Molecular mobility and physical stability of amorphous organic solids

This thesis study concerns molecular mobility and physical stability of amorphous organic solids. The surprisingly fast crystal growth at the free surface of amorphous organic solids and its inhibition by ultra-thin coating were studied in details. This fast surface crystal growth was suggested to be caused by the higher mobility of surface molecules. To understand this phenomenon, surface diffusion of organic glasses was studied for the first time using the method of surface grating decay. This study revealed that surface diffusion of an organic glass is at least 106 times faster than the bulk diffusion, indicting a mobile surface layer. This very fast surface diffusion may very well responsible for the fast surface crystal growth. Besides surface diffusion, bulk self-diffusion in glass forming liquid was studied with a novel method combining Raman microscopy and isotope labeling. Both single-component (terfenedine and polystyrene self-diffusion) and two-component (water diffusion in carbohydrate matrices) systems were studied. The measured self-diffusion coefficients of terfenedine and polystyrene agree with literature values, suggesting Raman microscopy is a valid method for measuring slow diffusion. Water diffusion in maltose matrices was measured over eight orders of magnitude with Raman microscopy and NMR, and it was found that water diffusion is significantly fast in maltose glasses, decouples from maltose diffusion, and is not strongly affected by the glass transition. Finally, the generality of stable organic glass formation by vapor deposition was studied using a simple sublimation glassware. This study, together with previous studies, demonstrates the generality of the phenomenon. We also found that substances forming stable glasses by vapor deposition tend to undergo surface-enhanced crystal growth, suggesting both phenomena could be linked to surface mobility. The findings of this thesis study will help people better understand and predict the stability amorphous organic solids.