Comment on the concentration and temperature dependences of the translational diffusion coefficient of flexible-coil macromolecules in solution: Poly(2-vinyl pyridine) in tetrahydrofuran

Concentration-dependence coefficients k_D^? of the mutual diffusion coefficient of poly(2-vinyl pyridine) in tetrahydrofuran in the temperature range 15–55°C are compared with predictions of recent theories of macromolecular diffusion. In this temperature range a change in local conformation of the polymer occurs. The accompanying changes in chain solvation and coil hydrodynamics result in a sizable decrease in the hydrodynamic interaction parameter X defined by Akcasu. The formulation of theory appropriate for comparison with frame-indifferent experimental diffusivities is discussed. We find that current theories predict qualitatively, but not quantitatively, the change in temperature dependence of k_D^? that occurs at the conformational transition. The discrepancies closely parallel those reported in recent independent comparisons of theoretical k_D^? versus experimental data for flexible-coil molecules. No theory can adequately predict k_D^? over a wide range of X.