A series of poly(methyl methacrylate) (PMMA/octavinyl polyhedral oligomeric silsesquioxane (POSS) blends were prepared by the solution-blending method and characterized with Fourier transform infrared, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis techniques. The glass-transtion temperature (T-s) of the PMMA-POSS blends showed a tendency of first increasing and then decreasing with an increase in the POSS content. The maximum T-g reached 137.2 degrees C when 0.84 mol % POSS was blended into the hybrid system, which was 28.2 degrees C higher than that of the mother PMMA. The X-ray diffraction patterns, transmission electron microscopy micrographs, and Fourier transform infrared spectra were employed to investigate the structure-property relationship of these hybrid nanocomposites and the T-g enhancement mechanism. The results showed that at a relatively low POSS content, POSS as an inert diluent decreased the interaction between the dipolar carbonyl groups of the homopolymer molecular chains. However, a new stronger dipole-dipole interaction between the POSS and the carbonyl of PMMA species formed at the same time, and a hindrance effect of nanosize POSS on the motion of the PMMA molecular chain may have played the main role in the T-g increase of the hybrid nanocomposites. At relatively high POSS concentrations, the strong dipole-dipole interactions that formed between the POSS and carbonyl groups of the PMMA gradually decreased because of the strong aggregation of POSS. This may be the main reason for the resultant T-g decrease in these hybrid nanocomposites. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2684-2690, 2009