Novel structure and dynamics of polymer thin films in supercritical fluids-effect of density fluctuation

Supercritical carbon dioxide (scCO₂) is being used increasingly as a green solvent in polymer processing. The major disadvantage thus far is that only a limited class of polymers, such as fluorinated or silicone-based polymers, can be dissolved in CO₂. Here I show that large density fluctuations in scCO₂ can significantly enhance the solubility of scCO₂ in polymer thin films even when the bulk polymers have very poor miscibility with CO₂. By using in situ neutron reflectivity, I found that a wide variety of polymer thin films can swell as much as 30-60% when exposed to scCO₂ within a narrow temperature and pressure regime, known as the "density fluctuation ridge", which defines the maximum density fluctuation amplitude in CO₂. Furthermore, the swollen structures induced by the density fluctuation could be frozen by a flash evaporation of CO₂ via the vitrification process of the polymer without a formation of void structures. X-ray reflectivity clearly showed that the scCO₂ process could be used to produce uniform low-density polymer thin films. I also found that other properties of the vitrified films, such as index of refraction, dielectric constant and glass transition, were correlated with the low-density density profile.