Quantitative study on the face shear piezoelectricity and its relaxation in uniaxially-drawn and annealed poly-l-lactic acid

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_g = 60 °C) to below melting (T_m = 170 °C) for 1 hour. The degrees of crystallinity X_c and orientation F_c by X-ray diffraction were controlled over a broad range to reach X_c = 0.8 and F_c = 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 e₁₄ and d₁₄, as well as the elastic shear compliance s₄₄ and stiffness c₄₄. At room temperature, e₁₄ was shown to be proportional to a product of X_c*F_c, whereas d₁₄ demonstrated saturation due to an increase in c₄₄. By extrapolating to X_c*F_c = 1, the e₁₄^c = 27 mC/m² 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 e₁₄ decreased in a similar manner to c₄₄ whereas d₁₄ increased slightly with increasing temperature. The temperature dispersions of e₁₄, d₁₄ and c₄₄ 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.