Topological Heterogeneity and Fracture of Bimodal Polymer Networks

Polymer networks comprising a bimodal chain-length distribution are widely used for applications that require high extensibility and toughness. While the increase in toughness arises primarily due to the extension of longer chains, topological defects such as primary loops and dangling ends formed during the curing process can significantly influence mechanical properties. In this work, coarse-grained simulations are employed to investigate topological heterogeneity in bimodal polymer networks and elucidate the role of defects on network elasticity and fracture. It is observed that cyclic defects in bimodal networks exhibit universal behavior similar to that of unimodal systems. Specifically, the primary-loop fraction versus the harmonically averaged local dimensionless concentration for various short-chain fractions collapses onto a master curve, highlighting that defect concentration is governed by a single parameter. Furthermore, the shear modulus obtained from simulations for an increasing short-chain fraction agrees reasonably well with predictions from real elastic network theory. Increasing the composition of short chains in the network leads to a decrease in both the ultimate stress and the ultimate strain of the material. This occurs due to a cumulative effect of short chains being more prone to breakage at low strains and their inherent tendency to form a large number of defects in the network. Nevertheless, the increase in the short-chain content enhances the strain hardening behavior of the material due to its finite extensibility, contributing significantly to the stress at low-to-intermediate strains. Additionally, it was observed that the Lake–Thomas framework, commonly used for monodisperse networks, can be used for bimodal networks by employing an effective bond-dissociation energy that is nearly 1.5 times the intrinsic bond-dissociation energy. Overall, this work provides insights into the topological heterogeneity and the resulting change in mechanical properties caused by disparate chain lengths in bimodal polymer networks.