Recently, a new experimental approach has been developed by the Ediger and Yu research groups to prepare glasses that are considerably more stable than "ordinary" glasses prepared by slow cooling of the liquid. In that approach, small organic molecules, such as 1,3,5-trisnaphthylbenzene (TNB) and indomethacin (IMC), are vapor-deposited onto a cold silica substrate. The resulting material, referred to as a "stable" glass, has a lower enthalpy (by as much as 10 J/g), a higher density (by 0.5-1.5 %) and a significantly higher on-set temperature, suggesting higher kinetic stability of such materials. By simple linear interpolation, an "ordinary" glass sample might need thousands of years of aging to obtain similar properties. The de Pablo research group has developed methods to study these materials using advanced molecular simulations that can enable the study of these materials in a more detailed manner. Using such advanced simulation methods, stable glasses of trehalose have also been prepared.
With the combination of experimental, theoretical and computational methods, a wide class of molecules can be used to study such significantly stable glasses. With the help of such extensive tools, we investigate several molecules that can be useful in many real-life applications. For example, stable glasses of liquid crystalline molecules like 4-Cyano-4'-pentylbiphenyl can enable us to prepare a highly stable glassy matrix with orientational order that can be useful in numerous engineering applications.Molecular Simulations of Stable Glasses
To understand stable glasses from a molecular perspective, molecular simulations can be of significant use. The de Pablo research group has developed a simulation method to prepare stable glasses. In particular, the method mimics different aspects of the physical vapor deposition. The simulations are performed in an NVT simulation box with a wall of Lennard Jones particles at one edge. The temperature of wall (TS) is kept constant using a thermostat. New molecule(s) are added in batches at an elevated temperature. The complete cyclic procedure can be summarized in following steps, which is described pictorially in Figure below.
New molecule(s) are added randomly at least 2 nm away from the free surface and the energy of system is minimized using a conjugate gradient scheme.
An NVT run is performed using two thermostats, a lower temperature for the substrate and already deposited molecules and a higher one for new molecules.
The new molecules are annealed to the substrate temperature.
The system is equilibrated in NVT ensemble with a single thermostat.
The energy of the system is minimized using a conjugate gradient scheme
Using the above described method, we have generated stable glasses of trehalose, a disaccharide useful in preservation of peptides for its anti-plastisizing properties. Below is a interactive 3-D configuration of a formed stable glass. Such a stable glass, similar to experimental observations for other molecules, shows a higher density, higher on-set temperature and a lower potential energy by as much as 9.2 J/g.
Model glassy systems have been used to study several important properties of glasses computationally. Study of such systems have been helpful in understanding several properties of glassy materials. We have used above described method to generate stable glasses of a binary glass model
Glass-forming molecules are deposited from the vapor phase onto a temperature-controlled substrate. The deposition process is carried out in a vacuum chamber (P ≈ 10−6 Pa). Hot molecules are evaporated from a crucible at Th, onto a substrate whose temperature is kept constant (at Ts ). The deposition rate is adjusted by controlling the temperature of the crucible, and is monitored with a quartz crystal microbalance. The most stable glasses are prepared when the substrate temperature is about 0.85 Tg.