Studies on Polymer Glasses, Melts, and Solutions

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TECHNICAL SUMMARY:This award supports research and education in studies of polymer glasses, melts, and solutions. The award is being support by the Division of Chemistry and the Division of Materials Research. The activity largely resides in two areas, polymer phase transitions and the impact of deuteration and local branching on the miscibility of polymer solutions and blends.Polymer thin films are studied which exhibit a glass transition where the polymer transforms from a glassy to a rubbery solid upon heating, a phenomenon which is usually explained through local relaxations. Though a glass transition is not itself unusual, the recent observation that the presence of a substrate and/or a free surface may have a long-range and significant impact on the glass transition of the polymer stimulates this research. The research develops theoretical models and computer simulations to address questions such as the why shifts in the glass transition of polymeric films may persist over tens of nanometers from the surface or substrate. There are two scenarios investigated. In one, the polymer film is supported by a substrate, in which case the glass transition of the film has been observed to occur tens of degrees above the bulk temperature. Conversely, the influence of a surface leads to a steep drop in the glass transition, particularly when the film is free-standing (has two free surfaces). In all cases the magnitude of the temperature shift depends on the choice of polymer and the film thickness. This proposal outlines a new model for the glass transition of thin film polymers, one which will address both the magnitude of the temperature shifts and the length scales over which they occur.The second area of research focuses on the impact of deuteration and local branching on the miscibility of polymer solutions and blends. The replacement of hydrogen for deuterium is employed in small angle neutron scattering experiments to study such systems. This isotopic exchange has an impact on miscibility which ranges from essentially nil to extremely significant. In the research carried out here, a theoretical analysis is outlined which will lead to a predictive tool for assessing the anticipated effect of deuteration. Another local change results in measurable shifts in miscibility is the incorporation of branches. These variations in local chain connectivity are characteristic of polyolefins. Research done here builds upon initial evidence that the theoretical methods developed to study deuteration effects can also apply for this category of structural change.The effort undertaken has broader impacts with both technological and educational consequences. In this work the theoretical tools developed by the PI to study complex fluids will be used to make testable predictions on polymer glasses and mixtures. The outcome is that a more sophisticated combination of strategies, accessible to the materials community, will be available in solving problems relating to understanding connections between microscopic structure and macroscopic behavior. The polymer thin films studied here are the subject of significant interest due to their potential for application in areas such as optics, catalysis, and electronics, as well as their ubiquitous presence in polymer nanocomposites. The polyolefins specifically are a class of polymers which have wide industrial application. The research activities involve graduates and postdoctoral fellows who are being trained in computer simulations methods and associated theory of polymer materials. Research results are disseminated through conference presentations by the PI and her research group at national meetings and in refereed publications. 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.NONTECHNICAL SUMMARY:This award supports research and education in studies of polymer materials. The award is being support by the Division of Chemistry and the Division of Materials Research. The activity largely resides in two areas, polymer phase changes and the mixing properties of polymer solutions and blends.Polymer thin films are studied which exhibit a glass transition where the polymer transforms from a glassy to a rubbery solid upon heating. Though a glass transition is not itself unusual, the recent observation that the presence of a surface may have a significant impact on the glass transition of the polymer stimulates this research. The research develops theoretical models and computer simulations to address questions such as the why shifts in the surface structure of polymeric films may persist deep into the film. There are two scenarios investigated. In one, the polymer film is supported by a substrate, in which case the glass transition of the film has been observed to occur tens of degrees above the usual temperature. Conversely, the influence of a surface leads to a steep drop in the glass transition onset, particularly when the film is free-standing (has two free surfaces). In all cases the magnitude of the temperature shift depends on the choice of polymer and the film thickness. This proposal outlines a new model for the glass transition of thin film polymers, one which will address both the magnitude of the temperature shifts and the length scales over which they occur.The second area of research focuses on the impact of deuteration and local branching on the mixing of polymer solutions and blends. In the research carried out here, a theoretical analysis is outlined which will lead to a predictive tool for assessing the anticipated effects. One factor investigated is the changes in mixing properties when the polymer molecules have side branches as opposed to being primarily long chains which is a characteristic of polyolefins. Research done here builds upon initial evidence that the theoretical methods developed in previous studies can also apply for this category of structural change.The effort undertaken has broader impacts with both technological and educational consequences. In this work the theoretical tools developed by the PI to study complex fluids will be used to make testable predictions on polymer glasses and polymer mixtures. The outcome is that a more sophisticated combination of strategies, accessible to the materials community, will be available in solving problems relating to understanding connections between molecular level structure and behavior of the material as a whole. The polymer thin films studied here are the subject of significant interest due to their potential for application in areas such as optics, catalysis, and electronics, as well as their ubiquitous presence in polymer nanocomposites. The polyolefins specifically are a class of polymers which have wide industrial application. The research activities involve graduates and postdoctoral fellows who are being trained in computer simulations methods and associated theory of polymer materials. Research results are disseminated through conference presentations by the PI and her research group at national meetings and in refereed publications. 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