IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 60, . 3, MARCH 2013 441 0885–3010/$25.00 © 2013 IEEE Electromechanical Properties of Relaxor Ferroelectric P(VDF-TrFE-CFE)-P(VDF-CTFE) Blends Lee J. Gorny, Sheng-Guo Lu, Sheng Liu, and Minren Lin Abstract—Electromechanical properties of the relaxor fer- roelectric poly(vinylidene fluoride–trifluoroethylene–chloroflu- oroethylene) [P(VDF-TrFE-CFE)] terpolymer blended with a small amount of poly(vinylidene fluoride-chlorotrifluoroethyl- ene) [P(VDF-CTFE)] copolymer, which possesses a much high- er elastic modulus than that of the neat terpolymer, were in- vestigated. It was observed that the presence of small amount of P(VDF-CTFE) does not affect the microstructure of the crystalline phase. However, the uniaxially stretched blended films show a slight increase in the crystallinity and increased or similar induced polarization at high electric fields compared with the neat terpolymer, likely caused by the interface effect. Consequently, for blends with P(VDF-CTFE) less than 5 wt%, the transverse strains S 1 along the stretching direction for uni- axially stretched blended films are nearly the same as those of neat P(VDF-TrFE-CFE), whereas the elastic modulus along the S 1 -direction increases with the P(VDF-CTFE) content. As a result, the blended films exhibit a higher elastic energy den- sity and electromechanical coupling factor k 31 compared with the neat terpolymer. I. I E  polymer materials which can generate high strain with high elastic energy density are attrac- tive for a broad range of applications [1]–[4]. This paper investigates a polymer blend approach to further improve the electromechanical response of the relaxor ferroelec- tric poly(vinylidene fluoride–trifluoroethylene–chlorofluo- roethylene) [P(VDF-TrFE-CFE)] terpolymer. P(VDF- TrFE)-based relaxor ferroelectric polymers exhibit large electrostrictive strains with high elastic energy densi- ties [5]–[9]. In this paper, it will be shown that blending P(VDF-TrFE-CFE) relaxor ferroelectric terpolymer with a small amount of P(VDF-CTFE) [chlorotrifluoroethylene (CTFE)] 91/9 mol% copolymer results in polymer films having an elastic modulus that is nearly doubled along the polymer film stretching direction. Transverse strains S 1 obtainable in the same direction of the blended films are similar to the neat terpolymer. Consequently, the elastic energy density and electromechanical coupling factor are improved compared with the neat terpolymer. It has also been observed that the blended films exhibit higher elec- trical breakdown strengths, which are also highly desirable for reliable electromechanical applications. II. E P(VDF-TrFE-CFE)/P(VDF-CTFE) blended films with 0, 2.5, 5, and 10 wt% copolymer were prepared using a so- lution-cast method. P(VDF-TrFE-CFE) 63/37/7.5 mol% terpolymers were synthesized by a suspension polymeriza- tion process and copolymer P(VDF-CTFE) 91/9 mol% was purchased from Solvay (Brussels, Belgium) [10]. Ter- polymer and copolymer solutions were mixed together by proper ratio (using N, N-dimethyl formamide as a sol- vent). The mixed solution was cast on a glass plate and dried at 70°C for 5 h. The green films were then stretched uniaxially to in- crease the polar phase and improve the electromechani- cal response. For most electromechanical applications using polymer films, the useful material characteristics are strain, elastic energy density, and electromechanical coupling factor. Stretching these films increases the trans- verse strain, elastic modulus along the stretching direc- tion, and breakdown strength markedly. After stretching, films were further annealed at 100°C for 15 h in a vacuum oven. This vacuum annealing process further removes the residual solvent and improves the crystallinity. Normal film thickness for this study is 25 μm after stretching. In this study, the electromechanical response of the blended films along the stretching direction with differ- ent stretching ratios was investigated. It was found that the transverse strain S 1 along the stretching direction in- creases with stretching ratio to about 4 to 5 times the original film length. Further stretching the film beyond that stretching ratio does not increase the strain levels un- der fields lower than 150 MV/m nor the elastic modulus. Hence, this paper will focus on the uniaxially stretched blended films with 5 times stretching ratio (the film is uniaxially stretched to 5 times the original length). Data presented in this paper were obtained using the following equipment. The microstructure of the blends was studied using X-ray diffraction (XRD) (Scintag Cu K α diffractometer, Scintag Inc., Cupertino, CA). Differen- Manuscript received February 2, 2011; accepted December 5, 2012. This work was supported by the National Institutes of Health under grant number R01-EY018387-02. L. J. Gorny, S.-G. Lu, and M. R. Lin are with the Materials Research Institute, The Pennsylvania State University, University Park, PA (e- mail: sglu@gdut.edu.cn). L. J. Gorny is also with the Mechanical and Nuclear Engineering De- partment, The Pennsylvania State University, University Park, PA. S.-G. Lu is also with the School of Materials and Energy, Guangdong University of Technology, Guangzhou, China. S. Liu is with the Electrical Engineering Department, The Pennsylva- nia State University, University Park, PA (e-mail: sul26@psu.edu). DOI http://dx.doi.org/10.1109/TUFFC.2013.2587