A series of carboxyl-terminated poly(ethylene glycol) adipate (CTPEGA) was synthesized by polycondensation of poly(ethylene glycol) (PEG) of various molecular weights ("2000," "4000," "6000," "8000; " "10,000" g/mol) and adipic acid. CTPEGA was incorporated into the epoxy by a prereaction method. The CTPEGA and modified epoxy samples were thoroughly characterized by Fourier transform infrared spectroscopy, H-1 NMR spectroscopy, differential scanning calorimetry, and gel permeation chromatography. The effects of molecular weight of CTPEGA on thermomechanical and viscoelastic properties of the modified epoxy networks were investigated. Maximum improvement in impact strength was found for the epoxy network modified with CTPEGA containing PEG of molecular weight 2000 g/mol. With further increase in molecular weight of CTPEGA, the impact strength of the modified network decreases. However, in case of higher molecular weight CTPEGA, the improvement in toughness was achieved without any reduction in T, due to the complete phase separation. The results were explained in terms of morphology studied by scanning electron microscopy. (c) 2006 Wiley Periodicals, Inc.