As the core material in film capacitors

graft modification has a significant impact on the properties of polypropylene (PP). In order to study the influence of branched chain structure on the crystallization and dielectric energy storage properties of PP

maleic anhydride (MAH) and polyetherimide (PEI) were grafted by a solid-phase grafting method

and cast films and biaxial films were prepared based on the production process of capacitor films. The results show that the introduction of polar groups can enhance the polarity of PP and increase the dielectric constant. Meanwhile

the branched chain structure introduced by grafting modification can reduce the interplanar spacing of cast films

while the long branched chain structure after grafting PEI makes the entanglement of PP’s molecule chains tighter and keeps the interplanar spacing at the minimum. After biaxial stretching

PP’s molecular chains can untangle and be arranged more orderly

causing an increase in crystallinity

which is manifested as an increase in the DC breakdown strength. Under the tensile stress

the long branched chain structure after grafting PEI leads to the presence of entanglement sites in PP’s molecular chains

causing them to break and the arrangement to loose. As a result

the interplanar spacing increases

which makes the increase of DC breakdown strength relatively small. Considering the variations of dielectric constant and DC breakdown strength

the biaxially oriented film grafted with MAH exhibits the optimal energy storage properties. Its discharge energy density is increased by 29.8% compared to PP

and the charge and discharge efficiency remains above 90%. This research provides a certain guidance for the selection of grafting modification groups for PP.