Hydrogen is one of the most promising energy carriers of the future. But this new perspective of a climate friendly energy storage possibility comes with a few challenges. Due to its size, the hydrogen molecule can pass easily through all different kinds of materials, in particular through polymeric materials. For an efficient transportation and use of hydrogen gas, it is necessary to develop functional polymers that can be used as sealants in high pressure applications and have a low permeability for small molecules. Polymer composites containing 2D nanofillers provide a solution to this problem as the fillers have a high aspect ratio and create a tortuous (i.e. longer path) for the gas molecules to diffuse. Graphene oxide (GO) is an inorganic 2D filler that has attracted significant attention due to its unique properties, including high mechanical strength and thermal stability. However, due to its highly polar surface properties, GO tends to agglomerate in technical rubbers such as polyurethanes, which decreases the barrier properties of GO filled polymer composites. In this thesis, the surface of GO was modified by a wet chemical silanization process to improve dispersion properties and to enhance filler matrix interaction in polyurethane composites. The surface properties were measured prior to and after surface modification with FT-IR spectroscopy, TGA, zeta potential analysis and XPS, confirming the attachment of the amino-functional organosilane. The modified filler as well as an unmodified reference were incorporated into a polyurethane resin and thermal as well as mechanical properties were studied as a function of the filler loading. The results clearly showed that an appropriate dispersion of GO within the polymer matrix is crucial for the mechanical properties of the composites.