Mechanistic Picture for the Slow Arrhenius Process in Glass Forming Systems: The Collective Small Displacements Model.

Recent experiments reveal increasing detail about the so-called slow Arrhenius process (SAP), which leads to molecular relaxation in glassy systems. The SAP operates over a temperature range that spans melt and glassy states and acts alongside the familiar α-relaxation. In this Letter, we propose a mechanistic picture for SAP relaxation wherein local amorphous packing is reshaped by molecular movements that are small in comparison to those in the α-modes. Our approach, the collective small displacements (CSD) model, captures a number of experimentally observed SAP characteristics, including its connection to rheology and shear flow. Using a statistical mechanics-based equation of state to obtain nonbonded segmental interaction energies, the CSD model predicts the SAP's activation energies a priori, based solely on thermodynamic analysis of material properties. The other parameters are all shown to be reasonably material-independent, which means that the model is able to predict SAP relaxation rates and SAP-driven equilibration rates (e.g., adsorption) without using any data on the SAP at all.