Relaxor ferroelectric polymers

Recent advances in PVDF-based materials have led to the elimination of the hysteretic behavior characteristic of ferroelectrics. For this reason, these PVDF-based materials are classified as relaxor ferroelectric polymers they will be discussed under the Electrostrictive Polymers heading. 

Relaxor ferroelectric polymers are intimately related to the ferroelectric polymers described above. All known relaxor ferroelectric polymers are based on the P(VDF-TrFE) copolymer. As the name suggests, these polymers behave as relaxor ferro-electrics, which is distinguished by a broad peak in dielectric constant and a strong frequency dispersion [99,100]. There are two major limitations of the P(VDF)-based ferroelectric actuators. First, the electrically induced paraelectric-ferroelectric transition that allows for actuation only occurs at temperatures above the Curie... 

Cheng ZY, Zhang QM, Bateman FB (2002) Dielectric relaxation behavior and its relation to microstructure in relaxor ferroelectric polymers high-energy electron irradiated poly(vinylidene fluoridetrifluoroethylene) copolymers. J Appl Phys 92 6749... 

Relaxor ferroelectric polymers produce large strains up to 5 % under large electric fields of 150 V/pm without the necessity of poling. Such polymers are very interesting for the development of tactile-feedback devices on various substrates a potential mass application are flexible touch screens in future mobile appliances (Ju et al. 2014). CXurently, such applications are hindered by the need of high voltages to drive the relaxor ferroelectric transducers. To overcome this limitation, it is necessary to develop multilayer ferroeleclric relaxor film structures (Fig. 8a). 

The large strain of relaxor ferroelectric polymers, coupled witii a high elastic modulus, makes these materials attractive to replace low strain piezoceramics in refreshable Braille displays (Ren et al. 2008). It is desirable to use small diameter... 

After the discovery of significant piezoelectricity in polymers in the late 60s, they were immediately considered for applications in the early 70s. Interest faded in the early 80s due to problems in producing reproducible devices. After solving these initial problems by developing efficient poling procedures and suitable electrical contacting methods, piezoelectric polymers became a commercial success in many niche applications. Science progresses in waves, and currently, we face a very active research phase where piezoelectric and relaxor ferroelectric polymers are employed... 

Lu SG, Rozic B, Zhang QM, Kutnjal Z, Pirc R, Lin M, Li X, Gomy L Comparison of directly and indirectly measured electrocaloric effect in relaxor ferroelectric polymer. Appl. Phys. Lett. 2010 97 202901. DOI 10.1063/1.3514255... 

Key Words Dipolar glasses, Ferroelectric relaxors, Conducting polymers, NMR line shape, Disorder, Local polarization related to the line shape, Symmetric/asymmetric quadrupole-perturbed NMR, H-bonded systems, Spin-lattice relaxation, Edwards-Anderson order parameter, Dimensionality of conduction, Proton, Deuteron tunnelling. 

Fig. 1.4 The alpha and beta phases of the ferroelectric polymer PVDF. The beta phase is stable at room temperature but can be reversibly changed to the alpha phase by heating above the Curie temperature. Above the Curie temperature, an electric field can be used to induce a change between the alpha and beta phases. For ferroelectric relaxor polymers, the Curie temperature is below room temperature so the alpha phase is stable. A change to the beta phase can be induced by an electric field [7], Materials Today 2007, reprinted with permission...

Finally, it is worth mentioning that a phenomenon analogous to the difference between the normal and giant flexoelectricity of calamitic and bent-core nematics, respectively, exists in crystals, ceramics and polymers too. The flexoelectric response (defined in Eq. (3.1)) of perovskite-type ferroelectrics, " of relaxor ferroelectric ceramics and polyvinylidene fluoride (PVDF) films are four orders of magnitude larger than the flexoelectricity of dielectric crystals. In those sohd ferroelectric materials the polarization induced by flexing is evidently of piezoelectric origin. 

In most substances, the electrostrictive strain (M < 10 m A ) is too small to be used in practical applications. The first useful electrostrictive strain ( 0.1 %) was found in relaxor ferroelectric ceramics [3], while newly developed electrostrictive polymers exhibit an electrostrictive strain of more than 5 % [1]. It is experimentally and theoretically proven... 

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). 

Zhang, Q.M., Bharti, V., Zhao, X. Giant electrostrictive response and ferroelectric relaxor baehavior in electron irradiated polyvinyUdene fluoride-trifiuoroethylene polymer. Science 280, 2101-2104 (1998)... 

Newnham RE, Sundar V, Yimnirun R, Su J, Zhang QM (1997) Electrostriction nonlinear electromechanical coupling in solid dielectrics. J Phys Chem 101 10141-10150 Ploss B, Ploss B, Shin FG, Chan HLW, Choy CL (2000) Pyroelectric or piezoelectric compensated ferroelectric composites. Appl Phys Lett 76 2776-2778 Poddar S, Ducharme S (2013) Measurement of the flexoelectric response in ferroelectric and relaxor polymer thin films. Appl Phys Lett 103 202901... 

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