The calorimetric glass transition (T_g) is measured for single polystyrene ultrathin films, 160 and 71 nm thick, using Flash Differential Scanning Calorimeter (DSC) as a function of cooling rate and film thickness. The glass transition temperature of 160 nm-thick film is consistent with the bulk data from conventional DSC. For 71 nm-thick film with grease between the film and the sensor, the T_g depression decreases with increasing cooling rate and its magnitude is similar to those for solid-supported films in the literature. The evolution of the T_g depression was investigated during annealing at 160°C, and we find that the depressed T_g value reverts to the bulk value within 20 hours due to hole growth and film thickening. On the other hand, for 71 nm-thick films directly spuncast on the sensor, no T_g depression is observed. The results show the importance of film preparation, interface, and annealing on the T_g depression of ultrathin films.
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