Impact of annealing and adsorption on the distribution of segmental mobility and tracer diffusivity of ultrathin films of polystyrene

We show experimental evidence that the changes ultrathin films undergo during annealing are strongly correlated to the amount of chains irreversibly adsorbed at the interface. A careful analysis of the time evolution of the dielectric function during annealing steps above Tg revealed three different regimes: at times much shorter than the adsorption time, the thickness of the adsorbed layer is constant and the interface mimics the effect of a free surface (packing frustration); upon increase of surface coverage, the films undergo a series of metastable states characterized by the largest changes in the deviations from bulk behavior; finally, when the thicknesses of the irreversibly adsorbed layer doubles its starting value, the system approach a new equilibrium whose properties are fixed by the new interfacial configurations. Our picture is further confirmed by the effect of annealing on the distribution of glass transition temperatures [1], dielectric relaxation strength and tracer diffusivity at different distances from the adsorbing interface. \\[4pt] [1] Rotella, Napolitano et al. Macromolecules, 2010, 43, 8686-8691