WATER SELF-DIFFUSION IN AQUEOUS BLOCK COPOLYMER SOLUTIONS

The water self-diffusion in aqueous EO99-PO65-EO99 (EO and PO being ethylene oxide and propylene oxide, respectively) solutions and gels has been studied with NMR. It was found that the water self-diffusion decreases monotonically with increasing polymer concentration, giving D/D0 almost-equal-to 0.5 at 40 wt %. In the analysis of the data, using the cell diffusion model, it was found that the decrease in D/D0 with increasing polymer concentration can be reproduced by taking into account both the obstruction due to excluded volume and the hydration of the polymer molecules. The analysis yields that somewhere between two and five water molecules per EO group are perturbed. On increasing the temperature, the water diffusion increases, following an Arrhenius-like equation, with E(A) = 18.6 kJ/mol at 20 wt % and E(A) = 17.8 kJ/mol at 8 wt %. Furthermore, the data are consistent with a gradual dehydration of the polymer molecules with increasing temperature. The dehydration, however, is uncorrelated with the occurrence of the gel region. We thus observe no anomalous dehydration of the polymer molecules on entering the gel region.