Threading-Unthreading Transition of Linear-Ring Polymer Blends in Extensional Flow

Adding small amounts of entangled ring polymers to entangled linear matrices yields unusual rheological properties. For example, it has been established that the zero-shear-rate viscosity of the blends is increased compared to their linear counterparts, due to linear-ring threading. In this work, we show that the linear-ring blends exhibit a stress overshoot in uniaxial extensional flow, and this overshoot is driven by a transient threading-unthreading transition of rings embedded within the linear entanglement network.

In the first part of this work, we focus on a symmetric linear-ring blend of polystyrene (PS), comprising constituents with the same molecular weight of 185 kg/mol. The mass fraction of ring PS is 30%. In extensional rheology measurements, the stress overshoot is observed for a range of stretch rates, in contrast to the two pure constituents which exhibit monotonic stress growth. Molecular dynamics simulations reveal that the stress overshoot is governed by the conformational change of ring molecules. Linear molecules also contribute to the overshoot but with a much weaker influence, which is also confirmed in small angle neutron scattering measurements.

In the second part, we investigate other symmetric and asymmetric PS blends with different ring fractions. The qualitative features and notably the stress overshoot are confirmed, however quantitative differences pointing to the role of molecular weight in ring threading are observed. This investigation provides a guidance on how the rheological properties of entangled linear polymers are influenced by the fraction and molecular weight of ring additives.