Stress-Biased Rearrangements and Preyield Behavior in Glasses

Abstract

On the basis of a physical picture of a glass as a mosaic of mesoscopic clusters differing in their yield characteristics, we propose a model for the preyield behavior in glassy materials; the model describes the stress-strain relationship at different strain rates in terms of one reduced variable. A test using experimental data for polycarbonate materials at different rates and temperatures appears favorable for the model. The model may be used to interpolate and extrapolate limited experimental data, and also provides a practical means to assess dynamic heterogeneities within polymeric glasses. When applied at different temperatures, the model gives insight into the dependence of the excitation energies on temperature (glassformer fragility) in the glassy state.

Heterogeneous Structure, Heterogeneous Dynamics, and Complex Behavior in Two-Dimensional Liquids

“…The similarly defined function for all atoms (including LLA) gives a description of the part of a configuration that keeps the local structure for a given time. To relate these internal microscopic characteristics to macroscopically observable quantities, one needs a micromechanical model of the material (an example of a micromechanical model for a stressed dynamically heterogeneous material can be found in ref 26). Construction and applications of appropriate micromechanical models to describe the response of the system to perturbations is an important but separate task that is beyond the scope of this Letter.…”

Rheology of Cellulose Ether Excipients Designed for Hot Melt Extrusion

“…The viscoelastic behavior of systems with multiple relaxation times are still poorly understood with the fractional Maxwell model (FMM) approach showing some promise in describing the mathematical formalism , and the “sticky Rouse model” capturing many crucial physical details. , It should be noted that underlying reptation of entangled chains also influences the hydrogen bond relaxation time scales . Patashinski and co-workers recently proposed a new formalism to describe the yield-stress behavior in systems with a distribution of effective bond energies; we imagine that a similar approach can be used to describe the yield stress of HPMC HME due to the hydrogen bond network. Thus, the apparent yield stress would be a function of the temperature, the hydrogen bond energy distribution, the density of hydrogen bonds, and potentially even the applied external stress (see, e.g., ref ).…”