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 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 °C. The f
inter-vs-T profiles obtained for Py(6.7)-PC18MA showed
a sharp transition between +10 and +15 °C, 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 f
inter for Py(6.7)-PC18MA at high temperatures, but at lower temperatures where
EP(SM) formed microcrystals, f
inter 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 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.
A poly(octadecyl methacrylate) sample fluorescently labeled with 6.7 mol% of pyrene (Py(6.7)-PC18MA) was used as a mimic of a pour point depressant (PPD) to investigate how Py(6.7)-PC18MA interacts with wax found in engine oils and ethylene-propylene (EP) copolymers used as mimics of viscosity index improvers (VIIs). The fluorescence spectra of Py(6.7)-PC18MA solutions in octane were acquired in octane at low and high concentrations of Py(6.7)-PC18MA and analyzed to obtain the molar fraction (finter) of pyrene labels, that formed excimer intermolecularly, a measure of the level of intermolecular interactions between Py(6.7)-PC18MA molecules in the solution. The finter-versus-T profile obtained for Py(6.7)-PC18MA alone in octane confirmed that Py(6.7)-PC18MA formed microcrystals at solution temperatures below 0 oC. The effect induced by the addition of wax and an amorphous (EP(AM)) and semicrystalline (EP(SM)) EP copolymer on the interactions experienced by Py(6.7)-PC18MA were characterized by monitoring finter as a function of temperature and comparing the different finter-versus-T plots obtained after the addition of the different components with the finter-versus-T plot obtained for Py(6.7)-PC18MA alone. These studies demonstrated that wax and EP(AM) increased the level of intermolecular interactions between the Py(6.7)-PC18MA molecules at all temperatures in octane. EP(SM) increased the interactions between Py(6.7)-PC18MA molecules at high temperature, where it was soluble in octane, but finter reverted to its value in the absence of EP(SM) at low temperatures, where EP(SM) had crystallized. These experiments illustrate how pyrene excimer fluorescence can be applied to probe the complex interactions taking place between the different components found in engine oils.
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