Using Pyrene Excimer Fluorescence To Probe the Interactions between Viscosity Index Improvers and Waxes Present in Automotive Oil

Abstract

A new methodology based on pyrene excimer fluorescence (PEF) was applied to quantitatively measure the actual level of intermolecular association between ethylene-propylene (EP) copolymers in toluene in the presence of wax, an intrinsic component of engine oils. EP copolymers are commonly used as viscosity index improvers (VIIs) in engine oils. Unfortunately waxes in engine oils thicken the oil and are suspected of associating with VIIs at low temperature.In this study, four EP copolymers were maleated to yield EP−MA and then fluorescently labeled with 1-pyrenemethylamine to yield Py-EP. Successful maleation and pyrene labeling were confirmed by Fourier transform infrared (FTIR) and UV-Vis absorption spectroscopy. The solution behaviour of the EP copolymers in the presence of wax was characterized by PEF, which was used to quantitatively measure the molar fraction of intermolecular interactions (finter) between EP copolymers in the absence or presence of wax. The fraction finter was determined through the analysis of the fluorescence spectra of the Py-EP solutions acquired as a function of temperature.Upon excitation, the Py-EP solutions generated excimer upon encounter between an excited and a ground-state pyrene. The ratio of the excimer fluorescence intensity (IE) over that of the monomer (IM) yielded the IE/IM ratio which was employed to determine finter. Plots of finter as a function of temperature provided a description of the solution behaviour of a given VII with or without wax present in solution. The results of this study indicate that wax in solution binds onto EP copolymers dissolved in toluene, which increases macromolecular associations as reflected by an increase in finter. The formation of microcrystals between semicrystalline EP copolymers however induces strong polymer-polymer interactions that result in the dissociation of the wax from the polymers.

Probing the Interactions between Mimics of Pour Point Depressants (PPDs) and Viscosity Index Improvers (VIIs) in Engine Oil Using Fluorescently Labeled PPDs

“…First, EP copolymers were found to interact with wax which are long chain hydrocarbons left in base oils to increase their viscosity, as long as they were soluble in toluene, but that EP(SM) would dissociate from wax upon crystallization of EP(SM) in toluene. 18 This effect is somewhat similar to that observed herein where amorphous and semicrystalline EP copolymers were found to interact with PC 18 MA as long as they were soluble in oil, whereas EP(SM) dissociated from PC 18 MA as soon as it crystallized in oil. It is thus rather interesting that the crystallization of PC 18 MA did not preclude interactions between PC 18 MA and EP copolymers whether they be amorphous or semicrystalline but that the crystallization of…”

“…The behavior observed for the Py(6.7)-PC 18 MA solutions in Figure 1 is exactly the same as the one observed for pyrenelabeled semicrystalline EP copolymers (Py-EP(SM)) in toluene. [17][18][19]22 As for Py(6.7)-PC 18 MA, the transition observed for Py-EP(SM) samples in toluene had been attributed to the crystallization of EP(SM). These results thus suggested that PEF reported on the crystallization of polymers in solution.…”

4,5,9,10‐Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two‐Photon Excited Emission

“…[19] In contrast, t-C 5 -PyDI has al ight yellow color with af luorescence quantum yield of 6.2 %i nt he solid state.T he formation of pyrene excimer is an important phenomenon that can be used in designing fluorescent-labeled probes and biosensors. [20] Recently,P ei [15] and Kçhler [21] have reported formation of excimer in pyrene-based small molecules and polymers, respectively.The excimer formation of t-C 5 -PyDI (Figure 4a) in cyclohexane was studied by recording the emission spectra under different concentrations (C 5 -PyDI was not studied owing to poor solubility). In the low concentration of 1 10 À7 mol L À1 ,t wo structural fluorescence bands peaking at 307 and 352 nm emerge.W hen the concentration gradually increases to 1 10 À6 mol L À1 ,anew broad band appears at 480 nm.…”

4,5,9,10‐Pyrene Diimides: A Family of Aromatic Diimides Exhibiting High Electron Mobility and Two‐Photon Excited Emission

“…In contrast, t‐C 5 ‐PyDI has a light yellow color with a fluorescence quantum yield of 6.2 % in the solid state. The formation of pyrene excimer is an important phenomenon that can be used in designing fluorescent‐labeled probes and biosensors . Recently, Pei and Köhler have reported formation of excimer in pyrene‐based small molecules and polymers, respectively.…”