Reconciliation of the molecular weight dependence of diffusion and viscosity in entangled polymers

New measurements of chain self-diffusion (D) in highly entangled hydrogenated polybutadiene solutions and melts indicate that D ∼ M^{-2.3 ± 0.1}, where M is the molecular weight. Although these results contradict the widespread belief that in melts D ∼ M^-2.0, we demonstrate that in fact the literature data for seven different polymers are consistent with D ∼ M^-2.28±0.05. This result largely removes a recalcitrant discrepancy between the M exponents for D and the viscosity. Important implications for theoretical approaches are also noted. 1999