In situ thermal studies of vapor-deposited molecular glasses: Anomalous endothermic changes accompanying structural relaxation

Thermal properties of vapor-deposited ethylbenzene glasses were studied by raising the temperature with a constant rate. We improved the sensitivity of the thermal sensor and data-accumulation method of the measurement used in our previous study [Chem. Lett., 39, 77 (2010)]. Thus an unexpected endothermic peak was observed in the relaxation process accompanying the volume shrinkage before the glass transition for the glass deposited at 80 K. This suggests that the glass deposited at the temperature much lower than the glass transition temperature (T_g) has a significantly inhomogeneous structure. For the glass deposited at 105 K, a sharp endothermic peak was observed accompanying a steep volume expansion before the glass transition. This supports our previous inference that the dense glasses deposited at temperatures close to T_g include a large amount of locally stable molecular conformations and they diminish in the structural relaxation preceding the glass transition.