Dynamic Bonds Drive Broad Fluctuations of Chain Stretch in Elongated Associative Polymer Melts

Associative polymer networks (APNs) form through the self-assembly of thermoreversible associative bonds, enabling them to be reprocessed at elevated temperatures. However, these fluctuating chemical bonds also produce complex chain dynamics during flow that are not well understood. Here we apply coarse-grained molecular dynamics simulations to model the nonequilibrium dynamics of APNs during nonlinear elongational flows for varying association strength, coordination, and strain rate. We observe that the coupling between chain and network relaxation drives a strong heterogeneity in chain elongation during deformation, producing broad distributions of chain stretch at all strain rates. This produces a remarkable plateau in extensional stresses over a wide range of strain rates and strong rate-thinning of the extensional viscosity. We show that the broad fluctuations in chain stretch are caused by a new form of extensional tumbling where chains cyclically fluctuate between collapsed and highly extended states, due to dynamic fluctuations in the associative bonds. These fluctuation-mediated dynamics cannot be described by established mean-field models for APNs but may enable the design of polymer networks with novel nonlinear viscoelastic properties.