Publications

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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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The effects of mixed functionality and degree of curing on the stress-strain behavior of highly cross-linked polymer networks are studied using molecular dynamics simulations. The networks are made dynamically in a manner similar to epoxy network formation, and the average functionality of the cross-linker, f av, is systematically varied from 3 to 6 by mixing cross-linkers with functionalities f = 3, 4, and 6. Stress-strain curves are determined for each system from tensile pull simulations. The range of strain of the plateau region (R P) in the stress-strain curve, failure strain (ε f), and failure stress (σ f) for fully cured networks are found to have a power law dependence on f av as ∼f av α. For R P and ε f, α is determined to be -1.22(3) and -1.26(4), respectively. The failure strain is equal to the strain needed to make taut the maximum of the minimal paths through the network connecting the two solid surfaces. The failure stress, however, shows two distinct regions. For f av α ≤ 4, σ f increases with increase in f av and α = 1.22(5). In this f av regime, the work to failure is constant. For f av α ≥ 4, the systems fail interfacially, σ f becomes a constant, and work to failure decreases with f av. These mechanical properties are also found to depend on the degree of curing. With decrease in percentage of curing, failure stress decreases and failure strain increases. The mode of failure changes from interfacial to bulk. © 2004 American Chemical Society.

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The fracture behavior of binary Lennard-Jones (LJ) glasses is studied by extensive molecular dynamics simulations. These LJ glasses represent a nonbond limit of polymer network glasses. We determine that the low strain behavior of the LJ and polymer glasses is similar. Two different LJ glasses are fractured under tensile strain without any preexisting crack. Void formation and resulting growth as strain increases is the mechanism through which the system fails. Void formation initiates at the yield strain of [Formula presented] which is approximately the same strain at which the yielding behavior is first observed in cross-linked network models of polymer adhesives. The yield stress increased only by small amounts with increased strain rate and with increased system size (from [Formula presented] atoms to [Formula presented] atoms). Within the ranges tested, the stress-strain behavior of these systems is independent of the temperature drop during quench and the initial molecular configuration. © 2003 The American Physical Society.