Transient
rheological properties of poly(ethylene glycol) diacrylate
(PEGDA) hydrogels under Couette flow conditions are determined via
molecular dynamics simulations and validated by high strain rate shear
experiments. Specifically, the influence of polymer concentration
on the transient-state shear-thickening behavior is studied for 20,
25, 50, and 70 wt % PEGDA hydrogels. The transient shear response
is characterized by the introduction of dimensionless variables and
the application of a self-similar solution to the power-law fluid
model. This enables meaningful comparison of the shear-thickening
exponent and the transient-state viscosity across the disparate length
and time scales between simulations and experiments. The momentum
diffusion exponent is found to increase with a decrease in PEGDA concentration,
which indicates higher shear-thickening behavior in lower PEGDA concentration
hydrogels. Shear-thickening mechanisms in hydrogels are explained
by computing the average mesh sizes in the polymer networks and the
distributions of junction separations. Two stages of PEGDA chain deformations
are observed under shear, where the first stage is associated with
chain conformational changes and the second stage is associated with
bond length and angular deviations. Hydrogels with lower PEGDA concentration
show a faster increase in the average mesh size, which offers higher
resistance to shear in both deformation stages, promoting greater
shear-thickening behavior.
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