What Happens When Threading is Suppressed in Blends of Ring and Linear Polymers?

Self-diffusivity of a large tracer ring polymer, [Formula: see text] , immersed in a matrix of linear polymers with [Formula: see text] monomers each shows unusual length dependence. [Formula: see text] initially increases, and then decreases with increasing [Formula: see text]. To understand the relationship between the nonmonotonic variation in [Formula: see text] and threading by matrix chains, we perform equilibrium Monte Carlo simulations of ring-linear blends in which the uncrossability of ring and linear polymer contours is switched on (non-crossing), or artificially turned off (crossing). The [Formula: see text] obtained from the crossing simulations, provides an upper bound for the [Formula: see text] obtained for the regular, non-crossing simulations. The center-of-mass mean-squared displacement ([Formula: see text]) curves for the crossing simulations are consistent with the Rouse model; we find [Formula: see text]. Analysis of the polymer structure indicates that the smaller matrix chains are able to infiltrate the space occupied by the ring probe more effectively, which is dynamically manifested as a larger frictional drag per ring monomer.