Spectroscopic studies of the model traction fluid bis(cyclohexyl) succinate and related molecules

Abstract

Fourier transform infra-red spectroscopy/microscopy and Raman scattering/microscopy have been used to try and relate the microscopic behaviour (i.e. the molecular properties) with the macroscopic behaviour (i.e. the viscosity) of the model traction fluid Bis(cyclohexyl) Succinate. Other structurally related compounds have also been studied to assist with interpretation. Variable temperature studies produced few conclusive changes, with the balance between density and temperature effects giving rise to few spectroscopically visible changes. Some correlation between the viscosity of the molecules studied and the changes observed was seen. The appearance of two distinct carbonyl stretching bands which change in intensity with temperature indicates an intramolecular conformational change. There is considerable broadening of all the vibrational bands of the molecules studied as the pressure is increased indicating an increase in the vibrational relaxation rate. Repulsive interactions are also seen to dominate as the pressure is increased. Analysis using the Kubo model has shown that the carbonyl stretching band is more likely to be formed in the slow modulation regime. Unusual behaviour is exhibited in some spectra obtained from the dynamic pressure studies including unusual band shapes and negative going bands. The combination of high pressure, shear rate, and temperature in the centre of the elastohydrodynamic contact only allows the bis(cyclohexyl) succinate molecules to assume a single conformation. As these conditions lessen, the molecules become less constrained and different behaviour is observed. Three dimensional pressure and film thickness profiles have been used to explain the changes seen in terms of a combination of pressure and film thickness changes. Solution studies show there are strong attractive dipole-dipole interactions, probably localised at the carbonyl group, between the bis(cyclohexyl) succinate molecules. Using the Eyring fluid model and viscosity data, the molecular behaviour has been related to the viscosity of the molecules.