Ferroelectric polymers PVDF-TrFE

We have shown how Electrostatic Force Microscopy can be an extremely useful tool to investigate and to modify the electric properties of sample surfaces on a microscopic and even nanoscopic scale and we have presented a phenomenological model to help relating the experimental data to the material properties. Ferroelectric domains can locally be reoriented and their time evolution can be followed, as was shown for PZT. We have also demonstrated how the ferroelectric polymer PVDF-TrFe could be locally modified which can be used to locally vary the optical properties of a LC cell. Finally, we have demonstrated that rubbing polymer substrates can indeed result in electrostatic charging, in particular for PMMA and PI, while no charging is found for PVA. 

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... 

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. 

First ferroelectric polymer - polyvinilidene fluoride (PVDF or PVF2) - was discovered in 1969. Extensive research has been focused on this substance and their copolymers withtrilluoroethylene (TrFE) since that time. Due to its resistivity to the harmful chemical substances is this polymer used in stractural coatings to prevent damage. Another excellent functional property is a veiy low value of the acoustic impedance, which allows for the better acoustic matching to water environment. Due to this property P(VDF/TrFE) copolymer is being applied mostly in hydrophones (Nalwa 1995) and ultrasound imaging transducers. PVDF polymer and its blends with TrFE are commercially available in the market. 

PVDF copolymers have been investigated for their piezo properties and for uses in various applications such as sensors. An example of a PVDF copolymer is polyvinylidenefluoride-co-trifluoroethylene [P(VDF-TrFE)], which is a ferroelectric, crystalline polar polymer that exhibits inherent piezoelectric and pyroelectric responses with low acoustic impedance. Such properties provide an optimistic approach towards the use of these polymers for various applications in the near future. Higashihata et al. (1981) compared the piezoelectric craistants of PVDF and P(VDF-TrFE) and observed that much larger values were obtained for P(VDF-TrFE) under the same polarizing conditions. The special interest in this copolymer is also due to the evidence reported by Furukawa et al. (1981) that the PVDF-TrFe copolymer can be annealed to 100% crystallinity, as opposed to 50% in PVDF. Other copolymers have also been explored to determine an enhanced piezo effect (Poulsen and Ducharme, 2010). 

Fig. 5 The ratio of the relevant component of the piezoelectric coefficient d and the respective polarization is plotted vs. the reciprocal of the respective elastic modulus Y (i.e., the relevant component of the elastic compliance). From left to right Literature data for inorganic ferroelectrics squares) barium titanate (BaTiOs), lead zirconate titanate (PZT), and lead zirconate niobate (PZN). Ferroelectric polymers triangles) polyamide-11 (PA-11), poly(vinylidene cyanide-vinyl acetate) (P(VDCN-VAc)), polyurea-5, poly(vinylidene fiuoride (PVDF)), poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene-hexafiuoropropylene) (P(VDF-HFP)). Polymer ferroelectrets circles) cellular polypropylene (cellular-PP) and tubular-channel poly (fluoro-ethylene-propylene) (FEP) (Qiu et al. 2014)...

Piezoelectric and ferroelectric polymers have been recognized as a new class of electroactive materials when the significant piezoelectricity in polyvinyhdene fluoride (PVDF or PVF2) was discovered by Kawai (1969). Since then, a variety of new piezoelectric polymers have been developed including copolymers of vinylidene fluoride and trifluoroethylene, P(VF2-TrFE), odd-numbered nylons, composite polymers, etc. These materials offer options of material selections for sensor and actuator technologies that need lightweight electroactive materials. 

However, this chapter is concerned with ferroelectric polymers. As PVDF and the VDF/TrFE copolymers are the main polymers known to exhibit ferroelectricity, their properties and applications will be discussed at length in the following sections. 

In linear dielectrics D is directly proportional to E, increasing and decreasing linearly with E. In ferroelectric polymers, such as PVDF and the VDF TrFE copolymers, the surface charge density increases with the field in a nonlinear manner, and exhibits hysteresis as the field is decreased, with a significant amount of surface charge remaining when the field is completely removed. Subsequent applications and reversals of the electric field produce the characteristic D-E hysteresis loops shown in Fig. 5.4. 

