Self-Diffusion Coefficients of Associating Polymers from Pulsed-Gradient Spin-Echo Nuclear Magnetic Resonance Spectroscopy

The self-diffusion coefficients of hydrophobic ethoxylated urethane associating polymers of different molecular weights but a constant length of the hydrophobic end cap were measured in aqueous solution by a pulsed-gradient spin-echo NMR technique. With increasing concentration, all polymers registered a decrease in their mean diffusion coefficient accompanied by an increase in the dispersion of the diffusion coefficient about the mean. Both effects correlated with the pronounced concentration-dependent viscosity increase characteristic of such polymers. The presence of hydrophobic end caps decreased the diffusion coefficient by more than an order of magnitude relative to that of control polymers of identical size but lacking such modifications. The results are consistent with the predictions of the transient micellar network model of network formation by such polymers and yield an estimate of the size (20-nm radius) and aggregation number (one micelle contains 20 hydrophobic chains) for the associated clusters.