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

• Main Authors: Zin, S. H. Mat, Velayutham, Thamil Selvi, Furukawa, T., Kodama, H., Gan, W. C., Chio-Srichan, Sirinart, Kriechbaum, M., Nakajima, T.
• Format: Article
• Published: Elsevier Sci Ltd 2022
• Subjects: QC Physics
• Online Access: http://eprints.um.edu.my/41858/

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 degrees C for a ratio of 2-6 and annealed at temperatures above glass transition (Tg = 60 degrees C) to below melting (Tm = 170 degrees C) for 1 hour. The degrees of crystallinity Xc and orientation Fc by X-ray diffraction were controlled over a broad range to reach Xc = 0.8 and Fc = 0.9. We measured broadband dielectric spectra where the piezoelectric resonance was observed superimposed on dielectric relaxation. Analyses of the resonance spectra for 45 degrees -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 Xc*Fc, whereas d14 demonstrated saturation due to an increase in c44. By extrapolating to Xc*Fc =1, the e14 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 temperaturefrequency 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.