Polymer foams with closed cells in the range of 90-200 nm and relative densities between 0.4 and 0.6 have been produced by CO2 gas dissolution foaming from neat poly(methyl methacrylate) (PMMA) and blends of PMMA and a triblock copolymer [poly(methyl methacrylate)-block poly(butyl acrylate)-block poly(methyl methacrylate), MAM]. Foams produced from these PMMA/MAM blends present a clear heterogeneous nucleation mechanism showing a clear relationship between the nanostructuration of the solid blend precursors and the nucleation efficiency of block copolymers during the saturation-expansion process. This paper discusses the role of the nanostructuration pattern of PMMA/MAM blends on the cells nucleation mechanism, showing that nucleation takes place preferably on the MAM nanostructures due to a combination of their high CO2-philicity, surface tension and morphology. (C) 2014 Elsevier B.V. All rights reserved.