Melt Rheology of Short-Chain Branched Ring Polymers in Shear Flow

We examined the general rheological characteristics of short-chain branched (SCB) ring polyethylene (PE) melts using atomistic nonequilibrium molecular dynamics (NEMD) simulations under shear in a wide range of flow strengths. To analyze the synergetic influence of the closed-loop ring geometry and short-chain branching on the structural and rheological behavior of polymeric systems, the results for the SCB ring PE melt were directly compared with those of the corresponding linear, ring, and SCB linear analogues. The ring geometry generally induces a more compact chain structure and a lower degree of chain orientation in the flow direction under shear. By means of the intrinsically fast random dynamics of short branches, short-chain branching also leads to a more compact structure and a lower degree of structural deformation against the applied flow. From the observation of their respective roles in the polymer structure and dynamics, the combination of the ring topology and short-chain branching resulted in distinctive synergistic characteristics. Specifically, relative to the rather small difference in shear-thinning behavior between the pure linear and ring PE melt systems, a noticeably large difference was observed between the corresponding SCB linear and ring PE systems, with this discrepancy becoming even stronger at higher flow strengths. The significant enhancement in shear viscosity for the SCB ring system is attributed to the existence of highly extended SCB ring chains in association with their tight topological linking (inter-ring threading) via enhanced interchain branch-branch and branch-backbone interactions.