Epoxy thermosets were modified with 0-6 wt.% of poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) or polystyrene-b-poly(ethylene oxide) (PS-PEO) diblock copolymers. Both polymers self-assembled into spherical micelles with PEO coronas and either a rubbery PEP or glassy PS core. The influence of the nature of the core on modulus, glass transition temperature, T-g, and critical strain energy release rate, G(1c), was investigated. As block copolymer was added up to 2 wt.% loading, both block copolymers provided similar improvements in G(1c). However, at 2 wt.% and above, the epoxies containing PEP-PEO diblock copolymers outperform those containing PS-PEO by approximately a factor of two. In addition, fractography of compact tension specimen surfaces revealed distinct features present on the PEP-PEO materials, which were not present on the PS-PEO-modified thermosets. (C) 2014 Elsevier Ltd. All rights reserved.