A series of polyurethane (PU) thin films with different hard-to-soft segment ratios were synthesized in our laboratory. The molecular and morphological structures of the PU films were characterized with Fourier transform infrared (FTIR), small-angle X-ray scattering (SAXS), wide-angle x-ray diffraction, dynamic mechanical analysis, and differential scanning calorimetry. The PU films showed a single glass transition when the hard-to-soft segment ratio varied from 1:2 to 1:8, suggesting no significant phase separation between the hard and soft segments. FTIR and SAXS results disclosed that the PU films had a network structure with the physical crosslinks formed via the intermolecular hydrogen bonds established between the hard segments. The fracture toughness of the ductile PU films was characterized with the essential work of fracture method under different conditions. It was found that the specific essential work of fracture was a function of the chain length between crosslinks and independent of the test temperature when fracture occurred at a temperature below the glass transition temperature. The physical meaning of this fracture parameter was proposed. (C) 2007 Wiley Periodicals, Inc.