Evolution of Property and Microstructure of P(VDF-TrFE) Copolymers Modified by Irradiation Introduced Defects
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Evolution of Property and Microstructure of P(VDF-TrFE) Copolymers Modified by Irradiation Introduced Defects Z.-Y. Cheng ([email protected]) and Zhimin Li Materials Research and Education Center, Auburn University, Auburn, AL 36849 Yanyun Ma and Q.M. Zhang The Pennsylvania State University, Materials Research Institute, University Park, PA 16802 Fred B. Bateman Radiation Interactions and Dosimetry, NIST, Gaithersburg, MD 20899, U.S.A ABSTRACT The effect of defects introduced by high-energy electron irradiation on microstructure and properties in poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] is reported. In studies of the copolymers, it is found that as defect concentration increases, the material can be changed from a normal ferroelectrics to a relaxor ferroelectrics (RFE) and then to a simple relaxor. Correspondingly, the crystalline morphology changes from a coexistence of polar and non-polar phases to a macroscopically uniform non-polar phase, as revealed by x -ray data. It was observed that the dielectric property in the copolymers with a different amount of defects was well described by the Vogel-Fulcher (V-F) relationship. Based on the experimental data, a critical size, which is the smallest size of crystal with ferroelectric phase, of about 5 nm was obtained for the copolymer. The RFE developed here exhibits a massive electrostrictive strain which is very attractive for many actuator and transducer applications and a high dielectric constant which is attractive for development of high-density energy storage capacitors and electronic packaging. INTRODUCTION High-energy irradiation is widely used in the polymer industry to modify material properties by means of introducing defects, such as double bonds, bulky pedant group, crosslinking, etc [1]. The effects of high-energy irradiation on piezoelectric properties, crystal structures, and polymer morphology of P(VDF-TrFE) were investigated [2]. P(VDF-TrFE) copolymers are normal ferroelectrics for compositions containing more than 50% VDF and less than 82% VDF. In this range of VDF content, the copolymer exhibits a typical ferroelectric-to-paraelectric (F-P) phase transition at a temperature lower than the melting point [3]. The ferroelectricity observed comes from the crystalline region in this semicrystalline copolymer. It is recently found that the P(VDFTrFE) copolymer can be converted to relaxor ferroelectrics (RFE) from normal ferroelectrics (FE) by means of high-energy electron irradiation. RFE phenomena have been observed in various inorganic ceramics and crystals and have been extensively studied due to the fact that RFE exhibit a very high dielectric constant and large strain response under an electric field, attractive for many applications. From a physics point of view, RFE are a partially ordered system, which is a very challenging research area in modern condensed matter physics [4,5]. However, it is very interesting to note that RFE behavior was not found in any organic/polymer system. Thus, the RFE phenomenon observed in de
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