Ferroelectric polymers properties

In this chapter piezoelectric crystals and polymers ferroelectric and ferromagnetic solids resistance of metals shock-induced electrical polarization electrochemistry elastic-plastic physical properties. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Chiral lc-polymers can be prepared by a proper functionalization of lc-polymers with chiral and reactive groups. These elastomers are interesting, because they combine the mechanical orientability of achiral lc-elastomers with the properties of chiral lc-phases, e.g. the ferroelectric properties of the chiral smectic C phase. The synthesis of these elastomers was very complicated so far, but the use of lc-polymers, which are functionalized with hydroxyl-groups, has opened an easy access to these systems. Also photocrosslinkable chiral lc-polymers can be prepared via this route. 

Conducting polymers also can be utilized to form core-shell structures with high dielectric constant particles. Fang et al. used PANl to encapsulate barium titanate via in situ oxidative polymerization. They examined the influence of the fraction of BaTiOs particles on the ER behavior, and found that the PANl/ BaTiOs compo-sites-based ERFs exhibit a better ER effect than does pure PANl, which result might be due to the unique ferroelectric properties as well as the high dielectric constant of BaTiOs nanoparticles. 

A number of other polymeric solids have also been the subject of much interest because of their special properties, such as polymers with high photoconductive efficiencies, polymers having nonlinear optical properties, and polymers with piezoelectric, pyroelectric and ferroelectric properties. Many of these polymeric materials offer significant potential advantages over the traditional materials used for the same application, and in some cases applications not possible by other means have been achieved. 

The introduction of a second chiral center of the same absolute configuration R into the spacer of polymer V-11 causes an important shift of the SmC phase of about 75 °C to lower temperatures and a decrease by half of the spontaneous polarization (Fig. 14). To prove whether there is an effect of partial compensation between the two asymmetric centers, with respect to the polarization, the configuration of the chiral center in the spacer was changed from R to S. It turned out that the ferroelectric properties were completely lost [21]. 

In order to achieve amorphous polar solids, some research groups have explored ferroelectric SCLCPs, and have shown that the basic rules governing the relationships between molecular structure and macroscopic ferroelectric LC properties are the same [225-228]. The main difference between low molar mass and polymer ferroelectric LCs, however, is the existence of stable glassy phases in most of the latter. 

Conduction and dielectric properties are not the only electrical properties that polymers can exhibit. Some polymers, in common with certain other types of materials, can exhibit ferroelectric properties, i.e. they can acquire a permanent electric dipole, or photoconductive properties, i.e. exposure to light can cause them to become conductors. Ferroelectric materials also have piezoelectric properties, i.e. there is an interaction between their states of stress or strain and the electric field across them. All of these properties have potential applications but they are not considered further in this book. 

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. 

Ferroelectric properties (much weaker than PVF2) are shown also by other polymers like poly(vinylchloride) (PVC) or poly(vinylfluoride) (PVF). (See Table 48.3 for... 

MV/m) than that of solid ferroelectrics (usually a few MV/m or smaller). The ferroelectric properties depend sensitively upon the details of sample preparation, for example the use of melt quenching or melt extrusion, the annealing temperature, or the details of the poling procedure. Polymer ferroelectrics are useful for soft transducers. 

There are a few publications on chiral smectic main-chain LC polymers as compared with side-chain polymers [86,87]. Watanabe et al. [88] seem to be the first to report ferroelectric properties of the main-chain FLCP... 

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. 

Chiellini, E., Galli, G., and Cioni, F., Chiral liquid crystal polymers with potential ferroelectric properties synthesis and characterization, Ferroelectrics, 114, 223-228 (1991). 

Bauer, F., PVFj polymers ferroelectric polarization and piezoelectric properties under dynamic pressure and shock wave action, Ferroelectrics, 49, 231-240 (1983). 

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]. 

The structural unit of the copolymer -(-CH2-CF2)n-(-CF2-CHF-)m- contains n and m corresponding monomer links. The ferroelectric properties are attributed to transverse dipole moments, formed by positive hydrogen and negative fluorine atoms. Below the temperature of the ferroelectric phase transition (about 80-100°C), the main chain of the polymer is in all-trans form and the dipole moments are parallel, at least, within ferroelectric domains separated fi-om each other by domain walls. The ferroelectric switching is due to an electric field induced, collective flip-flop of the dipoles around the backbone of the polymer. Several recent studies were devoted to a local ferroelectric switching of the domains in cast P(VDF-TrFE) films [6-8]. To this effect, a powerful technique, called Electrostatic Force Microscopy (EFM) [9] was used which was developed for studies of domains in thin ferroelectric films, see papers [10, 11] and references therein. 

The ferroelectric properties of the chiral smectic-C phase and the electroclinic effect of the smectic- phase appeared as a result of the symmetry breaking caused by the presence of chiral molecules. One can think of smectic phases in which nonchiral molecules arrange themselves in a polar order [86], and it seems that such phases were recently observed, indeed experimentally. The molecules which establish these phases are not chiral but possess a bent core resembling a bow- or banana-like shape [87] a second class of nonchiral liquid crystals showing polar ordering consists of certain polymer-monomer mixtures [88],... 

The occurrence of ferroelectricity in the chiral smectic C phase was already predicted by Meyer in 1975 [21]. From symmetry arguments, he concluded a polarization of a smectic layer perpendicular to the layer normal and to the director. Unwinding of the helicoidal structure causes macroscopic ferroelectric properties of the phase [22], [23]. Unlike low molar mass liquid crystals or linear liquid crystal polymers, the unwinding of the helicoidal structure is performed by applying an electric field or by surface effects. 

Galetti P et al (eds) (1988) Piezoelectric and ferroelectric properties of P(VDF-TrFE) copolymers and their pUcation to ultrasonic transducers. In Medical apphcations of piezoelectric polymers gordon and breach. New York... 

Gao Q et al (1999) Ferroelectric properties of nylon 11 and poly(vinylidene fluoride) blends. J Polym Sci Part B Polym Phys 37 3217... 

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). 

Tagami Y (1980) Effect of compressibility upon polymer interface properties. Ferroelectrics 30 115-116... 

---