Investigation of the Bound Layer in Thin Films of Hydrophilic Polymer and their Nanocomposites

The conformation of polymer chains is strongly affected in proximity to a solid, impenetrable surface. Depending on the interactions between the polymer and the surface, a distinct layer forms as the chains adsorb to the surface. Such a bound layer is known to significantly influence the structural and dynamic properties of polymer thin films and nanocomposites. In recent work, we introduced a novel approach for determining the thickness and density of this bound layer by measuring the swelling kinetics of polymer thin films exposed to solvent vapour. In this study, we have investigated the properties of the bound layer in annealed thin films of the hydrophilic polymer, Poly(vinyl alcohol) (PVA) further, and also in PVA/SiO₂ nanocomposites. The thickness of the bound layer (ds) obtained by analyzing the swelling kinetics was compared to that obtained using Guiselin's approach. The swelling kinetics of the films on exposure to water vapour was measured by in-situ spectroscopic ellipsometry and X-ray reflectivity. Intriguingly, Guiselin's approach revealed a noteworthy finding: the obtained ds values were essentially independent of the initial thickness of the films. These ds values were similar to those obtained using the swelling kinetics. In addition, the physical properties of the bound layer, as observed in PVA/SiO₂ nanocomposite films will be presented.