Implementing EAP materials as actuators requires the availability of a properties database and scaling laws to allow the actuator or transducer designer to determine their response at the operational conditions. A metric for the comparison of these materials' properties with other electroactive materials and devices is needed to support users in the implementation of these materials as actuators of choice. In selecting characterization techniques it is instructive to look at the various classes of electroactive Polymers and the source of their strain-field response. Generally, two main classes can be identified [Chapter 1 and Topic 3]:
(1) Electronic EAP materials â These are mostly materials that are dry and are driven by the electric field or Coulomb forces. This category includes piezoelectrics, and electrostrictive and ferroelectric materials. Generally these materials are polarizable with the strain being coupled to the electric displacement. The strain of electrostrictive and ferroelectric materials is proportional to the square of the polarization or electric displacement. In piezoelectric materials the strain couples linearly to the applied field or electric displacement. Charge transfer in these materials is in general electronic and at dc fields they behave as insulators. These properties have been studied for over a century in single crystals and for over three decades in polymers.
(2) Ionic EAP materials â These materials contain electrolytes and they involve transport of ionsâmolecules in response to an external electric field. Examples of such materials include conductive polymers, IPMCs, and ionic gels. The field controlled migration or diffusion of the various ionsâmolecules results in an internal stress distribution. These internal stress distributions can induce a wide variety of strains, from volume expansion or contraction, to bending. In some conductive polymers the materials exhibit both ionic and electronic conductivities. These materials are relatively new as actuator materials and have received much less attention in the literature than the piezoelectric and electrostrictive materials. At present, due to a wide variety of possible materials and conducting species, no generally accepted phenomenological model exists and much effort is underway to determine the commonalities of the various materials systems. A clearer understanding of the characterization techniques would help immensely in determining underlying theories and scaling laws for these actuator materials.
15.2 Characterization of EAP with Polarization-Dependent Strains
A significant body of knowledge is available for the characterization of polar polymer electromechanical materials.