Novel semi-interpenetrating polymer network (semi-IPN) membranes were synthesized from 6-O-carboxymethylchitosan (6-OCC) and waterborne polyurethanes (WPU) with ester or ether soft segments. Results from the dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) showed that miscibility and mechanical properties of the 6-OCC/WPU semi-IPNs were varied with their compositions and the cross-linking/grafting reactions. 6-OCC had better miscibility with the waterborne polyurethane containing ester soft segments, and could interfere the formation of hydrogen bonds between the ester soft segments and the NH groups in hard-segments. The 6-OCC/WPU composite membranes demonstrated microphase-separated structures in which the 6-OCC particles were dispersed in the matrix of the WPU-rich phase. The 6-OCC/WPU composites were cross-linked with glutaraldehyde or cross-linked/grafted with ethylene glycol diglycidyl ether (EGDE) to form 6-OCC/WPU semi-IPN membranes. Generally speaking, cross-linking and grafting altered the miscibility of the 6-OCC/WPU semi-IPN membranes, resulted in their different tensile strength and modulus. The growth of a model bacterial-Escherichia coli and the adhesion of platelets on the surfaces of the 6-OCC/WPU semi-IPN membranes indicated that the antibacterial ability and blood compatibility of WPU membranes were improved by the addition of 6-OCC, and strongly related to their cross-linking/grafting network structures. It was concluded that the 6-OCC/WPU semi-IPN membranes could be good candidates for biomedical applications. (c) 2005 Elsevier B.V. All rights reserved.