By the use of small angle neutron scattering, the chain conformation in a polymer nanocomposite was studied as a function of the nanoparticle fraction for two different molecular weights. A repulsive system was realized in mixing poly(ethylene-propylene) and hydrophobically modified silica. All nanocomposite SANS data exhibit a pronounced scattering peak at intermediate momentum transfers, which is closely connected with the particle structure. Even under contrast matching conditions, the hydrophobic surface layer of the nanoparticles was found to contribute significantly to the scattering signal. In particular, in the short chain matrix the peak origin is exclusively related to direct particle scattering. In the long chain matrix, an additional peak contribution is present. Possible origins include void correlation scattering or polymer correlation scattering. We show unambiguously that the conformation of short chains with a molecular weight of 3000 g/mol is not visibly disturbed by the presence of the nanoparticles. In contrast to that a polymer matrix with 50 000 g/mol chains is affected by the particle presence. The chain radius of gyration R-g decreases.