Films from castor oil-based polyurethane (PU) prepolymer and nitroguar gum (NGG) with different contents (10-70 wt %) were prepared through solution casting method. The networks of PU crosslinked with 1,4-butanediol were interpenetrated by linear NGG to form semi-interpenetrating polymer networks (semi-IPNs) in the blend films. The miscibility, morphology, and properties of the semi-IPNs coded as PUNG films were investigated with Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, wide-angle X-ray diffraction, density measurement, ultraviolet spectroscopy, thermogravimetric analysis, tensile, and solvent-resistance testing. The results revealed that the semi-IPNs films have good miscibility over the entire composition ratio of PU to NGG under study. The occurrence of hydrogen-bonding interaction between PU and NGG played a key role in improvement of the material performance. Compared with the pure PU film, the PUNG films exhibited higher values of tensile strength (11.7-28.4 MPa). Meanwhile, incorporating NGG into the PU networks led to an improvement of thermal stability and better solvent-resistance of the resulting materials. (c) 2007 Wiley Periodicals, Inc.