Relationship Between Structure and Dielectric Properties of Bi-oriented Isotactic Polypropylene Films for Capacitors

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semi-crystalline bioriented isotactic polypropylene (BOiPP) films is investigated using Wide Angle X-ray Diffraction (WAXD) and Polarized Optical Microscopy (POM). A beta-orthorhombic structure, with a crystallinity ratio of about 46%, and a "Crater" morphology of the beta-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy (BDS) over a wide temperature range (-150 degrees C to 125 degrees C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a a-relaxation peak associated to the glass transition temperature (Tg) at low frequency and temperature about -7 degrees C, and a broad beta*-relaxation at high frequency at about -60 degrees C, attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 mu m to 11.8 mu m) is correlated and explained by the increase of crystallinity ratio at larger thicknesses.