Polyurethane/clay nanocomposites have been synthesized using Na+-montmorillonite (Na+-MMT)/amphiphilic urethane precursor (APU) chains that have hydrophilic polyethylene oxide (PEO) chains and hydrophobic segments at the same molecules. Nanocomposites were synthesized through two different crosslinking polymerization methods. One is UV curing of melt mixed APU/Na+-MMT mixtures; the other is coalescence polymerization of APU/Na+-MMT emulsions. These two kinds of composites had intercalated silicate layers of Na+-montmorillonite by insertion of PEO chains in APU chains, which was confirmed by X-ray diffraction measurement and transmission electron microscopy. These composite films also showed improved mechanical properties compared to pristine APU films. Although the two kinds of nanocomposites exhibited the same degree of intercalation and were synthesized based on the same precursor chains, these nanocomposite films had the different mechanical properties. Nanocomposites synthesized using APU/Na+-MMT emulsions, having microphase-separated structure, had greater tensile strength than those prepared with melt-mixed APU/Na+-MMT mixtures. Location of intercalated Na+-MMT by PEO chains at the oil-water interface also could be confirmed by rheological behavior of the APU/Na+-MMT/water mixture. (C) 2003 Wiley Periodicals, Inc.