Examples of polymers that have a piezoactive response are poled poly(vinylidene fluoride) (PVDF) (151) and its copolymers with trifluo-roethylene co(VDF-TrFE) (152), and the family of odd nylons (153) (see Piezoelectric Polymers). These are partially crystalline materials in which the crystalline regions have a permanent electric dipole moment. These polymers show ferroelectric switching behavior indicating that after poling they have a net... 

In semicrystalline dipole electrets, polar crystallites are present in addition to the polar amorphous phase (Fig. 2b). In die technically most interesting semicrystalline dipole electrets such as polyvinyhdene fluoride (PVDF) and its copolymers with trifluoro ethylene (P(VDF-TrFE)) (Lovinger 1983) or hexafluoropropylene (P(VDF-HFP)), odd Nylons 7 and 11, polyureas, polyureflianes (PU), and some liquid crystalline polymers, the crystallites are ferroelectric (Vasudevan et al. 1979 Hattori et al. 1996). The terpolymer poly(vinyhdene-fluoride-trifluoroethylene— chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) has been shown to have relaxor ferroelectric properties as the CTFE group destabilizes die long-range order of the ferroelectric phase (Xu et al. 2001). 

Irradiated PVDF and poly(VDF-co-TrFE) copolymer possess ferroelectric properties that allow the use of such fluorinated polymer in the domain of captors, sensors, and detectors [47,194]. Another interesting property of crosslinked poly(VDF-co-HFP) copolymer is their insolubihty in organic solvent [195]. Cured fluorinated polymers can be processed as membranes for many electrochemical applications such as fuel cell and batteries [196]. For example, a poly(VDF-co-HFP) copolymer has been crossUnked with various systems such as polyols [197], by irradiation with electron beam or y-rays [197] or with aliphatic amines [198] in order to elaborate a solid polymer electrolyte for non aqueous lithium battery [197,198]. This electrolyte is particularly interesting for its ionic conductivity, its adhesion with an electro-conductive substrate and also remarkably enhanced heat resistance. 

The only polymers of practical ferroelectric use are PVDF, formed by the linking together of simple VDF (1,1-difluoroethylene) molecules, and VDF TrFE copolymer, which consists of both VDF and TrFE molecules linked together in the same chain. 

Values of tan can be calculated from conductance measurements or obtained directly using modern capacitance bridges. Polymers are comparatively lossy dielectrics. Typical room-temperature values of tan dg for PVDF range from 0.015 to 0.02 at 1 kHz and for VDF TrFE from 0.015 to 0.025, compared with values of 10 for some ferroelectric ceramics such as lithium tantalate and lead germanate. 

The results described here indicate that confining PVDF and P(VDF-TrFE) copolymers in cylindrical nanopores formed in alumina templates induces severe changes in the ferroelectric behavior of the polymer. In the case of the PVDF homopolymer, confinement into alumina templates induces an enhancement of ferroelectric-like features that might indicate the formation of an interfacial ferroelectric phase. In the case of the P(VDF-TrFE) copolymers, confinement into alumina templates yields inhibition of the ferro-para transition. In order to check whether this inhibition is due to purely spatial confinement or to the interaction with the confining walls, we confined P(VDF-TrFE) in nanospheres, where no interaction with walls is present. The results are discussed in the following section. 

Ferroelectric properties in PVDF and P(VDF-TrFE) copolymers are directly associated with their crystalline phase. Confinement may alter the stmcture of the crystalline phase. We remark that the changes in the ferroelectric behaviour cannot be solely attributed to spatial confinement (finite size effects). The occurrence of peculiar ferroelectric behavior is dictated by the presence of interfaces with solid walls, with whom the polymer chains interacts. To name some of the features discussed in this chapter when PVDF is confined into alumina cylindrical templates, an interfacial layer showing ferroelectric like behavior appears, whereas the bulk phase of this homopolymer is paraelectric under normal processing conditions. However, in P(VDF-TrFE) copolymers confined in alumina templates, due to the the severe confinement conditions, the transition from ferroelectric to paraelectric is inhibited, indicating that the interaction with the confining wall stabilizes de ferroelectric phase. Confinement without interfaces, like in the case of 3D confined nanospheres, does not affect the ferroelectric character of the polymer. The confinement in nanoparticles may decrease the crystallinity of the system, but the crystals, responsible for the ferroelectric character of the polymer, have essentially the same properties as in the bulk. 

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