Poly(methylmethacrylate) (PMMA)/montmorillonite clay nanocomposites were synthesized via the free radical polymerization of MMA in the presence of alkyl ammonium substituted polysilsesquioxane surfactant-modified clay and AIBN initiator in supercritical CO2 and ethanol. The reactions were also conducted by adding a small amount of water to observe the intercalation and exfoliation behavior of the clay and the properties of the nanocomposites. Initially, clay was cation exchanged with the surfactant to enhance its hydrophobicity and to expand the interlamellar spaces of silicate platelets. Organophilization with the three dimensional surfactant and a small amount of water molecules in the solvent reduced the Surface energy of clay dramatically, which promoted the miscibility of polymer/clay nanocomposites. The morphology of the nanocomposites was characterized by scanning electron microscopy. The intercalation and dispersion of the clay were quantified by both X-ray diffraction and transmission electron microscopy. Due to the three dimensional structure, alkyl ammonium Substituted polysilsesquioxane surfactant gives stable clay separation and dimension stability of the nanocomposites. Different distribution of the clay also plays an important role in physical properties. Thermogravimetric analysis and differential scanning calorimetry were employed to investigate the thermal properties and glass transition temperature of the nanocomposites. (C) 2009 Elsevier Inc. All rights reserved.