The glass transition and enthalpy recovery of a single polystyrene ultrathin film using Flash DSC

The kinetics of the glass transition are measured for a single polystyrene ultrathin film of 20 nm thickness using Flash differential scanning calorimetry (DSC). T_g is measured over a range of cooling rates from 0.1 to 1000 K/s and is depressed compared to the bulk. The depression decreases with increasing cooling rate, from 12 K lower than the bulk at 0.1 K/s to no significant change at 1000 K/s. Isothermal enthalpy recovery measurements are performed from 50 to 115 °C, and from these experiments, the temperature dependence of the induction time along the glass line is obtained, as well as the temperature dependence of the time scale required to reach equilibrium, providing a measure of the shortest effective glassy relaxation time and the longest effective equilibrium relaxation time, respectively. The induction time for the ultrathin film is found to be similar to the bulk at all temperatures presumably because the T_g values are the same due to the use of a cooling rate of 1000 K/s prior to the enthalpy recovery measurements. On the other hand, the times required to reach equilibrium for the ultrathin film and bulk are similar at 100 °C, and considerably shorter for the ultrathin film at 90 °C, consistent with faster dynamics under nanoconfinement at low temperatures. The magnitude of the "T_g depression" is smaller when using the equilibrium relaxation time from the structural recovery experiment as a measure of the dynamics than when measuring T_g after a cooling experiment. A relaxation map is developed to summarize the results.