Designing Stress-Adaptive Dense Suspensions using Dynamic Covalent Chemistry

TL;DR

This paper explores how dynamic covalent chemistry can be used to design dense suspensions with tunable stress-adaptive rheological behaviors, enabling programmable and switchable viscosity responses under shear.

Contribution

It introduces a novel approach using dynamic covalent bonds to control suspension rheology, demonstrating the ability to switch between shear thinning and antithixotropic behaviors.

Findings

Low Keq leads to shear thinning behavior.

High Keq results in antithixotropy, viscosity increasing with shear.

Polymer graft density influences the suspension's rheological response.

Abstract

The non-Newtonian behaviors of dense suspensions are central to their use in technological and industrial applications and arise from a network of particle-particle contacts that dynamically adapts to imposed shear. Reported herein are studies aimed at explor-ing how dynamic covalent chemistry between particles and the polymeric solvent can be used to tailor such stress-adaptive contact networks leading to their unusual rheological behaviors. Specifically, a room temperature dynamic thia-Michael bond is employed to rationally tune the equilibrium constant (Keq) of the polymeric solvent to the particle interface. It is demonstrated that low Keq leads to shear thinning while high Keq produces antithixotropy, a rare phenomenon where the viscosity increases with shearing time. It is proposed that an increase in Keq increases the polymer graft density at the particle surface and that antithixotropy primar-ily arises from partial debonding of the polymeric graft/solvent from the particle surface and the formation of polymer bridges between particles. Thus, the implementation of dynamic covalent chemistry provides a new molecular handle with which to tailor the macroscopic rheology of suspensions by introducing programable time dependence. These studies open the door to energy absorbing materials that not only sense mechanical inputs and adjust their dissipation as a function of time or shear rate but can switch between these two modalities on demand.