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May 26, 2004 Influence of Crystallization Conditions on the Microstructure and Electromechanical Properties of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) TerpolymersR.J. Klein1, J. Runt1,2, and Q.M. Zhang1,2,3 The crystallization process and its influence on the microstructure, ferroelectric, and electromechanical properties of a poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) terpolymer were investigated. By varying the isothermal crystallization temperature Tx of this terpolymer, the ratio of ferroelectric crystal concentration to the concentration of relaxor ferroelectric crystal (a defect-modified class of ferroelectrics exhibiting strong electrostrictive behavior) can be determined. The higher Tx samples show an increased polarization hysteresis and reduced electric field-induced strain response. In addition, the experimental results, in combination with other reported results, indicate that CFE units are included in the crystalline lattice. Consequently, the influence of CFE on the ferroelectric behavior of the polymer is through the defects it induces in the crystal lattice.
X-ray data were collected in order to interrogate interchain spacing and, via X-ray peak width, the crystalline order perpendicular to the chain direction. For PVDF-TrFE copolymers in the ferroelectric phase, this peak corresponds to the (110, 200) reflection. The data were acquired at selected temperatures between room temperature and Tx (taken on heating) in order to follow the evolution of the microstructure. The X-ray data in the 2θ range of the (110, 200) reflection acquired at room temperature for terpolymer films with different Tx are shown in Figure 1a.
The importance of these X-ray peak assignments is seen in the electromechanical strain data for samples at the extremes of the Tx range. As shown in Figure 2, the terpolymer sample with a Tx of 112°C exhibits a field-induced strain of -5.9% at 133 MV/m (megavolts per meter), while for the sample with Tx of 142°C, the strain is reduced to -4.2% under the same field. The lower field-induced strain is due to existing ferroelectric domains that do not undergo the local conformation change upon application of the electric field. Thus, for better performance, a sample should be annealed near 112°C. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
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Polymers for electromechanical applications offer many unique and
inherent advantages when compared with other materials, being
lightweight, flexible, and relatively easy to process and form into
complicated shapes or large areas. We have examined one of these
terpolymers, P(VDF-TrFE-CFE), and investigated how its ferroelectric
and electromechanical responses are influenced by polymer
crystallization processes in tandem with corresponding
microstructure changes. We found that, by varying the
crystallization temperature Tx, we can selectively vary the fraction
of polar and non-polar crystallites in the crystalline phase.
Crystallites formed during long periods of time at Tx exhibit less
polar behavior. Crystallites formed during the rapid cooling process
between Tx and room temperature exhibit stronger polar behavior.
This is established most quantitatively by wide-angle X-ray
diffraction (WAXD) and reinforced by Fourier transform infrared (FT-IR)
spectroscopy, polarization loop, and electromechanical strain
measurements. As will be shown, crystallizing the terpolymer at
higher Tx values has an adverse effect on its electromechanical
strain response. 
For the terpolymer films with Tx at 112, 122, and 127 °C, the
X-ray data are relatively well characterized by a single peak at
14.7°, which represents the non-polar phase. The data fitting can be
improved, however, by including a small peak at 15.3°, which is
close to the position of the diffraction peak expected for the polar
phase of the terpolymer’s corresponding copolymer, PVDF-TrFE. As Tx
is raised to 132 °C, this higher angle shoulder increases quite
markedly, and the best fitting to the data is achieved by including
two peaks, one near 15° and the other at 15.4°. Figure 1b summarizes
the evolution of the X-ray peak positions as Tx is raised from 112
to 142 °C. In comparison with the X-ray data for PVDF-TrFE, we
deduced that the peak at the lower angle arises from the non-polar
phase while the peaks at 15° and 15.4° indicate the presence of the
polar phase component in the crystallites.