APPLICATION OF MATCHED SPIN PROBES TO THE INVESTIGATION OF THE OVERALL MOTION OF MODEL LUBRICANT MOLECULES

EPR spectroscopy has been used to measure the fast-limit rotational co rrelation times (and their activation energies) of two spin probes whi ch closely match the structure of two model lubricants, namely a low-t raction fluid (di-n-octyl ether) and a high-traction fluid (2,4-dicycl ohexyl-2-methylpentane, DCMP). The data have been compared with those obtained from these fluids from C-13 NMR relaxation measurements. The latter provide values for the rotational correlation times and their a ctivation energies, but they give no insight into the isotropy of the motion. The EPR work showed that the spin probe for di-n-octyl ether e xecutes isotropic motion while that for DCMP is anisotropic. The value s obtained by EPR and NMR for the rotational correlation times and the ir associated activation energies are in reasonable agreement; however , the EPR results show that the spin probe for DCMP rotates with near axial symmetry and that the NMR rotational correlation time correspond s to the faster of the two motions.