We prepared two series of semiinterpenetrating polymer network (semi-IPN) films from cross-linked waterborne polyurethane (WPU) and carboxymethylchitin (CMCH) in the aqueous solution on the glass and Teflon as the hydrophilic and hydrophobic substrates, respectively, by casting method. The chemical compositions, structure and morphologies of the films were examined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). The miscibility, thermal stability and mechanical properties of the films were investigated by density measurement, dynamic mechanical analysis (DMA), ultraviolet (UV) spectroscopy, thermogravimetric analysis (TGA), tensile testing and solvent swelling testing. The results revealed that the semi-IPN films exhibited good miscibility when CMCH content was lower than 35 and 65 wt% for the films prepared on the glass and Teflon, respectively, resulting in higher light transmittance, thermal stability and tensile strength than the WPU film. Interestingly, the films prepared with the Teflon as the substrate possessed better miscibility and higher storage modulus, thermal stability, tensile strength and solvent-resistance than that with the glass as the substrate over the entire composition range studied here. This difference can be attributed that a strong intermolecular interaction occurred between WPU and CMCH to form a dense architecture, owing to that two kinds of macromolecules all were repulsed from the Teflon surface and forced to concentrate into inner surface. It has been confirmed that the hydrophility and hydrophobility of the solid substrate significantly influenced the structures and properties of the casting films, and using Teflon solid substrate can more effectively improve the miscibility and properties of the semi-IPN materials with hydrophilic character than glass one. We proposed a model describing the formation of WPU/CMCH semi-IPN films cast on the hydrophilic and hydrophobic substrates to illustrate the different structures of two types of films. (C) 2004 Elsevier Ltd. All rights reserved.