Enhancement of High-Temperature Breakdown Strength by Annealing Dielectric Polymers Near Melting Points

High-temperature dielectric polymers have received considerable attention, and their breakdown strength improvement has become a key point for the safe operation of electrical systems in harsh environments. Recent successes heavily rely on the dielectric properties at room temperatures utilizing nanocomposites, 2-D engineering, and multilayer construction. However, engineering the polymeric structure in terms of thermal treatment to further improve the breakdown strength at high temperatures remains a hurdle. Here, poly(ether ether ketone) (PEEK) and poly(phenylene sulfide) (PPS) films are revisited as model polymers. The effect of thermal annealing is demon-strated at various temperatures. Tuning the annealing conditions above glass transition temperatures, especially near the melting points, significantly enhances the break-down strength at high temperatures. The positive effect of critical annealing on the breakdown strength is found to originate from the reconstruction of molecular chains into a higher fraction of thinner lamellae at the consumption of the amorphous fraction, by means of differential scanning calorimetry (DSC), XRD, and small-angle X-ray scattering (SAXS). The increase in breakdown strength is found to stem mainly from the formation of thinner and more lamellae during annealing. The block of the motion of charge carriers at high temperatures also suppresses the dielectric loss at high temperatures. This method expands the dielectric polymer application temperature.