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 η_0,linear to their ring counterparts η_0,ring at isofrictional conditions is discussed as a function of the number of entanglements Z. In the unentangled regime η_0,linear/η _0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η_0,linear/ η_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, η_0,linear/η_0,ring ∼ Z ^1.2±0.3, is weaker than the scaling prediction (η_0,linear/η_0,ring ∼ Z ^1.6±0.3) and the simulations (η_0,linear/ η_0,ring ∼ 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.