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.

International audienceNanoparticles of TiO2 crystallizing as anatase and TiO2(B) phases with different ratios were synthesized by a three step treatment. including an initial reflux process in highly alkaline medium. For the first time, the ratio of each variety has been quantified by analysing the galvanostatic curves during the lithium electrochemical insertion. The weight percentage of TiO2(B) reaches 95% (110 m(2) g(-1) specific surface area). In order to get quantitative information based on the ratios deduced from electrochemistry, the same TiO2 samples have also been investigated by Raman spectroscopy. This technique shows a sensitivity seven times higher for anatase than TiO2(B) detection compared to electrochemical method. The equation deduced from Raman refinement and electrochemical quantification permits, for the first time, to quantify the anatase/TiO2(B) ratio simply by using Raman spectroscopy