Equilibrium and Nonequilibrium Viscosity of PbSiO ₃ Glass: Viscous Flow and Structural Relaxation

Viscosity measurements above ∼10 12 Pa·s are intrinsically time dependent, and thus equilibrium values can only be defined after structural relaxation is completed. This work investigates the evolution of nonequilibrium viscosity in PbSiO 3 glass below the glass transition region and its approach to equilibrium viscosity during structural relaxation. The penetration depth measured under a constant load was analyzed to quantify the time evolution of viscosity. The penetration rate and corresponding viscosity evolved toward steady‐state values during isothermal holds. At each temperature, the viscosity evolution was described using the Kohlrausch–Williams–Watts (KWW) function, yielding characteristic relaxation times and stretching exponents β . The values were comparable to the Maxwell stress‐relaxation time; however, because β < 1, the time required for near‐complete equilibration was much longer. A Tool–Narayanaswamy–Moynihan (TNM) analysis across all temperatures captured the nonlinear aging kinetics with a single set of parameters. The relaxation parameters derived from viscosity matched those previously obtained from refractive index relaxation measurements in the same glass batch, indicating that both observables probed the same α ‐relaxation process. Equilibrium viscosity was described using the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) model and incorporated into the Mauro–Allan–Potuzak (MAP) model to account for nonequilibrium initial states through their dependence on fictive temperature.