Fast temperature up steps as a test of the Tool-Narayanaswamy formalism

We investigated the aging dynamics of a glass-forming liquid triethyl-2-acetylcitrate, following fast temperature up steps with amplitudes ranging from 0.3 to 13.6 K. The initial states were either at equilibrium or prepared at increasing levels of out-of-equilibrium through a prior down step experiment. Our goal was to test the predictive power of the Tool-Narayanaswamy (TN) formalism, which assumes that the non-linear re-equilibration of a liquid can be linked to its linear response to a small perturbation. We determined the TN parameters for steps with small-to-moderate amplitude (≤3.3 K) and crucially took advantage of down step aging experiments below the glass transition temperature to constrain the determination of the equilibrium relaxation time. We tested the TN predictions and found very good agreement for differences in fictive temperature characterizing the distance from equilibrium as high as 10 K. For larger steps, however, the prediction progressively fails to capture the aging dynamics. This likely indicates that the re-equilibration mechanism is no longer related to the equilibrium dynamics. Finally, we discuss the possibility of obtaining a general criterion for the limit of validity of the TN formalism, which we compare to other systems from the literature.