Folding of sodium polystyrene sulfonate under shear in semi-dilute solutions

We apply explicit solvent, coarse-grained molecular dynamics simulations to the shear flow of sodium polystyrene sulfonate chains in dilute and semi-dilute, salt-free solutions. Simulations show that polyelectrolyte chains are first extended and aligned with the shear axis, and then a folding process occurs where a sharp bend folds the chain and the chain ends repeatedly switch front and rear positions. Folding only occurs for sufficiently long chains, i.e., not for 32 bead chains, but for 64 bead chains. This minimum chain length to observe chain folding is qualitatively consistent with extension occurring above a Weissenberg number of 1, assuming Zimm dynamics. The extension was shown by the appearance of a peak in the probability distributions of end-to-end distances of the polyelectrolyte chain P(Ree) at high Ree, and the folding in half appeared as a shoulder at low Ree. Folding times in the flow direction exhibited a power law scaling exponent of -2/3 in quantitative agreement with previously reported exponents. Folding times were also independent of polyelectrolyte concentration in the semi-dilute regime. Our simulations did not show evidence of counterion release under shear, in contrast to recent reports.