Tribological Study of Polyethylene Glycol Monolayers on Silica Substrate

Lubricating micro- and nano-electromechanical systems (MEMS and NEMS) has become increasingly challenging as devices are continuously miniaturized. Decreasing the dimensions of these systems causes their surface area to volume ratios to increase significantly, making frictional forces between moving parts dominant. Solid-state lubrication via self-assembled monolayers presents a promising alternative to traditional macro-scale lubricants, which are too bulky to use in such confined systems.^1,2 Previous studies have shown that self-assembled monolayers containing alkylsilanes and mixed alkylsilane and perfluoroalkylsilane chains reduce friction coefficients compared to equal length systems. Although, hydrocarbon-based monolayers have been widely studied and shown to improve frictional properties and device life, mixed monolayers impart the beneficial properties of perfluoroalkylsilanes to the film including low critical surface tension and high thermal and mechanical stability. Furthermore, a mixed length configuration with longer alkylsilane chains relative to the perfluoroalkylsilane chains leads to a liquid-like layer formed by the taller chains that fold over and align with the shearing direction.³

In this study, we investigate polyethylene glycol (PEG) monolayers using molecular dynamics simulations as a potential solid-state lubricant for NEMS. The presence of oxygen atoms within the chains render these chains more flexible than their alkyl- and perfluoroalkyl-counterparts. Thus, even sparsely functionalizing a surface leads to a uniform film. We have investigated the tribological properties of such films across a range of surface coverages and chain lengths. Preliminary results show the effects of surface coverage on friction coefficients is diminished as chain length is increased. This suggests that equivalent friction coefficients could be achieved whilst reducing separation distances by using longer chains on sparser surfaces.

1.    Booth, B. D., Vilt, S. G., Lewis, J. Ben, Rivera, J. L., Buehler, E. A., McCabe, C., & Jennings, G. K. (2011). Tribological Durability of Silane Monolayers on Silicon. Langmuir, 27(10), 5909–5917. doi:10.1021/la104778q 

2.    Lorenz, C. D., Chandross, M., Grest, G. S., Stevens, M. J., & Webb, E. B. (2005). Tribological properties of alkylsilane self-assembled monolayers. Langmuir, 21(25), 11744–8. doi:10.1021/la051741m

3.    Lewis, J. Ben, Vilt, S. G., Rivera, J. L., Jennings, G. K., & McCabe, C. (2012). Frictional properties of mixed fluorocarbon/hydrocarbon silane monolayers: a simulation study. Langmuir, 28(40), 14218–26. doi:10.1021/la3024315