On the compressibility of a glass-forming lubricant : experiments and molecular modeling

Abstract

A very large number of technologically important liquids, including lubricants, undergo a glass transition under increasing pressure, decreasing temperature, or increasing rate of deformation (one may consider the glassy solid to be a supercooled liquid). The compressibility of glass-forming lubricants has a strong influence on the modeling of elastohydrodynamic (EHD) contacts, where pressures (as high as several GPa) are sufficient to induce the glass transition. This paper presents both experimental and analytical studies of the compressibility of a low-molecular-weight synthetic organic lubricant known as 5P4E, which has a simple molecular structure amenable to molecular modeling. The experimental results are obtained using the compression Kolsky bar and pressure–shear plate impact techniques, and show that this lubricant has substantial compressibility under high pressures. An analytical and computational investigation of the nonlinear compressibility of this simple material based on estimates of the molecular structure and intermolecular interactions is then presented. The molecular structure and the various molecular conformations of the material are examined using relatively simple molecular mechanics calculations. An intermolecular interaction energy potential is obtained by examining the interactions of a molecule pair, and the molecular structure and interaction potential estimates are used together to provide a prediction of the materials nonlinear compressibility (although thermal effects are not completely accounted for in the model). All but one of the parameters in the model are obtained directly from the molecular mechanics computations; the one parameter that must be independently specified is the volume at room temperature and atmospheric-pressure, obtained from a simple density measurement. The predicted compressibility is found to be in remarkably good agreement with the experimental data.