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Chio-Srichan, Sirinart
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article
Quantitative study on the face shear piezoelectricity and its relaxation in uniaxially-drawn and annealed poly-l-lactic acid
Abstract
<p>Piezoelectric resonance spectroscopy was used to evaluate the face shear piezoelectricity and its relaxation for structurally controlled poly-L-lactic acid (PLLA) films. We prepared samples by uniaxial drawing at 80 °C for a ratio of 2–6 and annealed at temperatures above glass transition (T<sub>g</sub> = 60 °C) to below melting (T<sub>m</sub> = 170 °C) for 1 hour. The degrees of crystallinity X<sub>c</sub> and orientation F<sub>c</sub> by X-ray diffraction were controlled over a broad range to reach X<sub>c</sub> = 0.8 and F<sub>c</sub> = 0.9. We measured broadband dielectric spectra where the piezoelectric resonance was observed superimposed on dielectric relaxation. Analyses of the resonance spectra for 45°-cut square sample resulted in the determination of the face-shear piezoelectric constants e<sub>14</sub> and d<sub>14</sub>, as well as the elastic shear compliance s<sub>44</sub> and stiffness c<sub>44</sub>. At room temperature, e<sub>14</sub> was shown to be proportional to a product of X<sub>c</sub>*F<sub>c</sub>, whereas d<sub>14</sub> demonstrated saturation due to an increase in c<sub>44</sub>. By extrapolating to X<sub>c</sub>*F<sub>c</sub> = 1, the e<sub>14</sub><sup>c</sup> = 27 mC/m<sup>2</sup> of PLLA crystal was determined. As the temperature increases, piezoelectric relaxation due to non-crystalline segmental motion was observed as well as dielectric and elastic relaxation. It was found that e<sub>14</sub> decreased in a similar manner to c<sub>44</sub> whereas d<sub>14</sub> increased slightly with increasing temperature. The temperature dispersions of e<sub>14</sub>, d<sub>14</sub> and c<sub>44</sub> were reproduced using an equivalent three-spring model consisting of a crystalline piezoelectric spring connected by series and parallel non-crystalline relaxational springs based on the temperature-frequency reduction rule and the VTF-type dielectric relaxation time. The findings revealed key information on the ratio of noncrystalline phases connected in series and parallel to the oriented crystalline phase.</p>