FLUORESCENCE STUDIES OF ASSOCIATING POLYMERS IN WATER - DETERMINATION OF THE CHAIN-END AGGREGATION NUMBER AND A MODEL FOR THE ASSOCIATION PROCESS

Fluorescence probe experiments were carried out on aqueous solutions of methane-coupled poly(ethylene oxide) polymers containing C16H33O end groups. These HEUR polymers associate in water, giving rise to a sharp increase in zero-shear viscosity with increasing concentration above 0.2-0.5 wt % polymer and a pronounced shear thinning at modest shear rates. At very low concentrations (a few ppm), the hydrophobic end groups of these polymers come together to form micelle-like structures. We are interested in the mechanism of the polymer association and in determining the number of hydrophobic groups N-R that come together to form the micellar core. Fluorescence decay studies of pyrene excimer formation give values of N-R close to 20, independent of polymer concentration. This N-R value is a factor of 3 smaller than that found for typical nonionic micelles but larger than that inferred indirectly from different measurements on similar KEUR polymer systems. Steady-state fluorescence studies of intramolecular excimer formation in bis(1-pyrenyl)methyl ether (dipyme) solubilized in these polymers indicate that the micellar core is much more rigid than that of traditional surfactant micelles, with an estimated ''microviscosity'' an order of magnitude larger than that of sodium dodecyl sulfate micelles. A model is developed to accommodate these observations. In this model, the polymers form rosette-like micelles comprised of looped chains. At higher concentrations, larger structures are formed from aggregation of these micelles, held together by chains which bridge the micelles. The influence of dilution and of shear is to induce a bridge-to-loop transition, leading to a breakup of larger structures to smaller objects, micelles and smaller micelle aggregates.