Water-based polyurethane-urea (WPUU) nanoparticles containing 4,4'-methylenedi-p-phenyl diisocyanate (MDI) and isophorone diisocyanate (IPDI) were synthesized by a stepwise prepolymer mixing process, that is, the consecutive formation of hydroxyl-terminated and isocyanate-terminated polyurethane prepolymers. The reaction behavior, chemical structure, and consequent morphology of the polyurethane prepolymers and WPUU were investigated with Fourier transform infrared (FTIR), gel permeation chromatography, and NMR techniques with MDI concentrations ranging from 0 (pure IPDI) to 50% with respect to the total moles of isocyanate. Wide-angle X-ray diffraction and differential scanning calorimetry patterns showed that the crystallinity of WPUU, which mostly originated from crystallizable poly(tetramethylene adipate) polyol, was significantly affected by the MDI content. Both the crystallinity and melting temperature of WPUU decreased as the MDI content increased. Deconvoluted relative peak areas of the carbonyl region in the FTIR spectrum revealed that the effect of hydrogen bonding among the hard segments became favorable as the MDI content increased, whereas the hydrogen bonding of the soft segments significantly decreased. (C) 2004 Wiley Periodicals, Inc.