AC Calorimetric Study of Stable Glasses of Ethylcyclohexane - How Much Time is Needed to Form a Stable Glass?

摘要

Glasses of ethylcyclohexane produced by physical vapor deposition have been assessed by in situ AC chip nanocalorimetry. Consistent with previous works we observed most stable glasses for deposition at substrate temperatures around 0.85 Tg . The isothermal transformation of the vapor-deposited glasses into the supercooled liquid state is also measured. Its deposition rate dependency, covering four orders of magnitude, was measured for different substrate temperatures. The kinetic stability of the glasses (transformation time at 103K) shows strong deposition rate dependency for lower substrate temperatures. The transformation time dependency on deposition rate is well fitted by the Kohlrausch-Williams-Watts (KWW) function. The data provide an estimate for the substrate temperature dependent free surface residence time for the molecules in the assumed mobile surface layer needed to promote stable glass formation. Stable glasses are formed if this time is of the order of the ?-relaxation time of the ordinary glass, many orders of magnitude faster than the bulk ?-relaxation at the substrate temperature. Stable glasses are observed even for substrate temperatures below the Vogel and the Kauzmann temperatures, indicating a decoupling of the process of stable glass formation from the ?-relaxation.