Nanocellular Structures in Block Copolymers with CO_2-philic Blocks Using CO_2 as a Blowing Agent: Crossover from Micro-to Nanocellular Structures with Depressurization Temperature

摘要

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 eilect of block copolymer nanodomains in the conventional process.