Physical aging studied by a device allowing for rapid thermal equilibration

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.