Polyvinylidene piezoelectric properties

In this section, examples of films made from polyvinylidene fluoride (PVDF) are discussed. Although most of the pol5winylidene fluoride film is in the form of coating on metal substrates, stand-alone PVDF films and sheets are produced by extrusion and film blowing.1 ] ] Blends of PVDF and a number of other polymers such as polymethylmethacrylate (PMMA) are miscible. Films made from these blends have excellent piezoelectric properties. 

Two common piezoelectric materials are polymers (polyvinylidene fluoride, PVDF) and c mics (lead zirconate titanate, PZT). The polymer materials are soft and flexible however have lower dielectric and piezoelectric properties than ceramics. Conventional piezoelectric ceramic materials are rigid, heavy and can only be produced in block form. Ceramic materials add additional mass and stiffiiess to the host structure, especially when working with flexible/lightweight materials. This and their fragile nature limit possibilities for wearable devices. Comparisons of several piezoelectric materials are presented in Table 1. 

Kawai s (7) pioneering work almost thirty years ago in the area of piezoelectric polymers has led to the development of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers. Research on the morphology (2-5), piezoelectric and pyroelectric properties (6-70), and applications of polyvinylidene fluoride 11-14) are widespread in the literature. More recently Scheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numbered nylons (75-77). When examined relative to their glass transition tenq>erature, these nylons exhibit good piezoelectric properties (dai = 17 pCTN for Nylon 7) but have not been used commercially primarily due to the serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymers must have a noncentrosynunetric crystalline phase. In the case of PVDF and nylon, these polar crystals cannot be grown from the melt. The polymer must be mechanically oriented to induce noncentrosynunetric crystals which are subsequently polarized by an electric field. In such systems the amorphous phase supports the crystalline orientation and polarization is stable up to the Curie temperature. 

Piezopolymers are a fascinating piezoelectric group of materials, having good piezo performance and much higher flexibility than piezoceramics. Natural polymers such as polysaccharides, proteins and polynucleotides have shown some piezoelectric properties (Fukada, 2000). The polymers have superior benefits over other materials due to their ability to form yams and fabrics. Polyvinylidene fluoride (PVDF) is a piezoelectric polymer with remarkable piezo properties and is a highly suitable material for textile applications. 

Poly(3HB) and poly(3HB-co-3HV) are piezoelectric materials, whereas the piezoelectric properties of other PHAs have not been investigated (Steinbuchel 1996). The piezoelectric materials produce electric charges on parts of their surface when mechanical pressure is applied to the crystalline material, and an electric current will result from the charges if the crystal is short circuited. Conversely, application of a voltage between certain faces of the material produces a mechanical distortion (a deformation) of the material. Piezoelectric materials have important applications in electromechanical transducers, such as microphones. In medicine, chemically synthesized piezoelectric polymers such as polyvinylidene fluoride stimulated bone growth. The piezoelectric property of poly(3HB) may be important for some medical applications (Steinbuchel 1996). 

A detailed discussion on possible applications of PHB has been published [2], especially in medicine but considering also its optical activity and piezoelectric properties (by one order of magnitude lower than polyvinylidene fluoride but without interference from pyroelectricity due to temperature changes). Applications in various areas have been discussed not only for PHB but also for other important biodegradable plastics [77]. 

Commercial products based on copolymers of ethylene and TEE are made by free radical-initiated addition copolymerization.69 Small amounts (1 to 10 mol%) of modifying comonomers are added to eliminate a rapid embrittlement of the product at exposure to elevated temperatures. Examples of the modifying comonomers are perfluorobutyl ethylene, hexafluoropropylene, perfluorovinyl ether, and hexafluoro-isobutylene.70 ETFE copolymers are basically alternating copolymers,70 and in the molecular formula, they are isomeric with polyvinylidene fluoride (PVDF) with a head-to-head, tail-to-tail structure. However, in many important physical properties, the modified ETFE copolymers are superior to PVDF with the exception of the latter s remarkable piezoelectric and pyroelectric characteristics. 

The unique dielectric properties and polymorphism of PVDF are the source of its high piezoelectric and pyroelectric activity.75 The relationship between ferroelectric behavior, which includes piezoelectric and pyroelectric phenomena and other electrical properties of the polymorphs of polyvinylidene fluoride, is discussed in Reference 76. 

It is instructive to compare the basic properties of the piezoelectric polymer, polyvinylidene fluoride (PYDF) with those of PZT . The flexibility and low density of the polymer contrasts with the stiffness, brittleness and high density of PZT . On the other hand the piezoelectric d coefficient for PYDF is relatively small ( — 30pCN the mechanisms by which the polarisation in PVDF... 

As mentioned earlier, it is misleading to describe advances in the properties of polymers without also describing how such polymers are processed into thin films or crystals. Moreover, many of the physical properties of polymers are inextricably linked to their structural and orientational order. An excellent example of this is the piezoelectric and pyroelectric coefficients of the well-known polymer, polyvinylidene difluoride (PVdF)... 

Polyvinylidene fluoride is used extensively in the electronics industry because of its piezoelectric and pyroelectric properties. Only two room temperature solvents, n-methylpyrrolidone and dimethyl acetamide, have been found. Still-wagon (37) used n-methylpyrrolidone as the solvent for low-angle laser light-scattering measurement because of the larger refractive index increment value (dn/dc) in that solvent. No SEC work in these solvents was mentioned. 

A typical example for the frequency- and temperature-dependent dielectric properties of a piezoelectric polymer is given in Fig. 7 that displays the a-relaxation, related the dynamic glass transition, of a polyvinylidene fluoride (PVDF) film along with an upswing of the dielectric loss at low frequencies due to electrical conduction. 

Certain polymers, such as polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), possess special properties in the film form, caileApiezoelectricity and pyroelectricity. Piezoelectricity is electric polarization of a film produced by mechanical strain in some crystals. The polarization is proportional to the amount of strain and changes sign with it. The reverse is true and an electrical polarization induces a mechanical strain in piezoelectric sensors. Pyroelectricity is electric polarization of a film induced by thermal absorption in some polymer crystals. The induced polarization is proportional to the level of thermal change. These properties can be used in the manufacture of transducers, microphones, loudspeakers, pressure gauges, pickup heads, hydrophones, motion sensors, and other devices from biaxially oriented PVDF films. Table 13.37 gives the properties of a piezoelectric film of polyvinylidene fluoride. 

H. Ohigaahi, Ekctromechankal properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method. J, AppL Phys. 47M9 (1976). 

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