Self-diffusion of poly(propylene imine) dendrimers in methanol

The self-diffusion coefficients have been determined for five generations of poly(propylene imine) dendrimers in methanol at three different temperatures-5, 25, and 45 degreesC-over the whole concentration range. Pulsed field gradient spin echo NMR has been used. The Stokes-Einstein hard sphere radii have been calculated in the zero concentration limit. They were equal, within error,to the radii found from the viscosity. The high-generation dendrimers have three concentration regimes: a dilute, a semidilute, and a concentrated regime. For the lower generations, only a dilute and a semidilute regime can be found. In the dilute regime, the self-diffusion coefficient decreases as a function of the concentration. In the semidilute regime, this decrease continues. In part of the semidilute and in the concentrated regime diffusion was very slow, and we were not able to measure the long time self-diffusion coefficient. As the transition from semidilute to concentrated solutions corresponds to a decrease of the radius of the dendrimer, dendrimers in concentrated solutions can be considered as collapsed though still separate molecules. The behavior in the semidilute and the concentrated regimes is very different from polymer diffusion.