• Adding 15 phr PES-C to E51/DETDA epoxy increased Tg by 51.5°C and improved flexural strength, impact strength, and fracture toughness by 41.1%, 186.2%, and 42.7%, respectively.
• Phase separation from island to bi-continuous structures with increasing PES-C content is key to toughening.
• A 7 μm PES-C film interlayer improved mode-II fracture toughness (GIIC) of carbon fiber composites by 80.3%.
• The toughening mechanism involves cohesive failure and plastic deformation in the interleaving region.