Piezoelectric polymers mechanisms

For mechanical wave measurements, notice should be taken of the advances in technology. It is particularly notable that the major advances in materials description have not resulted so much from improved resolution in measurement of displacement and/or time, but in direct measurements of the derivative functions of acceleration, stress rate, and density rate as called for in the theory of structured wave propagation. Future developments, such as can be anticipated with piezoelectric polymers, in which direct measurements are made of rate-of-change of stress or particle velocity should lead to the observation of recognized mechanical effects in more detail, and perhaps the identification of new mechanical phenomena. 

Piezoelectric polymer film is usually partially crystalline and the crystallites are embedded in the amorphous phase, which exhibits mechanical relaxations. Therefore, the strain of each crystallite, S, may differ in both amplitude and phase from that of the film as a whole, S. In this case the complex piezoelectric constant of the film is written by putting S/S — K (complex quantity) in Eq. (62) as... 

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... 

An SFM based lithography technique by means of mechanical interactions as applied to surfaces of polystyrene (PS) and polymethylmethacrylate (PMMA) is described in Sect. 6. Similarly, some results of SFM-based poling experiments performed on a piezoelectric polymer are elucidated in Sect. 7.6. 

Koga, K. Ohigashi, H. "Piezoelectrisity and related properties of vinylidene fluoride and trifluoroethylene copolymers", J. Appl. Phys., Vol.59, No.6, pp.2142-2150, (1985) Lindner, M., Bauer-Gogonea, S., Bauer, S., Paajanen, M. Raukola, J. "Dielectric barrier microdischarges Mechanism for the charging of cellular piezoelectric polymers", J. Appl. Phys., Vol.91, No.8, pp.5283-5287, (2002)... 

Electrical Properties. The piezoelectricity of the technologically important polymer of vinylidene fluoride (PVDF) has been the subject of modeling for several decades (see Piezoelectric Polymers). An early example of the use of molecular mechanics to aid in the calculation of the mechanical and electrical properties of this polymer is found in the work of Tadokoro and co-workers (390,391). Subsequent investigation of PVDF by Karasawa and Goddard (83) focussed on the prediction of alternative crystal structures with use of the shell model to captiu-e polarization effects. The latter phenomenon was further explored by Carbeck and co-workers (84), who used the shell model to show that the induced moment because of neighboring dipoles in the crystal increases the dipole per repeat imit by about 50% over its value in the isolated molecule. 

Structural Requirements for Piezoelectric Polymers. The piezoelectric mechanisms for semicrystalline and amorphous polymers differ. Although the differences are distinct, particularly with respect to polarization stability, in the simplest terms, four critical elements exist for all piezoelectric polymers, regardless of morphology. These essential elements are (.1) the presence of permanent molecular dipoles, (2) the ability to orient or align the molecular dipoles, (5) the ability to sustain this dipole alignment once it is achieved, and (4) the ability of the material to undergo large strains when mechanically stressed (3). 

The work here summarized explores in vitro and in vivo use of a piezoelectric polymer for bone mechanical stimulation. 

An overview of the piezoeJectric activity in amorphous piezoelectric polymers is presented. The criteria required to render a polymer piezoelectric are discussed. Although piezoelectricity is a coupling between mechanical and electrical properties, most research has concentrated on the electrical properties of potentially piezoelectric polymers. In this work, we present con arative mechanical data as a function of temperature and offer a summary of polarization and electromechanical properties for each of the polymers considered. 

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. 

The final determining factor for a material s degree of piezoelectric response is the ability of the polymer to strain with applied stress. Since the remanent polarization in amorphous polymers is lost in the vicinity of Tg, the use of these piezoelectric polymers is limited to temperatures well below Tg. This means that the polymers are in their glassy state, and the further away from Tg the use temperature is, the stiffer the polymer. This also means that measurement of the bulk physical properties is crucial both for identifying practical applications and for comparing polymers. The electromechanical coupling coefficient, kai, is a measure of the combination of piezoelectric and mechanical properties of a material (refer to Table III). It can be calculated using the equation below ... 

Mechanical properties are often overlooked when investigating piezoelectric polymers. It is important to note that the piezoelectric response is a result of the coupling between the mechanical and dielectric properties in an amorphous polymer. The piezoelectric coefficient, d, is defined as... 

Lee et al. (2005) employed flie PEDOT/PSS treated with a dimethyl sulfoxide (DMSO) solvent as the electrodes of all polymer bimorph cantilevers in which the piezoelectric polymer poly(vinyUdene fluoride) (PVDF) was used as the active layers, as shown in Fig. 27. They compared flic mechanical output of the bimorph cantilevers with the PEDOT/PSS (DMSO) electrodes and that of the inorganic electrodes such as platinum (Pt) and indium tin oxide (TTO). The cantilever with... 

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