We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d approximate to 2 nm] and poly(methyl methacrylate) (PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to phi = 0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature T-g and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings phi < 0.10. At a loading phi = 0.05, the unmodified acrylic POSS induces a 4.9 degrees C decrease in the T, of PMMA, far less than the 17.4 degrees C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f(0) = 0.046 for PMMA at T-0 = 170 degrees C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f(0)=0.057, which corresponds to an addition of 0.47 nm 3 per added POSS molecule at phi = 0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at (phi = 0.20 and higher, but no significant phase separation is evident at phi = 0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings phi = 0.10, while no evidence of phase separation is present in the phi = 0.05 blend in DMA. (c) 2005 Elsevier Ltd. All rights reserved.