Monitoring Chemical Cross-Linking Reactions at Liquid Interfaces with Dilational Rheometry and General Stress Decomposition Analysis

A growing number of scientific challenges involve chemical cross-linking reactions at interfaces, forming membranes of nano- or micrometric thickness. The evolution of the mechanical properties of these interfaces can, in principle, be monitored with high sensitivity using interfacial rheology. However, classical dilational rheology methods, such as oscillating drop techniques, have long been limited by the rapid onset of nonlinear responses in solid-like interfaces. In this work, we demonstrate how General Stress Decomposition (GSD), derived from sensitive pressure measurements of an oscillating, spherical drop, can elegantly disentangle the mechanical response of a developing membrane into its linear viscoelastic and nonlinear components. Using a cross-linking reaction at a PEG-silicone interface as a model system, we demonstrate how GSD provides deeper insight into the impact of formulation parameters on membrane growth kinetics. This technique also allows us to put in evidence the presence of important internal mechanical stresses in the forming membrane which, in the past, have wrongly been interpreted as being due to the formation of copolymers via a side reaction.