CO2 foaming in thin films of block copolymer containing fluorinated blocks

A fabricating method of nanocellular structures in single-layer polymeric thin films was developed using supercritical carbon dioxide and fluorinated copolymer templates as reported in our previous communication [Li, L.; Yokoyama, H.; Nemoto, T.; Sugiyama, K. AdV. Mater. 2004, 16, 1226-1229]. This paper presents the details of the fabrication of nanocellular thin films. The morphology in the as-cast films depends on the solvent casting process and plays an important role for the final structures after the following the scCO(2) process. However, the nanocellular structures are not simply templated from the nanodomains; reconstruction of the structures during the CO2 processes is not negligible. The number density of nanocells is significantly smaller than that of PFMA nanodomains before the CO2 process. Starting from spherical domains followed by an scCO(2) process at a pressure from 8 to 20 MPa, nanocells appear with an average diameter ranging from 15 to 30 nm. SEM cross-sectional views in addition to in-plane views of the nanocellular thin films prove that not open porous but closed cellular structures are formed in films irrespective of thickness. Such nanocellular formation is primarily due to swelling of PFMA domains followed by gentle removal of CO2 at reduced temperature, but expansion on depressurization still influences the size and its distribution. The size distribution of cells is effectively reduced either by lowering depressurization temperature or by adding a low molecular weight homopolymer.