Packing-Density Effects on the Friction of n-Alkane Monolayers

Abstract

The classical molecular dynamics simulations presented here examine the tribology associated with the sliding of a hydrogen-terminated diamond counterface across a monolayer of n-alkane chains that are covalently bound to a diamond substrate. Two systems using chains of fixed length (18 carbon atoms per chain) on diamond (111) are examined: a tightly packed (2 × 2) arrangement and a loosely packed system with approximately 30% fewer chains. Both systems give a similar average friction at low loads. Under high loads, the tightly packed monolayer exhibits significantly lower friction than the loosely packed monolayer. While the movement of chains is greatly constricted in both systems, the tightly packed monolayer under high loads is clearly more uniform in geometry and more constrained with respect to the movement of individual chains than the loosely packed monolayer. This suggests that efficient packing of the chains is responsible for the lower friction for tight packing under high load. This is supported by the fact that sliding initiates larger bond-length fluctuations in the loosely packed system, which ultimately lead to more energy dissipation via vibration.

Molecular Origins of Elastomeric Friction

“…In ambient scanning force microscopy (SFM) experiments and computer models, such as molecular dynamic (MD) and Monte Carlo (MC) simulations, molecular scale friction is discussed in terms of mechanical relaxations and internal conformational changes [28][29][30][31] . These studies involved highly structured model systems, which were prepared by either self-assembly or Langmuir-Blodgett techniques 32 .…”

“…Such mono-or multilayered systems provide convenient access for investigations of molecular-scale dissipation mechanisms; e.g. load induced molecular tilts 28 , and reversible and irreversible conformation changes [28][29][30] . Simple frictional models, such as the Tomlinson-Prandtl model 33 , could be tested, and the corrugated molecular surface potential compared to the magnitude of discontinuous molecular stick-slip sliding 29,31 .…”

Confinement and compression of an oligomer brush

“…Harrison's argument, 24 that upon compression energetically costly gauche defects are inserted into the polymer brush, can thus be understood as follows. All those conformers of the polymer that are not eliminated by volume exclusion contribute to the brush; at large plate separation, the all-trans conformer is not the only one because at room temperature entropy demands that conformers with gauche defects contribute.…”

“…19 Experimentally, oligomer molecules are end grafted to a surface: a second surface ͑which may also be covered with a SAM or oligomer brush͒ is then brought in contact with the first, and the resulting compressional force is measured as a function of the separation via the surface force apparatus 20,21 or atomic force microscope ͑AFM͒. 19,22,23 Simple behavior of compressed SAMs can be understood using scaling arguments, 8 creation of gauche defects, 24 geometrical models ͑cooperative tilting͒, 25 or cubic lattice models. 26 However, these models neglect polymer-specific characteristics that can result in unexpected behavior.…”

Friction between solids

“…The creation of defects within the monolayer disrupts both the tight packing of the monolayer and the concerted response of the chains to the motion of the counterface, which leads to higher friction. As a result, the friction force as a function of sliding distance loses the periodicity associated with the (2!2) unit cell and the average friction is higher (Mikulski & Harrison 2001a). The effect of defect density on the friction between two SAMs composed of alkane chains attached to SiO 2 was examined by Chandross et al (2004) using MD and the COMPASS force field.…”