Fluorescence Studies of Associating Polymers in Water: Determination of the Chain end Aggregation Number and a Model for the Association Process

Abstract

Fluorescence probe experiments were carried out on aqueous solutions of urethane-coupled polyethylene oxide) polymers containing CieHasO 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 Nr that come together to form the micellar core. Fluorescence decay studies of pyrene excimer formation give values of Nr close to 20, independent of polymer concentration. This Nr 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 HEUR polymer systems. Steady-state fluorescence studies of intramolecular excimer formation in bis(l-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 objects, micelles and smaller micelle aggregates.

Shear-induced aggregation and break up of fibril clusters close to the percolation concentration

“…Structural changes of complex fluids under steady shear flow were investigated for different type of materials, such as suspensions, associating polymers and polymer solutions [1,2,3,4,5,6]. Under specific conditions, depending on the material, a shear-thickening regime is observed followed by shear-thinning behaviour [3,4,6].…”

“…Cluster-cluster collisions that occur along the streamlines of shear flow lead to the formation of larger clusters [7]. In polymer solutions, the size of these clusters depends upon the concentration [4]. Shear-induced stretching of the structures can also contribute to a shear-thickening effect [4,8,9].…”

“…In polymer solutions, the size of these clusters depends upon the concentration [4]. Shear-induced stretching of the structures can also contribute to a shear-thickening effect [4,8,9]. Increasing the shear rate increases the stress on these structures, and once a critical stress is exceeded, they break up, giving rise to shear-thinning behaviour [3,4,6].…”

“…Shear-induced stretching of the structures can also contribute to a shear-thickening effect [4,8,9]. Increasing the shear rate increases the stress on these structures, and once a critical stress is exceeded, they break up, giving rise to shear-thinning behaviour [3,4,6]. Abstract To probe the behaviour of fibrillar assemblies of ovalbumin under oscillatory shear, close to the percolation concentration, c p (7.5%), rheo-optical measurements and Fourier transform rheology were performed.…”

“…Under specific conditions, depending on the material, a shear-thickening regime is observed followed by shear-thinning behaviour [3,4,6]. Shear-thickening is often the result of shear-induced formation of intermolecular structures (clusters), leading to an increase in the effective volume fraction [1,2,3,5,7].…”