Abstract: A polymer foam is produced comprising a polymer and nanoparticles having a maximum dimension of 750 nm, which foam has cells with an average cell size of at most 1 ?m and a cell density of at least 1012 cells/ml, wherein polymeric grafts have been attached to the nanoparticles. The nanoparticles may be particles with a solid core or porous hollow core-shell particles. The foam can be manufactured by dispersing the nanoparticles in a polymer to yield a dispersion; by adding a blowing agent to the dispersion to obtain an expandable mixture; and by foaming the expandable mixture to obtain the polymer foam.