The PCL-b-PES-b-PCL triblock copolymer is used to incorporate into epoxy resin when the blends are cured with 4,4'-diaminodiphenylsulfone (DDS) to afford the nanostructured epoxy thermosets. The differential scanning calorimetry (DSC) and Fourier transform-infrared spectroscopy (FT-IR) show that the nanostructured PCL-b-PES-b-PCL thermosets are accessed through the formation of the intermolecular hydrogen bonding interactions. The nanostructures are further evidenced by means of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). By considering the miscibility of the subchains of block copolymer with epoxy before and after the curing reaction, it is judged that the formation of the nanostructures follows the mechanism of reaction-induced microphase separation. It is noted that the epoxy resin is significantly toughened in terms of the measurement of critical stress field intensity factor (K-IC). (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 424-432