Response of a Polyelectrolyte to Shear

The distribution of counterions around a model polyelectrolyte, sodium polystyrene sulfonate (NaPSS), under shear is investigated at single molecular level. A pH-responsive fluorophore is attached to the PSS- chain end, as an indicator of the local concentration of hydronium ions, serving partially as the counterions. Shear to the aqueous solution of NaPSS is generated either by a flow inside a small channel (low shear rate) or by a rheometer (high shear rate), and the fluorescence emission spectra are monitored as a function of the shear rate. The results have shown that the local concentration of hydronium ions decreases with the increased of shear rate, showing that the counterions of NaPSS depart from the charged main chain, induced by shear. It is demonstrated that the effective charge density of PSS- chain is increased when shear is applied to the solution, creating a higher recovery force back to the original equilibrium state.