This study aims at determining the compatibility behavior of nanoparticles surface with fluorinated matrices to obtain a homogenous dispersion and better composites properties. First, modified silica nanoparticles by C6F13I and C6F13-C2H4-SH led to various fluorinated silica of different massic concentrations and grafting rates. The dispersion of these nanoparticles (in 5 wt %) into molten poly(VDF-co-HFP) and poly(TFE-co-HFP) matrices were studied as well as the hydrophobic, mechanical, and thermal properties of both fluorinated copolymers and resulting composites. In both series, the storage modulus of nanocomposites increased while the melting (T-m) and decomposition (T-10%) temperatures varied with the polymer matrix. They increased for poly(VDF-co-HFP) composites (T-m= 134 to 144 degrees C and T-10%= 441 to 464 degrees C) but decreased for poly(TFE-co-HFP) nanocomposites (T-m= 276 to 268 degrees C and T-10%= 488 to 477 degrees C). (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1512-1522