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.