Influence of chain conformation on translational diffusion of poly(2-vinyl pyridine) in dilute solution

A comparative photon correlation spectroscopy study is reported of the concentration-dependent translational diffusion coefficient D_t of atactic poly(2-vinyl pyridine) in tetrahydrofuran and in aqueous solution, in the form of the poly(2-vinyl pyridinium)chloride salt (α = 0.4). The limiting Stokes radius of the polymer is observed to be identical within experimental error in tetrahydrofuran (THF) at temperatures below 30°C and in aqueous solutions at high ionic strength. This numerical value is comparable to expectation for an unperturbed atactic vinyl polymer chain and indicates a compact, possibly micellar, conformation. Raising the temperature in THF above 30°C and decreasing the ionic strength or increasing the ionization above α = 0.4 in aqueous solvents causes a discontinuous cooperative transition to a more expanded structure. The effect of the conformational change is also manifest in the concentration dependence of D_t. Using experimental estimates of the second osmotic virial coefficients obtained by total scattered intensity measurements, the experimental data for dD_t/dc are compared with prediction based on hydrodynamic theory. Substantial disagreement is found between theory and experiment, especially in the aqueous system. In 0.01M NaCl, decrease in polyion concentration induces the transition from the compact form to a highly extended structure. Angle-dependent quasielastic light scattering data from the expanded state provides information about the intramolecular chain dynamics.