Effect of Thermal Annealing on the Bound Layer in Polymer Thin Films: Insights from Swelling Kinetics and Guiselin’s Approach

Abstract

In supported polymer thin films, dependent on the polymer–substrate interactions, a distinct polymer layer forms next to the polymer–substrate interface. This distinct, buried layer is typically investigated using “Guiselin’s approach” which involves washing the thin film in a good solvent. Recently, we introduced a novel approach wherein the thickness of the distinct layer was obtained using only the swelling kinetics of the thin films. However, the relationship between the characteristics of the distinct layer as determined by the two approaches remains unclear. To address this, we have used spectroscopic ellipsometry and X-ray reflectivity measurements of spin-coated PVA thin films to systematically compare the characteristics of the “bound layer”, determined using an analysis of the time-dependent swelling, and the “adsorbed layer”, obtained using Guiselin’s approach. In particular, we investigated the effect of annealing above T_g for varying time durations, t_ann. The thickness of the bound layer, d_s, and that of the adsorbed layer, d_ads, were found to be nearly independent of the initial (dry) thickness of the thin film, d(0). For t_ann = 0, d_s ≈ 5 d_ads, suggesting that the bound layer included a significant fraction of chains that were weakly adsorbed. With increasing t_ann, d_s exhibited a monotonic decrease from ≈15 nm to a plateau value of ≈7 nm, while d_ads exhibited a rapid monotonic increase from ≈3 nm and approached a plateau value of ≈11 nm. Irrespective of t_ann and d_ads(0), the adsorbed layers exhibited a maximum increase in thickness of ≈2.7–4.5 nm when exposed to H₂O vapor. Interestingly, upon increasing t_ann, while the short-time diffusion coefficient of H₂O into the polymer thin film was found to decrease by approximately an order of magnitude, the corresponding effect on the adsorbed layer appeared to be significantly weaker.