Dielectric performance of high permitivity nanocomposites: impact of polystyrene grafting on BaTiO3 and TiO2

Polymer nanocomposites are a promising concept to improve energy storage density of capacitors, but realizing their hypothetical gains has proved challenging. The introduction of high permittivity fillers often leads to reduction in breakdown strength due to field exclusion, which intensifies the applied electric field within the polymer matrix near nanoparticle interfaces. This has prompted research in developing new nanoparticle functionalization chemistries and processing concepts to maximize particle separation. Herein, we compare the dielectric performance of blended nanocomposites to matrix free assemblies of hairy (polymer grafted) nanoparticles (HNPs) that exhibit comparable overall morphology. The dielectric breakdown strength of polystyrene grafted BaTiO3 (PS@BaTiO3) systems was over 40% greater than a blended nanocomposite with similar loading (similar to 25% v/v BaTiO3). Hairy nanoparticles with TiO2 cores followed similar trends in breakdown strength as a function of inorganic loading up to 40% v/v. Dielectric loss for PS@BaTiO3 HNPs was 2-5 times lower than analogous blended films for a wide frequency spectrum (1 Hz to 100 kHz). For content above 7% v/v, grafting the polymer chains to the BaTiO3 significantly improved energy storage efficiency. Overall this study indicates that polymer grafting improves capacitor performance relative to direct blending in likely two ways: (1) by mitigating interfacial transport to lower dielectric loss, irrespective of the dielectric contrast between matrix and nanoparticle, and (2) by restricting particle-particle hot-spots by establishing a finite minimum particle separation when the dielectric contrast between matrix and nanoparticle is large.