The structural effects of polydimethylsiloxane (PDMS) or polyhedral oligosilsesquioxanes (POSSs) on the thermomechanical properties of polyurethane (PU) networks were studied. An ester-amine-functionalized silsesquioxane and a PDMS macromer were synthesized, and the macromer (10 wt %) was crosslinked with the PU prepolymer to obtain PU networks. The synthesized macromers and hybrids were characterized with Fourier transform infrared, H-1-NMR, C-13-NMR, and Si-29-NMR spectroscopy techniques. The influence of POSS cubes on the thermal and mechanical properties of the polymer network films was studied comparatively with the similarly functionalized PDMS linear chain via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) measurements. The degradation pattern of the POSS-incorporated PU nanocomposites was almost the same as that of the PU network synthesized from the linear PDMS macromer. The differences in the char yields and activation energies of the hybrids reflected the enhancement of the thermal properties of the nanohybrids. The TGA and DSC curves of the macromers suggested that the thermal properties of the macromers not only depended on either the PDMS or POSS inorganic core but also depended on the organic peripheral attached to the inorganic core. The glass-transition temperatures of the nanohybrids were higher than those of the linear-PDMS-incorporated hybrids. The storage modulus values increased 3-fold upon the incorporation of POSS rigid groups into the PU hybrids in comparison with the flexible PDMS-chain-incorporated PU hybrids. The DMA measurements showed a long-range rubbery plateau region for all the PU hybrids, with high storage modulus and tan 8 values showing the structural homogeneity of the crosslinked networks. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 4052-4065, 2009