Understanding how small-molecule insights scale to bulk properties is a fundamental challenge in materials chemistry. It is widely accepted that the viscoelastic behavior of covalent adaptable networks (CANs) is dependent on the mechanism of bond exchange between dynamic cross-links. However, the development of mechanistic methods to study exchange in-situ has proven challenging. In this work, I establish solvent isotope effects (SIEs), in which an isotopically labeled solvent perturbs the equilibrium of reactive species, as a new tool for investigating molecular interactions in CANs. My efforts focus on elucidating how efficiently this small-molecule effect translates to bulk materials through a combination of small-molecule kinetics and rheological characterization.
Dithioalkylidenes, an emerging class of dynamic cross-links, are proposed to undergo reversible conjugate addition-elimination exchange with thiolates in water. Inspired by reports on enzyme-catalyzed reactions containing cystine residues, I hypothesized that swelling dithioalkylidene-based networks in D₂O instead of H₂O would shift the thiol/thiolate equilibrium to favor the reactive thiolate, promoting faster exchange and enhanced stress relaxation rates in bulk materials. Encouraged by a small-molecule model consistent with the expected SIE, I fabricated dithioalkylidene-based networks to examine the impact of the SIE on stress relaxation in bulk hydrogels. Gratifyingly, the SIE enabled deconvolution of the two emergent stress relaxation modes: a mode below 1 second attributed to dangling ends that slowed in D₂O due to the greater viscosity of the solvent, and a mode at varying time scales depending on the cross-linker structure attributed to bond exchange that was accelerated in D₂O due to the higher population of thiolate. These results demonstrate that SIEs present in small-molecule studies can be translated to bulk materials and used to resolve competing stress relaxation pathways.

Steven Chapman

Northwestern University

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