(698g) Examination of the Relationships between Terminal Group Chemistry and Tribological Properties in Monolayer Films through Molecular Dynamics Screening

The design space of nanoscale devices has been constrained by the friction and wear associated with surfaces in sliding contact. Self-assembled monolayer films have shown promise as a potential solution to these issues, providing a dense layer of surface-bound chains that prevent direct surface-surface contact and reduce both adhesive and frictional forces. A favorable characteristic of monolayer lubricants is that their tribological properties have been shown to be closely related to their chemical composition [1], presenting a near-infinite chemical parameter space that can be utilized to optimize monolayer chemistry for tribological application. However, examination of even a small subset of this vast parameter landscape poses a significant challenge for experiment, given that synthesis of films may be time-consuming and non-trivial and it may be difficult to fully control variation between different film chemistries. To this end, molecular simulation is a useful tool, providing precise control over system variables and molecular-level resolution.

Herein, we leverage the recently developed Molecular Simulation and Design Framework (MoSDeF) [2] to perform molecular dynamics screening of functionalized alkylsilane monolayer films on amorphous silica substrates, placed in contact and undergoing shear. Combinations of 16 distinct terminal group chemistries are evaluated, considering the cases where (1) the two monolayer films are uniform and identical, (2) the two monolayer films are chemically distinct, but each film itself is uniform, and (3) the case where one monolayer film is composed of a mixture two different terminal groups. The breadth of the data obtained from this screening is sufficient to utilize machine learning to develop predictive models for the tribology of functionalized monolayer films and to extract information on terminal group characteristics that most influence tribology. It is observed that molecular shape most strongly influences the coefficient of friction, whereby linear and planar molecules are preferable, while charge distribution is most closely linked to adhesion [3].

1. Brewer, N. J.; Beake, B. D.; Leggett, G. J. Friction Force Microscopy of Self-Assembled Monolayers: Influence of Adsorbate Alkyl Chain Length, Terminal Group Chemistry, and Scan Velocity. Langmuir 2001,17(6), 1970–1974.
2. http://github.com/mosdef-hub
3. Summers, A.Z.; Gilmer, J.; Iacovella, C.R.; Cummings, P.T.; McCabe, C. Examining Chemistry-Property Relationships in Lubricating Monolayer Films through Molecular Dynamics Screening, In Preparation.