Interfacial Polymer Rheology of Entangled Short-Chain Branched Ring Melts in Shear Flow

We present a detailed analysis of the rheological behavior of entangled short-chain-branched (SCB) ring polymers at interfaces via direct comparison with the corresponding pure (unbranched) ring polymers using atomistic nonequilibrium molecular dynamics simulations of confined polyethylene melt systems under shear flow. To elucidate the general structural and dynamical characteristics of interfacial polymer chains, we analyze various physical properties of the chains in the bulk and interfacial regions separately within the confined systems, such as the chain radius of gyration and its distribution, the average streaming velocity profile, and the degree of interfacial slip, with respect to the applied flow strength. The pure ring polymer melt has a highly extended and aligned chain structure along the flow (x-)direction at the interface, even under weak flow fields, indicative of the strong wall effects via the attractive polymer–wall interactions. In contrast, the interfacial SCB ring chains generally form a compact structure like that of the corresponding bulk chains in the weak flow regime, representing a significant role of the short branches to effectively diminish the wall effect. In conjunction with these structural characteristics, the entangled SCB ring polymer melt displays a markedly smaller degree of interfacial slip than the corresponding pure ring analogue in the weak-to-intermediate flow regimes. Furthermore, while both the pure ring and the SCB ring polymer melt systems reveal similar fundamental molecular mechanisms at the interface with respect to the flow strength (i.e., z-to-x rotation, loop wagging, loop migration, and loop tumbling mechanisms), the SCB ring polymer melt displays relatively weaker loop migration and loop wagging dynamics with highly curvy backbone structures in the intermediate flow regime. In the strong flow regime, both the pure ring and SCB ring systems exhibit the loop tumbling mechanism together with intensive collisions between the interfacial chains and the wall. However, the interfacial SCB ring chains execute substantial loop migration dynamics even at high flow fields, which facilitates interfacial slip.