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

New measurements of chain self-diffusion (D) in highly entangled hydrogenat ed polybutadiene solutions and melts indicate that D similar to M-2.3+/-0.1 , where M is the molecular weight. Although these results contradict the wi despread belief that in melts D similar to M-2.0, we demonstrate that in fa ct the literature data for seven different polymers an consistent with D si milar to M-2.28+/-0.05. This result largely removes a recalcitrant discrepa ncy between the M exponents for D and the viscosity. Important implications for theoretical approaches are also noted.