Optimizing and characterizing the dielectric and energy storage performance of PVDF/PMMA blend films via free volume regulation

Precise free volume regulation suppresses charge migration, doubling breakdown strength. • PVDF/PMMA blend films achieve a significant leap in energy storage density. • Free volume studied via positron annihilation lifetime (PAL) and simulation. Five series of PVDF/PMMA blend films were fabricated, and their energy storage performance was investigated via experimental and theoretical analyses of free volume characteristics. PMMA incorporation significantly enhances thermal stability of blend film. For instance, with 50  wt % PMMA, the glass transition temperature ( T g ) rises to 117.2 °C, much higher than that of pure PVDF (−45.2 °C). Increasing PMMA content decreases free volume size but increases its relative fraction, thereby enhancing breakdown strength and reducing material density. Although a moderate reduction in dielectric constant is observed, the marked improvement in breakdown strength leads to an overall increase in energy storage density. Specifically, at 40  wt % PMMA, the blend film achieves a maximum energy storage density of 8.13 J/cm 3 ( ƞ  = 67%), substantially exceeding the 3.01 J/cm 3 ( ƞ  = 67%) of pure PVDF, while exhibiting a high T g of 105.1 °C. These results highlight a synergistic effect derived from the complementary properties of the two polymers, offering an effective strategy for developing high-performance polymer-based energy storage films that combine the advantages of dipolar and linear dielectric materials.