Competing Relaxation Pathways in the Physical Aging of Glasses

Physical aging—the slow relaxation of non-equilibrium glasses toward equilibrium—remains a central challenge in condensed matter physics. Over the past fifteen years, advances in experimental characterization have revealed that aging is far more complex than a single structural relaxation process. Studies across polymer, molecular, metallic, and aqueous systems have shown that aging progresses through a hierarchy of coupled mechanisms where the contribution of mechanisms beyond the α-relaxation—long considered the dominant driver of aging—has been brought into focus. In this contribution, by critically presenting a wealth of result on physical aging by following the glass enthalpy evolution, mainly by calorimetric techniques, I aim to convey insights on the mechanisms underlying multi-step equilibrium recovery of glasses. These experimental insights demonstrate universal aging trends toward equilibration, underscoring the variety of kinetic paths in glass equilibration and paving the way to attain low energy glasses by exploiting non-α mechanisms of physical aging, active way below the glass transition temperature, T_g. Hence, this contribution synthesizes the major experimental breakthroughs of the past decade and a half, outlining how they collectively reshape our understanding of physical aging and enable more predictive control over the stability and long-term performance of glassy materials.