Origin of the broad endothermic peak observed at low temperatures for polystyrene and metals in Flash differential scanning calorimetry*

Abstract

The glass transition behavior of polystyrene as a function of different cooling rates with scanning to two different end temperatures, 30°C and −80°C, was investigated for four different substrate conditions using Flash differential scanning calorimetry, a fast scanning nanocalorimetry technique. In addition, structural recovery of polystyrene was performed at 20°C for aging times from 0.01 s to 8 h with scanning to −80°C for the same samples. A broad endotherm appears to grow at low temperatures (T << Tg) as cooling rate decreases and aging time increases, which is influenced by the substrate underlying the film, as well as by the end temperature condition in the scanning experiment. On the other hand, the endothermic overshoot associated with Tg is not influenced by substrate or scan end temperature. In addition, indium and vapor-deposited gold, both crystalline materials, show the growth of a very similar broad endotherm at low temperatures as cooling rate decreases and aging time increases indicating that the low-temperature endotherm is an artifact and not a relaxation associated with the material under investigation. Several potential explanations are put forward.