(303d) Calorimetric Tg of Single Polystyrene Ultrathin Films
Tg depressions are generally observed for supported and freely standing polystyrene ultrathin films. However, the field is controversial with nanocalorimetric studies made by other researchers over the past ten years showing no changes in Tg for polystyrene. Suggestions have even been made that the Tg depressions observed in polystyrene thin films are due to artifacts caused by sample preparation, plasticization, and/or degradation. In order to address this controversy, the calorimetric glass transition (Tg) is measured for single polystyrene ultrathin films using Flash Differential Scanning Calorimeter (DSC) as a function of cooling rate, molecular weight, film preparation method, and film thickness. The Tg values of films greater than approximately 100 nm in thickness are in good agreement with conventional DSC bulk data. For thinner films, a Tg depression occurs when films are spuncast and placed on the calorimetric chip with a thin layer of grease between the film and sensor; these "liquid-floating" samples show a Tg depression which decreases with increasing cooling rate and its magnitude is similar to values found in the literature for the most mobile solid-supported films, which might be expected since our films are "liquid floating" on a thin layer of grease. On the other hand, when polystyrene is spuncast directly on the chip sensor, no Tg depression is observed. These results explain the lack of a Tg depression observed in prior nanocalorimetric studies where films where spuncast directly on the chip sensor. The role of annealing above Tg is also examined. We find for all polystyrene ultrathin films that show a Tg depression, that Tg values revert to the bulk value after long-time annealing at 160 °C. Based on atomic force microscopy studies, the recovery process is due to dewetting and hole growth which results in a thickening of the film to above 100 nm. The annealing results confirm that the Tg depression is not due to degradation or artifacts involving plasticization by small molecules.