Low-Barrier Onset of Physical Aging Across Bulk and Nanopore-Confined Poly(methyl methacrylate)

Physical aging in amorphous polymers reflects the slow structural relaxation toward equilibrium below the glass transition. Isoconversional analysis suggests that progressively larger activation barriers are explored during aging, approaching that of α-relaxation near equilibrium. Yet the molecular origin of the low-barrier regime that defines the onset of aging, where the recovered enthalpy is only a few percent of the total, remains poorly understood. Here, we combine calorimetry and broadband dielectric spectroscopy on bulk and nanopore-confined poly(methyl methacrylate) (PMMA) to isolate the earliest stage of aging through the identification of the induction time of the kinetics (t_ind). We find that t_ind follows a clear Arrhenius dependence with an activation energy that, for both bulk and confined PMMA, is more aligned with that of the slow Arrhenius process, rather than that of the β-relaxation mode resolved in the dielectric spectra. These findings challenge the prevailing assumption that Arrhenius-like behavior is a unique signature of the β-relaxation, suggesting that additional localized modes play key roles in the dynamics of disordered solids and calling for a reassessment of how low-barrier processes are assigned.