Dense Glass Packing Can Slow Reactions with an Atmospheric Gas
J. Phys. Chem. B
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Previous work utilizing crystal polymorphs has established the importance of the molecular packing environment for modulating solid-gas reactivity. Here we show, for the first time, that the chemical stability of an amorphous material in contact with a reactive gas can be significantly improved by controlling glass packing. We utilize the reaction of indomethacin with ammonia as this system has been well-characterized for crystalline polymorphs. For these experiments, physical vapor deposition (PVD) is used to prepare glasses of indomethacin with a range of densities and thermal stabilities. The indomethacin-ammonia reactivity is assessed through the increase in mass of glassy thin films exposed to ammonia gas, as characterized by a quartz crystal microbalance (QCM). Indomethacin glasses vapor-deposited at substrate temperatures below the glass transition temperature (Tg) show unprecedently decreased reaction rates relative to the liquid-cooled glass, by as much as one order of magnitude, with the densest glasses having the slowest reactions. The diminished solubility of ammonia in dense PVD glasses is found to be a major factor in their remarkable chemical stability. As chemically stable amorphous solids are in demand for applications including pharmaceuticals and organic electronics, this work provides a strategy to improve performance of these materials.
Glasses, Amorphous materials, Physical vapor deposition, Ammonia