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Electroactive polymers may be broadly classified into electronic and ionic based on their drive mechanism.
Electric field or Coulomb forces generally drive electronic EAPs, while the primary driver for ionic EAPs is the mobility or diffusion of ions. Examples of the electronically driven EAPs include ferroelectric and dielectric polymers, electrostrictive paper and electrostrictive, electro-viscoelestic and liquid crystal elastomers. Ionically driven EAPs include ionic polymer gel, ionomeric polymer metal composite, conductive polymers and carbon nanotubes.
Ferroelectric Polymers- Among the electronically driven polymers, those with ferroelectric characteristics have shown much
promise for a variety of applications. One of the best examples of this class of materials is polyvinylidene difluoride or PVDF. It is a long chain semicrystalline polymer of the repeat unit (CH2-CF
2). The monomer, vinylidene fluoride units, CH2=CF2, polymerize in an orderly fashion to produce greater than 90 percent head-to-tail configuration; i.e. – CH2 – CF2 – CH
2 – CF2
-, and exhibits an unusually high net dipole moment. As with ceramic piezoelectric materials it produces a voltage when strained and deforms when subjected to an electric field. The properties of PVDF, including piezoelectricity, are highly influenced by the degree and type of crystalline structure. There are at least three distinct crystalline forms. The most common—and the one obtained when the polymer is cooled from its melt—is the nonpolar, centrosymmetric unit cell called the alpha phase. Deformation of alpha-type crystallites, such as the stretching of extruded film at temperatures below 80°C, causes packing of unit cells in parallel planes to produce the polar beta phase. A third configuration is the gamma phase, which, while polar, is intermediate in terms of centrosymmetry between alpha and beta phase configurations.
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