Studies on Polymeric Glasses, Melts, and Mixtures: Connecting Microscopic Character with Observable Behaviour

Dartmouth College, Hanover NH

Investigators

Abstract

Nontechnical Summary Most of what we see in the world is made up of a relatively small number of elements. The fact that they combine together to create such a variety of matter with such an enormous range of behavior provides evidence that the line between chemical constituency and observable physical properties is neither short nor straight. The research targeted by this study focuses largely on macromolecules and their mixtures. Macromolecules, or polymers, are large molecules that result from the connection of small molecule 'repeat units'. One example is polyethylene, made from ethylene, which is used to produce (among other things) plastic bags, films, and bottles. Polyethylene contains only two elements, carbon and hydrogen; other polymers, having different properties relative to polyethylene, can be made from the same two elements. Even for this simple example, it is not straightforward to predict the properties of all of these polymers from their chemical 'recipe', alone. In addition to the chemical nature of a substance, the properties of a material can depend on how it is formulated. For example, whether it is cast in a film, made into a membrane, or processed in bulk, influences how the material responds to temperature, pressure, and the presence of other constituents. In the research funded through this proposal a combination of theory and computer simulation will be used to create new connections between the microscopic nature and characterization of complex materials and their bulk and film properties. Developed theoretical tools will be capable to lead from the characterization of a pure component (using experimental data) to analysis and prediction of how that substance will behave under varying conditions, and mixed with different partners. Another aspect of this research deals with how the behavior of a macromolecule changes when dealing with a thin film or even a membrane, relative to a bulk sample. For example, there is evidence that some polymers melt at significantly lower temperatures when they are thin films than when in the bulk; intriguingly, this effect can be nullified or even reversed when the film is loaded onto a solid substrate, depending on the chemical nature of the polymer and the substrate. The PI will explore how the physical format of the sample, and the choice of its neighbors, affects some of its observable properties; such insight is key for the myriad applications that involve thin film polymers. In terms of human resources, the research will create continuing opportunities for involvement by undergraduates, graduate students, and postdoctoral fellows, with particular efforts aimed towards undergraduate women. This work will foster new opportunities for connecting with the scientific public, via talks, posters, and publications, and for outreach to the general public This research is co-funded by the Division of Materials Research and the Chemistry Division Technical Summary Polymeric components may be blended, layered, or phase separated in a controlled fashion, in order to produce sophisticated new materials. The properties of such systems depend both on the microscopic chemical nature of the polymers chosen, as well as the form in which they are used, for example, in the bulk, or as films, membranes, or composites. It is therefore important to understand how both chemical constituency and formulation contribute to macroscopic properties. This research will provide fundamental insight as to the molecular features that help drive a variety of condensed matter transitions under a range of circumstances. The tools combine analytic statistical mechanical theory with simulation methods. The systems of interest comprise molecules ranging in size from small to polymeric, states ranging from glassy to melt to (where applicable) vapor, The setups range from single to multicomponent systems, from supported to layered films, from solutions to blends. The properties encompass both equilibrium and dynamic, the latter associated with the process of glassification. Systems of particular interest include glasses, and polymer melts, solutions, and blends. In the case of thin films, supported, freestanding films, and multi-layered films will all be investigated. The use of multiple approaches will provide opportunity for cross checking the different strategies, as well as comparing the results of each to experiment. The research will create new insight regarding the properties of complex systems in different environments, and will produce tools for making substantive predictions about mixture behavior based on pure component properties, alone. In addition, progress in the different areas described will create opportunities in areas where they overlap. Examples include: understanding the changes in polymer mixture behavior going from the bulk to a thin film, and studying the solubility of supercritical carbon dioxide in ionic liquids. Societal benefits aimed both at the larger scientific community and the more general public will accrue from the work described here. New methods will expand the range of soft matter communities able to apply the results of the work proposed. This extended reach will be aided by computational tools written with casual, scientific users in mind, posted on the group website. In terms of human resources, the research will create continuing opportunities for involvement by undergraduates, graduate students, and postdoctoral fellows, with particular efforts aimed towards undergraduate women. This work will foster new opportunities for connecting with the scientific public, via talks, posters, and publications, and for outreach to the general public. This research is co-funded by the Division of Materials Research and the Chemistry Division