(612h) Anomalous Swelling of Thin Films with CO2

What has become obvious in the last year or so is that we do not have a clear understanding of why thin polymer films appear to anomalously ?swell? in CO2 under certain conditions. This is an important problem within the context of using CO2 as a processing solvent in microelectronic applications. One potential adverse effect would be the de-lamination of the film from the substrate. We would like to provide a definitive answer to this problem by using a combination of Sanchez-Lacombe Equation of State and interfacial tension theory. We will investigate the following hypothesis: The anomalous swelling is NOT caused by an increase of solubility of CO2 in the polymer film. Our position, based on a thermodynamics argument, is that CO2 becomes less soluble in a polymer when CO2 becomes more compressible. One explanation is that in the very compressible region of CO2, the expulsion of CO2 is not uniform within the thin film. The presence of the substrate could act as an attractive and nucleating surface for the accumulation of CO2 at the polymer/substrate interface. The accretion of relative low density CO2 at the interface would increase the overall film thickness (volume expansion). This effect would not be a significant percentage wise in a thick film, but it would be in a thin film. We plan to use compressible model of a CO2/polymer (PDMS and PMMA) and calculate the affect of an attractive surface on the distribution of CO2 within the film. Under conditions where CO2 is very compressible, it is expected that the bulk solubility of the CO2 in the film will decrease; the expulsion of the solvent will not be uniform and a CO2 rich layer will form at the substrate-film interface. This is the hypothesis that we wish to check by carrying out this calculation.