NMR Imaging of Water Uptake in Multilayer Polymeric Films: Stressing the Role of Mechanical Stress

The penetration of water into two-layer polymeric films of a hydrophilic base layer and hydrophobic top layer plays an important role in their performance. Little is known about the coupled effects of water uptake and stress in such films. To study such interactive phenomena, time-dependent distributions in the film are needed, which cannot be provided by traditional techniques. In this study, high-resolution NMR imaging was used to measure water profiles, showing that applied stress increased both the uptake rate and the amount of absorbed water. A model was formulated to describe the process, using the barrier properties of and the chemical potential differences over the top coat. On the basis of this, the diffusivity in the top layer and stress contribution were estimated. The results show that external stresses significantly influence water penetration into multilayer films.