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 eta(0,linear) to their ring counterparts eta(0,ring) at isofrictional conditions is discussed as a function of the number of entanglements Z. In the unentangled regime eta(0,linear)/eta(0,ring) is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation eta(0,linear)/eta(0,ring) = 2. In the entanglement regime, the Z-dependence of ring 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 < Z < 20, eta(0,linear)/eta(0,ring) similar to Z(1.2 +/- 0.3), is weaker than the scaling prediction (eta(0,linear)/eta(0,ring) similar to Z(1.6 +/- 0.3)) and the simulations (eta(0,linear)/eta(0,ring) similar to Z(2.0 +/- 0.3)). Nevertheless, the present collection of state-of-the-art experimental data unambiguously demonstrates that rings exhibit a universal trend clearly departing from that of their linear counterparts, and hence it represents a major step toward resolving a 30-year-old problem.