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

The level of interpolymeric interactions experienced by a pyrene-labeled poly(octadecyl methacrylate) labeled with 6.7 mol % of 1-pyrenebutanol (Py(6.7)-PC(18)MA) 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)-PC(18)MA solutions in engine oil were acquired and analyzed to obtain f(inter), the molar fraction of pyrene labels that formed excimer intermolecularly upon encounter between an excited- and a ground-state pyrene. The fraction f(inter) was measured as a function of solution temperature between -30 and +25 degrees C. The f(inter)-vs-T profiles obtained for Py(6.7)-PC(18)MA showed a sharp transition between +10 and +15 degrees C, indicating an increase in intermolecular interactions at temperatures lower than the transition. This behavior was attributed to the crystallization of the PC(18)MA octadecyl side chains. Addition of EP(AM) resulted in an increase in interpolymeric interactions between Py(6.7)-PC(18)MA macromolecules at all solution temperatures studied. Addition of EP(SM) increased f(inter) for Py(6.7)-PC(18)MA at high temperatures, but at lower temperatures where EP(SM) formed microcrystals, f(inter) for Py(6.7)-PC(18)MA returned to its original value when no EP(SM) was present in solution. These trends indicated that PC(18)MA interacted with the EP copolymers as long as the copolymers remained solvated. Crystallization of EP(SM) released the polyolefin from PC(18)MA, and PC(18)MA 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 f(inter) 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.