Measurement of the limiting fictive temperature over five decades of cooling and heating rates

The fictive temperature (T_f) was defined by Tool in the 1940s as a measure of glassy structure. T_f is generally measured on heating and can be calculated from the enthalpy overshoot in calorimetric studies using a method developed by Moynihan. Prior work has demonstrated that the limiting fictive temperature (T_f′) is similar to T_g (measured on cooling) and depends on the cooling rate in a manner consistent with the Williams-Landel-Ferry (WLF) relationship. Theoretically, the limiting fictive temperature should not depend on heating rate, but this has been experimentally verified only for a very limited range of heating rates. Here, rapid-scanning chip calorimetry and conventional differential scanning calorimetry (DSC) are combined to investigate T_f′ for polystyrene over a broad range of heating rates ranging from 0.017 to 3000 K/s after cooling at different rates. The results show that T_f′ depends on cooling rate following the WLF equation. On the other hand, T_f′ is not a function of heating rate, consistent with theoretical predictions, in spite of the change in the magnitude and placement of the enthalpy overshoot.