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Abstract: Nanocomposites made from inorganic nanoparticles and polymers have many applications in optics, electronics and biomaterials. However, the glass transition temperature (Tg ) of a nanocomposite is very difficult to measure accurately by conventional thermal analysis such as DSC or TMA when the concentration of the nanoparticle reaches a threshold of the percolation network. At this threshold stage, the phase transition in the nano domains of the matrix is too small to be detected by macroscale thermal analysis. We have developed a methodology basis on thermal atomic force microscope (AFM) to monitor the nanophase transition of the nanocomposite in situ upon heating. This method has demonstrated the capability in determining the Tg of a nanocomposite made by spherical SiO2 nanoparticles dispersed in polyacrylate. The threshold of the percolation network of this nanocomposite is at 40wt% of SiO2 nanoparticles according to the results of refractive index, AFM, nanoindentation, DSC, TMA and TGA. [Copyright &y& Elsevier]