Shear Response of Polyelectrolytes in Aqueous Solutions

The microscopic dynamics of individual polymer chains under shear flow are characterized by remarkable conformational changes on length and time scales that differ from their inherent motion in quiescent solutions. For polyelectrolytes in solutions, the response to shear reflects a balance of chain elasticity and electrostatic interactions, resulting in distinct microscopic conformational dynamics whose correlation with macroscopic rheological properties, remains an open question. Here, the conformational dynamics under shear of a fully sulfonated polystyrene polyelectrolyte in aqueous solutions, as their concentration is varied from just above the overlap concentration c* to the concentrated regime, is followed by large-scale fully atomistic non-equilibrium molecular dynamics simulations. The polyelectrolyte chains are observed to undergo tumbling with a characteristic time that scales with the shear rate as , observed experimentally for large bio molecules. Despite the high density of non-screen charges along the backbone that results in stretching of the chains in their quiescent state, under shear the tumbling occurs by chains folding back on themselves with minimal extension in the transverse direction. The folding occurs along chain segments where the counterions are mostly condensed.