Electrostrictive poly(vinylidene ̄uoride-tri ̄uoroethylene) copolymers

High energy electron (1.0±2.55 MeV) irradiation was used to modify the phase transitional behavior of poly(vinylidene ̄uoridetri ̄uoroethylene) (P(VDF-TrFE)) copolymers in an attempt to signi®cantly improve the electromechanical properties of the copolymers. It is found that the copolymers under a proper irradiation treatment exhibit very little room temperature polarization hysteresis and very large electrostrictive strain (the longitudinal strain of ÿ5% can be achieved). Because of the large anisotropy in the strain responses along and perpendicular to the polymer chain, the transverse strain can be tuned over a broad range by varying the ®lm stretching condition. For unstretched ®lms, the magnitude of transverse strain is approximately about/less than 1/3 of that of the longitudinal strain, and for stretched ®lms, the transverse strain along the stretching direction is comparable to the longitudinal strain. In addition to the high strain response, the irradiated copolymers also possess high elastic energy density and mechanical load capability as indicated by the relatively high elastic modulus of the copolymer and the high strain response of the transverse strain even under 40 MPa tensile stress. The high strain and high elastic modulus of the irradiated copolymer also result in an improved electromechanical coupling factor where the transverse coupling factor of 0.45 has been observed. The frequency dependence of the strain response was also characterized up to near 100 kHz and the results show that the high electromechanical response can be maintained to high frequencies. Several unimorph actuators were fabricated using the modi®ed copolymer and the test results demonstrate high performance of the devices due to the high strain and high load capability of the material. # 2001 Published by Elsevier Science B.V.