Acoustic Wave Propagation as a Long-Range Mechanism for Interactions in Glasses: Experimental Evidence from Studies on Glassy-Rubbery Polymer Bilayers

Over the years several different experiments in nanoconfined systems have demonstrated unexplained long-ranged interactions of interfacial effects. For example, our group has observed profiles in local glass transition temperature Tg(z) across glassy-rubbery polymer-polymer interfaces that can span ~200+ nm. We have found that the range and magnitude of this effect correlate with the interfacial width between the two polymer domains and the modulus of the neighboring domain. More recently, we have investigated shear wave propagation in glassy polystyrene (PS) and rubbery poly(butadiene) (PB) bilayers using a quartz crystal microbalance (QCM), finding that a broad gradient in modulus G(z) emerges upon formation of the ~5 nm PS/PB compositional interface, consistent with the broad Tg(z) profile in this system. We propose that how interfaces impact the propagation of acoustic waves across the system, reflecting or transmitting at interfaces, may be key to understanding this behavior and other long-range effects in glasses. In particular, the propagation of acoustic waves ~5 nm that can hybridize with quasilocalized excitations (QLEs) associated with collective oscillations, which have been computationally linked to cooperative alpha-relaxation events. These ideas are supported by existing literature demonstrating changes to the vibrational density of states (VDoS) near the boson peak in thin films. This talk will summarize what evidence exists in support of such a long-range mechanism.