The Effect of Annealing on the Breakdown Strength of BOPP Dielectric Films

The advancement of high‐energy‐density capacitors necessitates dielectric films with superior breakdown strength. Although enhancing crystallinity is a common strategy to improve dielectric breakdown performance, it is not the sole determinant of breakdown strength. This work systematically investigates the effect of annealing (60°C–140°C) on the microstructure and electrical breakdown behaviour of biaxially oriented polypropylene (BOPP) films. Annealing above 110°C is found to not only increase crystallinity and lamellar thickness but also promotes a rearrangement of the amorphous phase. A combination of in situ fourier transform infrared spectroscopy (FTIR), low‐field solid‐state nuclear magnetic resonance (LF‐SSNMR), and Positron annihilation lifetime spectroscopy (PALS) reveals that annealing reduces free volume and significantly suppresses molecular chain mobility in the amorphous phase, as the annealing temperature raise to 140°C, the fractional free volume decreased from 0.15% to 0.09%, and the segment mobility was significantly suppressed. Consequently, the energy storage density has increased from 1.27 J/cm 3 to 2.7 J/cm 3 , and the charge‐discharge efficiency of the 140°C‐annealed film still reached as high as 94.4%, meanwhile, the breakdown filed strength of the metallised 140°C‐annealed film has also increased by 12.6% (≈67 MV/m). These findings demonstrate that amorphous phase mobility including mobility of functional groups, in addition to crystallinity, is a dominant factor governing breakdown behaviour. The constrained chain mobility impedes charge carrier transport and limits energy accumulation, thereby suppressing the initiation of electron avalanche breakdown. This study establishes a new structure–property relationship centred on the mobility of amorphous chains, identifying it as a key descriptor for design of high‐performance dielectric polymers via annealing.