The mechanism of enhancing material insulation performance through homogenizing free volume distribution mediated by a micro-branched structure.

The development of advanced polymer dielectrics for electronic and energy storage devices demands precise nanoscale molecular design. Departing from conventional strategies that merely augment free volume fraction, this study proposes a robust approach to optimize dielectric performance by homogenizing nanoscale free volume distribution via a tailored micro-branched architecture. Maleic anhydride-capped HDI trimer (HM) and TDI trimer (TM) were chemically grafted into a bismaleimide (BMI) resin to construct a micro-branched polymeric network with well-defined nanoscale topology. Positron annihilation lifetime spectroscopy (PALS) and molecular dynamics simulations collectively confirm that the optimized micro-branched structure enables a homogeneous distribution of nanoscale free volume holes (radius reduced to 0.256 nm), which efficiently restricts the mean free path of charge carriers through a prominent nanoscale confinement effect. Synergistically with the enhanced crosslinking density derived from the branched architecture, this nanoscale engineering strategy yields extraordinary dielectric performance: the breakdown strength is drastically enhanced by 48.2% to 35.62 kV/mm, the relative permittivity is reduced to 2.90, and the glass transition temperature is significantly elevated. Complementary theoretical calculations further verify that the micro-branched units reinforce electron-trapping capability at the nanoscale. This work establishes a versatile nanoscale design strategy for advanced polymer dielectrics, emphasizing the importance of precise free volume engineering at the molecular-nanoscale interface. The findings hold broad prospects for advancing the application of high-performance dielectric materials in nanodevices, flexible electronics, and high energy density storage systems.