AbstractOur limited success in toughening methylene dianiline (MDA)‐cured Epon 828, using varying rubber types, led to a study of the role of the matrix viscoelasticity in the toughening process. Two rubber types, with different interfacial bonding capabilities, poly(n‐butyl acrylate)/15 wt acrylonitrile/2 wt acrylic acid and poly(n‐butylacrylate)/15 wt acrylonitrile, were incorporated into systems containing varying amine concentrations to control crosslink density. Impact strengths of controls and rubber‐modified compositions increased with excess amine concentrations up to 70. The impact strengths for the poly(n‐butyl acrylate)/15 wt acrylonitrile/2 wt acrylic acid rubber‐modified compositions were greater than their equivalent controls, with the effect being greater at a lower crosslink density. This study confirmed that the matrix viscoelasticity is the controlling parameter in the toughening process. The degree of rubber–epoxy interfacial bonding is also an important parameter to consider, if the matrix viscoelasticity permits toughening. A modified stress response model was used to explain the tougheni
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