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Distinct Thermophysical Properties Associated with Low Molecular Weight Linear and Cyclic Polymers in Bulk and Confined States: Tg, Fragility, and Thin Film Stability

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Significant deviations in the glass transition temperature (Tg) of nanoconfined polymer films from the bulk have been studied for over twenty years with the focus on high molecular weight (MW), linear polymers. This thesis explores low MW polymers, which represent an important class of materials widely used as coatings, detergents, and resists, and polymers with cyclic/ring architectures. An understanding of the thermophysical and interfacial properties of such polymer films provides valuable guidance for their practical use as nanomaterials. We first developed a fundamental understanding regarding the role of “chain-end structure” by noting that the chemical structure of the chain-end moiety per se may influence polymer relaxation dynamics in low MW polymers. Dramatic tunability in the Tg and fragility for low MW linear polymers is achieved by initiator fragments located at chain ends. The perturbation of Tg by the combined effects of a reduction in MW and chain-end structure correlates one-to-one within error with the perturbation of fragility. In addition to bulk properties, this work investigates the physics of polymer thin films. Alternative approaches are presented to suppress or eliminate the Tg-confinement effect of low MW linear PS (l-PS) by incorporating strong hydrogen-bonding interactions at the interface. Using low MW l-PS, we found that the strength of the m-confinement effects decreases with decreasing bulk polymer fragility. In stark contrast to the substantial Tg-confinement effects in high MW l-PS, nearly completely suppressed confinement effect is discovered in low MW cyclic PS (c-PS). Upon nanoscale confinement, high MW l-PS films exhibit significantly reduced fragility compared to bulk. Despite having similar bulk fragility as high MW l-PS, low MW c-PS films show major suppression in fragility reduction with decreasing thickness. This result indicates a strong correlation between the susceptibility of fragility perturbation and the susceptibility of Tg perturbation, caused by chain topology and/or by confinement. Lastly, c-PS thin films exhibit superior thermal stability against dewetting compared to their linear analogues, making novel cyclic polymer architectures attractive materials for practical usage. Taking advantage of different chain topologies and their specific attributes may be the next step in materials design and technology breakthrough.

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  • 05/14/2019
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