The two-phase structure of polymer blends or block copolymers prepared with polyurethane macroazo initiators and methyl methacrylate or styrene have been studied. The synthesis of these block copolymers was reported in a previous article (Cheikhalard, et al. J Appl Polym Sci 1998, 70, 613-627). One of the difficulties encountered with the copolymers is their characterization in order to know if the products of the syntheses of monomers A and B are true block copolymers P(A-b-B) or homopolymer blends Polymer A/polymer B (PA/PB) with a certain concentration of block copolymers. Preceding studies have shown that the polymers obtained previously with polyurethane macroazo initiators (reacted with methyl methacrylate or styrene) are primarily block copolymers. In the present study, the morphology, the thermal and the rheological properties of the crude block copolymers and of some of their fractionated products are investigated and compared with their corresponding blends of homopolymers having the same composition. Results displayed in this report show clearly that the shear storage modulus G＇ of our polymers at the rubbery plateau are higher than the corresponding polymer blends. They confirm previous analyses (by steric exclusion chromatography and nuclear magnetic resonance) (Cheikhalard, et al. J Appl Polym Sci 1998, 70, 613-627) aimed at proving the copolymer morphology of our polymers. The morphology of blends was first studied by optical microscopy. A decrease in the dispersed phase size is observed when the percentage of polyurethane content increases. Transmission electron micrographs obtained for mixtures and pure block copolymers show a microheterogeneous structure as seen by optical microscopy. These two structures were also characterized by differential scanning calorimetry and by dynamic viscoelastic measurements. A decrease in the glass transition temperature of hard blocks (polymethyl methacrylate or polystyrene), in comparison with the pure homopolymers, in polymer blends and in block copolymers was observed and can be attributed to the presence of a small amount of soft blocks in the hard phase. A quantitative evaluation of the degree of phase separation was obtained by differential scanning calorimetry showing the presence of an interphase. (C) 2000 John Wiley & Sons, Inc.