Connecting Dynamics and Thermodynamics to Predict Mobility and Glassiness

Dartmouth College, Hanover NH

Investigators

Abstract

Non-technical abstract This research will lead to new understanding of how certain very widely-used materials behave and why they sometimes misbehave. Some solid materials are crystalline; table salt and sugar are two familiar examples. In these cases the atoms or molecules that make up the solid are regularly positioned and highly organized, and their structure and properties do not change with time - they are at equilibrium. In many other cases the solids are glassy, as if the particles in the liquid state were stuck in place. Window glass is just one example; it is made of small molecules containing silicon and oxygen. A large fraction of specially designed materials, which often involve very large molecules known as polymers, are also in the glassy solid state. Although glassy solids have been part of our surroundings for centuries, there are still aspects of their behavior that are not well understood. Examples from daily life include protective coatings, photoresists, polymers reinforced with nanoparticles, membranes, and filters. In each of these cases a significant fraction of the glassy solid molecules are near an interface; different kinds of experiments have led to the conclusion that these molecules can behave very differently from neighboring molecules ensconced in the bulk. A related problem is that, where glassy solid properties are carefully designed to optimize performance, as the material ages its properties can change in undesirable ways. These situations reflect the fact that glassy solids, unlike crystalline solids, are not at equilibrium; and their properties can shift over time. These changes can affect their performance, and therefore cause problems. This project will produce new ways of modeling glassy solids that will account for the influence of a neighboring surface on how molecules pack. The models will work together with experimental data and lead to new methods for understanding and predicting the ways that glassy material behavior can change when interfaces are present. Technical abstract Dynamic relaxation of material begins locally through segmental motion; its progression over short time and length scales drives the longer and larger response. Dynamic behavior also reflects the local structural and energetic characteristics that determine thermodynamic properties. The research proposed here will reveal deep connections between these realms by using and advancing thermodynamic (Locally Correlated Lattice) and dynamic (Cooperative Free Volume) models originated in the PI's research group. This new set of tools will be applied to predict glassy material behavior over a wide range of conditions. Two areas of particular focus involve the presence of interfacial regions and the effect of different experimental pathways used to mimic the effects of long-time aging. This research involves interrelated projects: (a) A model will be developed for the interfacial region from very near (nm) the surface to distances where bulk behavior is recovered. It will lead to predictions for local density, mobility (mobile layer thickness), segmental relaxation times, and the changing lengthscale of cooperative motion, as functions of distance from the interface, temperature, and film thickness. (b) A new model will be introduced for nanocomposites that reflects differences between the interfacial region (next to the particles) and the matrix. This model, informed by thermodynamic properties, will lead to predictions for segmental dynamics and changes in cooperative length scales as functions of temperature, pressure, and nanoparticle loading. (c) Advances from (a) and (b) will lead to new ways for connecting the thermodynamic and dynamic properties of glass formers to their long term stability. This work will also lead to insight about how those connections depend on the experimental path to glassiness. The materials targeted by this project serve an extremely wide range of functions, and this research will lead to new insight about how their molecular nature links to the particular properties that make them uniquely suited to their applications. The new models resulting from this research will be accessible and generally useful for linking thermodynamic properties to dynamic behavior. The PI will further successful efforts to involve and encourage STEM participation among women and will continue working with graduate and undergraduate students to add new resources on polymer-related science to the public knowledge base. The PI will also continue energetic efforts to introduce fundamental concepts in physical science to the general public. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.