Soft Matter (contact Bobby Sumpter)
The field of soft matter spans a number of important systems including liquids, colloids, foams, gels, polymers, and biological materials. Due to the enormous chemical diversity, compositions based on earth abundant elements, and the ability for large scale production, many of these systems offer attractive cost effective alternatives to traditional materials, an advantage that can clearly help drive our nations industrial competitiveness. As such, soft matter systems are increasingly called upon to help solve or complement current and future needs in science and technology. While the intrinsic chemical diversity of soft matter systems is a recognized advantage in the materials world, it nonetheless poses a grand challenge in regard to the development of adequate structure-property-transport-processing relationships capable of narrowing the design space to one that is tractable. In this regard, it is imperative that theoretical and simulation tools can be utilized in a tightly integrated manner with experiments in order to develop a better understanding of the underlying physicochemical processes that control the critical physical, mechanical and electrical properties, and to enable capabilities for reliable design and prediction of soft matter systems that can meet desired performance metrics. The goal of our work is to use scale spanning computational methods to help understand and guide the development of new/improved soft materials with robust morphologies.
Chemical Kinetics (contact Ariana Beste)
Chemical kinetics is the study of reaction rates and rate constants for chemical processes. Reaction rates are measured under varying conditions from which rate laws and reaction mechanisms are derived. Typically, the depletion of a reactant or the formation of a product involves multiple sequential and competing reaction steps. Whereas it is difficult to analyze individual reaction steps experimentally, computational methods can focus in on single chemical events. Rate constants for elementary reactions are determined by entropic and energetic changes during reaction progress. Commonly, an energetic barrier has to be crossed before reaction can occur. The point of highest potential or Gibbs free energy along the minimum energy path is termed transition state and is the object of interest in computational chemical kinetics. Within transition state theory, the rate constant depends exponentially on the energy difference between transition state and reactants. This implies that the accurate computation of energies is imperative to successful kinetic predictions. However, as computational power and thereby accessible system size increases, entropic contributions gain importance. In particular, we are interested in transition states of organic molecules containing about 50 atoms that are characterized by a number of low-frequency modes with significant deviation from harmonic behavior, the latter being typically assumed. Since low-frequency motion contributes the most to the vibrational entropy, we address this issue in our computational work.
Pyrolysis of Lignin Model Compounds (Contact Ariana Beste)
Lignin is an underused but major component of biomass. In order to make biomass a viable alternative to fossil fuels it is of great importance to develop effective processing techniques of all components of biomass, in particularly lignin, not only for energy but also chemicals and materials production. Due to the complexity of lignin itself and its degradation pathways, the study of model compounds is common practice. Model compounds of lignin represent typical linkages found in the three-dimensional polyphenolic network. The most abundant linkage is the β-O-4 ether linakge for which phenethyl phenyl ether (PPE) is the simplest model compound. Our efforts have been focused on the thermal degradation of PPE complementing experimental research conducted in the Chemical Sciences Division at ORNL. Experimental conditions foster a free radical chain mechanism for the pyrolysis of PPE for which we investigate key mechanistic steps using computational methods. We address questions that are difficult to answer with experimental techniques: how do substituents influence individual parts of the reaction mechanism, what is their effect on reaction energies and barriers, and how does the electronic structure change with substituent pattern. Since experiment measures overall product distributions, not all reaction steps are visible to experiment; e.g., when the reactions yield identical products, but are investigated by us with computational methods. Kinetic information is obtained by locating transition states and calculating rate constants for the corresponding reactions. The rate constants are used in simplified kinetic models based on rate determining reaction steps only and kinetic simulations of the complete mechanism. Our approach gives, on one hand, valuable insight into the details of lignin model degradation that is not attainable by experiment while, on the other hand, returning product distributions that are directly comparable to experimental values and validate our methodology.