The effects of high-energy proton irradiation on the structure and properties of 56/44 mol % poly(vinylidene fluoride-trifluoroethylene) copolymer were studied with differential scanning calorimetry (DSC), X-ray diffraction (XRD), relative permittivity, and polarization hysteresis measurements. Copolymer films prepared by hot compression molding were irradiated with a broad range of proton dosages (10107 Mrad) at room temperature. The DSC results showed that the ferroelectric transition was strongly affected by the proton dosages. The XRD data indicated the reduction of polar ordering in the copolymer by the proton-irradiation treatment. From the relative permittivity and polarization behavior, the copolymer film was found to be converted from a normal ferroelectric material to a relaxor ferroelectric material as the proton dosage was increased to 50 Mrad. The electrostrictive coefficient of the 56/44 mol % copolymer was enhanced after irradiation, and the optimized proton dosage for attaining the highest electrostrictive strain response was determined. (c) 2005 Wiley Periodicals, Inc.