Supercritical carbon dioxide (scCO2) was considered as an environmental friendly medium for grafting the surface of silica nanoparticles with a series of reactive imidazolium-based ionic liquids (IL), denoted as IL-silanes. The effects of the reaction conditions on the resulting IL-grafted layers on the silica surface were characterized from thermogravimetric (TGA) and transmission electronic microscopy (TEM) analyses. The IL-grafted nanoparticles were introduced into poly(methyl methacrylate) (PMMA) via melt mixing in order to design nanocomposites and to demonstrate the ability of IL-grafted nanoparticles to be dispersed into a polymer medium. The role of the IL-grafted surfaces on the morphology of PMMA-based nanocomposites was evidenced by TEM and small angle neutron scattering (SANS). It was demonstrated that longer the alkyl chain length of the IL-silane, better the state of dispersion of nanoparticles within PMMA. In addition, better thermal stabilities and mechanical performances, as well as new surface properties were observed for the designed PMMA-based nanocomposites.