  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.
