Progress in the Rheology of Cyclic Polymers

The ability to purify cyclic polymers with liquid chromatography at the critical condition and the recent advances in theoretical modeling and advanced simulations have opened the route for determining and understanding their rheological properties, and hence resolving a nearly 30-years old controversy. In the unentangled regime, the zero-shear viscosity of cyclic polymers is half that of their linear counterparts and their rheology conforms to the Rouse model. The behavior of entangled rings conforms to the lattice animal configuration and is characterized by a self-similar stress relaxation with a power law exponent of about 0.4 extending over 3 decades in time. No entanglement plateau is observed. Their viscosity remains much smaller in comparison to linear polymers. Tiny amounts of linear chain contaminants (of the order of 0.1%, below their overlap concentration) dramatically alter the rheology of rings as they entropically penetrate them, forming a transient network of linear chains bridged by rings. Despite the significant progress, many outstanding challenges lie ahead and make this topic fascinating.