Physical aging in molecular glasses beyond the α relaxation

Abstract

The description of kinetics of physical aging, namely the slow evolution of a glass thermodynamic state toward equilibrium, generally relies on the exclusive role of the main α relaxation. Here, we study the kinetics of physical aging over a wide temperature range in five small molecules interacting via van der Waals forces monitoring the time evolution of the glass enthalpic state. To this aim, we employ fast scanning calorimetry, which permits exploring a wide range of aging times. To challenge the role of the α relaxation in the description of physical aging, we employ a model-independent approach, based on the time to reach equilibrium, and a modified version of the single parameter aging model. The latter accounts for the non-linearity of aging making use of the so-called density scaling approach to describe the dependence of the α relaxation time on the glass thermodynamic state. We show that the α relaxation is generally adequate to describe aging at temperatures close to the glass transition and, for lower temperatures, the latest stages of equilibration. In contrast, at low aging temperatures, it fails to catch a wide portion of the time-dependent evolution of the glass thermodynamic state, which is found to be much faster than predicted considering only the α relaxation. Hence, our results and analysis provide compelling arguments that the description of glass equilibration under a wide range of aging conditions is conveyed by different molecular mechanisms, beyond the mere role of the α relaxation.

The slow Arrhenius process in small organic molecules

“…Growing experimental evidence shows, however, that systems can evolve towards more stable, less energetic states also through faster pathways, characterized by a constant activation barrier. For instance, as shown by dewetting, 3 physical aging, 4–7 crystallization, 8,9 shear relaxation 10 experiments and surface diffusion measurements, 11,12 materials can equilibrate within timescales much shorter than that predicted by models based on the α-relaxation only.…”

Length Scale of Molecular Motions Governing Glass Equilibration in Hyperquenched and Slow-Cooled Polystyrene

“…Similar VFT-to-Arrhenius crossover in temperature dependence of aging kinetics was previously observed in poly(vinyl chloride), 47 poly(vinyl acetate), 48 conjugated polymer of poly[2,5-bis(3-dodecylthiophen-2-yl)thieno [3,2-b]thiophene], 49 polyurethane, 50 and also in nonpolymeric molecular glasses. 51 This raises a fundamental question concerning the molecular origin of this crossover and the microscopic mechanism underlying the glass equilibration in different temperature ranges.…”

“…Instead, when T a < T c , a thermally activated Arrhenius behavior with an apparent activation energy E a = (290 ± 20) kJ/mol is observed (Figure S2 demonstrates this unique E a value obtained by TTS of aging data), suggesting the dominance of an alternative molecular mechanism at low temperature decoupled from the high- T cooperative motion. Similar VFT-to-Arrhenius crossover in temperature dependence of aging kinetics was previously observed in poly(vinyl chloride), poly(vinyl acetate), conjugated polymer of poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2- b ]­thiophene], polyurethane, and also in nonpolymeric molecular glasses . This raises a fundamental question concerning the molecular origin of this crossover and the microscopic mechanism underlying the glass equilibration in different temperature ranges.…”

Crossing the Frontier of Validity of the Material Time Approach in the Aging of a Molecular Glass

“…A typical aging experiment consists in setting a system out-of-equilibrium, for instance, by the application of a mechanical load, a static electric field, or a change in temperature, − and observing its reequilibration dynamics through the evolution of a quantity such as its enthalpy, , density, refractive index or dielectric quantities. − In an ideal step experiment, care is taken to destabilize the system quickly enough so that no relaxation occurs, thus simplifying the interpretation of aging dynamics. ,,,, When the amplitude of the perturbation is extremely small, the response is linear (i.e., simply proportional to the perturbation amplitude) . However, temperature step experiments are usually not in this regime, as an amplitude of only a few kelvin already yields a nonlinear response.…”

“…5 From a fundamental perspective, the study of aging provides information on the out-of-equilibrium relaxation mechanisms. 6,7 A typical aging experiment consists in setting a system outof-equilibrium, for instance, by the application of a mechanical load, 8 a static electric field, 9 or a change in temperature, 10−18 and observing its reequilibration dynamics through the evolution of a quantity such as its enthalpy, 16,18 density, 19 refractive index 18 or dielectric quantities. 12−15 In an ideal step experiment, care is taken to destabilize the system quickly enough so that no relaxation occurs, thus simplifying the interpretation of aging dynamics.…”