Irreversibly adsorbed layer in supported ultrathin polymer film investigated by local dielectric spectroscopy

Polymer chains can adsorb onto a solid substrate without the formation of chemical bonds. Because this mechanism of adsorption is driven by the weak dipolar interactions and requires simultaneous pinning of many repeating units of the chain, its kinetics can be extremely slow, especially for polymer melt. As a consequence, polymer chains at the interface with a substrate can reside for very long times in non-equilibrium states, before reaching the equilibrium configuration. Remarkably, recent works verified that the deviations from the bulk behavior in the dynamics of nanoconfined polymers are strongly affected by those non-equilibrium configurations assumed in adsorbed layers. In this Chapter, we report experimental evidences on the existence of an irreversibly adsorbed layer in poly(vinyl acetate) (PVAc) films in contact with different substrates. The presence of such a layer is proved through atomic force microscopy imaging of the residual layer remaining on the substrate after washing the polymer film in a good solvent. Moreover, we demonstrate that the evolution of the irreversibly adsorbed layer is unambiguously related to the change in relaxation dynamics of polymer films under annealing at a high temperature (~T_g + 60 K). Finally, we demonstrate the direct effect of this adsorbed layer on the maximum moisture uptake of supported ultrathin PVAc films, hence providing a simple approach for controlling the moisture absorption of the nanosized polymer films.