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

A very large number of technologically important liquids, including lubrica nts, undergo a glass transition under increasing pressure, decreasing tempe rature, or increasing rate of deformation (one may consider the glassy soli d to be a supercooled liquid). The compressibility of glass-forming lubrica nts has a strong influence on the modeling of elastohydrodynamic (EHD) cont acts, where pressures (as high as several GPa) are sufficient to induce the glass transition. This paper presents both experimental and analytical stu dies of the compressibility of a low-molecular-weight synthetic organic lub ricant known as 5P4E, which has a simple molecular structure amenable to mo lecular modeling. The experimental results are obtained using the compressi on Kolsky bar and pressure-shear prate impact techniques, and show that thi s lubricant has substantial compressibility under high pressures. An analytical and computational investigation of the nonlinear compressibil ity of this simple material based on estimates of the molecular structure a nd intermolecular interactions is then presented. The molecular structure a nd the various molecular conformations of the material are examined using r elatively simple "molecular mechanics" calculations. An intermolecular inte raction energy potential is obtained by examining the interactions of a mol ecule pair, and the molecular structure and interaction potential estimates are used together to provide a prediction of the material's nonlinear comp ressibility (although thermal effects are not completely accounted for in t he 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-p ressure, obtained from a simple density measurement. The predicted compress ibility is found to be in remarkably good agreement with the experimental d ata. (C) 1998 Elsevier Science Ltd. All rights reserved.