Mechanism for Viscosity Reduction in High Molecular Weight Polymer Nanometer Films

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TECHNICAL ABSTRACT:While the dynamics of polymers in a melt is well studied and understood, little is known about the counterpart problem for polymers confined in nanometer films. It is broadly believed that a thin mobile layer exists at the free surface where polymer meets the air. As the thickness of a polymer film decreases, the influence of this mobile layer increases with the surface-to-volume ratio and may bring about visible enhancement to the mobility of the film as a whole. But this picture is difficult to reconcile for high molecular weight (Mw) films, where the perpendicular dimension of the polymer coils exceeds the thickness of the surface mobile layer, which is about 3 nm. Specifically, this necessitates the surface chains to be highly stretched with a stretching energy that can be 100 kBT per chain. The PI's conjecture is that a new mechanism involving slippage of the polymer chains on the substrate and exhibition of Rouse dynamics by the chains, possibly facilitated by nano-confinement and the free surface, may explain the data of the high-Mw films. As for the two-layer model, she surmises that it is operative and in competition with the new mechanism, and dominates the new mechanism in the low-Mw regime. In this program, experiments are proposed to test the validity of the two-layer model and the new mechanism in different Mw regimes. The main purpose is to gain insight about the origin of the viscosity reduction exhibited by the high-Mw films. In addition, this project will include counterpart studies on a different polymer film system to explore the generality of the new phenomena observed in the viscosity of the polystyrene films.NON-TECHNICAL ABSTRACT:Many existing and emergent technological applications employ polymer nanometer films. These include lubrication, adhesives, biomedical technologies and nano-electronics, etc. Viscosity is an important engineering property that specifies how fluidic a film is and often also determines how durable a film would be under thermal stress. While the science about the viscosity of polymers in a bulk sample is well understood, little is known for that of polymers under confinement in nanometer films. This program aims for a better understanding of the new properties found in the viscosity of some polymer nanometer films. It will have an impact on academic fields as polymer physics and materials science, and technological fields that will benefit from better strategic design or adaptation of polymeric materials enabled by the new knowledge gained through the studies. Another important impact is the training of personnel in the STEM field. Participants of this program will not only receive training on the state-of-the-art characterization and processing techniques of polymer films, they will also be given ample opportunities to present talks and posters at professional meetings. Two graduate students will be trained. In addition, the PI will recruit high-school students to the program. In the past six years, she has been a mentor of the Boston University Research Internship in Science & Engineering (BU RISE) program that brings in high-school students nationwide to conduct research with a faculty mentor, and will continue to do so.