Moisture absorption into ultrathin hydrophilic polymer films on different substrate surfaces

Abstract Moisture absorption into ultrathin poly(vinyl pyrrolidone) (PVP) films with varying thickness was examined using X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements. Two different surfaces were used for the substrate: a hydrophilic silicon oxide (SiO x ) and a hydrophobic hexamethyldisilazane (HMDS) treated silicon oxide surface. The total equilibrium moisture absorption (solubility) was insensitive to the surface treatment in the thickest films (≈150 nm). However, strong reductions in the equilibrium uptake with decreasing PVP film thickness were observed on the HMDS surfaces, while the SiO x surface exhibited thickness independent equilibrium absorption. The decreased absorption with decreasing film thickness is attributed a depletion layer of water near the polymer/HMDS interface, arising from hydrophobic interactions between the surface and water. The diffusivity of water decreased when the film thickness was less than 60 nm, independent of the surface treatment. Changes in the properties of ultrathin polymer films occur even in plasticized films containing nearly 50% water.

Moisture Absorption into Ultrathin Hydrophilic Polymer Films on Different Substrate Surfaces

Swelling of poly(3-alkylthiophene) films exposed to solvent vapors and humidity: Evaluation of solubility parameters

X-ray scattering investigations of surfaces and interfaces in soft-condensed matter are reviewed. Both high-resolution structural determinations in the direct space and investigations of fluctuations with long-range correlations requiring a high resolution in the Fourier space are discussed. All the scattering cross-sections for diffraction or diffuse scattering are derived within a unified frame, and the experimental aspects related to their measurement are discussed in detail. The general principles are illustrated by various examples of studies of the liquid-vapour interface, Langmuir and Langmuir-Blodgett films, wetting films, polymer films, liquid crystals and liquid-liquid interfaces.

High-resolution x-ray scattering measurements: I. Surfaces

Our previous publications have demonstrated that thin glassy polymer films respond to highly sorbing penetrants, such as CO2, quite differently than thick films. These studies focused on CO2 permeation behavior and revealed that, for thin films, CO2 permeability at constant CO2 pressure goes through a maximum followed by a continual decrease in permeability owing to physical aging. So far, thick and thin glassy polymer films have been compared in the context of permeability, but lack of substantial means of obtaining thin film sorption data has prevented adequate comparison of thick and thin films in the context of gas solubility. In this paper, spectroscopic ellipsometry is used to obtain simultaneously the film thickness and CO2 sorption capacity for thin glassy polymer films. This allows a more comprehensive look at CO2 permeability, sorption, and diffusivity as a function of both CO2 pressure and exposure time. The evidence reported here suggests that thin film sorption behavior is substantially diffe...

Carbon Dioxide Sorption and Plasticization of Thin Glassy Polymer Films Tracked by Optical Methods

Water absorption in thin films (<1000 A) of a commercial polyimide was evaluated by monitoring dimensional changes induced by a humid environment. Film thickness was measured using x-ray reflectivity, which is a nondestructive technique offering angstrom resolution in the measurements of thin film or multilayer thickness. The effect of several variables on the absorption of moisture were monitored in polyimide films adhered to polished silicon substrates, including total dry film thickness, exposure time, and the contribution ofa coupling agent. The percentage increase in film thickness due to moisture uptake is found to be a weak function of dry film thickness, decreasing as dry film thickness increases, and to be somewhat affected by the use of an interfacial coupling agent. The observed behavior points to the polymer/substrate interface as a strong factor controlling the absorption of moisture in the polyimide/ silicon system, and is believed to reflect the presence of a highly moisture-saturated interfacial layer. A bilayer model is proposed, and the feasibility of using this model to describe the observed behavior is considered.

W. E. Wallace

Papers

Interface effects on moisture absorption in ultrathin polyimide films