We investigate the physical foaming process of glassy poly(ether imide) and poly(ether sulfone) using carbon dioxide and report temperature-concentration diagrams ("foam diagrams") marking out the foaming envelope in which dense CO2-saturated films expand and microvoids are introduced. Two types of porosities are observed. Closed microcellular structures occur at carbon dioxide saturation levels below 50 cm3 (STP)/cm3 (polymer); nanoporous bicontinuous (open) structures with pore sizes as small as 40 nm occur above this CO2 concentration threshold, which is identical for both polymers. The cellular-to-bicontinuous transition is characterized in detail on the basis of gas permeation measurements and is represented as a separate window inside the foaming diagram. In this paper, the transition to bicontinuous structures is reported for the first time, and its generic physical basis is critically reviewed.
Krause, B., Münüklü, P., van der Vegt, N. F. A., Wessling, M., & Sijbesma, H. P. (2001). Bicontinuous nanoporous polymers by carbon dioxide foaming. Macromolecules, 34(25), 8792-8801. https://doi.org/10.1021/ma010854j