In polymer physics, the dewetting of spin-coated polystyrene ultrathin films on silicon remains mysterious. By adopting a simple top-down method based on good solvent rinsing, we are able to prepare flat polystyrene films with a controlled thickness ranging from 1.3 to 7.0 nm. Their stability was scrutinized after a classical annealing procedure above the glass transition temperature. Films were found to be stable on oxide-free silicon irrespective of film thickness, while they were unstable (<2.9 nm) and metastable (>2.9 nm) on 2 nm oxide-covered silicon substrates. The Lifshitz-van der Waals intermolecular theory that predicts the domains of stability as a function of the film thickness and of the substrate nature is now fully reconciled with our experimental observations. We surmise that this reconciliation is due to the good solvent rinsing procedure that removes the residual stress and/or the density variation of the polystyrene films inhibiting thermodynamically the dewetting on oxide-free silicon.

The effect of increasing pressure and two-dimensional (2D) confinement on the dynamics of glass-forming polymer poly­(methylphenylsiloxane) (PMPS) was investigated with the use of dielectric spectroscopy. We demonstrate that the glass-forming polymer confined to nanoporous alumina might obey the density scaling relation similar to that in the bulk and that the same value of the scaling exponent is used to superimpose the α-relaxation time measured under different thermodynamic conditions. Our comprehensive analysis of the relaxation processes detected in the dielectric loss spectra of PMPS allows us to identify the Johari–Goldstein β-relaxation which for a bulk polymer shows up as a well-resolved peak while under 2D nanoconfinement only as an excess wing. In contrast to previous studies, we provide dielectric evidence of an additional α′-relaxation, slower than the segmental (α-) dynamics, which is related to the chain dynamics of PMPS.

International audienceAfter more than 2 decades of intense research, the density variation in confined polymer films still remains a puzzling problem subject to controversy as the methods utilized to determine the density are often model dependent. Here, we propose a direct and model independent method to detect the density/refractive index variations in polymer thin films through the adsorption of ceria nanoparticles (NPs) onto their surface. The amount of adsorbed NP scales with the polymer film refractive index; hence, any increase/ decrease in the NP surface coverage directly indicates an increase/decrease in the film refractive index and density. Experimenting our proposed novel approach on two well-studied polymers, we found that the density of polystyrene (PS) thin films deposited on oxide-free Si substrate increases with a reduction of the film thickness. On the contrary, poly(methyl methacrylate) (PMMA) films deposited on wafers with native silicon oxide show a decrease of their density when the film thickness is reduced