Rubber‐modified epoxies. II. Morphology and mechanical properties

A wide range in morphology paralleling a spectrum of phase separated and dissolved rubber, can be developed in rubber‐modified epoxies through control of rubber‐epoxy compatibility and cure conditions. These morphologies result in different stress response mechanisms. Dissolved rubber promotes plastic deformation and necking at low strain rates that provide large increases in the elongation to break. Dissolved rubber is ineffective in providing improvement at impact rates. Phase separated rubber domains can also increase the elongation to break since cavitation is promoted at the interfacial boundary. The elongation is limited to the extent of cavitation and therefore large increases in the energy to break are not found. The presence of rubber domains was found to be a necessary but not sufficient condition for impact energy improvement. Optimum materials contained a relatively large amount of dissolved rubber and a low volume fraction of phase separated rubber. These combine high elongations at low strain rates with improved impact properties.