Polymers are an extremely versatile material and the addition of filler particles can further enhance their performance. In the case of nano-sized filler particles, these have a particularly strong impact on the overall mechanical properties of the material. The observed reinforcement of the material is primarily attributed to the so-called interphase, which forms between the matrix polymer and the filler particles. The interphase refers to the zone in which the presence of the filler particles alters the properties of the bulk polymer. In order to accurately predict the mechanical behavior of such nanocomposites, it is, therefore, necessary to combine the material behavior of the matrix and filler with an accurate constitutive model of the interphase. However, due to the small dimensions, the material parameters of the interphase are hardly accessible experimentally. Therefore, we present a methodology to determine the property gradients within the interphase based on molecular dynamics simulations combined with the finite element method. To this end, we employ a domain-decomposition multiscale method [1] to deform and investigate samples containing two nanoparticles. For exemplary silica-reinforced polystyrene, we characterize the interphase as elasto-plastic and identify the gradients of the individual material parameters within the interphase [2]. This interphase model allows us to investigate the effects of filler size and filler separation in more detail using finite element simulations. This study is thus a necessary preliminary step for the systematic investigation of suitable filler configurations by means of representative volume elements. [1] Pfaller, S., Kergaßner, A., & Steinmann, P. (2019). Optimisation of the capriccio method to couple particle-and continuum-based simulations of polymers. Multiscale Science and Engineering, 1(4), 318-333. [2] Ries, M., Possart, G., Steinmann, P., & Pfaller, S. (2021). A coupled MD-FE methodology to characterize mechanical interphases in polymeric nanocomposites. International Journal of Mechanical Sciences, 106564.