Characterization of the Polar Core of PIBSI-Polyamine Dispersants by Pyrene Excimer Formation (PEF)

A polyisobutylene sample terminated with a succinic anhydride (PIBSA) was modified via succinimide formation with diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) either alone or as an equimolar mixture of DETA and TETA to form either mono-(m-) or bis-(b-) PIBSI-polyamines (PIBSI-PA). The residual free amines of the PIBSI-PA samples were labeled with 1-pyrenebutyric acid to enhance the signal and characterize the conformation of the small polyamine segments of the pyrene-labeled PIBSI-PA (PIBSI-PA-Py) constructs via pyrene excimer formation (PEF) between an excited and a ground-state pyrenyl label. The pyrene monomer and excimer fluorescence decays of dilute solutions of the PIBSI-PA-Py samples in tetrahydrofuran (THF) were globally fit with the model free analysis (MFA) to obtain the average rate constant (⟨k⟩) for PEF and the molar fractions of the pyrenyl species involved in PEF. ⟨k⟩ was plotted as a function of the local pyrene concentration (Pyloc) calculated by assuming that the segments connecting every pair of pyrenyl labels in the PIBSI-PA-Py samples obeyed Gaussian statistics. The linear ⟨k⟩-vs-Pyloc plot, that passed through the origin, validated the calculation of Pyloc for the PIBSI-PA-Py samples. It confirmed the assumption that the conformation of the polyethyleneamine segments is governed by the same Gaussian statistics whether it is linked or not to a large polyisobutylene block and thus retains its conformational flexibility after attachment of a large polyisobutylene chain. This flexibility enables the polyethyleneamine segment to latch effectively onto the uneven surface contour of carbonaceous particles created in oil during engine operation, conferring the well-known dispersing capability of PIBSI molecules. In summary, this study illustrates how PEF can be applied to characterize the conformation of the small subsection of a much larger macromolecule, a task that remains challenging for most current experimental techniques.