Flat-On Secondary Crystals As Effective Blocks To Reduce Ionic Conduction Loss In Polysulfone/Poly(Vinylidene Fluoride) Multilayer Dielectric Films

Recently, poly(vinylidene fluoride) (PVDF)- based multilayer films have demonstrated good potential as high energy density, high temperature, and low loss polymer dielectrics for advanced electrical and power applications. However, impurity ion conduction in the PVDF layers can cause significant dielectric loss at high temperatures. In this study, we discovered a facile melt-recrystallization method to suppress ionic conduction loss in polysulfone (PSF)/PVDF 50/50 (v/v) 33-layer films. By use of combined differential scanning calorimetry, broadband dielectric spectroscopy, and simultaneous small-angle X-ray scattering/wide-angle X-ray diffraction techniques, the underlying mechanism for the suppression of ionic conduction was unraveled. Basically, the growth and hierarchical organization of primary and secondary PVDF crystals confined in 400 nm layers played an important role. When the cooling rate during melt-recrystallization was high (e.g., >= 500 degrees C/min), small and poorly oriented secondary crystals between orderly stacked edge-on primary crystals allowed free transport of impurity ions in PVDF layers. At low to moderate cooling rates (i.e., <100 degrees C/min), growth of flat-on secondary crystals between the edge -on primary crystalline lamellae blocked the transport of impurity ions, suppressing the dielectric loss from ionic conduction. On the basis of this study, we propose a modified multilayer coextrusion method with controlled cooling rates to achieve flat -on secondary crystals for the reduction of high temperature dielectric loss in PVDF-based multilayer dielectric films.