Open nanoporous morphologies from polymeric blends by carbon dioxide foaming

We report the formation of open nanoporous polymer films composed of homogeneous polysulfone/polyimide blends. Porosity is introduced by expansion of carbon dioxide-saturated films at elevated temperatures. To interpret details of the porous morphologies in terms of the experimental conditions during expansion, the glass transition temperature and carbon dioxide solubility of the dense film were examined at various blend compositions. We find that above a critical threshold of the carbon dioxide concentration the porous structure obtained changes from microcellular into open nanoporous. This critical carbon dioxide concentration is independent of the blend composition. Remarkably, it resembles the value previously reported on different polymers.