Surface self diffusion of organic glasses measured by surface grating decay.

摘要

Surface diffusion influences many important processes in condensed matter, including crystal growth, catalysis, corrosion, and sintering. While diffusion has been studied extensively on metals and semiconductors, no data existed for organic materials until recently. Organic glasses have certain characteristics that make them ideal, and understanding the mobility of organic glasses is important as they are developed for pharmaceutical, electrical, and food applications. Facilitating the study of surface diffusion on organic glasses is the desire to understand the processes of fast surface crystallization, vapor deposition to create stable glasses, and the nature of surface mobility on polymer glasses.;To study these processes, the surface mobility of three organic glasses, Nifedipine (NIF), Indomethacin (IMC), and Sucrose Benzoate (SB), was experimentally measured. NIF and IMC are pharmaceutical materials who exhibit fast surface crystallization and can be deposited as stable glasses. We studied their surface diffusion to relate to surface crystal growth rates, and found a direct correlation between the two processes. Extrapolation of surface diffusion values to 0.85 Tg, an ideal temperature for vapor deposition of stable glasses, revealed that molecules could move tens of nanometers before being covered by a newly deposited layer. A comparison of the surface diffusion values with the surface relaxation times calculated from two models that seek to explain surface mobility showed that improvements need to be made to the models as neither correctly predicted the faster surface mobility of NIF.;An important aspect of glasses is their relaxation, or aging, with time towards the equilibrium liquid. In order to understand if this bulk relaxation has any effect on surface diffusion, gratings of IMC and NIF were aged for up to two months. This aging time will increase the bulk relaxation time by four orders of magnitude, but observations of surface grating decay show no change in surface diffusion values. This finding shows that surface and bulk dynamics are not coupled, although the depth of this decoupling could not be determined from our experiments.;Finally, the surface grating decay of Sucrose Benzoate was studied to learn about the transition of behaviors from low to high molecular weight glasses. This study, together with previous work from IMC, showed that the near-surface viscosity calculated from grating decay experiments is higher than bulk viscosity. This is in agreement with both polymer and silicate glasses that have been measured in the same way, and presents an interesting dependence on surface structure. Early evidence also suggests that surface diffusion is the main mechanism of grating decay for Sucrose Benzoate at temperatures below Tg.