(671i) Ultra-High Dielectric Strength and Capacitive Energy Density in Ultrathin Glassy Polymer Films

Ultrathin polymer films present unique opportunities to understand the physics and properties of polymers at the nanoscale when the film thicknesses become comparable to the polymer dimensions. Ultrathin polymer films have been shown to demonstrate differences in properties such as glass transition temperature, mechanical properties, polymer mobility, etc. as compared to their bulk counterparts. In this work, we demonstrate that ultrathin glassy polymer films (~100 nm) show an order of magnitude higher dielectric strength (E_BD) and two orders of magnitude higher capacitive energy density (U_max ∝ E_BD²) of ~100 J/cm³ in standard localized point-contact measurements, as compared to the bulk polymer films when used as dielectric capacitors. The dielectric capacitors demonstrate 3-4 order of magnitude higher power densities as compared to electrochemical energy storage devices such as batteries, thus making them useful as high-power density energy storage devices. We believe that the enhancement of the dielectric strength and capacitive energy density is due to the tighter chain packing of polymers, cast from a good solvent into thin films, dried under conditions of slow evaporation of the good solvent. We test the density of thin polymer films by optical measurements and observe that the ultrathin films show higher densities and thus lower free volume as compared to their bulk counterparts, which might be a governing factor for the enhancement of the dielectric strength. Furthermore, we observe that the dielectric strength of annealed ultrathin polymer films decreases as compared to the as-cast films, which might be attributed to the surface segregation of chain ends upon annealing, which act as defect sites for the dielectric breakdown of the polymer films. Moreover, the ultrathin films of polymers having sub-room temperature glass transition don’t show ultra-high dielectric strength and capacitive energy density, which might be due to the presence of excess free volume in these polymer films that act as defect sites for breakdown. This enhancement of dielectric strength in ultrathin polymer films might help in increasing the understanding of polymer behavior in ultrathin films.