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

Abstract

The level of interpolymeric interactions experienced by a pyrene-labeled poly(octadecyl methacrylate) labeled with 6.7 mol% of 1-pyrenebutanol (Py(6.7)-PC18MA) was characterized by fluorescence in the presence of an amorphous (EP(AM)) and a semicrystalline (EP(SM)) ethylene propylene copolymer. These polymers belong to different families of engine oil additives. The fluorescence spectra of Py(6.7)-PC18MA solutions in engine oil were acquired and analyzed to obtain finter, the molar fraction of pyrene labels that formed excimer intermolecularly upon encounter between an excited and a ground-state pyrene. The fraction finter was measured as a function of solution temperature between 30 and +25 oC. The finter-vs-T profiles obtained for Py(6.7)-PC18MA showed a sharp transition between +10 and +15 oC indicating an increase in intermolecular interactions at temperatures lower than the transition. This behavior was attributed to the crystallization of the PC18MA octadecyl side chains. Addition of EP(AM) resulted in an increase in interpolymeric interactions between Py(6.7)-PC18MA macromolecules at all solution temperatures studied. Addition of EP(SM) increased finter for Py(6.7)-PC18MA at high temperatures, but at lower temperatures where EP(SM) formed microcrystals, finter for Py(6.7)-PC18MA returned to its original value when no EP(SM) was present in solution. These trends indicated that PC18MA interacted with the EP copolymers as long as the copolymers remained solvated. Crystallization of EP(SM) released the polyolefin from PC18MA and PC18MA resumed its original level of intermolecular interactions as if no polyolefin was present in the solution. These experiments extended our earlier finding established with pyrene-labeled EP copolymers that finter is a powerful analytical means to quantify the level of intermacromolecular interactions taking place during the crystallization of polymers and monitor their interactions with other macromolecules in solution.