Temperature-Dependent Rotational Dipole Mobility and Devitrification of the Rigid Amorphous Fraction in Unpoled and Poled Biaxially Oriented Poly(vinylidene fluoride)

• Published in: Macromolecules 2022-11, Vol.55 (21), p.9705-9714
• Main Authors: Rui, Guanchun, Allahyarov, Elshad, Thomas, John J., Taylor, Philip L., Zhu, Lei
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.2c01110

In addition to the pronounced ferroelectric property from polar crystalline phases, poly­(vinylidene fluoride) (PVDF) and its random copolymers also exhibit high dielectric permittivities as a result of highly mobile dipoles in the amorphous phase. In this study, we developed a new theoretical approach to determine the dipole concentration, dipole moment, dipole–dipole interaction, and rotational dipole mobility for PVDF by means of broadband dielectric spectroscopy. From the permittivity of molten PVDF, the Kirkwood–Fröhlich g-factor and its temperature dependence were determined and used as global parameters for the simulation of unpoled and highly poled biaxially oriented PVDF (BOPVDF). It was found that the concentration of active dipoles substantially increased as the temperature was increased from −30 to 40 °C, indicative of the devitrification of the rigid amorphous fraction (RAF). Both the calculated dipole moment and the g-factor of poled BOPVDF were higher than those of unpoled BOPVDF, resulting in a higher permittivity and piezoelectric performance. In addition, the dipole–dipole interaction was found to increase substantially upon increasing the temperature from −30 to 40 °C, leading to an increase of over 4 orders of magnitude in the rotational dipole mobility. This was direct evidence for the devitrification of the RAF in both unpoled and poled BOPVDF.