Real-space observation of the temporal evolution of nanoscale mechanical heterogeneities in deeply aged polymer glasses

Molecular mechanism driving non-equilibrium polymer glasses toward more thermodynamically stable configurations is commonly attributed to the α-relaxation associated with cooperative segmental motions. However, growing experimental evidence has revealed the presence of a much faster relaxation process that enables partial equilibrium recovery deep in the glassy state. This faster process has also been identified as a universal feature across different classes of glasses. Recent studies have sought to attribute this relaxation to subtle configurational rearrangements of localized ensembles of loosely packed molecular segments embedded in a tightly packed glassy matrix; however, direct experimental visualization of these rearrangements remains extremely challenging. Here, we propose a novel experimental strategy using bimodal atomic force microscopy to observe the temporal evolution of nanoscale mechanical heterogeneities (NMHs) in an epoxy polymer glass under aging below the glass transition temperature (Tg). Our results indicate that even deep in the glassy state, approximately 70 K below Tg, the modulus variations of NMHs persist, albeit in a slow and localized manner, in sharp contrast to the fast and highly cooperative changes observed during aging near Tg. This finding provides real-space evidence suggesting a correlation between the structural rearrangements and a relaxation process much faster than the α relaxation. This paper uses atomic force microscopy to observe the temporal evolution of nanoscale mechanical heterogeneities in an epoxy polymer glass under aging below the glass transition temperature. It is found that nanoscale mechanical heterogeneity persists even 70 K below the glass transition, albeit in a slow and localized manner.