(Dr. Jacek Jarzynski, advisor)
"Investigation of the Electromechanical Properties of Electrostrictive Polymers"
Piezoelectric polymers such as polyvinylidene fluoride (PVDF) and its copolymers are widely used in electromechanical applications such as sonars, actuators and loudspeakers. Another type of polymer, a polyurethane, has recently received some attention for its potentially more advantageous use in similar applications, because of the large electrostrictive strains that it exhibits. However, the electrostrictive behavior of this material has not been completely characterized and the fundamental mechanisms responsible for electrostriction in polyurethanes are poorly understood. Furthermore, the existing empirical techniques to measure electrostrictive coefficients cannot be adequately used on polyurethanes because the very soft nature of the materials makes measurements very sensitive to experimental conditions. The goals of this thesis are (1) to develop a new method suitable for measurements of electrostrictive coefficients of polyurethanes, (2) to use this method to quantify the electrostrictive behavior of several types of polyurethanes, and (3) to use this data to gain new insights into the origins of electrostriction in these materials.
In the new method, electrically-induced strains in a rubber-encapsulated polymer film are optically measured by a laser Doppler vibrometer, and the results of these measurements are input into a Rayleigh-Ritz energy minimization procedure implemented symbolically in MATHCAD. This technique yields the values of the three tensile electromechanical coefficients of the polymer, and can be used both for piezoelectric and electrostrictive materials. The method is first validated on PVDF, and it is then applied to the characterization of four structurally different polyurethanes. It is found that, at the macroscopic level, electrostrictive coefficients are directly related to the dielectric and the elastic properties of the materials, and that, at the microscopic level, electrostriction is most consistent with the space-charge theory. Contributions of this thesis include: (1) the development of an original tool for polymer measurements; (2) the measurement of a complete set of true electrostrictive coefficients on four types of polyurethanes; (3) a better understanding of the phenomenon of electrostriction in polyurethanes.