The structure and physicomechanical properties of nylon 6 (N-6) modified with polyethylene oxide-polyisoprene-polyethylene oxide (PEO-PI-PEO) copolymers via activated anionic polymerization have been investigated by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), thermogravimetric analysis (TGA), dynamic mechanical analysis (DTMA), and by optical microscopy. This study deals with the influence that the N-6/PI ratio and the length of the PI segments have on the physicomechanical behavior, thermal stability, and the structural changes of the block copolymers. Literature does not report on previous investigations of this kind. (C) 2004 Wiley Periodicals, Inc.