Pyro-electric polymers

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E. Giannetti, 2001, A. Lovinger, 1982 N. S. Nalwa, 1995) ... 

The newest market for KYNAR resins is perhaps the most exciting and most challenging. This is the KYNAR Piezo Film. By definition, a piezo electric material is one that can change polarization in response to mechanical stress. The CH - CF repeat unit was found to exhibit the strongest piezo electric and pyro electric activity of all known polymers. This property was first reported in 1969 based on experiments using KYNAR PVDF. Since that time the piezo and pyro electric properties of PVDF have been the subject of many publications. 

One of the most extensively studied pdymers of medium crystallinity is poly-(vinylidene fluoride). The interest in this polymer results from its use as a piezoelectrical and pyro-electrical material (Nakamura and Wada, 1971 Hiqrakawa and Wada, 1973 Murayama, 1975 Murayama and co-workers, 1975 Muiayama and Hashizume, 1976 Mopsik and Broadhurst, 1975). Many dielectric studies have been made and recent work includes that of Koizumi and co-workers (1969), Sasabe and co-workers (1969), Yano (1970), Kakutani (1970), Osaki and cowoikers(1971), Nakagawa and Ishida (1973), Uemura (1974), Osaki and Iriiida (1974), Yano and co-workers (1974) and Brereton and co-workers (1977). It is evid it from these and eadier studies that the dielectric behaviour of this polymer is one of the most complex of the linear polymer systems. Studies have been made of samples of different (i) ciystal forms, (ii) orientations, (iii) crystallinity, (iv) thermal and electrical histories. At least four relaxation regions are observed, being labelled here as a, Oc,... 

Electriad and dielectric behaviour of polymers reflect macromolecular structure and motion, both in solution and the solid state. Some polymers whidt have special electrical properties may have commaical potential. Mention need ordy be made of polymer electrets, pyro-electrk polymers, photo-conductive polymers as used In ctro-imaging, and conductive poly mas to indicate tR expansion of use over that of insulators. The separation of electrical behaviour into didectric and btdk conductive properties is convenient and has been followed in this review. 

Althou a study of the piezo- and pyro-electricity of a polymer can provide valuable information bearing on polymer morphology and properties, this review will concentrate on the applications of these materials with particular reference to PVDF. For a fuller discussion of the baac theory the reader is directed to the excellent reviews by Hayakawa and Wada ... 

This polymer is of special interest due to its commercial importance as an electroactive material e.g. as piezo- or pyro-electric films). The molecule is highly polar and the material may exist in five crystal forms in three of which the molecular dipoles are parallel.It is evident from a study of the early literature (see ref. 10 for a review) that the dielectric behaviour of this polymer is the most complex of all the linear polymer systems. Studies have been made for partially crystalline samples having different crystal forms, different degrees of orientation and crystallinity, and electrical and thermal histories. Unoriented materials may have degrees of crystallinity up to 50% so they are composites of crystalline and amorphous regions with the attendant complications of relating the measured permittivities to the volume fractions of the phases, the permittivities of the phases, including and for the crystals, and the orientation distribution function of the crystallites, as has... 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

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-... 

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