects in liquid crystals. Phys Rev Lett 22 918-921 Newnham RE (2005) Properties of materials anisotropy, symmetry, stmeture. Oxford University Press, Oxford/New York... 

The definition of piezoelectricity extended considerably in time. Originally it was used only for crystals in connection with compressions, but later it was generalized to polymers and other materials for any strains and stresses, including shear. For historical reasons, in most cases direct and converse piezoelectric effects are distinguished. When electric polarization is produced by... 

Electrical stimulation of cell activities is an independent problem of tte piezoelectric effect in polymers, althouf the initial work was closely related to the stress>inducCd electric potential in bone. The enhancemem of cell activities in bone, cartilage, and other tissues by the action of electric current has been investigated in great detail. The stress-induced potential in bone is believed to be caused mostly by the ionic streamitig potential, which is proportioiial to the potential in canaliculi. 

Present researdi on physical properties, performance, and passible applications of electroactive polymers has oonceatraled < cooducting potymers because of their peculiar electrical properties, but the interest in polymers originally arose firom the rlieleciric ones. The strong piezoelectric effect in polyfvinylidene fluoride) (PVDF or PVFi) poled under high electric field was observed the first time by Kawai (1) in 1969, while its pyro-... 

An nnusual phenomenon exhibited by a few ceramic materials (as well as some polymers) is piezoelectricity—WiexaWy, pressure electricity. Electric polarization (i.e., an electric field or voltage) is induced in the piezoelectric crystal as a result of a mechanical strain (dimensional change) produced from the application of an external force (Figure 18.36). Reversing the sign of the force (e.g., from tension to compression) reverses the direction of the field. The inverse piezoelectric effect is also displayed by this group of materials—that is, a mechanical strain results from the imposition of an electrical field. 

Piezoelectric Effects and ElectretS. When certain polymers, eg, PTFE and poly(vinylidene fluoride), are placed while hot in a d-c electric field (polarized), and cooled while the d-c field is still applied, stable trapped charges remain (71). Such permanently polarized materials, called electrets, may possess piezoelectric properties, which are useful in microphones and small loudspeakers. Heating an electret produces electrical discharges between opposed and shorted electrodes... 

The adsorption of molecules into a sorbent layer (e.g. a polymer or a molecular film) produces a change of mass and the measurement of these mass shifts can allow the evaluation of the amount of adsorbed molecules. The measure of small mass changes is made possible by piezoelectric resonators. A piezoelectric resonator is a piezoelectric crystal properly cut along a well specified crystalline axis. Due to the piezoelectric effect, the mechanical resonance of the crystal is coupled with an electric resonance. Since crystal resonance is extremely efficient the electric resonance is characterised by a very large quality factor. This property is largely exploited in electronics to build stable oscillators as clock references. The same effect is exploited for chemical sensing adopting particularly shaped crystals such... 

Polymers exhibit creep and hysteresis as previously indicated in Chapter 4, Section 4.3. The hysteresis results in energy loss and the polymers are frequency sensitive under cyclic loading. There is also a corresponding hysteresis piezoelectric effect under cyclic electric AC current or the reverse by the current resulting from cyclic stress. Figure 16.4 shows a hysteresis loop for PVDF at room temperature (20 C) with a cyclic voltage E at a frequency of 1 MHz. Takase et al. also shows plots of hysteresis at lower temperatures down to -100°C well below the glass transition temperature of-50 C. The residual polarization remained nearly constant at all tanperatures, abont 50-60 mC/m. ... 

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