Solvent diffusion probe volume dependence

Translational motions of solvent and other small molecules in polymer solutions are quite different from their behaviors in viscous liquids. The self-diffusion coefficient of the solvent has a transition at a polymer volume fraction 0.4. At smaller (j), Ds follows a simple exponential exp(-a) in polymer concentration, but at larger Ds(c) follows a stretched exponential with large exponent. The exponential factor a is independent of polymer molecular weight, while rj depends strongly on M, so Ds and A must be nearly independent of solution rj. Probes somewhat... 

The addition of a polymeric solute to a small-molecule solvent affects translational diffusion, viscosity, and rotational diffusion of solvent and other small molecules in solution. For polymer concentrations 4> < 0.4, the solvent selfdiffusion coefficient follows D exp(—a). The constant a is linear in the probe s molecular volume but is independent of polymer molecular weight. At larger concentrations (/) > 0.4, the simple-exponential dependence of D on concentration is replaced by a stretched-exponential concentration dependence, D and dD/dc both appearing continuous through the transition. The effects of added polymer on solvent self-diffusion and on the diffusion of small-molecule probes are clearly not the same. 

---