Nanocomposites based on sequential semi-interpenetrating polymer network (semi-IPN) of cross-linked polyurethane and linear poly(2-hydroxyethyl methacrylate) with 0.25 and 3 wt % of nanosilica filler were prepared and investigated. The unmodified silica, carboxyl-modified, and amino-modified silica were used in an attempt to control the microphase separation of the polymer matrix by polymer-filler interactions. A variety of experimental techniques were used to study morphology, thermal transitions, mechanical properties, and polymer dynamics of the nanocomposites. Special attention was paid to the investigation of the hydration properties of the nanocomposites in the perspective of biomedical applications. The results show that the good hydration properties of the semi-IPN matrix are preserved in the nanocomposites. Effects of water on polymer dynamics were found to be particularly pronounced for the secondary (sw),(PHEMA) and the (PU) relaxations, in agreement with interpretations in terms of hydrogen bonding interactions with specific groups in the structure of the two polymers. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 397-408