Anomalously High\nDielectric Strength and Capacitive\nEnergy Density of Thin Entangled Glassy Polymer Films

The influence of high-intensity electric fields on the\nstability\nof polymeric materials is a problem of interest in the design of next-generation\nenergy storage and electronic devices, and for understanding the limits\nof stability of polymer films exposed to large electric fields generally.\nHere, we show that the dielectric strength of entangled glassy polymer\nfilms increases as an inverse power-law of the film thickness <i>h</i> for “ultrathin” films below a micron in\nthickness. The dielectric strength enhancement in these polymer films\nbecomes as large as ≈2 GV/m in films thinner than 100 nm, but\nin this thickness regime, the increase of the dielectric strength\ndepends strongly on the polymer mass, sample aging time, and the method\nof film preparation. The enhancement of the dielectric breakdown strength\nis attributed to the mechanical instability of the elastic film subjected\nto sufficiently large electric fields and a large, but not generally\nwell-understood, enhancement of effective stiffness of entangled glassy\npolymer films subject to large deformations, an effect that has previously\nbeen observed to become greatly enhanced when such films are made\nthinner. As a proof of principle regarding applications, we utilize\nultrathin glassy polymer films of the type studied in our paper to\nfabricate polymeric nanocapacitors having ultrahigh discharge energy\ndensities (<i>U</i><sub>d</sub><sup>max</sup>) as large\nas 27 J/cm<sup>3</sup> and having efficiencies greater than 80%. These\nefficiency values at comparable charge densities are significantly\nhigher than those of competing ferroelectric polymer materials, and\nwe anticipate that our observations will inspire the creation of practical\nhigh-energy density nanocapacitor devices for advanced energy storage\napplications.