The effect of density fluctuations in supercritical fluids: new science and technology for polymer thin films

Supercritical carbon dioxide (scCO2) is being used increasingly as a green solvent in polymer processing. However, the major disadvantage thus far is that only a limited class of polymers, such as fluorinated or silicone-based polymers, can be dissolved in scCO2. Here we show that large density fluctuations in scCO2 can significantly enhance the solubility of scCO2 with polymer thin films even when the bulk polymers have very poor miscibility with scCO2. In addition, by utilizing quick solvent evaporation of CO2, we could preserve the swollen structures, resulting in low-density polymer thin films where nanometer-scale porosity was significantly introduced. The use of the low-density polymer thin films could allow us to develop a new gas membrane that could selectively permit a large flow of small molecule gases, such as O2 and CO2, while completely blocking out larger gases or particulates.