Supercritical fluid processing of polymer thin films: An X-ray study of molecular-level porosity

This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO₂) on polymer thin film processing. The key phenomenon is excess sorption of CO₂ molecules into polymer thin films even when the bulk miscibility with CO₂ is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO₂ near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO₂, we can preserve the "expanded" structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally "green" manner, without organic solvents, and are not specific to any class of polymers.