ABSTRACT
Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.
ABSTRACT
A strongly adsorbed, tightly bound polymer layer can exist at the polymer/substrate interface in polymer thin films and polymer nanocomposites. The characteristics of the tightly bound layer have long been of interest because of its effect on physical properties. However, direct investigations are challenging as the layer is buried deep within the sample. A common approach to access the tightly bound layer is by rinsing or washing away the loosely bound polymer using a good solvent. While this enables direct investigations of the tightly bound layer, it is unclear if the layer remains unperturbed by the preparation process. Therefore, in situ techniques that can probe the tightly bound layer without strongly perturbing it are preferable. In previous work (P. D. Lairenjam, S. K. Sukumaran and D. K. Satapathy, Macromolecules, 2021, 54, 10931-10942), we introduced an approach to estimate the thickness of the tightly bound layer at the chitosan/silicon interface using swelling of nanoscale thin films when exposed to solvent vapour. To determine the general validity of the approach, in this work we investigated the swelling of poly(vinyl alcohol) (PVA) thin films using two independent techniques: spectroscopic ellipsometry and X-ray reflectivity. We found that the swelling kinetics for thin films of initial thickness in the range 18-215 nm could be described by a single time-dependent swelling ratio, c(t), provided we account for a tightly bound layer of thickness 15 nm at the polymer/substrate interface. Consistent with the conclusions from the swelling measurements, electron density profiles determined by modeling X-ray reflectivity data clearly indicated the existence at the polymer/substrate interface of a 15 nm thick layer of a slightly higher density than the rest of the film. The early-time diffusion coefficient of H2O in PVA determined from the temporal evolution of the mass uptake of the solvent vapour was found to decrease by 3-4 orders of magnitude when the film thickness decreased by approximately an order of magnitude.
ABSTRACT
We report an interplay between the desorption of intrinsic water and relaxation of polymer chains resulting in an unusual thermomechanical response of a hydrogel, wherein the elastic modulus increases in a certain temperature range followed by a sharp decrease with a further increase in temperature. We establish that, in a hydrogel, the desorption of disparate water types having distinct binding energy affects the consolidation and relaxation behaviour of the matrix, which in turn affects the mechanical properties at different temperature ranges. Using temperature-dependent dielectric relaxation spectroscopy and nanoindentation techniques, the chain dynamics and mechanical properties are investigated.
