Nanocellular Structures in Block Copolymers with CO2-philic Blocks Using CO2 as a Blowing Agent: Crossover from Micro- to Nanocellular Structures with Depressurization Temperature

Uniform nanocellular structures are successfully formed within the spherical nanodomains of CO 2 -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 (CO 2 ). 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 CO 2 -philic block nanodomains: T d must be well below the glass transition temperature ( T g ) of the skeleton PS domains in the presence of CO 2 . As T d is raised and approaches T g, 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 T d 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 CO 2 either into the growing microcells or to the surface to evaporate. When T d is set above T g, 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.