Fracture toughness is a key property of epoxy resins with a high glass transition temperature (T), used in carbon fiber/epoxy composites for aerospace applications. Conventional toughening methods rely on adding toughening agents, often compromising the processibility and thermal stability. This study introduces a simple self-toughening approach that enhances the fracture toughness without sacrificing other properties by controlling the cured epoxy network structure. Tetraglycidyl 4,4'-diaminodiphenylmethane (TGDDM) epoxy resin was cured using mixtures of structural isomeric curing agents, 3,3'- and 4,4'-diaminodiphenyl sulfone (3,3'- and 4,4'-DDS), at ratios of 7:3, 5:5, and 3:7. The optimal 7:3 ratio produced a resin with 30% higher fracture toughness compared to TGDDM/3,3'-DDS and 100% higher than the TGDDM/4,4'-DDS system. The T of the self-toughened resin ranged from 241 to 266 °C, which was intermediate between the T values of the TGDDM/3,3'-DDS and TGDDM/4,4'-DDS systems. This improvement is attributed to the higher crosslink density and reduced free volume of the epoxy network. These findings demonstrate that simply mixing isomeric curing agents enables self-toughening, providing a practical and efficient strategy to enhance the performance of high-T epoxy resins in advanced composite applications.
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