Ultra-High Strain Response of Elastomeric Polymer Dielectrics
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elastomers and has performance comparable to that of natural muscle, we refer to this approach as electrostrictive polymer artificial muscle (EPAM). This paper is organized as follows: We first describe the basic principal of operation of the technology. Next, we describe the measured performance of several materials, including a recently identified acrylic that is capable of extremely large strains (more than 200%). We then describe issues related to the fabrication of devices based on this technology. Next, the design, fabrication, and performance of specific actuator embodiments are described. Finally, we summarize and discuss the potential applications of this technology and the research challenges that remain. Backeround
The principle of operation of the EPAM technology is shown in Figure 1. An elastomeric polymer is sandwiched between two compliant electrodes. When a voltage difference is placed across the top and bottom electrodes, the polymer is squeezed in thickness and stretched in area. We have previously shown that the principal cause of this stress condition and the resultant deformation of the polymer is the electrostatic forces of the free charges on the electrodes.'" It is useful to introduce an analytical model that relates the observed stresses and strains to the applied voltage. V
Polymer Film Compliant El
(on
Top and Bottom Surfaces)
(a) Voltage Off
(b) Voltage On
Figure 1. Principle of Operation of EPAM; Film Expands in Area and Contracts in
Thickness The derivation of the electrostatic model is described by Pelrine, Kormbluh, and Joseph.15
The actuation pressure, p, is given by p = f F, E' = , co (V/z ,(1) where E is the electric field, e is the dielectric constant, co is the permittivity of free space, Vis the voltage, and z is the polymer thickness. Note that this pressure is greater by a factor of 2 than that arising from the commonly used equation for Maxwell's stress in a dielectric of a rigid plate capacitor. The greater pressure is due to the compliance of the electrodes, which allows both the forces of attraction between the oppositely charged electrodes and the forces tending to separate the charges on each electrode to couple into the effective pressure normal to the plane of the film. For small strains with free boundary conditions, the polymer thickness strain, sz, is given by (2) S, = -p/y = _F, & (V/z)2/y , where Yis the modulus of elasticity. The model for large strains with more realistic constrained boundary conditions, such as those required to drive a load, is more complex. However, this simple case illustrates the influence of the electrical and mechanical properties of the polymer on
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actuation performance. The model also assumes that the elastomer is an ideal rubber, that is, that the rubber is incompressible and has a Poisson's ratio of 0.5. One of the more useful metrics for comparing actuator materials, independent of size, is the energy densities of the materials. The actuator energy density is the maximum mechanical energy output per cycle and per unit
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