Jaeger Tamara D, Simmons David S
Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325, USA.
Department of Chemical, Biological, and Materials Engineering, University of South Florida, Tampa, Florida 33620, USA.
J Chem Phys. 2022 Mar 21;156(11):114504. doi: 10.1063/5.0080717.
A central feature of the non-equilibrium glassy "state" is its tendency to age toward equilibrium, obeying signatures identified by Kovacs over 50 years ago. The origin of these signatures, their fate far from equilibrium and at high temperatures, and the underlying nature of the glassy "state" far from equilibrium remain unsettled. Here, we simulate physical aging of polymeric glasses, driven much farther from equilibrium and at much higher temperatures than possible in experimental melt-quenched glasses. While these glasses exhibit Kovacs' signatures of glassy aging at sufficiently low temperatures, these signatures disappear above the onset T of non-Arrhenius equilibrium dynamics, suggesting that T demarcates an upper bound to genuinely glassy states. Aging times in glasses after temperature up-jumps are found to obey an Arrhenius law interpolating between equilibrium dynamics at T and at the start of the temperature up-jump, providing a zero-parameter rule predicting their aging behavior and identifying another unrecognized centrality of T to aging behavior. This differs qualitatively from behavior of our glasses produced by temperature down-jumps, which exhibit a fractional power law decoupling relation with equilibrium dynamics. While the Tool-Narayanaswamy-Moynihan model can predict the qualitative single-temperature behavior of these systems, we find that it fails to predict the disappearance of Kovacs signatures above T and the temperature dependence of aging after large temperature up-jumps. These findings highlight a need for new theoretical insights into the aging behavior of glasses at ultra-high fictive temperatures and far from equilibrium.
非平衡玻璃“态”的一个核心特征是其趋向于向平衡态老化，遵循科瓦奇在50多年前识别出的特征。这些特征的起源、它们在远离平衡态及高温下的情况，以及远离平衡态的玻璃“态”的本质仍未确定。在此，我们模拟了聚合物玻璃的物理老化，其驱动条件比实验熔体淬火玻璃更远离平衡态且温度更高。虽然这些玻璃在足够低的温度下呈现出科瓦奇玻璃老化的特征，但这些特征在非阿累尼乌斯平衡动力学的起始温度T以上消失，这表明T划定了真正玻璃态的上限。发现温度跃升后玻璃中的老化时间遵循阿累尼乌斯定律，该定律在T时的平衡动力学与温度跃升开始时的平衡动力学之间进行插值，提供了一个零参数规则来预测其老化行为，并确定了T对老化行为的另一个未被认识到的核心作用。这在性质上不同于我们通过温度下降产生的玻璃的行为，后者表现出与平衡动力学的分数幂律解耦关系。虽然Tool-Narayanaswamy-Moynihan模型可以预测这些系统的定性单温度行为，但我们发现它无法预测T以上科瓦奇特征的消失以及大温度跃升后老化对温度的依赖性。这些发现凸显了对超高虚构温度下且远离平衡态的玻璃老化行为进行新的理论洞察的必要性。
