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 (T_g) 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 T_g close to 250 °C due to the bulky adamantyl group attached to the polystyrene backbone. Although high-T_g blocks (polymers) lack processability in general, CO₂ effectively plasticizes and reduces the T_g of the high-T_g 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 (T_s), CO₂ pressure and depressurization temperature (T_d), to introduce nanoscopic cells in such high-T_g matrices, and achieved porosities as high as 0.25, average diameter about 20 nm and remarkable thermal robustness up to more than 200 °C.