Single-parameter aging in molecular glasses*

Physcial aging is the study of how the properties of a system change over time as it relaxes toward equilibrium. Physical aging can be observed in glasses just below the glass transition, where relaxation is slow, but still fast enough to be observed. Physical aging, the glass transition and the puzzles of supercooled liquids are thus closely linked phenomena.

In a series of papers we have shown how physical aging can be described by a single-parameter scenario through precise measurements and careful temperature protocols. Single-parameter aging is a reformulation of the Tool-Narayanaswamy idea of a fictive temperature controlling the state of the system - as well as the property monitored - during aging.

In addition, our measurements show that: 1) the high-frequency shear elastic modulus governs the relaxation rate during aging for DC704 (tetra-phenyl tetra-methyl trisiloxane), thus suggesting that in equilibrium the temperature dependence of could explain the non-Arrhenius behavior of the relaxation time (the shoving model); 2) a terminal relaxation rate exists in nonlinear relaxation curves of squalane, which is difficult to resolve in linear spectroscopic measurements; 3) non-linear aging can be predicted from equilibrium fluctuations in the molecular liquids VEC (4-vinyl-1,3-dioxolan-2-one) and NMEC (N-methyl-ε-caprolactam), as well as in computer simulations of a binary Lennard-Jones liquid.

These findings may be general for supercooled liquids.