Influence of Tailored Blend Morphology on Resulting Dielectric Properties in PP-Cast Film

The investigated approach to increase storage capacity while not compromising dielectric loss or processability is to compliment the properties of a commonly used dielectric polymer, isotactic polypropylene (PP) with a material with higher permittivity, polyvinylidenfluorid (PVDF), in the blend phase. It is known that by a combination of materials with different dielectric constants a common mean value can be achieved. Further, the effect of the shape of inclusions on the resulting permittivity is described in literature. Specifically modified morphologies were processed by shifting the blends into narrow shear rate ranges using skin-layers in multilayer cast film. The complex viscosities were measured to select specific viscosity ratios in the blends and multilayer films. The development of the morphology was examined in extruded cast film via laser scanning microscopy (LSM) and dark field microscopy in respect to filler content, viscosity ratios and layer-structure. The shown rheological influence on the morphology is in good agreement with the theoretical expectations. The elongation of blend particles was suppressed when extruding blends in the core of multilayer cast films with low viscosity skin-layers. The resulting relative permittivity of blends was calculated using the Maxwell-Garnett approximation with form factors that take into account how the structure and internal geometry of the mixture affect permittivity. The measured relative permittivity increased for specimen with a mainly spherical blend phase, which is in good agreement with the mathematical EMA model.