The microstructure and mechanical properties of a block copolymer modified commercial thermoset plastic formed from a bisphenol-A based epoxy and a bio-derived amine hardener (Cardolite VR NC-541LV) were investigated. A series of poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock copolymers was synthesized at fixed composition (31+/-1% by volume PEO) and varying molecular weight expanding on a commercially available PEO-PBO compound marketed by the Dow Chemical Company under the trade name FORTEGRA (TM) 100; direct application of any of these block copolymers resulted in little improvement of the poor fracture toughness of the cured material. Modification of the resin formulation and curing protocol led to the development of well-defined spherical and branched worm-like micelles containing a PBO core and PEO corona in the cross-linked products as evidenced by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements. Maximum fracture toughness (K-1c) and a ninefold increase in the critical strain energy release rate (G(1c)) over the unmodified neat epoxy was achieved at 5 wt % loading of intermediate molecular weight PEO-PBO, without measureable reductions in modulus, glass transition temperature or transparency. This study provides new strategies for engineering improved performance in thermoset materials using block copolymer additives that exhibit challenging mixing thermodynamic characteristics with the component monomers. (C) 2015 Wiley Periodicals, Inc.