Piezoelectric PVDF fluoride

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.)... 

Particularly relevant is the case of some copolymers of PVDF. Already small amounts (5-20% by mol) of a fluorolefinic comonomer (vinyl fluoride (VF) [89-90], trifluoroethylene [91-93], tetrafluorethylene [94, 95]) can force the polymers to a melt crystallization in the piezoelectric P form. (We recall that the homopolymer crystallizes in the non-piezoelectric a form, by melt crystallization). 

A close correlation between the polarities of piezoelectricity and pyroelectricity was found for PVC and poly (vinylidene fluoride) (PVDF) films (Nakamura and Wada, 1971). However, it must be emphasized that the polarity of piezoelectricity is determined not only by the polarity of the charge distribution but also by that of heterogeneous strain. The origin of heterogeneous strain in the elongation of film may derive from heterogeneity in the structure of the film. 

Kawai (1) and (2) (1969) found that polar polymer films such as PVDF, poly (vinyl fluoride), PVC, nylon 11, and polycarbonate exhibit a strong piezoelectricity when they are drawn and then polarized under a high cLc. field Ep at a high temperature Tp and cooled keeping the d.c. field. The piezoelectricity thus obtained depends on Ep, Tp, and poling period. An improved poling technique was reported by Edelman, Grisham, Roth, and Cohen (1970). 

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... 

This paper presents details of a new, low-cost, continuous process in which pellets of poly(vinylidene fluoride) (PVDF) are directly converted into piezoelectric tube. Kawai s paper in 1969 (1 ) described a batch process for making PVDF piezoelectric by stretching filmB at an elevated temperature and subsequently applying a high electric field to the electroded and heated films (thermal poling). Since then many variations of this technique have been described. Southgate in 1976 (2) demonstrated that... 

In this paper we have addressed the issue of piezoelectric performance of PVDF and copolymers of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) over temperature ranges simulating the LEO environment, and examined the effects of radiation (gamma and vacuum ultraviolet) and atomic oxygen. 

The effects of simultaneous AO/VUV exposure of the two vinylidene fluoride based polymers were also examined. In both cases significant weight loss and surface erosion resulted from AO attack. Erosion yields were 2.8xl0 24 cm3/atom for PVDF and 2.5x1 O 24 cm3/atom for P(VDF-TrFE), consistent with previous literature data for similar materials. The film orientation of PVDF samples was reflected in the surface topology features after exposure, while the less orientated P(VDF-TrFE) samples had less regular surface patterning after exposure. Significantly, neither AO nor VUV irradiation dramatically altered the piezoelectric properties and we propose that these materials should perform satisfactorily under moderate LEO conditions. 

There are various operation modes for piezoelectric sensors, depending on the crystallographic orientation of the plate and the material [1]. These modes include transversal compression, thickness or longitudinal compression, thickness shear action and face shear action. Also available are piezoelectric polymeric films, which are very thin, lightweight and pliant, such as polyvinylidene fluoride (PVDF) [3,4]. These films can be cut easily and adapted to uneven surfaces. Resonance applications are not possible with PVDFs because of their low mechanical quality factor. However, they can be used in acoustical broad-band applications for microphones and loudspeakers. 

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. 

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. 

A large number of apphcations have been proposed for piezoelectric polymers. The types of applications can be grouped into live major categories sonar hydrophones, ultrasonic transducers, audio-frequency transducers, pyroelectric sensors, and electromechanical devices. The principal polymers of interest in these applications are PVDF and copolymers of vinylidene fluoride and trifluoroethylene. 

Ferroelectric composites are alternatives to standard piezoelectric and pyroelectric ceramics such as lead zirconate titanate (PZT) and BaHOs (BT). They combine the strong ferroelectric and dielectric properties of ceramics with the easy processing and good mechanical properties of polymers. Dispersion of micrometer-sized ferroelectric particles in an electrically passive epoxy matrix was first published by Furukawa et al. [1976] and later extended to ferroelectric matrices such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-3-fluoroethylene) (PVDF-TrFE) [Hsiang et al., 2001 Hilczer et al., 2002 Gimenes et al., 2004 Lam et al., 2005 Beloti et al., 2006]. However, the necessity of miniaturization of electronic components and... 

Finally, it is worth mentioning that a phenomenon analogous to the difference between the normal and giant flexoelectricity of calamitic and bent-core nematics, respectively, exists in crystals, ceramics and polymers too. The flexoelectric response (defined in Eq. (3.1)) of perovskite-type ferroelectrics, " of relaxor ferroelectric ceramics and polyvinylidene fluoride (PVDF) films are four orders of magnitude larger than the flexoelectricity of dielectric crystals. In those sohd ferroelectric materials the polarization induced by flexing is evidently of piezoelectric origin. 

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. 

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