Polymer Glass, Melt, and Mixture Thermodynamics in the Bulk and in Thin Films

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TECHNICAL SUMMARYThe Division of Materials Research and the Division of Chemistry contribute funds to this award. It supports theoretical reearch and education that involves the application of statistical thermodynamic methods to problems of both fundamental and applied interest.Polymeric thin films have potential applications in areas such as electronics, optics, and protective membranes; in addition, they are a ubiquitous component in nanocomposites. Maintenance of structural integrity is key, thus it is essential to ensure that operating conditions do not approach the glass transition temperature. Evidence from a variety of experimental techniques indicates that a supporting substrate and a free surface can each have a profound impact on the glass transition temperature of a polymeric thin film, relative to the bulk value. However, the interpretation of these results is controversial. Theory can play an essential role in resovlving the controversy through the development of clearly elucidated models with well-defined assumptions, and the application of them to determine what physical behaviour would ensue. A focus of this project builds on the PI's previous work that yielded a model of thermodynamic properties that appears able to predict shifts in the glass transitions of freestanding polymeric films that match with experimental data. The PI aims to exploit this and related models in order to predict both glass transitions and phase separation in polymer films.An important question is whether it is possible to capture the thermodynamic behavior well enough - not just the free energy, but also the entropic and enthalpic contributions to it - so that predictions regarding any mixture may be made using only pure component experimental data as input. The PI's lattice-based theory of polymer fluids and mixtures in analyzing dozens of polymers and blends has yielded intriguing connections between pure component properties and thermodynamic behaviour. The PI plans to explore whether strategic choices of components will result in control of both the enthalpy and entropy of mixing in polymeric blends and solutions, allowing for true predictions regarding miscibility based on pure component properties, alone. Theoretical models developed in the course of the research have potential for widespread use. The ability to correlate characteristic pure component behaviour with the enthalpic and entropic contributions to mixture thermodynamics will yield routes to the development of new materials. Another outcome of this research will be a thermodynamic model for how both the local and averaged behaviour of polymers and their blends changes in going from bulk to film. The impact of this research will be magnified through the involvement of graduate students and postdoctoral fellows, both via publications and also through their presentations at national meetings. The PI's efforts in research and teaching mentorship have resulted in an increase in the number of women in the ranks of graduate and postgraduate students and in faculty; it is expected that such effects will continue. NONTECHNICAL SUMMARYThe Division of Materials Research and the Division of Chemistry contribute funds to this award. An enduring ambition for a theorist is to solve complex problems with conceptually transparent tools. The PI will focus on research areas from studies on glassy polymer thin films to thermodynamic properties of novel, untested, polymer-containing mixtures. Preliminary evidence indicates that tools from the same basic set may be effective across this span of interests. When a simple tool is shown to be convincing in tackling a series of complicated questions, it not only reveals something about the essential elements of the problems, but also illuminates the extent to which seemingly very different problems may be related.A typical small molecule liquid, when cooled, can turn into an amorphous glass or a crystal. What is usually thought of as 'glass,' as found in windows and drinking vessels, has as its majority component silicon dioxide, with other small molecules mixed in. However, large chain-like molecules such as polymers also form glassy materials. Among the many common examples are polycarbonate eyeglasses and windshields. The temperature below which a polymer system turns into a glass is called the 'glass transition temperature'. Recent experimental studies have revealed that when polymers are formed into very thin films, ten to one hundred nanometers thick, where a nanometer is a billionth of a meter, the presence of either a free surface or a supporting underlayer may result in a shift in the glass transition that can be quite dramatic - up to thirty degrees Celsius.There is some evidence that more than one physical driving force may be at play in the evident shifts of glass temperature with film thickness. The PI's investigation begins a thought experiment: imagine that a 'slice' of material is extracted from a bulk sample to form an ultrathin slab. The formation of two free surfaces creates layers of polymeric segments for whom neighbouring interactions with other segments have been greatly diminished. By describing the implications in mathematical terms the PI finds that missing interactions result in a film glass transition temperature lower than that for the bulk sample. Test calculations are in reasonable agreement with experiments.This focus on how neighbouring interactions affect the physical behaviour of a polymeric sample also motivates the PI's studies on what happens when polymers of two types are mixed. In that case a polymer segment will not face missing interactions, but altered ones, due to the presence of another kind of segment. The ability to predict the extent to which polymers of different types may mix is crucial in the design of new materials, and will also lead to better manipulation of poorly-understood systems already being used. Combining results for the two kinds of problems described above will allow the PI to 'create' mixtures in both thin film and bulk form, with the potential for controlling the extent to which different materials may mix as a function of sample thickness. The scientific discoveries arising from this research will be magnified through the involvement of graduate students and postdoctoral fellows, both via publications and also through their presentations at national meetings. In addition, the PI's efforts in research and teaching mentorship have resulted in an increase in the number of women in the ranks of graduate and postgraduate students and in faculty; it is expected that such effects will continue.