Piezoelectric polyvinylidene fluoride

Table 3.78. Characteristics of Piezoelectric Polyvinylidene Fluoride Film ...

As discussed in Section 10.2.3, PVP and DNA have been used to wrap and water-solublize SWNTs. For specific actuator, electrical and electro-optic applications, SWNTs have been wrapped by piezoelectric polyvinylidene fluoride and trifluor-oethylene copolymer [50] or with conjugated polymers [51, 52]. The conjugated polymer used to form a composite with MWNTs and an electron-transport layer in light-emitting diodes is poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV) [53]. Wrapping coupled with electron doping has been achieved with polyethylene imine to form p-n junction devices ([40], see footnote 1). 

Piezo- Piezoelectric Polyvinylidene fluoride Lead zirconate titanate... 

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... 

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. 

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. 

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. 

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. 

A recent publication by Clements et al. [36] describes the effect of applied tensile stress on the proton spectra of polyvinylidene fluoride. The spectra consist of a narrow singlet superimposed on a broad doublet, which are assigned to amorphous and crystalline material, respectively. When stress is applied, the proportions of these components change, the fractional crystallinity increasing when the stress is parallel to the draw direction and decreasing when perpendicular. These measurements were all carried out with the draw direction parallel to the magnetic field direction because the crystalline doublet was most apparent in this case. The authors offer this crystallinity change as a partial cause of the piezoelectric response of the material. 

Piezoelectric materials can be grouped into the class of natural crystals, such as quartz or tourmaline, into one of polymers, such as polyvinylidene fluoride (PVDF) or that of polycrystalline ceramics. 

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. 

Examples of optical fibres based on the DSSCs have also been reported (Toivola et al., 2009 Ramier et al., 2008). Recently, a fibre based on a piezoelectric material such as polyvinylidene fluoride has been developed, which can convert both mechanical and light energies (Siores et al., 2010). The authors of the patent claim that the fibre is flexible and can be incorporated into textiles. The commercialisation of these PV fibres is, however, likely to be some time away. 

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. 

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