Viscosity dependence of polystyrene local dynamics in dilute solution

Variable-temperature H-2 NMR T-1 measurements have been performed on backbone-deuterated atactic polystyrene at two Larmor frequencies in four solvents: toluene, cis-decalin, dibutyl phthalate, and dioctyl phthalate. The time integral [sigma] of the C-D vector orientation correlation function is extracted from the T-1 data without assuming a specific model for C-D vector reorientation. The hydrodynamic Kramers' theory in the high-friction limit cannot describe the viscosity and temperature dependence of [sigma]. In contrast, [sigma] is found to have a power law dependence on solvent viscosity with an exponent of 0.76 +/- 0.05, whether the viscosity is varied by changing solvent or temperature. The internal energy barrier for polystyrene C-D vector orientation is determined to be 14 +/- 3 kJ/mol using the power law viscosity dependence. The inapplicability of Kramers' theory is attributed to the lack of a clear separation between the time scales of polymer and solvent motions. As expected on the basis of this explanation, the viscosity exponents for polystyrene and five other polymers are found to correlate with the molecular weight of the side groups.