Electrostrictive poly(vinylidene fluoride-trifluoroethylene) copolymers

High energy electron (1.0-2.55 MeV) irradiation was used to modify the phas e transitional behavior of poly(vinylidene fluoride-trifluoroethylene) (P(V DF-TrFE)) copolymers in an attempt to significantly improve the electromech anical properties of the copolymers. It is found that the copolymers under a proper irradiation treatment exhibit very little room temperature polariz ation hysteresis and very large electrostrictive strain (the longitudinal s train of -5% can be achieved). Because of the large anisotropy in the strai n responses along and perpendicular to the polymer chain, the transverse st rain can be tuned over a broad range by varying the film stretching conditi on. For unstretched films, the magnitude of transverse strain is approximat ely about/less than 1/3 of that of the longitudinal strain, and for stretch ed films, the transverse strain along the stretching direction is comparabl e to the longitudinal strain. In addition to the high strain response, the irradiated copolymers also possess high elastic energy density and mechanic al load capability as indicated by the relatively high elastic modulus of t he copolymer and the high strain response of the transverse strain even und er 40 MPa tensile stress. The high strain and high elastic modulus of the i rradiated copolymer also result in an improved electromechanical coupling f actor where the transverse coupling factor of 0.45 has been observed. The f requency dependence of the strain response was also characterized up to nea r 100 kHz and the results show that the high electromechanical response can be maintained to high frequencies. Several unimorph actuators were fabrica ted using the modified copolymer and the test results demonstrate high perf ormance of the devices due to the high strain and high load capability of t he material. (C) 2001 Published by Elsevier Science B.V.