Role of interfacial interactions on the anomalous swelling of polymer thin films in supercritical carbon dioxide

Abstract

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO 2 (Sc-Co 2 ) in the vicinity of the critical point of CO 2 . The adsorption isotherm of CO 2 on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anomalous maxima under similar CO 2 conditions. It is believed that because CO 2 possesses a low cohesive energy density, there would be an excess amount of CO 2 at the surfaces of these materials and hence the CO 2 /polymer interface. This might cause excess CO 2 in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO 2 would reside at the polymer/substrate and polymer/CO 2 interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO 2 in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anomalous swelling of polymer thin films of varying initial thicknesses, h 0 , exposed to Sc-CO 2 . We examined three homopolymers, poly(1,1′-dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO 2 , and the diblock copolymer of PS- b -PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO 2 at interfaces. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1313–1324, 2007