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
In daily life we enjoy the benefits of material design – from clothing, to furniture, to communications and computer devices, to transportation. Synthetic polymers play a large role and for both mundane and extreme applications the knowledge of how these important constituents behave is critical. In our group we focus on several key properties of polymers, and we work to understand how the molecular features of a substance translate into some of its material behaviour. One example is the category of polymer blends. Sometimes the best design for a particular application will involve mixing two polymers to make a polymer blend, or alloy. In the small molecule world it is often very easy to determine whether two constituents will combine to make a homogenous mixture – think oil and water, compared to sugar and water. In that case it is straightforward to do the experiment and observe the definitive answer to the question: Will these two substances mix and stay mixed? However, mixing two viscous polymer melts is not so easy, and it is even more challenging to determine whether mixing will persist. Indeed, it may take months or much longer to conclude whether the mixed state is the equilibrium state, and therefore going to be stable indefinitely. However, there are theoretical methods that will yield an answer in the absence of experimental results, and we use statistical thermodynamics to formulate these tools. Our theory allows us to predict whether ‘mixed’ or ‘unmixed’ is the long-term outcome when two given components are combined. We can also predict the effects of changing composition, temperature, or pressure. The theory we have derived is relatively straightforward; the thermodynamic equations can be easily written out, and solved on a laptop. What makes this possible is that we simplify the reality of our continuous world by mapping it on to a lattice. Imagine taking a pearl necklace and tossing it over and over again onto a small wooden table, measuring various properties for each toss, and then averaging them. Alternatively, imagine that the tabletop has on it a square lattice – like a checkerboard – and that the only necklace positions allowed are ones that put one of the connected pearls on each site. Vastly fewer possible arrangements ensue and we have greatly simplified the task of averaging, however, at the ‘price’ of eliminating the contributions from many of the molecular arrangements. Are the two methods equivalent? One longtime goal in the group was finally realized during this grant when we were able to compare two versions of our theory – one lattice, one for the continuum – with experimental and simulation data for a variety of liquid and vapour properties. We showed that the easier lattice model produced results in superb agreement with the data, demonstrating that simplifying the amount of molecular level detail did not translate into less sophisticated predictions about fluid behaviour. Another property of practical interest is the temperature at which a polymer fluid, or melt, turns solid. Some polymers cool and crystallize, but many more become amorphous solids, or glasses. While window glass – made of small molecules - is ubiquitous in our lives, polymer glassy materials are widely used; polycarbonates, alone, appear in electronic applications, data storage, safety glass, etc. For materials that crystallize the transition from disorganized melt to highly organized solid is accompanied by a tremendous increase in molecular-level ordering. Think of how much more efficiently a small space can be packed with metal chairs if neatly stacked, relative to a haphazard pile. The ‘sloppy’ packing of molecules in a glass means that there is room for local motions of small sections of a molecule to persist, even as the material is solidifying and continues to cool. This behaviour is one identifying feature of a glass. A relatively recent discovery is that the temperature of the melt-glass transition can be greatly affected by whether the sample is in the bulk or is an ultrathin (tens of nanometers) film. The consequences of thin film format are still not well understood, nor are some of the fundamental features even of the melt-glass transition in the bulk. During the support of this grant we developed a number of theories focused on thin film glasses, and were successful in capturing particular features of interest. However, no current single theory – ours or anyone’s - can explain the collection of experimental observations. We are now working on two very different approaches, each of which has a simple physical picture for how the cooling/cooled material differs from the melt. By tackling this complicated problem using several methods, which make different kinds of approximations, we anticipate working our way to a deeper and better understanding of how molten systems ‘go glassy’.
