GLASSY AGING DYNAMICS

1. Introduction

When a glassforming liquid is cooled fast enough, below its glass temperature T_g it becomes a glass - a "frozen liquid" - that is out of thermodynamic equilibrium. Keeping then the glass at a constant temperature, the so-called "physical aging" takes place, i.e. its physical properties vary with time when the material tries to regain the equilibrium state. This is of considerable technical importance in glass fabrication and for the degradation of glassy materials in applications as glass fibers, optical components or polymers.

Schematic plot of the temperature dependence of a material property around the glass temperature (solid line). When crossing Tg, a weaker temperature dependence is found. The dashed line shows the equilibrium curve. The vertical arrow indicates the approach of equilibrium under aging at constant temperature [from: P. Lunkenheimer and A. Loidl, Glassy dynamics: From millihertz to terahertz, in The scaling of relaxation processes, edited by F. Kremer and A. Loidl (Springer, Cham, 2018), p. 23.]

2. Examples:

We have performed aging experiments, measuring time-dependent dielectric properties in a large variety of different glass formers [3-6]. We showed that the aging of glasses can be described much simpler than by the many-decades old models used so far. We found that the time characterizing the rapidity of the aging is identical to the time characterizing the microscopic molecular motions (the "α relaxation") and that this time itself varies with time during aging. We found a self-consistent recursive formula to describe the time-dependence of the physical properties of the glass, which is straightforward to apply and involves less parameters than the models used so far by glass physicists and engineers [3,6] (see Figure). Overall, the aging of glasses is governed by the same mechanisms as the dynamic equilibrium properties of the corresponding liquids.

Time dependence of dielectric loss (a-c) or modulus (d) at various frequencies for four glassformers after quickly cooling from the liquid into the glassy state. The lines are fits with the new approach. For each material, the main fit parameters are the same for all frequencies and fully consistent with the results at temperatures above the glass transition. [P. Lunkenheimer, R. Wehn, U. Schneider, and A. Loidl, Glassy aging dynamics, Phys. Rev. Lett. 95, 055702 (2005).]

In addition, we have performed time-dependent measurements of the excess-wing region, showing up in the broadband dielectric spectra of many glassforming liquids as a second power law at the high-frequency flank of the α-peak [1,2]. Its physical origin is a long-standing riddle of glass physics. Applying maximum aging times of up to five weeks, at temperatures below the glass temperature the excess wing develops into a shoulder. These results clearly indicate that the excess wing is due to a secondary relaxation process. Click here to learn more.

3. Some relevant publications from our group: