ANOMALOUS DIFFUSION IN AN AQUEOUS SYSTEM OF A POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)-POLY(ETHYLENE OXIDE) TRIBLOCK COPOLYMER DURING GELATION STUDIED BY PULSED-FIELD GRADIENT NMR

The molecular motion of poly(ethylene oxide)-poly(propylene oxide)-pol y(ethylene oxide) block copolymer in water was investigated during the temperature-induced sol-gel transition with the aid of pulsed field g radient NMR. The strong slowing down of the diffusive motion in the ge l regime is accompanied by an increased nonexponentiality of the spin echo attenuation function. The value of the stretched exponent, descri bing the decay of the spin echo attenuation, decreases with increasing temperature, and this trend suggests a broader distribution of diffus ion coefficients when approaching the gel state. Ordinary diffusion be havior is observed in the sol phase, whereas anomalous diffusion is de tected as the gel evolves, and the mean square displacement [z(2)] sho ws a power law dependence on time ([z(2)] proportional to t(alpha)) wi th alpha < 1. The value of a, decreases as the gelation process procee ds, and alpha is around 0.5 in the gel zone, but a wave vector depende nce of alpha can be traced at temperatures in the gel regime.