Investigation of readily processable thermoplastic‐toughened thermosets. III. Toughening BMIs and epoxy with a comb‐shaped imide oligomer

This is the third in a five‐part series describing the preparation of tough, high‐performance thermosets from low viscosity, autoclave‐processable prepolymers. The first 2 articles described toughening of bismaleimides (BMI) and epoxy with linear imide thermoplastics of ∼ 1000 g/mol. Highly processable prepolymers were obtained, which resulted in increases in fracture toughness for BMI of ∼ 75–100%, while the fracture toughness of epoxy was increased by up to 220%. This article describes the preparation of a low‐molecular‐weight comb‐shaped imide oligomer (∼ 4100 g/mol) and the effect of the oligomer architecture and end‐group on BMI and epoxy prepolymer viscosity and fracture toughness. When an unreactive comb‐shaped oligomer was incorporated in a BMI prepolymer (10% thermoplastic loading in the thermoset), the fracture toughness increased by 67% over that of an untoughed control, while a reactive oligomer increased the fracture toughness by 150% over an untoughened control. At 55°C, the viscosity of the solution of the reactive comb‐shaped imide in B was only 6.2 Pa · S. When the oligomer was dissolved in epoxy resin, the viscosity was less than 0.2 Pa · S at 90°C, and the fracture toughness increased by 110 and 133% (at ∼ 13% loading in the thermoset), relative to an untoughened control, depending on the reactivity of the end group. The T g and high‐temperature modulus of BMI and epoxy remained approximately the same relative to the untoughened controls. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 943–951, 1998