Physical aging studied by a device allowing for rapid thermal equilibration

TL;DR

This study introduces a rapid thermal equilibration device enabling detailed observation of aging in organic glasses, revealing universal features such as a material-dependent internal clock and exponential long-time relaxation.

Contribution

The paper presents a novel device that extends observable aging times by two orders of magnitude, allowing detailed analysis of aging dynamics in multiple organic glasses using the Tool-Narayanaswamy formalism.

Findings

All five liquids have an internal clock controlling aging.

No expansion gaps observed between relaxation rates.

Long-time structural relaxation is exponential.

Abstract

Aging to the equilibrium liquid state of organic glasses is studied. The glasses were prepared by cooling the liquid to temperatures just below the glass transition. Aging following a temperature jump was studied by measuring the dielectric loss at a fixed frequency using a microregulator in which temperature is controlled by means of a Peltier element. Compared to conventional equipment the new device adds almost two orders of magnitude to the span of observable aging times. Data for the following five glass-forming liquids are presented: Dibutyl phthalate, diethyl phthalate, 2,3-epoxy propyl-phenyl-ether, 5-polyphenyl-ether, and triphenyl phosphite. The aging data were analyzed using the Tool-Narayanaswamy formalism. The following features are found for all five liquids: 1) Each liquid has an "internal clock", a fact that is established by showing that the aging of the structure is controlled by the same material time that controls the dielectric properties. 2) There are no so-called expansion gaps between the long-time limits of the relaxation rates following up and down jumps to the same temperature. 3) At long times the structural relaxation is not stretched, but a simple exponential decay. 4) For small temperature steps the rate of the long-time exponential structural relaxation is identical to that of the long-time decay of the dipole autocorrelation function.