Enhanced 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. In this work, we demonstrate that ultrathin glassy polymer films (~100 nm) show an order of magnitude higher dielectric strength (E_BD) and capacitive energy density (U_max ∝ E_BD²) of ~27 J/cm³ as compared to the bulk polymer films when used as dielectric capacitors. We believe that the enhancement of the dielectric strength and capacitive energy density is due to the tighter chain packing of polymers in ultrathin films. 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. 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. The high dielectric strength of ultrathin polymer films might help in increasing the understanding of polymer behavior in ultrathin films.