RESUMO
Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm(3) whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films.
RESUMO
The glass-transition temperature, Tg, of isotactic PMMA thin films has been measured for four thicknesses by enhanced Raman spectroscopy and ellipsometry. This was made possible by inserting a silica spacer layer between the film and the substrate. The use of such a spacer drastically improves the sensitivity of Raman scattering measurements. The improvement in the sensitivity allows us to study phenomena involving changes in molecular dynamics, such as the phase transition, and to probe the existence in very thin films of several thickness-dependent transition temperatures, Tg(h). This in turn is interpreted as the occurrence in the film of a layered structure. The influence of the polymer concentration on the conformation of the surface adsorbed polymer layer and therefore on Tg(h) is discussed.
