Structural recovery of a single polystyrene thin film using nanocalorimetry to extend the aging time and temperature range

The structural recovery of a single polystyrene thin film is investigated using nanocalorimetry at aging times as short as 0.01 s, as well as aging at temperatures as high as15 K above the nominal glass transition temperature for high fictive-temperature glasses obtained at high cooling rates. The results indicate that structural recovery progresses as expected when the aging temperature is low compared to the initial fictive temperature; in this case, the fictive temperature evolves smoothly towards the aging temperature at a rate that depends on the aging temperature and initial fictive temperature (i.e., on the cooling rate prior to aging), and at equilibrium, the fictive temperature T_f = T_a. For the case when the aging temperature is much higher than the initial fictive temperature, no relaxation occurs during isothermal aging. For the intermediate case when the aging temperature is in the vicinity of the initial fictive temperature, relaxation occurs both during isothermal aging and during cooling after aging, with the result that the equilibrium fictive temperature is lower than the aging temperature. The use of a new equation for the relaxation time in the framework of the Tool-Narayanaswamy-Moynihan (TNM) model allows description of the data with a single set of model parameters.