Addition of rubber particles to epoxy thermosets has been successful for toughening these brittle materials. However, a complete description of the toughening mechanisms is still elusive. Of particular interest is understanding the enhanced toughness achieved upon adding small amounts of block copolymers to commercial epoxy resins. Simultaneous small-angle X-ray scattering (SAXS) and tensile experiments were performed on epoxies modified with 30 nm diameter rubbery and glassy core block copolymer micelles. The SAXS data revealed efficient cavitation of the spherical rubbery cores and absence of cavitation in the glassy nanodomains. These results are quantitatively anticipated by theory that accounts for cavitation in rubber toughened plastics as a function of particle size.
