Glasses ‘R’ Us!
Glass means more than “window glass” or optical fibers. Almost any chemical substance can be transformed into a solid that lacks long-range periodicity – a glass! Examples include organic glasses (used as OLEDs in phone displays), polymer glasses (used to construct the Boeing 787 Dreamliner), and metallic glasses (used in electrical transformers). Our lab has two glass project areas, as described in this section. Our research attempts to develop a molecular-level understanding of dynamics in polymeric materials and low molecular weight glass formers. As devices move closer to the nanometer length scale, the knowledge obtained from our molecular level experiments will become more essential to the correct functioning of these devices. Our research is funded by the National Science Foundation and the Department of Energy.
Introduction to supercooled liquids and glasses
If crystallization is avoided upon cooling, a liquid will become more viscous and eventually transform into a glassy solid. Thus a new type of matter is produced by kinetics with no significant change in structure. Glasses of many types (polymeric, low molecular weight organic, inorganic, saccharide,...) play important roles in technologies.
We have shown that, as the glass transition is approached, dynamics become increasingly spatially heterogeneous, i.e., the dynamics in one region of the sample may be orders of magnitude faster than the dynamics a few nanometers away. While heterogeneous dynamics exist at equilibrium in bulk supercooled liquids, in other situations the heterogeneity can be much larger. For example, we have shown that dynamics within a few nanometers of the free surface of a polymer film can be 4 orders of magnitude faster than the interior of the film. When a stable glass transforms into a supercooled liquid, there is a sharp propagating interface where the transformation occurs. At this interface the dynamics change by many orders of magnitude (at least 5) in a few nanometers.