We studied the effects of functional groups on the physicochemical properties, such as rheological behavior, thermal stability, glass-transition temperature, and electrical and mechanical properties, of 4,4'-diaminodiphenyl methane (DDM) cured methyl-substituted and trifluoromethyl-substituted difunctional epoxy resins [i.e., 2-diglycidylether of toluene (DGET) and 2-diglycidylether of benzotrifluoride (DGEBTF)]. The DGEBTF/DDM system showed higher thermal stability and lower dielectric constant values than the DGET/DDM system. The impact strength of the DGEBTF/DDM system was significantly higher than that of the DGET/DDM system. The increase in intermolecular interactions because of the increasing polarity of the fluorine in the epoxy backbone was probably the key to these results. (c) 2005 Wiley Periodicals, Inc.