Rheological Isotope Effects for Molecular Insight in Covalent Adaptable Networks

Understanding how small-molecule reactivity translates to bulk properties remains a fundamental challenge in polymer science. It is widely accepted that the viscoelastic behavior of covalent adaptable networks (CANs) is related to the kinetics of dynamic covalent bond exchange, but the nature of this relationship is complicated by the network environment. However, there are a lack of molecular tools to study exchange directly in polymer networks. In this work, we establish rheological isotope effects (RIEs) as a new tool for studying dynamic covalent exchange in bulk networks. By swelling dithioalkylidene-based CANs in H₂O or D₂O, we perturb the thiol/thiolate equilibrium to favor the reactive thiolate in D₂O, accelerating exchange and thus stress relaxation rates. We found that bulk networks composed of thiol-terminated, multi-arm poly(ethylene glycol) cross-linked by dithioalkylidenes exhibit a RIE between 0.31 and 0.64 for stress relaxation, depending on the cross-linker structure, cross-link density, and polymer topology. Monte Carlo simulations of network formation and evolution indicate that differences in RIE magnitude likely arise from topological defects including bridging chains. These defects reduce effective network size, decreasing the dependence of stress relaxation on bond exchange. We believe RIEs present new opportunities to study mechanisms of exchange in bulk networks, deconvolute modes of stress relaxation in complex CANs, and provide insight into the impact of defects on viscoelasticity.