Novel amphiphilic block copolymer modifiers based on chain-extended polyester for improved toughness of epoxy resins were synthesized by attaching polyols having different structures and compatibilities with a dianhydride chain-extender. Used polyols in this research were polytetrahydrofuran as miscible and hydroxyl-terminated polybutadiene as immiscible segment. Generated carboxyl groups will be prepolymerized with an excess of epoxy resin to exclusively form epoxy groups between the polyol spacer. Resulting morphologies in the prepared diglycidyl ether of bisphenol-A-based epoxy thermosets cured with dicyandiamide and urea were strongly dependent on the initial parameters of the block copolymerization process. Changing the dianhydride concentration as well as relation between miscible and immiscible polyol spacer showed various particle sizes including unimodal and bimodal distributions. The best mechanical performance in terms of fracture toughness (K-1C) could be achieved with bimodal particle size distributions indicating synergistic effects between the different particle sizes in the range of 0.1-7 mu m. POLYM. ENG. SCI., 59:E216-E223, 2019. (c) 2018 Society of Plastics Engineers