Semicrystalline Structure–Dielectric Property Relationship and Electrical Conduction in a Biaxially Oriented Poly(vinylidene fluoride) Film under High Electric Fields and High Temperatures

Abstract

Poly(vinylidene fluoride) (PVDF)-based homopolymers and copolymers are attractive for a broad range of electroactive applications because of their high dielectric constants. Especially, biaxially oriented PVDF (BOPVDF) films exhibit a DC breakdown strength as high as that for biaxially oriented polypropylene films. In this work, we revealed the molecular origin of the high dielectric constant via study of a commercial BOPVDF film. By determination of the dielectric constant for the amorphous phase in BOPVDF, a high value of ca. 21-22 at 25 °C was obtained, and a three-phase (i.e., lamellar crystal/oriented interphase/amorphous region) semicrystalline model was proposed to explain this result. Meanwhile, electronic conduction mechanisms in BOPVDF under high electric fields and elevated temperatures were investigated by thermally stimulated depolarization current (TSDC) spectroscopy and leakage current studies. Space charge injection from metal electrodes was identified as a major factor for electronic conduction when BOPVDF was poled above 75 °C and 20 MV/m. In addition, when silver or aluminum were used as electrodes, new ions were generated from electrochemical reactions under high fields. Due to the electrochemical reactions between PVDF and the metal electrode, a question is raised for practical electrical applications using PVDF and its copolymers under high-field and high-temperature conditions. A potential method to prevent electrochemical degradation of PVDF is proposed in this study.

Primary and secondary crystallization of fast-cooled poly(vinylidene fluoride) studied by Flash DSC, wide-angle X-ray diffraction and Fourier transform infrared spectroscopy

“…In Fig. (b), FTIR spectra show strong bands at 975, 795, 764 and 615 cm −1 , which belong to α‐phase crystals as well . Apparently, PVDF crystallites formed at low temperatures upon fast cooling still exhibit the dominant α‐phase, as their diffraction patterns and FTIR spectra remain almost unchanged in comparison to the crystallites yielded under slow cooling.…”

Rational Co‐Design of Polymer Dielectrics for Energy Storage

“…Biaxially oriented polypropylene (BOPP), with a high breakdown field of about 700 MV m −1 and a dielectric constant of about 2.2, is the current state‐of‐the‐art polymer dielectric in high‐energy‐density (metalized) film capacitors. Attempts to improve upon BOPP have been based on poly(vinylidene fluoride) (PVDF) and its copolymers, polymer nanocomposites, multilayers, and so on. All such potential replacements have suffered from one or another weakness, like high loss or low parallel‐plate breakdown field.…”

Characteristic analysis of biaxially stretched PVDF thin films

“…After stretching, the biaxially oriented PVDF thin film would have balanced piezoelectric activities on the film plane compared to uniaxially oriented one, that would preserve larger piezoelectric coefficient . The highest β‐phase and piezoelectric response ( d 33 ) of biaxially stretched PVDF film was found with stretch ratio of R = 4 × 4 . The aim of this article is to fabricate the PVDF thin film by the equi‐biaxial stretching process in both stretching directions with same stretch ratio; then characterize and evaluate the function of the biaxially stretched PVDF thin film by means of various equipment, and make further comparison with the results of uniaxial stretching under different stretch ratio as well as temperature.…”