Testing the Material Time Concept in Nanoconfined Glasses: Equilibration Kinetics of 5PPE in Nanopores

Abstract

Glassy materials undergo physical aging, changing their physical properties as they approach thermodynamic equilibrium. When confined in nanopores, glass-forming systems exhibit peculiar behavior, including out-of-equilibrium phenomena observed on a molecular time scale much faster than in the bulk. This occurs during rapid cooling, when the fraction of molecules located near the confining walls becomes kinetically frozen at the experimental time scale. As a result, the α-relaxation of the core fraction becomes faster than the equilibrium bulk liquid at the same temperature. Upon prolonged annealing, confinement-induced changes in the glass transition dynamics can be progressively eliminated, and the α-relaxation time recovers its bulk value. Recently, it has been demonstrated that a “material time” concept─the idea that structural recovery is governed by an internal clock whose rate changes as the system ages─can capture the out-of-equilibrium response of nanoscale confined glasses. Here, we test this framework for a model van der Waals liquid, 5-ring polyphenyl ether (5PPE), confined within self-ordered nanoporous alumina templates of straight cylindrical nanochannels. We show that the equilibration kinetics can be successfully predicted for pores of different sizes and various thermal protocols. Our results provide new experimental evidence that the single-parameter aging concept remains valid not only for macroscopic glasses but also for nanoscale-confined glass-forming liquids.