Nanocellular foaming of polystyrene (PS) and a polystyrene copolymer (PS-b-PFDA) with fluorinated block (1,1,2,2-tetrahydroperfluorodecyl acrylate block, PFDA) was studied in supercritical CO2 (scCO2) via a one-step foaming batch process. Atom Transfer Radical Polymerization (ATRP) was used to synthesize all the polymers. Neat PS and PS-b-PFDA copolymer samples were produced by extrusion and solid thick plaques were shaped in a hot-press, and then subsequently foamed in a single-step foaming process using scCO2 to analyze the effect of the addition of the fluorinated block copolymer in the foaming behaviour of neat PS. Samples were saturated under high pressures of CO2 (30 MPa) at low temperatures (e.g., 0 degrees C) followed by a depressurization at a rate of 5 MPa/min. Foamed materials of neat PS and PS-b-PFDA copolymer were produced in the same conditions showing that the presence of high CO2-philic perfluoro blocks, in the form of submicrometric separated domains in the PS matrix, acts as nucleating agents during the foaming process. The preponderance of the fluorinated blocks in the foaming behavior is evidenced, leading to PS-b-PFDA nanocellular foams with cell sizes in the order of 100 nm, and bulk densities about 0.7 g/cm3. The use of fluorinated blocks improve drastically the foam morphology, leading to ultramicro cellular and possibly nanocellular foams with a great homogeneity of the porous structure directly related to the dispersion of highly CO2-philic fluorinated blocks in the PS matrix. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012