In situ investigation of vapor-deposited glasses of toluene and ethylbenzene via alternating current chip-nanocalorimetry

Abstract

Vapor-deposited glasses of toluene and ethylbenzene have been characterized by in situ ac chip-nanocalorimetry. The high sensitivity of this method allows the detection of small changes in the heat capacity of nanogram size samples. We observe that vapor-deposited glasses have up to 4% lower heat capacities than the ordinary glass. The largest heat capacity decrease and the most kinetically stable glasses of toluene and ethylbenzene are observed in a range of deposition temperatures between 0.75 Tg and 0.96 Tg. Compared to larger molecules, deposition rate has a minor influence on the kinetic stability of these glasses. For both toluene and ethylbenzene, the kinetic stability is strongly correlated with the heat capacity decrease for deposition temperatures above 0.8 Tg. In addition, ac-nanocalorimetry was used to follow the isothermal transformation of the stable glasses into the supercooled liquid at temperatures slightly above Tg. Toluene and ethylbenzene stable glasses exhibit a constant transformation rate which is consistent with the growth front mechanism recently demonstrated for tris-naphthylbenzene and indomethacin. The kinetic stability of the most stable toluene and ethylbenzene glasses is comparable to that observed for other stable glasses formed by vapor deposition.

The role of thermodynamic stability in the characteristics of the devitrification front of vapour-deposited glasses of toluene

“…23 Experimentally, this phenomenon has also been observed using a variety of techniques in several systems, e.g. secondary mass ion spectroscopy (SIMS), 13,24 ellipsometry, 25 AC-calorimetry, 6,26 differential scanning calorimetry 17 and fast-scanning nanocalorimetry. 17,27 Both theory and experiments show a direct relation between the growth front velocity and the mobility of the supercooled liquid.…”

“…However, according to previous work this change in the deposition rate should not represent a major change in the stability of toluene thin film glasses. 5,6 We therefore assume that the characteristics of their glass will lie in between the ones we deposited at 104.2 K and 110.4 K. We also plot in Fig. 2b a representation of the function v gr = Ct Àg using gamma = 0.92 and a prefactor C that fits both our 104.2 and 110.4 K data and the 105 K data from Ahrenberg et al 6 within a tolerance of AE13%.…”

“…Prepared by physical vapour deposition, some properties of these glasses can be easily tuned by changing the substrate temperature and the deposition rate. When the substrate temperature is around 0.85 times the glass transition temperature (T g ), the produced glasses, also named as ultrastable glasses (UG), exhibit enhanced properties, such as low enthalpy, [3][4][5] high kinetic stability, 5,6 low heat capacities 6,7 or high density. 8,9 Apparently, at this particular substrate temperature and at low deposition rates, when molecules reach the substrate they become part of a mobile surface layer having time to explore their best configuration state before getting buried by the following ones.…”

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

“…Discovered using model glass formers and labeled "stable glasses," these glasses have lower enthalpies (20), higher densities (21), and resist structural reorganization to higher temperatures than is possible with any other preparation route (22)(23)(24). The properties of stable glasses are explained by the high mobility of the free surface during the vapor deposition process (20,25).…”

Communication: Surface-facilitated softening of ordinary and vapor-deposited glasses

“…8 of Ref. 14). In non-isothermal experiments, where the temperature of devitrification relates to the heating rate, SF softening can occur 15,16 at tens of K above T g .…”