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• Flexible etherified methylene chains and rigid phenyl structures were introduced into the cross-linking network. • The film coupled hardness with softness improved high-field polarization and high-temperature energy storage efficiency. • The film with side-chain flexible segments showed more outstanding dielectric and thermodynamic properties. The introduction of highly polarized flexible segments into polymer molecular chains is an effective means to improve the dielectric constant and mechanical flexibility of polymers, which is important for improving the energy storage characteristics and applicability for the roll-to-roll process of metallic film capacitors. However, the introduction of flexible segments may adversely affect the heat resistance of polymer materials, which limits their application at high working temperatures. In this work, we have prepared a novel epoxy film with excellent comprehensive properties by introducing rigid phenyl and flexible etherified methylene side chains. For instance, the optimal epoxy film showed an energy storage density of 7.06 J cm–3 and 85% charge-discharge efficiency at room temperature and 420 kV mm–1. At 200 kV mm–1 and 110 °C, a working condition for the application of the electric vehicle, the prepared film still showed an energy storage density of 1.5 J cm–3 and charge-discharge efficiency of 86%, which is 3 times that of BOPP film. This work provides an idea for material designs of high-performance polymer film capacitors. [ABSTRACT FROM AUTHOR]