Electrostrictive Properties of Poly(vinylidenefluoride- trifluoroethylene-chlorotrifluoroethylene) G. S. Buckley, †,‡ C. M. Roland,* ,† R. Casalini, § A. Petchsuk, | and T. C. Chung | Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, Chemistry Department, George Mason University, Fairfax, Virginia 22030, and Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 30, 2001. Revised Manuscript Received April 9, 2002 Terpolymers of vinylidene fluoride (VDF), trifluoroethylene (TrFE), and chlorotrifluoro- ethylene (CTFE) were synthesized as potential materials for electromechanical transduction. These terpolymers had relatively high molecular weights (∼30 kg/mol) and CTFE levels in the range of 5-10 mol %. The presence of the bulky CTFE units disrupts the sequence length of the crystal, which lowers both the melting and Curie transitions; however, the degree of crystallinity remains high. The formation of smaller, more mobile polar domains gives rise to good electromechanical response. At low electric fields (7 MV/m), longitudinal strains as high as 0.5% are attained. This is significantly higher than the strains achieved with the same terpolymer obtained by bulk polymerization. The present materials exhibit a low mechanical modulus (ca. 0.2 GPa) relative to other VDF-TrFE copolymers. This might limit their use, depending on the application. Introduction Piezoelectric and electrostrictive materials convert electrical energy into mechanical energy and have important advantages (e.g., low power consumption and fast response) over electromagnetic motors. Although electroactive ceramics are widely used in piezoelectric devices, polymeric materials are lower in weight, have greater toughness and better processibility. Thus, sub- stantial efforts have been made to develop electroactive polymers. The only commercially significant polymer is polyvinylidene fluoride (PVDF) and its copolymer with triflouorethylene [P(VDF-TrFE)], and most work has focused on them. A common feature of various approaches for improv- ing the electrostrictive properties of PVDF is reduction of the crystallite size through the introduction of defects. This can be accomplished through the formation of a network structure, either by radiation 1,2 or by chemical cross-linking, 3,4 or through quenching of the polymer from the amorphous state. 5 Smaller ferroelectric do- mains are more effectively oriented by an applied field, potentially yielding better electromechanical properties. Another method for achieving the same objective is incorporation of a disparate monomer into the polymer backbone. If this monomer is poorly accommodated within the unit cell, the crystal sequence length is reduced, yielding smaller, more easily oriented crystal- line structures. Some promising results have been obtained with terpolymers of hexafluoropropylene with VDF and TrFE. 5-7 An interesting variation is terpolymers of VDF and TrFE with chlorotrifluoroethylene (CTFE). The presence of the chlorine atom imposes a large steric hindrance, which favors the (ferroelectric) trans conformation of the polymer backbone (to alleviate steric repulsions between the chlorine and flourine atoms). 8,9 This is important because, in the trans conformation, the dipoles all add, enabling a higher polarization to be achieved. The electrical energy that can be stored in the material is directly proportional to this polarization and thus determines, in combination with the transduction ef- ficiency, the electromechanical performance. At the same time, the random placement of the CTFE units disrupts the polar coupling, resulting in more easily oriented domains. Recently, promising results were reported for bulk polymerized P(VDF-TrFE-CTFE) terpolymer. 10 In this paper, we describe characterization of the electro- mechanical properties of similar terpolymers syn- thesized using a novel borane/oxygen initiator. This process affords control over the monomer addition, yielding compositionally homogeneous (nonblocky) ma- terials. * Corresponding author. † Naval Research Laboratory. ‡ Permanent address: School of Science and Technology, Cameron University, Lawton, OK 73505. § George Mason University. | The Pennsylvania State University. (1) Daudin, B.; Dubus, M. J. Appl. Phys. 1987, 62, 994. (2) Zhang, Q. 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