Factors that Influence Long-Range Dynamical Gradients in Glasses and How They May Be Understood by Acoustic Waves

Over the past two decades, there has been mounting experimental evidence demonstrating that glassy dynamics can be correlated over exceedingly long distances from tens to hundreds of nanometers. These observations defy current theoretical formulations suggesting we are missing some physics in our understanding of glasses. This talk will summarize various experimental observations that defy current understanding and discuss how they may be understood in the context of acoustic waves that interact with the vibrational density of states around the boson peak. Work from our lab on glassy-rubbery polymer domains provides strong evidence suggesting the long-range dynamical gradient that forms across the glassy-rubbery interface arises from acoustic impedance matching of ~5 nm waves resulting in broad local glass transition temperature Tg(z) and modulus G(z) gradients. Using QCM, we demonstrate that the formation of the broad G(z) gradient can be accelerated by explicit addition of a 5 nm impedance matching layer. We also show that the long-range Tg(z) gradient observed a glassy-rubbery polymer interfaces can be reduced to the same shorter Tg(z) gradient observed near a free surface by limiting the overall domain size of the polymer. In addition, we find long-range Tg(z) gradients are obtained by end-grafted silica substrates suggesting commonalities in these phenomena.