DYNAMIC CROSSOVER EFFECTS OBSERVED IN SOLUTIONS OF A HYDROPHOBICALLY ASSOCIATING WATER-SOLUBLE POLYMER

The dynamics of aqueous solutions of a hydrophobically associating pol ymer is studied by means of pulsed field gradient NMR and dynamic ligh t scattering (DLS). The results from the NMR measurements at higher co ncentrations reveal an anomalous diffusion behavior. However, when the observation time is increased the classical Fickian diffusion is reco vered. These features are discussed in terms of effects of fractal str ucture and in the light of a coupling model. The concentration depende nce of the self-diffusion coefficient is well described by a stretched exponential function. These results constitute the basis for the eval uation of the concentration-dependent coupling parameter. The time cor relation function data obtained from the DLS experiments are analyzed, at all concentrations, with the aid of a model, where the initial dec ay is described by a single exponential followed by a nonexponential r elaxation function, which is characterized by a fractional exponential of the Kohlrausch-Williams-Watts type. In the initial stage, an uncou pled relaxation time tau(f) is recognized, which decreases slowly with concentration. The wave vector (q) dependence of tau(f) indicates a c rossover from diffusive to a Zimm-like behavior as the concentration i ncreases. At longer times, the analysis yields a mean relaxation time tau(s), associated with the release of coupled clusters. The width of the distribution of relaxation times is given by a parameter beta. Thi s quantity, which is independent of q, decreases from 0.97 at low conc entrations to about 0.4 at high concentrations. The parameter tau(s) i ncreases dramatically (several decades) in the studied concentration r ange. The q dependence of tau(s) is considerably stronger, at higher c oncentrations, than that of tau(f). All these features can be rational ized in the framework of the coupling model.