Relationships in a bismaleimide resin system. Part II: Thermomechanical properties

A range of bismaleimide resins of differing network structure was produced both by varying the ratios of N,N′‐bismaleimido‐4,4′‐diphenylmethane (BMI) and methylene dianiline (MDA), and also by the use of different thermal processing cycles. Characterization of the resins by a variety of mechanical, physical, and micrographic techniques showed that properties such as modulus and yield strength varied directly with crosslink density and its associated free volume. Fracture toughness was found to increase as the crosslink density decreased, and microscopic investigation of the resin fracture surfaces identified characteristics which correlated with the resin fracture toughness.