Counterions Redistribution of a Polyelectrolyte Induced by Shear Flow

Charged macromolecules under flow are ubiquitous, and the important physics inside is not yet well understood. In this study, single-molecule fluorescence spectroscopy methods are adopted to investigate the counterion distribution and effective charge state of a model polyelectrolyte, sodium polystyrenesulfonate (NaPSS), under shear flow. Taking advantage of the stability of the flow device, the investigation has accessed the response of the charged chain to shear of very low shear rate as a continuation of the previous study at a high shear rate using a rheometer. By examining the emission spectra of a pH-responsive probe attached to the chain end of NaPSS using a home-built confocal microspectrometer, the starting point of the shear-induced departure of the counterions has been exposed. The results have shown a strong dependence on the molecular weight and salt concentration of the shear effect and have clearly demonstrated that the shear-induced departure of the counterion brings in an increase of effective charge density of the main chain, creating an enhanced electrostatic interchain repulsion and helping the sheared solution to recover its equilibrium structure.