On the application of single parameteraging to glass equilibration

The kinetics of physical aging in glasses is typically described by models accounting for the material time concept, which, via the time dependent relaxation time-the material "inner clock"-linearizes the response. Although this description has been shown to be successful for relatively small temperature jumps, typically close to the glass transition temperature, Tg, its validity for large jumps is a matter of open debate. Here, we implement the material time concept within the single parameter aging model with a single time scale, that is, the one of the main α relaxation, and apply it to isothermal aging data after both temperature down- and up-temperature jumps. Our results show that a single parameter falls short of capturing the entire phenomenology of physical aging. Specifically, a single parameter can either describe aging after temperature up-jumps and relatively small down-jumps or capture the entire range of aging kinetics after temperature down-jumps while falling short of predicting aging after up-jumps. This outcome highlights the necessity of incorporating multiple time scales in the description of physical aging.