Dilute Aqueous Poly(ethylene oxide) Solutions:  Clusters and Single Molecules in Thermodynamic Equilibrium

Dilute aqueous solutions of broad and narrow distribution poly(ethylene oxide) fractions were studied using dynamic (DLS) and static (SLS) light scattering, as well as gel permeation chromatography (GPC). It was found that above a critical self-association concentration, which depends on the molecular weight of the PEO, polymer clusters and free polymer coils coexist in a thermodynamic equilibrium. The polymer clusters can be removed by filtration and begin to reform spontaneously in solution within 30 min after filtration. A steady-state amount of PEO in clusters is attained within about 24 h. At steady-state the narrow-distribution and broad-distribution PEO samples have 7−10% wt/wt and 12−18% wt/wt of the polymer present as clusters in solution, respectively, at PEO concentrations of about 250 mg/L. It was found that the PEO clusters were composed of a few hundred polymer chains per cluster. From DLS the polymer cluster diameters were found to be independent of molecular weight and to decrease, with time, from 0.90 to 0.45 μm. In dynamic light scattering studies of the PEO solutions containing clusters, the light scattered from the clusters is mixed with light scattered from freely dissolved molecules. This mixing of light with different Doppler-shifted frequencies leads to quasi-heterodyning.