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. 

There are mainly two kinds of piezoelectric polymer materials as mentioned before. First, the polymer materials intrinsically have the piezoelectric effect. This kind of polymer materials mainly are PVDF and its copolymer of trifluoroethylene (PVDF-TrFE) (Furukawa, 1989), nylon-11 (Newman et al., 1980), and polyuria (Hattori et al., 1996). However, most polymer-based piezoelectric generators are fabricated from PVDF and its copolymers. The other polymer materials might endow the generator with thermo-resisting properties, while it has not been verified yet. 

Electrostrictive polymers PVDF based copolymers e.g. Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) [81... 

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). 

In general, the polymers evaluated and discussed here are poled piezoelectric films of 30 pm PVDF from MSI and 28 pm P(VDF-TrFE) from... 

The effects of simultaneous AO/VUV exposure of the two vinylidene fluoride based polymers were also examined. In both cases significant weight loss and surface erosion resulted from AO attack. Erosion yields were 2.8xl0 24 cm3/atom for PVDF and 2.5x1 O 24 cm3/atom for P(VDF-TrFE), consistent with previous literature data for similar materials. The film orientation of PVDF samples was reflected in the surface topology features after exposure, while the less orientated P(VDF-TrFE) samples had less regular surface patterning after exposure. Significantly, neither AO nor VUV irradiation dramatically altered the piezoelectric properties and we propose that these materials should perform satisfactorily under moderate LEO conditions. 

The piezoelectric effect stems from hydrogen and fluorine atoms in the VDF, which are positioned perpendicularly to the polymer backbone. Fig. 5.7A shows a typical molecular structure of PVDF with different crystalline phases (Chang et al., 2012). The piezoelectric performance of PVDF is dependent on the nature of the crystalline phase (Crossley et al., 2014). Typically, PVDF has three crystalline phases, namely a, p, and y, and it is the a-phase that typically forms in most situations. While it is polar p-phase that shows the strongest piezoelectric behavior so this material needs to be electrically poled using an electric field with the order of 100 MV m or mechanically stretched. A higher P-phase crystalline can lead to a higher piezoelectric coefficient Note that the copolymer of P(VDF-TrFE) [(CH2-CF2) -(CHF-CF2)ml crystallizes more easily into the P-phase due to steric factors (Furukawa, 1989). So, the most applied material in piezoelectric generator is P(VDF-TrFE). 

In order to improve mechanical properties for different applicatitMis, many pyroelectric polymer composites have been prepared, such as BaTiOs-PVC composites, (BioJS[ao.5)o.94Bao.o6Ti03/P (VDF-TrFE), 65PMN-35PT/P(VDF-TrFE), and DTGS PVDF. 

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... 

State of the Art. Pioneering work in the area of piezoelectric polymers (9) led to the development of strong piezoelectric activity in poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) and tetrafluoroethylene (TFE). These semicrystalline fluoropolymers represent the state of the art in piezoelectric pol5uners and are currently the only commercial piezoelectric polymers. 

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 polymer is used in copolymers with TrFE in certain range of its molar ratio. This procedure will increase crystallinity ratio of the polymer up to 90%. Therefore such copolymers exhibit much stronger piezoelectric activity. The most interesting molar ratio range is 60-80% of PVDF. In that range the thickness electromechanical coupling factor kt reaches its maximum value k is a measure for the electromechanical energy conversion). Copolymerized TrFE units decrease the Curie temperature of the polymer (e.g. c = 80°C for P(60%VDF/40%TrFE) polymer). 

It was found that in several polymers, such as stretched and poled poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), a strong polarization effect is observed under influence on mechanical stress and temperature. This means that piezoelectric and pyroelectric gas sensors can also be designed based on polymers (see Chap. 13 [Vol. 1]). 

The comparison of piezoelectric properties of several of the major piezoelectric ceramics discussed above is given in Table 13.3. hi this table, PZT 4 is hard PZT (PZT doped with acceptor ions, such as K or Na at the A site, or Fe % Al % or Mn at the B site), PZT 5H is soft PZT (PZT doped with donor ions, such as La " at the A site, or Nb or Sb at the B site), LF4T is (K Na Li, ) (Nbj j Ta, jSbj f )Oy and PVDF is piezoelectric polymer synthesized using copolymerization of vini-lydene difluoride with trifluoroethylene (TrFE). 

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