Crystallization Regulation by Functionalized Norbornene toward High-Temperature Energy Storage Characteristics of Polypropylene

Newly emerging electronic and electrical devices are placing increased demands on biaxially oriented polypropylene (BOPP) for enhanced high-temperature energy storage performance in dielectric film capacitors. Herein, polypropylene (PP) is modified with functionalized norbornene, nadic anhydride (NA), using a soft solid-phase grafting method to produce NA-grafted PP (PP-g-NA). The NA enhances the dipole moment and introduces deep traps for the electrons. Unexpectedly, the strong van der Waals interactions between PP and NA not only restrict the motion of molecular chains but also result in grain refinement while maintaining high crystallinity in the modified BOPP after blending with 10 wt % PP-g-NA. It leads to the formation of more micrograin boundaries, thus further enhancing the scattering effect on charge carriers. Both effects improve the DC breakdown strength of BOPP up to 738.96 kV/mm at room temperature and 456 kV/mm at 125 °C. Due to the simultaneous increases in dielectric constant and breakdown strength, modified BOPP demonstrates a 250.7% increase in discharge energy density of 2.49 J/cm³, while maintaining a discharge efficiency of 96.0% at 125 °C. This work proposes a strategy to modify PP by manipulating its grain characteristics toward excellent high-temperature energy storage performance.