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

Abstract

The molecular motion of poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer in water was investigated during the temperature-induced sol−gel transition with the aid of pulsed field gradient NMR. The strong slowing down of the diffusive motion in the gel regime is accompanied by an increased nonexponentiality of the spin echo attenuation function. The value of the stretched exponent, describing the decay of the spin echo attenuation, decreases with increasing temperature, and this trend suggests a broader distribution of diffusion coefficients when approaching the gel state. Ordinary diffusion behavior is observed in the sol phase, whereas anomalous diffusion is detected as the gel evolves, and the mean square displacement 〈z2〉 shows a power law dependence on time (〈z2〉 ∝ tα) with α < 1. The value of α decreases as the gelation process proceeds, and α is around 0.5 in the gel zone, but a wave vector dependence of α can be traced at temperatures in the gel regime.