Devitrification properties of vapor-deposited ethylcyclohexane glasses and interpretation of the molecular mechanism for formation of vapor-deposited glasses.

Abstract

We constructed an adiabatic calorimeter adapted for the preparation and in situ thermal characterization of vapor-deposited glasses and reported the investigation of the enthalpic states and dynamic properties of ethylcyclohexane (ECH) glasses prepared by vapor deposition in the temperature range of (0.71-0.96)Tg,liq; Tg,liq = (101 ± 1) K is the calorimetric glass transition temperature of the bulk liquid. It was verified that the ECH glasses deposited at temperatures immediately below Tg,liq were characterized by lower enthalpies and higher devitrification temperatures (Tdev), as compared to the glass obtained by supercooling the bulk liquid. The deposition temperature (TD) expected to yield experimentally the entity with the highest Tdev and the lowest enthalpic state was estimated to be 0.93Tg,liq. A model potentially elucidating the fundamental mechanism of formation and devitrification for the glasses prepared via the physical vapor deposition method as a function of TD was proposed. The fundamental point is that the glass is formed by deposition in a molecule-by-molecule fashion and the molecule deposited is frozen in a certain configuration determined by its being itself on the surface. For amorphous entities prepared at a TD much lower than Tg,liq, the surface molecule is frozen mostly as they are deposited. For the entity deposited at TD = 0.93Tg,liq in the case of ECH, the surface molecule is mobile immediately after the deposition to look for its stable configuration only on account of the intermolecular interactions with the molecules beneath and in the same surface layer as itself and freezes in a certain reasonably stable configuration; the molecules below the surface layer have already frozen in and get more stabilization energy through the additional interactions with the surface molecules. As a result, the intermolecular interaction of the molecules accumulated in such a way is stronger than that in the bulk liquid glass. It is argued that this is the fundamental reason why the glass formed immediately below Tg,liq has a lower enthalpy and a higher devitrification temperature than those of the liquid-cooled one.