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The thermal, dynamic mechanical analysis, morphology and mechanical properties of semi-interpenetrating polymer networks based on crosslinked polyurethane (PU) and poly(2-hydroxyethyl methacrylate) (PHEMA) synthesized by photopolymerization and by thermopolymerization have been investigated. The thermal analysis has evidenced the two glass temperature transitions in the semi-IPNs and this is confirmed by the thermodynamic miscibility investigation of the systems. The Dynamic Mechanical Analysis spectra have shown that the phase separation is more significant in the thermopolymerized semi-IPNs: the tan δ peaks of constituent polymers are more distinct and the minimum between the two peaks is deeper. The calculated segregation degree values of semi-IPN's components are significantly higher for thermopolymerized semi-IPNs, thereby the process of phase separation in the thermopolymerized semi-IPNs is more developed. The structures of two series of samples investigated by SEM are completely different. The mechanical properties reflect these changes in structure of semi-IPNs with increasing amount of PHEMA and with the changing of the method of synthesis. The results suggest that the studied semi-IPNs are two-phase systems with incomplete phase separation. The semi-IPN samples with early stage of phase separation demonstrate higher mechanical characteristics. POLYM. ENG. SCI., 48:588-597, 2008. © 2008 Society of Plastics Engineers, INTRODUCTION Interpenetrating polymer networks (IPNs) are materials based on two or more polymers, each of them is chemically crosslinked or at least one network is synthesized in the presence of [...]