Crossover between cooperative and fractal relaxation in complex glass-formers

Kinetics of physical aging at different temperatures is studied in situ in arsenic selenide glasses using high-precision differential scanning calorimetry technique. A well-expressed step-like behaviour in the enthalpy recovery kinetics is recorded for low aging temperatures. These fine features disappear when the aging temperature (Ta) approaches the glass transition temperature (Tg). The overall kinetics is described by stretched exponential function with stretching exponent close to 3/5 at Ta > ~0.95 Tg almost independent on glass composition, and 3/7 when the aging temperature drops to ~0.9 Tg. These values are consistent with the prediction of Phillips' diffusion-to-traps model. Further decrease in aging temperature to ~0.85 Tg leads to the appearance of step-like behaviour and stretching exponent of 1/3 for the overall kinetics, which is the limiting value predicted by random walk on the fractal model. Such behavior is explained as crossover from homogeneous cooperative relaxation of non-percolating structural units to high-dimensional fractal relaxation within hierarchically-arranged two-stage physical aging model.