The experimental method used in this study was described
in detail in previous publications.17,18 Its main
features are briefly summarized next. Small square
polymer films (3 mm2) were sandwiched between an
aluminum plate (acting as a rigid backing and as an
electrode) and a piece of aluminum foil (acting as a
moving electrode) with a soft silicone rubber. The
samples were subjected to an alternating-current driving
voltage combined with a large direct-current bias
voltage, which induced electromechanical strains.
These strains were optically measured in three directions
by a laser Doppler vibrometer and the data were
input into a Rayleigh–Ritz energy minimization procedure
implemented symbolically in Mathcad. By considering
the appropriate energy functional (which includes
elastic, electrostrictive, and electrostatic contributions),
the method provides the three tensile
electrostrictive strain coefficients of the polymer