Fabrication of High Temperature Polyphenylsulfone Nanofoams Using High Pressure Liquid Carbon Dioxide

In this paper, we describe fabrication of micro- and nano-structured foams in high temperature polyphenylsulfone (PPSU) with a T g of 219°C using a solid-state carbon dioxide (CO 2 ) foaming process. We have fabricated microcellular foams with cell size in 1–10 μm range, and for the first time, nanocellular foams with cell size in 20–40 nm range. We discovered a critical CO 2 concentration window 10.5%–11.8%, where a rapid 2–3 orders of increase in cell nucleation density is observed and cell size drops to below 100 nm. Two approaches are demonstrated to create PPSU nanofoams with a cell size of 20–40 nm: increasing saturation pressure to 7 MPa or decreasing saturation temperature to 10°C. Both approaches use high pressure liquid CO 2 and the CO 2 concentration in the polymer are above the critical concentration window, which result in the formation of nanocells. At high CO 2 concentration of 11.8% and 13%, as foaming temperature increases, the nanostructures transition from closed structure to an open porous structure.