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International audience; Beyond the peculiar glass transition temperature (Tg) studied extensively for over two decades, we have investigated in this work the variation of the mass density of polymer thin films, a key property barely put forward in the literature. We were able to directly measure the mass density of polystyrene (PS) and poly (methyl methacrylate) (PMMA) thin films as a function of their thickness from accurate Quartz Crystal Microbalance (QCM) dissolution experiments. Depending on the chemical nature of the polymers, the results showed an unambiguous variation of the mass density when the film thickness was reduced: an increase for PS and a decrease for PMMA films. In the last part, we measured the coefficient of thermal expansion (CTE) of both polymers to rationalize the apparent inconsistency of an increase/decrease in the mass density with the commonly observed depression/rise of the Tg of PS/PMMA films. Surprisingly, the coefficient of thermal expansion and the mass density showed a similar variation with thickness, pointing out a clear correlation between the glass transition temperature, the coefficient of thermal expansion and the free volume of the polymer.

We report here the swelling and relaxation properties of confined poly(n-butyl methacrylate) (PBMA) films having thicknesses of less than 70 nm under supercritical carbon dioxide (scCO2) using the X-ray reflectivity technique. Swellability is found to be dominant in thinner films compared to thicker ones as a consequence of the confinement-induced densification of the former. Swellability is proportionately increased with the density of the film. PBMA films exhibit a more significant swelling than do PS films, and their differences become more prominent with the increase in film thickness. A comparison between the results obtained for polystyrene (PS) and PBMA ultrathin films reveals that the swellability is dependent upon the specific intermolecular interaction between CO2 and the chemical groups available in the polymers. Owing to strong Lewis acid-base interactions with scCO2 and the lower glass-transition temperature (bulk Tg ≈ 29 °C), PBMA films exhibit a greater amount of swelling than do PS films (bulk Tg ≈ 100 °C). Though they reach to the different swollen state upon exposition, identical relaxation behavior as a function of aging time is evidenced. This unprecedented behavior can be ascribed to the strong bonding between trapped CO2 and PBMA that probably impedes the release of CO2 molecules from the swollen PBMA films manifested in suppressed relaxation.