Viscosity of ring polymer melts.

We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts to their ring counterparts at isofrictional conditions is discussed as function of the number of entanglements . In the unentangled regime is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation . In the entanglement regime, the -dependence of rings viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available experimental data in the rather limited range 1