Molecular Glasses with High Fictive Temperatures for Energy Landscape Evaluations

Abstract

With an interest in obtaining data on laboratory glass samples to compare with simulated glasses produced by molecular dynamics computer simulations, we have explored, using calorimetric techniques, the fictive temperatures that can be obtained using different laboratory quenching methods. We describe some useful analytical methods for characterizing quenched samples and, in the process, demonstrate a modified graphical treatment of DSC data that directly yields the m fragility index ("steepness" index) and permits the assembly of enthalpy relaxation data on different liquids in a fragility plot. Using these methods, we provide evidence for the trapping of high-T g molecular glasses at fictive temperatures up to 1.16T g and show that fictive temperatures up to and even beyond the crossover temperature for fragile glass formers should be possible using refined electrospray and fiber-spinning techniques. We discuss the relation of the low-T/T g enthalpy relaxation, found in all hyperquenched glasses, to topographic features of the energy landscape for glassforming liquids.

Intermolecular Forces and the Glass Transition

“…With some cleverness cooling rates as fast as 10 6 K/s have been reached for melt-spun glasses 75 . In dimensionless simulation units cooling rates as small as 10 −7 = dT * /dt * and as large as 10 −3 = dT * /dt * have been used in Lennard-Jones simulations, where t * = t 48…”

An electrospray technique for hyperquenched glass calorimetry studies: Propylene glycol and di-n-butyl phthalate

“…Then the fictive temperature can be assessed by calorimetric measurements performed on the quenched glass during its reheating, as described in detail in recent papers [1,3] which relate back to early studies by Moynihan and coworkers [28]. We will illustrate these features in the present work in which we report on the adaptation of the electrospray technique for differential scanning calorimetry studies of the hyperquenched glassy systems.…”

“…The standard scan [1,27,34] is one in which the heat flow is measured during an upscan at the standard scan rate of 20 through the glass transformation range at 20 K/min. For the comparison, the quenched sample is upscanned at the standard rate, and the two scans are superposed using the data at temperatures (i) below that of the relaxation onset of the quenched sample, and (ii) above the glass transformation range, T > 1.1T g , (where all traces of the sample history disappear) to adjust any slope discrepancies.…”

“…In recent studies we [1,2], and others [3,4], have stressed the importance of laboratory hyperquenching strategies in clarifying the physics of glassforming systems. Not only does vitrification on very short time scales help bridge the current gap between computer simulation investigations of supercooled liquids and their experimental counterparts, but it provides glassy materials in much higher states of configurational excitation than have previously been studied in appropriate detail.…”

“…By studying the onset of relaxation of the hyperquenched glasses during reheating, the depth of the trap in which the glass was arrested (or the energy barrier to be overcome in structural relaxation) can be determined [2]. Then, by appropriate thermochemical methods (detailed below and elsewhere [1,3]), the temperature at which the system was arrested during the hyperquench can be obtained. Thus hyperquenching studies (which in the past have been used almost exclusively to vitrify systems that normally crystallize) can provide information on the energetics of the 'liquid landscape' [13,14] in energy ranges approaching those involved in computer simulation studies.…”