Uniform nanocellular structures are successfully formed within the spherical nanodomains Of CO2-philic fluorinated blocks in Poly [styrene-block-4-(perfluorooctylpropyloxy)styrene] (PS-PFS) and Poly[styrene-block-perfluorooctylethyl methacrylate] (PS-PFMA) monoliths using supercritical (SC) carbon dioxide (CO2). The nanocells have a very small surface area, indicative of the closed cell structure. Temperature of depressurization (T-d) is the key to the uniform nanocellular formation in the CO2-philic block nanodomains: Td must be well below the glass transition temperature (T,) of the skeleton PS domains in the presence of CO2. As Td is raised and approaches T, the nanocellular structure crosses over to the microcellular structure on the order of micrometers, which is the typical cell structure formed via the conventional foaming mechanism. Two independent cell formation mechanisms coexist when Td is in the vicinity of T-g The nanocells have upper limit diameter of ca. 40 nm, which is apparently determined by the balance between the expansion of nanocells and diffusion Of CO2 either into the growing microcells or to the surface to evaporate. When Td is set above T, we simply find conventional microcells only slightly smaller than those formed in homopolystyrene, which are indicative of the negligible effect of block copolymer nanodomains in the conventional process.