Influence of molecular weight of polymer matrix on nanocomposites rheology is not yet well understood. Herein dynamic rheological responses of fumed silica (FS)/polyethylene oxide (PEO) nanocomposites are investigated as a function of viscosity-averaged molecular weight (M-eta) of PEO, volume fraction (phi) and surface characteristics (hydrophilic or hydrophobic) of FS. In the nanocomposites, FS does not influence the glass transition and crystallinity of PEO in the mobile PEO phase while the interfacial interactions tend to immobilize a small fraction of PEO chains that could not undergo glass transition. In spite of the common observation that the reinforcement decreases with increasing M-eta of PEO and improving hydrophobicity of FS, linear rheological responses are well reproduced by the two-phase model, revealing the crucial contribution of the non-Newtonian matrix undergoing microscopic strain amplified by the filler. Furthermore, nonlinear rheological responses of the nanocomposites are collapsed into master curves plotted against local strain of the matrix. Analyzing the nonlinear rheology by Fourier transform and stress waveform methods reveal the dominating contribution of the matrix and the role of strain amplification played by the filler. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 397-405