By foaming plastics, unique product properties can be realized in addition to weight and cost reduction. With an increasing number of cells and small cell diameters, foams can be used, for example, as insulating materials with low thermal conductivity. For the production of foamed plastics by foam extrusion, the gas can be introduced into the polymer melt either by chemical or physical blowing agents. For the manufacture of low density products, nucleating agents are added to the physical blowing agent to increase the number of nucleation sites. However, as the proportion of gas cavities increases, the proportion of polymer responsible for mechanical stability decreases. This can be counteracted by crosslinking the polymer. The aim of this work was to modify the surface of a nucleating agent in such a way that both the foaming process and the crosslinking of polyethylene are supported. The first part of the work consisted of binding two types of initiators to the surface of the nucleating agent. These initiators do not only release gas to support the foaming process, but also form stable radicals to support silane crosslinking. With the help of various analytical methods, it could be demonstrated that the initiators can be bound to the surface of the nucleating agent. Moreover, the influence of the modified filler on the crosslinking reaction was investigated. For this purpose, crosslinked samples were produced both on a laboratory scale and by reactive extrusion. Both methods showed that the use of the modified filler led to a higher crosslinking density. The influence of the molecular structure of two polyethylene types on crosslinking was also investigated. It turned out that the type with the higher number of short chain branches and an even distribution of comonomers led to a higher crosslinking density. In the last part of this thesis the influence of the modified nucleating agents on the foam structure was investigated. From a certain amount on, a foam structure with a smaller mean cell diameter and lower density compared to the use of the pristine nucleating agent was realized with the help of one of these modified nucleating agents.