Polymer dielectrics, an insulating material ubiquitous in electrical power systems, must be ultralight, mechanically and dielectrically strong, and very thermally conductive. However, electric and thermal transport parameters are intercorrelated in a way that works against the occurrence of thermally conductive polymer electric insulators. Here, we describe how solution gel-shearing-strained polyethylene yields an electric insulating material with an outstanding in-plane thermal conductivity of 10.74 W m −1 K −1 and an average dielectric constant of 4.1. The dielectric constant and loss of such sheared polymer electric insulators are nearly independent of the frequency and a wide temperature range. The gel-shearing aligns ultrahigh-molecular weight polymer crystalline chains for the formation of separated and aligned nanoscale fibrous arrays. Together with lattice strains and the presence of boron nitride nanosheets, the dielectric polymer shows high current density carrying and high operating temperature, which is attributed to greatly enhanced heat conduction.

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