Multiwalled carbon nanotubes (MWNTs) were functionalized by a free-radical reaction of vinyltriethoxysilane and were blended with poly(urea urethane) (PUU) containing poly(dimethylsiloxane) as a soft segment. PUU was end-capped with aminopropyltriethoxysilane (A-silane) or phenyltriethoxysilane (P-silane). A-silane-end-capped PUU was covalently bonded to functionalized MWNTs, whereas P-silane-end-capped PUU was noncovalently bonded to pristine MWNTs by a pi-pi interaction. Fourier transform infrared, Raman spectra, and thermogravimetric analysis confirmed the functionalization of MWNTs. The results showed that the optimal reaction time of the functionalization of MWNT was 8 h, and the organic content of the modified carbon nanotubes reached 35.22%. Solid-state nuclear magnetic resonance and dynamic mechanical analysis were used to investigate the molecular structure and molecular mobility of the carbon-nanotube/PUU nanocomposites. A-silane PUU covalently bonded to MWNTs showed a considerable reduction in the molecular motion of the soft segment, which led to the glass-transition temperature decreasing from -117 to -127 degrees C as MWNTs were incorporated. (c) 2005 Wiley Periodicals, Inc.