Thermodynamic and Dynamic Behaviour in Polymer Melts, Glasses, and Mixtures: Links to Structure Using Theory and Simulation

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NONTECHNICAL SUMMARYThis award supports theoretical and computational research and education on polymeric systems and related materials. There are many applications for synthetic materials such as polymers, ranging from coatings for electronics, to clothing, furniture, and airplane components. Ideally, the material properties will be well matched to the intended use: e.g. tough and abrasion-resistant for hard-shell suitcases, resistant to tear for garbage bags. New synthetic strategies are yielding impressive ability to design a large molecule down to the level of which atoms are incorporated and how they are locally connected. In this project, the PI and her group will use theory and simulation to develop new tools for predicting how molecular composition controls bulk material properties. Because so many applications involve thin films, the effects of interfaces will also be of strong interest. One of the important themes that runs through much of the proposed research involves the "unused" volume captured within a solid or liquid; this will change with pressure and temperature. For example, a liquid will become more dense as temperature is lowered; a solid glassy state has less "free" volume than the liquid. Prior research in the PI's group shows that theoretical predictions for free volume correlate with materials properties, e.g. its melting temperature, how it absorbs energy, and its tendency to mix. This work will focus on linking free volume and molecular structure. The aim is to complete the path connecting the microscopic design of large molecules to their bulk and film properties. The process will introduce new strategies to the scientific community for materials design. Education and training of all undergraduate students and postdocs involved in the research will be broad since a strong emphasis in the PI's research is to hold simulation and theory directly accountable to experimental results. The PI has a strong track record of involving women in research, and also maintains an interest in public outreach.TECHNICAL SUMMARYThis award supports theoretical research and education aimed at discovering links between structure, equilibrium properties, and dynamic response in polymer melts, mixtures and glasses, in the bulk and confined through interfaces. Synthesis experts are developing methods that allow for increasing control of molecular content, and the ability to draw bright lines between chemical constituency and behavior is crucial. Materials of interest here include macromolecules in melt and solution, and glassy solids. Both analytic theory as well as simulations will be brought to bear, and the two routes will overlap for some of the studies of interest. The research will generate testable predictions that will be held accountable to experimental data. The result will be new strategies for choosing molecular constituents so as to produce desired physical properties. The goals of research include: (1) Connections between thermodynamic characterization and dynamic response. Preliminary evidence shows that the Locally Correlated Lattice (LCL) model developed under prior NSF support yields a well-defined thermodynamic quantification of free volume. Further, the free volume predictions correlate with the glass transition, and suggest an explanation for the temperature and volume dependence of dynamic relaxation in polymeric and small molecule systems. The PI aims to extend these results to a large variety of polymeric and other glassy systems, using relaxation data that spans many decades, as well as a broad range of temperatures and pressures. Thermodynamic scaling using the LCL free volume will collapse the entire data set for each system to a single line, requiring only a single optimization parameter. Application to many systems will allow correlation between local chemical structure, which can be synthetically controlled, and the material dependent scaling parameter. The result will be an ability to predict how a designed structure will dynamically relax. The scaling analysis will also lead to predictions about the pressure dependence of dynamic relaxation, given only ambient pressure experimental data. Both bulk and thin film systems will be studied.(2) Free Volume and Cohesive Energy Density as Orthogonal Controls on Miscibility. The cohesive energy density has a poor history of predicting miscibility in polymer solutions and blends. Application of the LCL model to study mixing behavior has shown the LCL free volume to be an orthogonal metric to cohesive energy density; there is evidence that it can serve as a predictive tool where the cohesive energy density fails. The LCL theory can predict both quantities, which will allow this hypothesis to be tested. Application to many systems will allow correlations with molecular properties, a particular interest being chain stiffness. (3) A Coarse-grained Simulation Method for Studying the Effects of Interfaces. Introduction and control of interfaces plays an increasingly important role in material design. The PI's Limited Mobility simulation approach can model a range of experimentally observed behavior. Key features include decoupling between local density and local mobility, and incorporating nearest-neighbor facilitation for local moves. Initial results indicate that simulation parameter values connect closely with experimentally measured characteristic molecular properties. In this work, the limited mobility model will be applied to capture the disruptive influences of a wide variety of interfaces, including anti/plasticizing additives and multilayer systems.