The tribological behavior of oriented poly(tetrafluoroethylene) (PTFE) sliding surfaces is examined as a function of sliding direction and applied normal load in classical molecular dynamics (MD) simulations. The forces are calculated with the second-generation reactive empirical bond-order potential for short-range interactions, and with a Lennard-Jones potential for long-range interactions. The range of applied normal loads considered is 5-30 nN. The displacement of interfacial atoms from their initial positions during sliding is found to vary by a factor of seven, depending on the relative orientation of the sliding chains. However, within each sliding configuration the magnitude of the interfacial atomic displacements exhibits little dependence on load over the range considered. The predicted friction coefficients are also found to vary with chain orientation and are in excellent quantitative agreement with experimental measurements.

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.

Crosslinks occur in polymers following irradiation and are used in computational simulations to mimic the effects of chain tangling. Here, the effect of crosslink density on the tribological behavior of atomic-scale models of polyethylene and polytetrafluoroethylene is determined using classical molecular dynamics simulations. In the simulations, oriented crosslinked surfaces are slid in different directions over a range of applied normal loads. The results indicate that, at the same normal load, the friction force increases with increased crosslink density. In addition, the influence of randomized versus ordered crosslinking on the simulated tribological behavior is investigated. Finally, the influence of crosslink density on the simulated wear mechanisms of polyethylene and polytetrafluoroethylene is elucidated. The results have important implications for the atomic-scale modeling of friction at the interfaces of polymers that have been irradiated or contain entangled chains.

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The tribological behavior of oriented, selfmated poly(tetrafluoroethylene) (PTFE) sliding surfaces is examined as a function of temperature from 25 to 300 K. Three distinct sliding configurations are considered: sliding perpendicular to the chain alignment in both PTFE surfaces, sliding parallel to chain alignment in both PTFE surfaces and sliding parallel to the chain alignment on one surface but perpendicular to chain alignment on the other. Our simulations reveal a general trend of increasing friction coefficient with decreasing temperature for configurations where the chains are aligned in the direction of sliding or crossed. However, for conditions where the chains are aligned but the sliding motion is perpendicular to this alignment, gross deformation and athermal friction behaviors are observed. The atomic-level processes associated with these increases in friction at low temperatures are characterized.