Fabricated by high-pressure or supercritical CO2 gas dissolution foaming process, nanocellular and microcellular polymer foams based on poly(methyl methacrylate) (PMMA homopolymer) present a controlled nucleation mechanism by the addition of a methylmethacrylate-butylacrylate-methylmethacrylate block copolymer (MAM), leading to defined nanocellular morphologies templated by the nanostructuration of PMMA/MAM precursor blends. Influence of the CO2 saturation temperature on the foaming mechanism and on the foam structure has been studied in 90/10 PMMA/MAM blends and also in the neat (amorphous) PMMA or (nanostructured) MAM polymers, in order to understand the role of the MAM nanostructuration in the cell growth and coalescence phenomena. CO2 uptake and desorption measurements on series of block copolymer/homopolymer blend samples show a competitive behavior of the soft, rubbery, and CO2-philic block of PBA (poly(butyl acrylate)) domains: fast desorption kinetics but higher initial saturation. This competition nevertheless is strongly influenced by the type of dispersion of PBA (e.g. micellar or lamellar) and a very consequent influence on foaming. CO2 sorption and desorption were characterized in order to provide a better understanding of the role of the block copolymer on the foaming stages. Poly(butyl acrylate) blocks are shown to have a faster CO2 diffusion rate than poly(methyl methacrylate) but are more CO2-philic. Thus gas saturation and cell nucleation (heterogeneous) are more affected by the PBA block while cell coalescence is more affected by the PMMA phases (in the copolymer blocks + in the matrix). (C) 2014 Elsevier B.V. All rights reserved.