Annealing Matters: Shear Wave Propagation through a Dissimilar Polymer-Polymer Interface at MHz Frequencies Measured by QCM

Recent work by our group has demonstrated that small changes in the composition profile between dissimilar polymer domains associated with annealing can strongly alter the dynamical coupling across these domains. In contrast, other studies have reported that polymer-liquid interfaces behave similar to the interface between a polymer and a hard substrate. Collecting these ideas, we hypothesize that the mechanism underlying strong dynamic coupling across dissimilar polymer-polymer interfaces may be related to impedance matching, where similar moduli and densities and larger interfacial widths translate to more transmission of phonon modes or acoustic waves through the interface. We show using a quartz crystal microbalance (QCM) that MHz-frequency shear waves are transmitted differently through a polystyrene (PS) / poly(n-butyl methacrylate) (PnBMA) interface depending on whether the interface has been minimally annealed or annealed to equilibrium. Using a simple continuum model of shear wave reflection from an interface, we show that the data are inconsistent with the notion of only the compositional interfacial width increasing, and that there is likely a longer-ranged altered viscoelastic profile produced during the interface annealing.