Synergistic enhancement of toughness and breakdown strength in all-organic epoxy composites and its mechanism

Synergistically enhancing toughness, breakdown strength (Eb), and glass transition temperature (Tg) of epoxy resin (EP) remains challenging. To address this, we introduce a reactive toughening agent of carboxyl-terminated butadiene-acrylonitrile rubber (CTBN) and small-molecule polyvinyl carbazole (PVK) into the epoxy network to construct all-organic epoxy composites. Results show that tensile and flexural strengths first increase and then decrease with the rising CTBN content, peaking at 5 wt. % and 10 wt. % CTBN, corresponding to increases of 27.8% and 5% over neat EP, respectively. CTBN monotonically reduces Tg slightly, but further addition of PVK partially recovers Tg without affecting thermal stability. With increasing CTBN content, DC and AC Eb are both first increased and then declined, whereas high-frequency Eb shows little variation; meanwhile, dielectric loss and permittivity rise significantly. Introducing PVK further enhances DC and AC Eb by up to 15.4% and 15.7% over neat EP, respectively, and notably reduces dielectric loss and permittivity of EP/CTBN composites, especially at elevated temperatures, by hindering molecular chain motion. Mechanistically, CTBN introduces deep traps and PVK increases trap density, both of which improve Eb, which can be validated by phase-field simulation. This study provides theoretical and experimental support for developing all-organic epoxy composites with high toughness, high Eb, and high Tg.