Emergence of Effective Entanglements in Unentangled Linear Polymers by Adding Rings

Abstract Topological constraints (i.e., chain noncrossing) play a key role in determining the physical properties of polymer materials in industry or the biological functions of biomacromolecules in life, with the entanglement effect in a long-chain linear polymer melt or concentrated solution being a famous demonstration. In this work, we carried out extensive molecular dynamics simulations of the Kremer–Grest model and computationally investigated dynamic features of the intrinsically unentangled (short) linear polymers in melts when a fraction of small rings is added, aiming to uncover an essential role, if existing, of linear-ring threading, a special topological constraint, in polymer dynamics of those intrinsically unentangled linear polymers. Our simulations reveal that, unexpectedly and interestingly, a dynamic slowing down clearly results from the presence of ring molecules in the system of the intrinsically unentangled linear polymers. This conclusion is strongly supported by our simulation results, including mean-square displacement, diffusion constant, and stress relaxation in the linear-ring mixture systems with varying fractions of rings. Furthermore, it is essentially implicated from our simulation results that effective entanglements between those intrinsically unentangled linear polymers are formed by adding rings, which is then confirmed by topological analysis. The underlying physical origin of the effective entanglements being formed, and thus the dynamic slowing down of those short linear chains, lies in the fact that rings act as the bridging molecules, which, via linear-ring threading, topologically link short linear chains together, i.e., forming percolated structures finally.