Transient-State Rheological Behavior of Poly(ethylene glycol) Diacrylate Hydrogels at High Shear Strain Rates
K Luo and K Upadhyay and G Subhash and DE Spearot, MACROMOLECULES, 52, 5860-5871 (2019).
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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