Effect of Biaxial Stretching on the Dielectric Performance of Long-Chain Branched Polypropylene for Metallized Film Capacitors

In this paper, the polypropylene after long chain branching was subjected to simultaneous biaxially stretching at different multiplicities to form biaxially oriented modified PP films. Compared with pure PP, LCBPP has more molecular chain entanglement points, which can inhibit the thermal motion of molecular chains at high temperatures, so it can improve the thermal stability of dielectrics. However, the tangling of molecular chains also suppresses the development of perfect spherical crystals, which limits the performance of LCBPP at room temperature. Higher multiplicity of biaxial stretching can optimize the connections between spherical crystals by stress-induced crystallization and promote the regular arrangement of molecular chains to form dense crystals. The compact aggregated state structure reduces the average free volume and suppresses the acceleration process of carriers within the free volume, so the stretched samples exhibit superior breakdown properties. Therefore, molecular orientation based on the introduction of long-chain branched structures allows further improvement of the dielectric properties of the films.