Structural recovery of a single polystyrene thin film using Flash DSC at low aging temperatures

• Enthalpy recovery of a single 1-μm thick polystyrene film is investigated using Flash DSC.
• Enthalpy evolves smoothly over 6 decades for the aging temperatures studied between 50 and 100 °C.
• An induction time at early aging times is related to the relaxation time along the iso-Tf glass line.
• The normalized apparent activation energy along the glass line is Ea/R = 13 kK.
• The normalized apparent activation energy in the equilibrium state is much greater at 104 kK.

The structural recovery of a single polystyrene thin film is studied using Flash differential scanning calorimetry (DSC) for aging temperatures ranging from 50.5 to 100.5 °C and for aging times of up to 18,000 s (300 min). A high fictive temperature glass with Tf′ = 117.0 ± 0.5 °C, obtained after cooling at 1000 K/s, is employed in this study. Structural recovery is investigated by monitoring the evolution of the fictive temperature, Tf, which initially remains unchanged and then decreases smoothly and approximately linearly with logarithmic aging time. Equilibrium is reached when Tf = Ta at the highest aging temperatures. The length of the initial plateau is longer for the lower aging temperatures and is related to the increase in the relaxation time with decreasing temperature along the glass line, increasing approximately one decade for every 21 K. This temperature dependence yields a normalized apparent activation energy (Ea/R) of approximately 13 kK for the mobility along the glass line. On the other hand, the apparent activation energy for the mobility along the liquid line is 102 kK.

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