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

Fluorescence probe experiments were carried out on aqueous solutions o f methane-coupled poly(ethylene oxide) polymers containing C16H33O end groups. These HEUR polymers associate in water, giving rise to a shar p 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 gro ups of these polymers come together to form micelle-like structures. W e are interested in the mechanism of the polymer association and in de termining the number of hydrophobic groups N-R that come together to f orm the micellar core. Fluorescence decay studies of pyrene excimer fo rmation give values of N-R close to 20, independent of polymer concent ration. This N-R value is a factor of 3 smaller than that found for ty pical nonionic micelles but larger than that inferred indirectly from different measurements on similar KEUR polymer systems. Steady-state f luorescence studies of intramolecular excimer formation in bis(1-pyren yl)methyl ether (dipyme) solubilized in these polymers indicate that t he micellar core is much more rigid than that of traditional surfactan t micelles, with an estimated ''microviscosity'' an order of magnitude larger than that of sodium dodecyl sulfate micelles. A model is devel oped to accommodate these observations. In this model, the polymers fo rm rosette-like micelles comprised of looped chains. At higher concent rations, larger structures are formed from aggregation of these micell es, held together by chains which bridge the micelles. The influence o f dilution and of shear is to induce a bridge-to-loop transition, lead ing to a breakup of larger structures to smaller objects, micelles and smaller micelle aggregates.