Fast Dynamics in Glass Forming Liquids: Interrelated Measures and Time-scales
Glass-forming liquids exhibit upon cooling a striking divergence of their characteristic time-scale of long-time structural relaxation from that of local elastic vibration. Historically, the structural relaxation time has received a majority of attention due to its central role in defining glass formation. However, recent experimental results in the context of sugar-glass formulations for protein preservation have indicated that the size-scale of local vibration may in fact determine many important glass properties. In particular, the preservation time of proteins in these formulations has been shown to correlate strongly, not with the structural alpha relaxation time, but with the Debye-Waller factor, a measure of local rattle space. As an apparent consequence of this effect, the addition to the formulation of .antiplasticizer. additives that reduce the Deybe-Waller factor extends protein preservation time. These findings highlight the need to better understand the nature of this .fast dynamics. regime, to develop a broader metrology for its characterization, and to understand its connection to long-time dynamic processes such as protein degradation. Accordingly, we employ theory and simulation to suggest an expanded metrology of this .fast dynamics. timescale and to clarify its physical nature. Based on the Gaussian approximation for the distribution of particle displacements, we establish several straightforward interrelations between the Debye-Waller factor and other fast dynamics properties, including fast beta relaxation time and non-ergodicity parameter. We propose a new colloid-inspired model for fast glassy relaxation, based on the percolation of elastic clusters upon cooling, and find that this model provides an excellent description for the entire fast relaxation regime. Finally, we examine the connection between the Debye-Waller factor and the long-time structural relaxation time of the glassy matrix. We propose a new free-volume model interrelating these scales and examine its success in comparison to present transition-state theories for this relation. This new model provides a physically meaningful fit to data from several simulated and experimental systems, with potentially broad implications for the physics and metrology of glass-formers.
Simmons, David, "Fast Dynamics in Glass Forming Liquids: Interrelated Measures and Time-scales" (2011). Polymer Engineering Faculty Research. 1363.