Nanocalorimetery is a powerful method that allows us to measure the heat capacity of very small samples, down to nanograms of material. Vapor deposition is an ideal way to place a few nanograms of material of the nanocalorimeter. We can compare heat capacities of a stable glass to the ordinary liquid cooled glass. We find that stable glasses generally have a lower heat capacity, indicating a different molecular packing than the ordinary glass. We can also track the transformation from glass to super cooled liquid by monitoring the change in heat capacity. The delayed transformation of a stable glass relative to an ordinary glass is a feature of its kinetic stability.
Katie Whitaker has recently shown that we can make stable glasses of mixtures. When mixtures of cis- and trans-decalin are vapor deposited (solid lines), they show lower heat capacities and higher transformation temperatures than the corresponding ordinary glasses (dashed lines).
Alfonso Sepulveda and Steve Swallen used secondary ion mass spectrometry (SIMS) to show that stable glasses tend to transform via a growth front from the free surface. By capping the free surface of an IMC stable glass with a higher Tg material, Alfonso was able to delay the transformation of the stable glass into the liquid. Mike Tylinski used nanocalorimetry to show this effect with toluene stable glasses. The capping of the stable glass delayed the transformation, indicating that the capped sample is even more kinetically stable.
Nanocalorimetry is important in exploring the range of possible stable glasses, understanding their behavior, and learning how to manipulate their properties. Nanocalorimetry and dielectric relaxation are particularly important for analyzing glasses prepared by low Tg glassformers since these techniques have been implemented into one of our vacuum chambers.