Properties of ferroelectric polymers

For some ferroelectric applications, polymer sheet of 1-2 mm thickness is required. Commercially this can be produced by extruding molten polymer through a die of the appropriate cross-sectional shape and size. The hot extrudate can be led over or through pairs of cooling rollers or into a cooling liquid before it is cut into individual sheets or wound into a roll. 

On a small scale, compression moulding, where the requisite quantity of polymer pellets is introduced into a hot mould of the desired dimensions, then pressed into a homogeneous mould-sized sheet, has been found satisfactory. 

Neither of these methods produces any significant orientation. This is immaterial in the case of copolymer that is self-orienting, but a stretching stage would be essential after sheet formation in the case of PVDF. 

In the case of the VDF TrFE copolymer advantage can be taken of its self-orienting property to fabricate potentially electrically active structures. Attempts to thermally form oriented PVDF inevitably lead to loss of orientation, which is not easily recovered. Copolymer is more useful in this respect, since it reverts to the oriented form on cooling. Simple hemicylinders can be formed from copolymer sheet by heating and bending round a former. Shapes such as domes can be made by using heat and vacuum to force the sheet to conform to a shaped master. 

More complex shapes are possible by the use of injection moulding, although the need to pole the active area with high fields must always be considered. 

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C.-C. Wang, J.-F. Song, H.-M. Bao, Q.-D. Shen, and C.-Z. Yang, Enhancement of electrical properties of ferroelectric polymers by polyaniline nanofibers with controllable conductivities, Adv. Funct. Mater., 18, 1299-1306 (2008). 

In ferroelectric polymers, permincnl polarization is an intrinsic behavior depending on the crystal structure. Thus, permancnl polarization can be achieved by ensuring proper crystallization or rccrystallization conditions. For comprehensive recent reviews about structure and properties of ferroelectric polymers, see Nalwa [6] and Kepler and Anderson (7). 

Peculiar properties of ferroelectric polymers, namely, their intrinsic broad bandwidth and high mechaiiical-to-elecirical conversioo efficiency, easy confomubility to biological structures, low cost, and the almost ideal mechanical impedance matching to hunun soft... 

M. Doi, Rheological properties of rodlike polymers in isotropic and liquid crystalline phases, Ferroelectrics, 30, 247 (1980). 

Gao Q, Scheinbeim JI, Newman BA (2000) Dipolar intermolecular interactions, structural development, and electromechanical properties in ferroelectric polymer blends of nylon-11 and poly(vinylidene fluoride). Macromolecules 33 7564... 

Lovinger, A. J., Recent developments in the structure, properties, and applications of ferroelectric polymers, Jpn. J. Appl. Phys. Suppl. 2, 24, 18 (1985). 

The investigation of combined FLCPs was initiated by Zentel et al. [91-93] as a part of their approach to ferroelectric LC elastomers [94]. Figure 12 shows typical structures of combined FLCPs and cross-linkable chiral combined LC polymers. Poths et al. [67] used that approach to synthesize combined polymers with axially chiral mesogenic side groups (similar to the acrylic side-chain polymer above). The smectic C structure of polymers has been identified by optical microscopy and x-ray data, but no ferroelectric properties of the polymers have been reported yet. 

Mechanical Properties of Ferroelectric LC-Elastomers. The cross-linking reactions of a series of copolymers analogous to polymer P2, but differing in the amoimt of cross-linkable groups, were studied by ftir spectroscopy (17). These measurements show a decrease of the acrylamide double bond on irradiation. Conversions between 60 and 84% were observed. The imcertainty of the conversion, however, is high because only very few double bonds are present in pol5nner P2 and they are visible in the infrared spectrum at rather low intensity. 

Recently, PVDF has been intensively studied by many authors as a polymer matrix for ceramic nanopowders such as BaTiOs [212,214-216], PbTiOs [217], CaCOs [218], and Pb(Zro.5TiOo.5)03 [215] because they combine the excellent ferroelectric properties of ceramics with the flexible mechanical properties of the polymer. The PVDF polymer composites with electroactive ceramic nanoparticles were prepared by sol-gel processes [214,217], a natural adsorption action between the nanosized BaTiOs and PVDF particles, and then a hot press process [216]. 

In this eontext, ferroelectrets actually represent a third elass of piezoelectric polymers in the sense that these cellular materials share features of charge electrets (real eharges) with those of ferroelectric polymers (hysteresis-type ED characteristics) (Bauer et al. 2004 Lekkala et al. 1996). Moreover, ferroelectrets are characterized by extremely high piezoelectric 33 coefficients as well as an extreme anisotropy in their mechanical and electromechanical properties, which partially require speeifie characterization techniques (Dansachmiiller et al. 2005). 

The elearical and mechanical properties of piezoelectric polymers make them a suitable alternative to conventional ferroelectric ceramics (such as PZT) to transducer design, particularly as sensing probes. The lower electromechanical coupling factor makes the polymer inferior as a transmitttog material and has limited its use as a projector or pulse-echo transduction material. 

The electrical and mechanical properties of piezoelectric polymers make them interesting also in the development of electroacustk or ultrasonic transducers for medical applications. Comparison of tbe representative PVDF material characteristics with a convenlional ferroelectric ceramic as PZT shows several features of piezoelectric polymers which make them attractive in transducer design (TU>le 1). 

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