Spatial heterogeneities in structural temperature cause Kovacs expansion gap paradox in aging of glasses

TL;DR

This paper demonstrates that spatial heterogeneities in structural temperature explain the Kovacs expansion gap paradox in glass aging, using a lattice model to capture asymmetric relaxation and domain dynamics.

Contribution

It introduces a lattice model that reproduces the Kovacs paradox by incorporating spatial heterogeneity in structural temperature and domain nucleation processes.

Findings

The model captures asymmetric relaxation upon heating and cooling.

Spatial heterogeneity in structural temperature explains the expansion gap.

Relaxation involves nucleation and growth of warmer domains.

Abstract

Volume and enthalpy relaxation of glasses after a sudden temperature change has been extensively studied since Kovacs seminal work. One observes an asymmetric approach to equilibrium upon cooling versus heating and, more counter-intuitively, the expansion gap paradox, i.e. a dependence on the initial temperature of the effective relaxation time even close to equilibrium when heating. Here we show that a distinguishable-particles lattice model can capture both the asymmetry and the expansion gap. We quantitatively characterize the energetic states of the particles configurations using a physical realization of the fictive temperature called the structural temperature, which, in the heating case, displays a strong spatial heterogeneity. The system relaxes by nucleation and expansion of warmer mobile domains having attained the final temperature, against cooler immobile domains maintained at the initial temperature. A small population of these cooler regions persists close to equilibrium, thus explaining the paradox.