Giant Electrostrictive Response in Poly(vinylidene-fluoride hexafluoropropylene) copolymer
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Giant electrostrictive strains were first observed in a polyurethane elastomer [1]. The polyurethane exhibited strains greater than 3% under electric fields of up to 40 MV/m and an elastic modulus on the order of 0.01 GPa. Recently, strains up to 4% were observed in a copolymer of PVF 2, Poly(vinylidene fluoride/ trifluoroethylene), [P(VF 2/TrFE)] upon application of electric fileds up to 150 MV/m. These films were subjected to a two step process in which the materials were first melt-pressed and slow cooled and then irradiated with a high energy electron beam [2]. The irradiated P(VF 2/TrFE) films were observed to have an elastic modulus of approximately 0.4 GPa. In the present study, the electrostrictive strains were measured in poly(vinylidenefluoride/hexafluoropropylene), P(VF2 /HFP), random copolymers under electric fields up to 60 MV/m. The strain response of ice water quenched, air quenched and slow cooled samples was compared for 5% and 15% HFP copolymer compositions. The thickness strain constant, dt, was then calculated from the strain vs. electric field curve. Dielectric constant of the materials, elastic modulus, d 31 and e31 were studied, as well as the material's melting behavior and crystallinity observed from DSC and X-Ray Diffraction data.
61 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
EXPERIMENTAL Films of the 5% and 15% mol % HFP copolymers of PVF 2/HFP were prepared in a Carver Laboratory Press@. The copolymers were obtained in pellets from Soltex, and converted then into powder in a Freezer Mill. The powder was melted at 190 'C and pressed at 4,000 psi. For each composition, three different thermal treatments were used: ice water quench, air quench and slow cool. All samples were 50 to 60 jim in thickness and were cut into strips of 3 x 2 cm. Two strips of the same film were cut for each kind of sample. Gold electrodes 30 nin in thickness were deposited on the two sides of the films using a Sputter Coater EMS 650. The electrode area was 2.5 x 1.5 cm. The two films were placed on the sides of an air-gap capacitor, sandwiched between the capacitor plates (Fig.1). One electrode of each film was connected to the high voltage supply and the other electrode was grounded. Electrostrictive strains were measured as a function of dc electric field up to 60 MV/m using the air-gap capacitor.
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Fig. 1- Schematic of the air-gap capacitance system. Air-gap electrode area is 4.0 cm 2 . The capacitance of the air-gap, measured using an HP 4192A Impedance Analyzer, was then related to the strain change in the film. The measured capacitance is related to the air-gap thickness through equation 1. [1]
t = co.A/C
where co is the permittivity of air, A is the air-gap electrode area and C is the measured capacitance. Given the arrangement of the polymer films sandwiched between the capacitor plates, the air-gap thickness is
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