Anomalously High Dielectric Strength and Capacitive Energy Density of Thin Entangled Glassy Polymer Films

The influence of high-intensity electric fields on the stability of polymeric materials is a problem of interest in the design of next-generation energy storage and electronic devices, and for understanding the limits of stability of polymer films exposed to large electric fields generally. Here, we show that the dielectric strength of entangled glassy polymer films increases as an inverse power-law of the film thickness h for "ultrathin" films below a micron in thickness. The dielectric strength enhancement in these polymer films becomes as large as approximate to 2 GV/m in films thinner than 100 nm, but in this thickness regime, the increase of the dielectric strength depends strongly on the polymer mass, sample aging time, and the method of film preparation. The enhancement of the dielectric breakdown strength is attributed to the mechanical instability of the elastic film subjected to sufficiently large electric fields and a large, but not generally well-understood, enhancement of effective stiffness of entangled glassy polymer films subject to large deformations, an effect that has previously been observed to become greatly enhanced when such films are made thinner. As a proof of principle regarding applications, we utilize ultrathin glassy polymer films of the type studied in our paper to fabricate polymeric nanocapacitors having ultrahigh discharge energy densities (U d max) as large as 27 J/cm3 and having efficiencies greater than 80%. These efficiency values at comparable charge densities are significantly higher than those of competing ferroelectric polymer materials, and we anticipate that our observations will inspire the creation of practical high-energy density nanocapacitor devices for advanced energy storage applications.