Electrostrictive polymers

EAPs can be broadly divided into two categories based on their method of actuation ionic and field-activated. Further subdivision based on their actuation mechanism and the type of material involved is also possible. Ionic polymer-metal composites, ionic gels, carbon nanotubes, and conductive polymers fall under the ionic classification. Ferroelectric polymers, polymer electrets, electrostrictive polymers, and dielectric elastomers fall under the electronic classification. 

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. 

Electrostrictive polymers have a spontaneous electric polarization. Electrostriction results from the change in dipole density of the material. These polymers contain molecular or nanocrystaUine polarizations that align with an applied electric field. PVDF copolymers with nano-sized crystalline domains, electrostrictive graft copolymers, and liquid crystal elastomers fall under this category. 

Nam JD, Hwang SD, Choi HR, Lee JH, Kim KJ, Heo S (2005) Electrostrictive polymer nanocomposites exhibiting tunable electrical properties. Smart Mater Struct 14 87... 

Liu Y, Ren KL, Hofmann HF, Zhang Q (2005) Investigation of electrostrictive polymers for energy harvesting. IEEE Trans Ultrason Ferroelectr Freq Control 52(12) 2411-2417... 

Coupling of highly conductive CNT network with a soft polymer matrix with large coefficient of thermal expansion (CTE) leads to a new class of electrothermal CNT-polymer composite actuators,which generates significant strains reversibly at applied electric fields at least two orders of magnitude lower than those reported for electrostrictive polymer nanocomposites. 

R. Kornbluh, R. Pelrine, J. Eckerle, J. Joseph, Electrostrictive Polymer Artificial Muscle Actuators. IEEE 1998, 3, 2147-2154. 

This equation shows that such materials exhibit a quadratic dependence of the strain on the applied field, as it happens for electrostrictive polymers. However, in comparison with these polymers, dielectric elastomers are capable of significantly larger deformations, even though at reduced forces, as reported in the following subsection. 

Several different PAMs have been developed to operate in the bending mode and some have been used to fabricate simple bio-inspired robots, as described in the next section. Here, the mechanisms of operation for conducting polymers, IPMCs, and electrostrictive polymers are briefly reviewed. 

Heydt, R., Kornbluh, R.D., Pelrine, R., Mason, V. Design and performance of an electrostrictive-polymer-film acoustic actuator. Journal of Sound and Vibration 215, 297-311 (1998)... 

This book intends to provide a comprehensive and updated insight into both the fundamentals of each class of EAP, and examples of the most significant applications of EAP actuators in the biomedical field, either already demonstrated or currently under development. Eor this purpose, the book comprises five sections devoted to the most technologically mature EAPs, namely polymer gels, ionic polymer-metal composites, conjugated polymers, piezoelectric/electrostrictive polymers and dielectric elastomers. Each section is... 

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... 

Figure 16.1 Schematic view of strain response in piezoelectric and electrostrictive polymers. When a small AC electric field superimposed on a DC bias field is applied on an electrostrictive polymer, an apparent piezoelectric response is observed.

When a small AC electric field is imposed on a DC electric bias field, an apparent piezoelectric effect can be obtained in an electrostrictive polymer, as shown in Figure 16.1. For the field along the 3-direction, the dominating AC strain response term (Ar) is obtained using Equation (16.9) as ... 

Equation (16.12) is valid for both linear and nonlinear dielectric polymers as long as AE is small. Considering the similarity between Equations (16.1) and (16.12), an effective piezoelectric coefficient is introduced for electrostrictive polymers under a DC bias ... 

The elastic energy density characterize the elastic energy stored in the E-M materials and are defined as Wv = and Wg = where p is the density of the material. Wy is related to the volume of the device, while Wg is related to the mass of the device. A device made of an E-M material with a higher elastic energy density would have a smaller size/mass. As shown in Tables 16.1 and 16.2, the newly developed electrostrictive polymers exhibit a much higher elastic energy density than piezoelectric ceramics and polymers. 

The temperature has a very complicated influence on the E-M performance of polymers. The Young s modulus of polymer decreases with the temperature, but the temperature dependence of its dielectric properties can be very different. For some of the piezoelectric and electrostrictive polymers, there is a phase transition at temperatures close to room temperature. At the phase transition temperature, the dielectric permittivity reaches its maximum and the polymer exhibits a high dielectric loss. Additionally, at temperatures around the glass transition temperature, all polymers exhibit some dielectric relaxation and elastic relaxation, resulting in a frequency dependence of material properties and a high dielectric and elastic loss. 

Many electrostrictive polymers have been developed in the last decade [1] and these newly developed electrostrictive polymers exhibit a high electric-field-induced strain, as shown in Figiure 16.5a, where the maximum thickness strain response of the polymer at different fields is given. A typical relationship between the strain response and the electric field observed in these polymers is shown in Figure 16.5b. All these electrostrictive polymers are polar polymers that contain polar units in the polymer chain. The electrostrictive strain response reflects the change in these polar units due to an electric field. 

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