Publications

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We perform molecular dynamics simulations of coarse-grained ionomer melts with two different architectures. Regularly spaced charged beads are placed either in the polymer backbone (ionenes) or pendant to it. The ionic aggregate structure is quantified as a function of the dielectric constant. The low wave vector ionomer scattering peak is present in all cases, but is significantly more intense for pendant ions, which form compact, discrete aggregates with liquidlike interaggregate order. This is in qualitative contrast to the ionenes, which form extended aggregates. © 2011 American Physical Society.

Ionomers--polymers containing a small fraction of covalently bound ionic groups--have potential application as solid electrolytes in batteries. Understanding ion transport in ionomers is essential for such applications. Due to strong electrostatic interactions in these materials, the ions form aggregates, tending to slow counterion diffusion. A key question is how ionomer properties affect ionic aggregation and counterion dynamics on a molecular level. Recent experimental advances have allowed synthesis and extensive characterization of ionomers with a precise, constant spacing of charged groups, making them ideal for controlled measurement and more direct comparison with molecular simulation. We have used coarse-grained molecular dynamics to simulate such ionomers with regularly spaced charged beads. The charged beads are placed either in the polymer backbone or as pendants on the backbone. The polymers, along with the counterions, are simulated at melt densities. The ionic aggregate structure was determined as a function of the dielectric constant, spacing of the charged beads on the polymer, and the sizes of the charged beads and counterions. The pendant ion architecture can yield qualitatively different aggregate structures from those of the linear polymers. For small pendant ions, roughly spherical aggregates have been found above the glass transition temperature. The implications of these aggregates for ion diffusion will be discussed.

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We have conducted a molecular dynamics (MD) simulation study of water confined between methyl-terminated and carboxyl-terminated alkylsilane self-assembled monolayers (SAMs) on amorphous silica substrates. In doing so, we have investigated the dynamic and structural behavior of the water molecules when compressed to loads ranging from 20 to 950 MPa for two different amounts of water (27 and 58 water molecules/nm{sup 2}). Within the studied range of loads, we observe that no water molecules penetrate the hydrophobic region of the carboxyl-terminated SAMs. However, we observe that at loads larger than 150 MPa water molecules penetrate the methyl-terminated SAMs and form hydrogen-bonded chains that connect to the bulk water. The diffusion coefficient of the water molecules decreases as the water film becomes thinner and pressure increases. When compared to bulk diffusion coefficients of water molecules at the various loads, we found that the diffusion coefficients for the systems with 27 water molecules/nm{sup 2} are reduced by a factor of 20 at low loads and by a factor of 40 at high loads, while the diffusion coefficients for the systems with 58 water molecules/nm{sup 2} are reduced by a factor of 25 at all loads.

A molecular-scale interpretation of interfacial processes is often downplayed in the analysis of traditional water treatment methods. However, such an approach is critical for the development of enhanced performance in traditional desalination and water treatments. Water confined between surfaces, within channels, or in pores is ubiquitous in technology and nature. Its physical and chemical properties in such environments are unpredictably different from bulk water. As a result, advances in water desalination and purification methods may be accomplished through an improved analysis of water behavior in these challenging environments using state-of-the-art microscopy, spectroscopy, experimental, and computational methods.

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We present the results of massively parallel molecular dynamics simulations aimed at understanding the nanotribological properties of alkylsilane self-assembled monolayers (SAMs) on amorphous silica. In contrast to studies with opposing flat plates, as found in the bulk of the simulation literature, we use a model system with a realistic AFM tip (radius of curvature ranging from 3 to 30 nm) in contact with a SAM-coated silica substrate. We compare the differences in response between systems in which chains are fully physisorbed, fully chemisorbed, and systems with a mixture of the two. Our results demonstrate that the ubiquitous JKR and DMT models do not accurately describe the contact mechanics of these systems. In shear simulations, we find that the chain length has minimal effects on both the friction force and coefficient. The tip radius affects the friction force only (i.e., the coefficient is unchanged) by a constant shift in magnitude due to the increase in pull-off force with increasing radius. We also find that at extremely low loads, on the order of 10 nN, shearing from the tip causes damage to the physisorbed monolayers by removal of molecules. © 2008 American Chemical Society.

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In this study, we perform molecular dynamics simulations of adhesive contact and friction between alkylsilane Si(OH){sub 3}(CX{sub 2}){sub 10}CX{sub 3} and alkoxylsilane Si(OH){sub 2}(CX{sub 2}){sub 10}CX{sub 3} (where X = H or F) self-assembled monolayers (SAMs) on an amorphous silica substrate. The alkylsilane SAMs are primarily hydrogen-bonded or physisorbed to the surface. The alkoxylsilane SAMs are covalently bonded or chemisorbed to the surface. Previously, we studied the chemisorbed systems. In this work, we study the physisorbed systems and compare the tribological properties with the chemisorbed systems. Furthermore, we examine how water at the interface of the SAMs and substrate affects the tribological properties of the physisorbed systems. When less than a third of a monolayer is present, very little difference in the microscopic friction coefficient {mu} or shear stresses is observed. For increasing amounts of water, the values of {mu} and the shear stresses decrease; this effect is somewhat more pronounced for fluorocarbon alkylsilane SAMs than for the hydrocarbon SAMs. The observed decrease in friction is a consequence of a slip plane that occurs in the water as the amount of water is increased. We studied the frictional behavior using relative shear velocities ranging from v = 2 cm/s to 2 m/s. Similar to previously reported results for alkoxylsilane SAMs, the values of the measured stress and {mu} for the alkylsilane SAM systems decrease monotonically with v.

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