Simulations of High-Strain Electrostrictive Chlorinated Terpolymers
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Simulations of High-Strain Electrostrictive Chlorinated Terpolymers George J. Kavarnos and Thomas Ramotowski1 Department of Chemistry, University of Rhode Island, Kingston, RI 02881, U.S.A. 1 Transduction Materials Branch, Naval Undersea Warfare Center, Newport, RI 02841, U.S.A.
ABSTRACT Chlorinated poly(vinylidene fluoride/trifluoroethylene) terpolymers are remarkable examples of high strain electrostrictive materials. These polymers are synthesized by copolymerizing vinylidene fluoride and trifluoroethylene with small levels of a third chlorinated monomer. The electromechanical responses of these materials are believed to originate from the chlorine atom, which, by its presence in the polymer chains and by virtue of its large van der Waals radius, destroys the long-range crystalline polar macro-domains and transforms the polymer from a normal to a high-strain relaxor ferroelectric. To exploit the strain properties of the terpolymer, it is desirable to understand the structural implications resulting from the presence of the chlorinated monomer. To this end, computations have been performed on model superlattices of terpolymers using quantum-mechanical based force fields. The focus has been on determining the energetics and kinetics of crystallization of the various polymorphs that have been identified by x-ray diffraction and fourier transform infrared spectroscopy. The chlorinated monomer is shown to act as a defect that can be incorporated into the lamellar structures of annealed terpolymer without a high cost in energy. The degree of incorporation of the chlorinated monomer into the crystal lattice is controlled by annealing conditions and ultimately determines the ferroelectric behavior of the terpolymers. INTRODUCTION Polyvinylidene-trifluoroethylene [P(VDF-TrFE) where VDF is vinylidene fluoride and TrFE is trifluoroethylene] with low levels of chlorine can be processed into high-strain films by solvent casting and annealing.1,2 The presence of chlorine in these terpolymers evidently exerts a profound effect on their mechanical behavior, converting them from normal ferroelectric into electrostrictive polymers. Electrostriction is a property exhibited by many materials where under high electric fields, the electromechanical response can be described by equation 1:3 S = Q ⋅ P2
(1)
Where S is the strain response, Q is the electrostrictive coefficient, and P is the polarization. The presence of chlorine is believed to play a significant role since, by virtue of its large van der Waals radius, i. e., 1.81 Ǻ vs. 1.35 Ǻ for fluorine and 1.2 Ǻ for hydrogen, it influences the kinetics of crystallization from the melt, the degree of crystallization, as well as the morphology of the crystallite structures. Unlike P(VDF-TrFE) films, which can be processed and annealed into highly crystalline polar all-trans crystallites (Form I) that exhibit normal ferroelectric
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Figure 1. Chain packing in Forms I (left), II (middle), and III (right). Large filled circles represent fluorine atoms. Small unfilled circl
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