Thermally robust nanocellular thin films of high-Tg semifluorinated block copolymers foamed with supercritical carbon dioxide

Abstract

Nanoscopic porous and cellular materials with high thermal stability are demanded for a variety of applications. We fabricated thermally robust nanocellular thin films using block copolymer templated carbon dioxide foaming (BCTCF). We synthesized semi-fluorinated block copolymers having a CO2-philic fluorine containing block and high glass transition temperature (Tg) blocks, such as poly[4-(1-adamantyl)styrene-b-perfluorooctylethyl methacrylate] (P(AdSt-FMA)) and poly[α-methylstyrene-b-perfluorooctylethyl methacrylate] (P(AMSt-FMA)). In particular, PAdSt has a Tg close to 250 °C due to the bulky adamantyl group attached to the polystyrene backbone. Although high-Tg blocks (polymers) lack processability in general, CO2 effectively plasticizes and reduces the Tg of the high-Tg blocks, swells the fluorinated block domains, and leaves empty nanocells in the fluorinated block domains after CO2 was removed. We optimized process conditions, such as saturation temperature (Ts), CO2 pressure and depressurization temperature (Td), to introduce nanoscopic cells in such high-Tg matrices, and achieved porosities as high as 0.25, average diameter about 20 nm and remarkable thermal robustness up to more than 200 °C.