Mechanical properties ferroelectric polymers

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... 

Ferroelectric liquid crystals (FLC) have attracted attention because of their high speed response and memory effect (7-5). The characteristics of fast response and memory effect make them suitable in electro-optical device applications, such as display, light valve and memory devices. Ferroelectric side chain liquid crystalline polymers (FLCPs) exhibit desirable mechanical properties of polymers and electro-optical properties of low molecular weight FLC, which have been investigated extensively Corresponding author. 

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. 

Li Z, Grimsditch M, Xu X, Chan SK (1993) The elastic, piezoelectric and dielectric constants of tetragonal PbTiOs single crystals. Ferroelectrics 141 313-325 MarraSP, Ramesh KT, Douglas AS (1999) The Mechanical properties of lead-titanate/polymer 0-3 composites. Compos Sci Technol 59 2163-2173 Materials Data Sheets of APC International, Tokin, Ferroperm, Morgan Matroc, Siemens Mattiat OE (1971) Ultrasonic transducer materials. Plenum Press, Tokyo McLachlan DS, Blaszkiewicz M, Newnham RE (1990) Electrical resistivity of composites. J Am Ceram Soc 73 2187-2203... 

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]. 

PVDF was discovered by Dr. Heiji Kawai in 1969 [32]. Although PVDF is a piezoelectric system, it is a ferroelectric cum piezoelectric material as explained earlier, with a Curie point of 103 °C. PVDF possesses various phases such as a, p, y, and 5, among which p-phase has the most responsive piezoelectric properties. Compared with all ferroelectric polymers, PVDF has a dielectric constant with a reasonable chemical and mechanical durability [4,32]. In general, the physical properties of PVDF make it the most valuable material for application in sensors. 

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 most interesting properties of polymers are their high mechanical and electrical strength and low electrical conductivity and acoustic impedance, whereas the ferroelectric ceramics exhibit good dielectric, pyroelectric, and piezoelectric properties (4,8,67]. 

The electrical and mechanical properties trf piezoelectric polymers make them a possible alternative to ferroelectric ceramics such as lead zirconate titanate. For several reasons, they are attractive for transducer design. The mechanical flexibility and conformability of thin-film PVDF means that it can be configured into a wide range of transducer products. The low acoustic impedance of PVDF is companrf>le to body tissues, which makes it useful for acoustic imaging applications. Short impulse response and high axial resolution in acoustic imaging systems arc possible with PVDF-featured devices because of the robustness and broadband characteristics of the polymer. 

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. 

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... 

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). 

The rest of this chapter is structured as follows. Section 5.2 gives a brief historical overview together with a description of the synthesis, structure and preparation of ferroelectric polymers. Section 5.3 defines the properties required to evaluate these materials, and includes typical values and brief accounts of the measurement methods. Section 5.4 details the applications of ferroelectric polymers, covering sound transducers, biomedical, pyroelectric and mechanical applications. Conclusions are briefly described in section 5,5. 

Ferroelectric polymers are characterized by a range of electrical, thermal and mechanical properties. This section begins with a brief description of the characteristics unique to ferroelectric materials before detailing the... 

It can be seen from the equations in section 5.3.3 that the mechanical properties of ferroelectrics are closely related to the piezoelectric properties. The more widely used mechanical constants and their effect on the ferroelectric polymers will be discussed here. 

The huge variety of nanocomposite materials containing versatile nanofillers offers a wide range of potential applications. Depending on the specific properties of the nanofiller, the polymer materials may be reinforced chemical and thermal stability can be enhanced or completely new electrical, ferroelectric, magnetic, or diverse optical properties may be introduced to the material. Since the reinforcement of polymers is taking an immense research field including CNT composite materials and clay nanocomposites, the mechanical properties of these composite materials have been discussed elsewhere extensively. Hierefore, this contribution will focus on optical and thermal properties of nanocomposite and hybrid materials. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

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