Polymeric Oil Additives and their Small Molecule Analogues Studied via Pyrene Excimer Fluorescence

Abstract

Several series of pyrene-labeled small molecules (PySMs) and large macromolecules (PyLMs) were studied with pyrene excimer formation (PEF) through the analysis of their monomer and excimer fluorescence decays with either the model free analysis (MFA) or the fluorescence blob model (FBM). In these studies, the conformation and dynamics of the pyrene side chains of the PySMs were characterized with parameters such as the average rate constant (<k>) of PEF or the number (Nblob) of structural units inside a polymeric blob. These parameters were then compared to those obtained for the same PySMs after they had been incorporated into much larger macromolecules to generate PyLMs, whose specific pyrene-labeled subsections were probed by PEF. Since <k> reports on the local pyrene concentration ([Py]loc), the <k> values were compared for each PySM or PyLM in several organic solvents. Similarly, block copolymers (BCPs) were prepared where pyrene was randomly incorporated in a specific block, whose homopolymer analog had been characterized beforehand by PEF. Comparison of the PEF response of the homopolymer alone and incorporated as one block of a BCP provided information on the flexibility and dynamics of each specific block within the pyrene-labeled BCPs. The linear relationship between <k> and [Py]loc established earlier for a variety of PyLMs was studied for a set of linear diols (Py2-DOs) and branched polyols (Py-POs), which were labeled through ester linkages with two or more pyrenes, respectively. <k> increased more quickly with increasing [Py]loc for the Py2-DOs having a lower [Py]loc than for the Py-POs taking higher [Py]loc values, resulting in a clear breakpoint in the <k>-vs-[Py]loc trends when transitioning between the two classes of PySMs. The effect was observed in the more polar solvents N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and the relatively less polar solvents tetrahydrofuran (THF) and dioxane. The decreased sensitivity of <k> toward [Py]loc for the branched Py-POs was attributed to the quickly diffusing pyrene side chains, which sterically hindered access to an excited pyrene from pyrenyl labels located further away on the chain. This effect reduced the effective [Py]loc sensed by an excited pyrene and yielded lower <k> values. These results were further supported by calculating the theoretical <k>th for the Py-PO samples based on the <k> values of the Py2-DO samples. <k> determined experimentally for the branched Py-POs was indeed lower than <k>th as predicted from the lower [Py]loc sensed by excited pyrenes. A series of PySMs was synthesized by using amide linkages to attach 1-pyrenebutyric acid to diamines and polyethyleneamines to yield linear (Py2-DAs) and branched (Py-PA) molecules with two and more than two pyrenyl labels, respectively. Their <k> values, determined in the same solvents used in the Py2-DO and Py-PO study, revealed that the polar solvents DMF and DMSO yielded a linear <k>-vs-[Py]loc relationship without a breakpoint between the Py2-DAs and Py-PAs. However, a breakpoint was found in the <k>-vs-[Py]loc trends in the less polar solvents THF and dioxane when transitioning between the Py2-DAs and Py-PAs, as had also been noted earlier for the Py2-DOs and Py-POs but in all solvents. This effect was explained by amide-solvent interactions which were favoured in the more polar solvents that enhanced the double bond character of the C-N bond in the amides, slowing their pyrene side chain motion and restoring access to all ground-state pyrenes of a Py-PA sample to an excited pyrene. The preceding results were used for comparison of the PEF response of a series of succinimide terminated polyisobutylenes (PIBSIs) having polyethyleneamine ends which were labeled with pyrene (PIBSI-PA-Pys) to resemble the Py-PAs of the previous study. Labeling the polyamine subdomains of the PIBSI samples enabled their detection through PEF and their <k> vs-[Py]loc trends were compared with those of the Py-PAs. Since the PIBSI-PA-Pys were not soluble in the more well-behaved DMF and DMSO, their behavior was assessed in THF. Much lower <k> values were obtained for the PIBSI-PA-Py samples than for the Py-PAs, but a linear <k>-vs-[Py]loc trend was obtained that passed through the origin as dictated by the <k>=kdiff×[Py]loc relationship. This linear trend provided a means for characterizing the polyamine blocks of PIBSI dispersants, which are often difficult to study by standard methods. The final study focused on poly(alkyl methacrylate) (PAMA) di-block BCPs synthesized through atom transfer radical polymerization with a poly(stearyl methacrylate) (PC18MA) and poly(butyl methacrylate) (PC4MA) block with pyrene randomly incorporated in either block. The flexibility of either block was characterized with the number (Nblob) of structural units a pyrene label could sense during its fluorescence lifetime through analysis of the fluorescence decays with the FBM. In good solvents for the BCPs, the Nblob values matched those predicted theoretically while in hexane, octane, and dodecane, which are poor solvents for the PC4MA block, the process of PEF was hindered whether pyrene was in the well-solubilized PC18MA block or the poorly solubilized PC4MA blocks. This effect suggested that interactions between the two blocks slowed dynamics compared to their well solubilized behaviour in the better solvents THF, toluene, and o xylene. The crystallization behaviour of the stearyl side chains in the BCPs was probed through their fluorescence spectra above and below their crystallization temperature. The results indicated crystallization-driven micellization of the BCPs through a closed-association mechanism. In summary, this thesis has probed an interesting set of PySMs with PEF to report on their internal dynamics and interactions with different solvents, while also illustrating how PEF can be applied to amplify the signal and probe the conformation and dynamics of subdomains within larger PyLMs in solution. These methods have a particular application for the study of polymeric oil additives which see widespread use in the lubricant industry.