Diffusion of low-molecular weight poly(ethylene oxide) in water

Abstract

The diffusion coefficients in water of ethylene glycol and several poly(ethylene oxides) of low molecular weight have been measured at 15, 25 and 35°C. The activation energies of diffusion are in the range 19–21 kJ mol^–1 in agreement with published results on more rigid molecules. The Stokes-Einstein diffusion radii increase with temperature probably as a result of conformational changes in the polyoxyethylene chain. After allowing for hydrogen bonding of water molecules at the terminal hydroxyl groups, the diffusion coefficients are found to be inversely proportional to the root mean square distance (s²)^½ of a chain element from the molecular centre of mass. This relationship holds over the whole molecular weight range studied, and no evidence for free-draining is found. Comparison with the theory of Kirkwood and Riseman shows that the actual value of (s²)^½ is 1.95 times its free rotational value. Such a high value is consistent with the association of water molecules with the polymer in its diffusional movement.