CAREER: Free Surface Mobility and its Role in the Formation of Exceptionally Stable Glasses

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

Technical Summary Molecular glasses with high densities and exceptional kinetic stabilities have been recently produced by means of physical vapor deposition (PVD). In these experiments, the substrate temperature was held at a temperature below the glass transition temperature, Tg, where the bulk relaxation dynamics are extremely slow. In order for these PVD glasses to overcome the kinetic barriers preventing bulk glasses from reaching such near-equilibrium states at low temperatures, molecules must have access to enhanced mobility during vapor deposition. It is hypothesized that this enhanced mobility is caused by a layer of increased mobility close to the air interface. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, this hypothesis will be tested by studying the dynamical properties of the surface of organic molecular glasses using a nanoparticle probe technique developed by the PI and others. The proposed improvements to this technique will allow simultaneous studies of the properties of the bulk glass and its free surface, making it an ideal method for investigating the correlation between surface properties and stable glass formation. This technique will be applied on a broad range of organic molecules to investigate whether this phenomena is universal for all glasses, or a chemical effect specific to particular molecular structures. These studies will be important in advancing our fundamental understanding of the glass transition phenomena. Non-Technical Summary: Glasses, out of equilibrium solids with structures that resemble that of equilibrium liquids, are ubiquitous in our daily life, and are widely used in the electronic and medical industries. Despite this, developing new useful glassy materials, or improving the properties of known glassy materials has proven to be difficult due to extremely slow molecular motion within the glass. For example, in order to make high-density glasses via aging, the process by which a glass naturally becomes more dense, one would have to wait a few hundred thousand years. A recent discovery, however, shows that glasses with highly desirable properties, such as increased density and stability, can be produced in a few hours. It is hypothesized that this is caused by the presence of a layer at the air/glass surface of most organic and polymeric glasses that is a few nanometers thick and behaves like a liquid rather than an out of equilibrium solid. Our proposed studies aim at understanding the origins of the liquid-layer and its effect on the structure of glasses. Understating the properties of the liquid-layer can help design and produce materials with improved properties for organic electronic, pharmaceutical, lubrication and coating technologies. Furthermore, we will combine these studies with educational efforts aimed at introducing concepts of glassy polymer dynamics and structure to a wide audience. Polymers are widely used in everyday life. Products such as bullet proof glass, silly putty and tires are examples of materials with similar design concepts, but widely varying properties. We will design experimental modules to highlight the importance of chemical composition, structure and dynamics on the final properties of a polymeric system. These experiments will be presented with adjustable levels of technical detail so that students of all backgrounds can learn from them. The PI will also work closely with institutions at the University of Pennsylvania and science teachers from elementary and high schools in the Philadelphia area to develop, implement, and disseminate these modules. To reach an even larger audience, videos of these modules will be made available online.