Publications

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Poly(ethylene oxide) (PEO) is the quintessential biocompatible polymer. Due to its ability to form hydrogen bonds, it is soluble in water, and yet is uncharged and relatively inert. It is being investigated for use in a wide range of biomedical and biotechnical applications, including the prevention of protein adhesion (biofouling), controlled drug delivery, and tissue scaffolds. PEO has also been proposed for use in novel polymer hydrogel nanocomposites with superior mechanical properties. However, the phase behavior of PEO in water is highly anomalous and is not addressed by current theories of polymer solutions. The effective interactions between PEO and water are very concentration dependent, unlike other polymer/solvent systems, due to water-water and water-PEO hydrogen bonds. An understanding of this anomalous behavior requires a careful examination of PEO liquids and solutions on the molecular level. We performed massively parallel molecular dynamics simulations and self-consistent Polymer Reference Interaction Site Model (PRISM) calculations on PEO liquids. We also initiated MD studies on PEO/water solutions with and without an applied electric field. This work is summarized in three parts devoted to: (1) A comparison of MD simulations, theory and experiment on PEO liquids; (2) The implementation of water potentials into the LAMMPS MD code; and (3) A theoretical analysis of the effect of an applied electric field on the phase diagram of polymer solutions.

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As electronic and optical components reach the micro- and nanoscales, efficient assembly and packaging require the use of adhesive bonds. This work focuses on resolving several fundamental issues in the transition from macro- to micro- to nanobonding. A primary issue is that, as bondline thicknesses decrease, knowledge of the stability and dewetting dynamics of thin adhesive films is important to obtain robust, void-free adhesive bonds. While researchers have studied dewetting dynamics of thin films of model, non-polar polymers, little experimental work has been done regarding dewetting dynamics of thin adhesive films, which exhibit much more complex behaviors. In this work, the areas of dispensing small volumes of viscous materials, capillary fluid flow, surface energetics, and wetting have all been investigated. By resolving these adhesive-bonding issues, we are allowing significantly smaller devices to be designed and fabricated. Simultaneously, we are increasing the manufacturability and reliability of these devices.

This report is divided into two parts: a study of the glass transition in confined geometries, and formation mechanisms of block copolymer mesophases by solvent evaporation-induced self-assembly. The effect of geometrical confinement on the glass transition of polymers is a very important consideration for applications of polymers in nanotechnology applications. We hypothesize that the shift of the glass transition temperature of polymers in confined geometries can be attributed to the inhomogeneous density profile of the liquid. Accordingly, we assume that the glass temperature in the inhomogeneous state can be approximated by the Tg of a corresponding homogeneous, bulk polymer, but at a density equal to the average density of the inhomogeneous system. Simple models based on this hypothesis give results that are in remarkable agreement with experimental measurements of the glass transition of confined liquids. Evaporation-induced self-assembly (EISA) of block copolymers is a versatile process for producing novel, nanostructured materials and is the focus of much of the experimental work at Sandia in the Brinker group. In the EISA process, as the solvent preferentially evaporates from a cast film, two possible scenarios can occur: microphase separation or micellization of the block copolymers in solution. In the present investigation, we established the conditions that dictate which scenario takes place. Our approach makes use of scaling arguments to determine whether the overlap concentration c* occurs before or after the critical micelle concentration (CMC). These theoretical arguments are used to interpret recent experimental results of Yu and collaborators on EISA experiments on Silica/PS-PEO systems.